Patent Publication Number: US-2022236241-A1

Title: Distance measuring device and method for measuring distance

Description:
TECHNICAL FIELD 
     The present invention relates to a distance measuring device and a method for measuring a distance. 
     BACKGROUND ART 
     In the field of the petrochemical industry, many catalytic reactions such as a decomposition reaction, a reforming reaction, an oxidation reaction, an ammoxidation reaction, and a reduction reaction of hydrocarbons, using a reactor with multiple tubes, are performed. A reactor used for these reactions is provided with a few thousands to several tens of thousands of reaction tubes, and a reaction tube is filled with a solid in a granular shape such as a catalyst and an inert substance suitable for each catalytic reaction (hereinafter, a granular solid such as a catalyst filled in a reaction tube may be simply referred to as a “solid” or a “filling”). For example, Patent Literature 1 discloses a method in which a layer filled with an inert substance is provided between a layer filled with a pre-stage reaction catalyst and a layer filled with a post-stage reaction catalyst, and acrylic acid is produced from propylene by a two-stage catalytic gas-phase oxidation reaction using one heat-exchange type reactor with multiple tubes. 
     To perform reaction in a preferable state using a reactor with multiple tubes as described above, it is important to keep a filling height of a filling within a predetermined control range. When a filling height of a catalyst is not constant for each of reaction tubes, reaction varies for each of the reaction tubes. This may cause a decrease in a reaction rate or a decrease in a yield as a whole, or may cause runaway of reaction in some of the reaction tubes. For example, Patent Literature 2 discloses a method for filling a catalyst in which each reaction tube is filled with a catalyst such that a difference between a filling height of each reaction tube and an average value of the filling heights is within ±20% of the average value of the filling heights. Thus, during catalyst filling operation, operation of measuring a filling height of an object filled in each reaction tube is usually performed. 
     Measurement of a filling height of a filling is performed by measuring a distance (hereinafter, also referred to as a “space length”) from an opening of a reaction tube to the filling. 
     CITATION LIST 
     Patent Literatures 
     
         
         Patent Literature 1: JP 11-130722 A 
         Patent Literature 2: JP 2003-340267 A 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     In general, a reactor with multiple tubes has a structure in which an upper end portion of a reaction tube of the reactor with multiple tubes is inserted into a hole of a perforated plate, and a connection portion between the perforated plate and the reaction tube is welded and joined. The perforated plate to which the reaction tube is joined is referred to as a tube sheet, and a surface of the tube sheet is referred to as a tube sheet surface. A surface of the perforated plate itself before welding and joining the reaction tube is smooth. However, the tube sheet surface after the reaction tube is welded and joined is uneven due to welding marks such as weld beads and spatter, and thus causing a non-smooth portion. 
     When the space length is measured from above an upper tube sheet of a reactor using, for example, a laser distance meter attached to a normal tripod as a length measuring instrument, one or more legs of the tripod may be placed on a welding mark or may come immediately above an opening of a reaction tube. For this reason, when the space length is measured for a plurality of reaction tubes, a measurement direction of the laser distance meter cannot be always parallel to an axial length direction of the reaction tubes. This causes a problem in that even when the space length can be measured for any one of the reaction tubes, measurement may not be performed for the other reaction tubes, and thus the measurement direction needs to be adjusted each time. 
     The inventor has found that when a base member such as a rail is installed on a tube sheet and a measuring member such as a tripod to which a length measuring instrument such as a laser distance meter is attached is disposed on the base member, the space length can be stably measured for any reaction tube without being affected by unevenness due to a welding mark. The inventor has also found that space lengths of a plurality of reaction tubes can be continuously measured by sliding the tripod along the rail in this method. 
     As described above, the inventor has found out that the space length can be stably and quickly measured in a non-contact manner by allowing a measuring member holding the length measuring instrument to be movable on the base member, and thus have made the present invention. 
     Thus, an object of the present invention is to provide a distance measuring device and a method for measuring a distance, being capable of simply and quickly measuring a distance from an opening of a reaction tube to a solid in a granular shape filled in the reaction tube. 
     Solution to Problem 
     A distance measuring device according to an aspect of the present invention measures, in a reactor in which a plurality of reaction tubes arranged parallel to each other is joined to a tube sheet, a distance from an opening formed at an end of a reaction tube in an axial length direction to a solid in a granular shape of a catalyst and/or an inert substance filled in the reaction tube in a non-contact manner for at least some of the plurality of reaction tubes. The distance measuring device includes a measuring member holding a length measuring instrument, and at least one base member on which the measuring member is movably disposed. When a straight line parallel to a direction in which the measuring member disposed on the base member moves is defined as a reference line, an angle formed by a straight line parallel to an axial length direction of the reaction tube and the reference line, which are on an identical plane, is constant for the plurality of reaction tubes disposed side by side along the reference line. A measurement direction of the length measuring instrument is parallel to the axial length direction of the reaction tube when the measuring member is disposed on the base member. The measuring member is disposed on the base member to be able to sequentially move from a position where the distance of one of the reaction tubes is measured to a position where the distance of another of the reaction tubes is to be measured. 
     A method for measuring a distance according to another aspect of the present invention is configured for a reactor in which a plurality of reaction tubes arranged parallel to each other is joined to a tube sheet, and the method includes the step of measuring a distance from an opening formed at an end of a reaction tube in an axial length direction to a solid in a granular shape of a catalyst and/or an inert substance filled in the reaction tube in a non-contact manner for at least some of the plurality of reaction tubes. In this method for measuring a distance, a measuring member holding a length measuring instrument is movably disposed on a base member, a straight line parallel to a direction in which the measuring member disposed on the base member moves is defined as a reference line, an angle formed by a straight line parallel to the axial length direction of the reaction tube and the reference line, which are on an identical plane, is constant for the plurality of reaction tubes disposed side by side along the reference line, a measurement direction of the length measuring instrument is parallel to the axial length direction of the reaction tube in a state where the measuring member is disposed on the base member, and the distance is sequentially measured by sequentially moving the measuring member from a position where the distance of one of the reaction tubes is measured to a position where the distance of another of the reaction tubes is to be measured. 
     Advantageous Effect of the Invention 
     According to the present invention, it is unnecessary to repeat adjustment work of allowing a measurement direction measured by the length measuring instrument to be parallel to the axial length direction of a reaction tube, and a distance from the opening of the reaction tube to the solid in a granular shape filled in the reaction tube can be simply and quickly measured in a non-contact manner for the plurality of reaction tubes by sequentially moving the measuring member along the base member. Thus, a construction period at work of filling or replacing a filling can be shortened, so that cost associated with the work can be reduced, and thus this can also contribute to improvement of a plant operation rate. 
     To allow the reactor to perform reaction in a preferable state, it is important to keep a filling height of the filling within a predetermined control range without damaging the filling in the reaction tube. According to the present invention, the distance can be quickly measured without damaging a filling such as a fragile catalyst, so that the filling height of the filling can be quickly kept within a predetermined control range, and thus the reaction can be stably performed for a long period of time. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1(A)  is a perspective view schematically illustrating a distance measuring device of a first embodiment, and  FIG. 1(B)  is a sectional view taken along line  1 B- 1 B of  FIG. 1(A) . 
         FIG. 2  is a view schematically illustrating a state where a distance from an opening formed at an end of a reaction tube in an axial length direction to a solid in a granular shape of a catalyst and/or an inert substance filled in the reaction tube is sequentially measured in a non-contact manner by a distance measuring device. 
         FIG. 3A  is a schematic view for illustrating an angle formed by a straight line parallel to an axial length direction of a reaction tube and a reference line, which are on an identical plane, when a straight line parallel to a direction in which a measuring member disposed on a base member moves is defined as the reference line. 
         FIG. 3B (A) is a perspective view illustrating a relationship between a contact surface of a base member and an axial length direction of a reaction tube, and  FIG. 3B (B) is an enlarged perspective view illustrating a portion  3 B surrounded by a two-dot chain line in  FIG. 3B (A). 
         FIGS. 3C (A),  3 C(B),  3 C(C), and  3 C(D) are sectional views schematically illustrating various modes in which a base member and a measuring member are in contact with each other. 
         FIG. 4(A)  is a sectional view illustrating a tube sheet to which a plurality of reaction tubes is joined, and  FIG. 4(B)  is a top view of  FIG. 4(A)  illustrating the tube sheet. 
         FIG. 5  is a view schematically illustrating a reactor including a plurality of reaction tubes and schematically illustrating a state where a distance from an opening of a reaction tube to a solid is measured. 
         FIG. 6  is a perspective view schematically illustrating a distance measuring device according to a second embodiment. 
         FIG. 7  is a sectional view taken along line  7 - 7  in  FIG. 6 . 
         FIG. 8  is a perspective view schematically illustrating a distance measuring device according to a third embodiment. 
         FIG. 9(A)  is a sectional view taken along line  9 A- 9 A in  FIG. 8 , and  FIG. 9(B)  is an enlarged sectional view illustrating a portion  9 B surrounded by a two-dot chain line in  FIG. 9(A) . 
         FIG. 10  is a sectional view taken along line  10 - 10  in  FIG. 8 . 
         FIG. 11(A)  is a sectional view illustrating a distance measuring device according to a fourth embodiment and corresponding to  FIG. 9(A) , and  FIG. 11(B)  is an enlarged sectional view illustrating a portion  11 B surrounded by a two-dot chain line in  FIG. 11(A) . 
         FIG. 12  is a perspective view schematically illustrating a distance measuring device according to a fifth embodiment. 
         FIG. 13  is a sectional view illustrating a distance measuring device according to a sixth embodiment and corresponding to  FIG. 1(B) . 
         FIG. 14  is a sectional view illustrating a distance measuring device according to a seventh embodiment and corresponding to  FIG. 2 . 
         FIG. 15  is a sectional view illustrating a distance measuring device according to an eighth embodiment and corresponding to  FIG. 2 . 
         FIG. 16  is a perspective view schematically illustrating a state of embodying Reference Example 1 of a method for measuring a distance. 
         FIG. 17  is a view schematically illustrating a state of embodying Reference Example 2 of a method for measuring a distance. 
         FIG. 18(A)  is a sectional view illustrating a tube sheet to which a plurality of reaction tubes is joined, and  FIG. 18(B)  is a top view of  FIG. 18(A)  illustrating the tube sheet. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The distance measuring device of the present invention, in a reactor in which a plurality of reaction tubes arranged parallel to each other is joined to a tube sheet, can measure a distance from an opening formed at an end of a reaction tube in an axial length direction to a solid in a granular shape of a catalyst and/or an inert substance filled in the reaction tube in a non-contact manner. Measurement of a distance can be performed only for some of the reaction tubes contained in the reactor or for all the reaction tubes contained in the reactor. That is, the measurement of a distance can be performed for at least some of the plurality of reaction tubes. The distance measuring device includes a measuring member holding a length measuring instrument, and at least one base member on which the measuring member is movably disposed. When a straight line parallel to a direction in which the measuring member disposed on the base member moves is defined as a reference line, an angle formed by a straight line parallel to an axial length direction of the reaction tube and the reference line, which are on an identical plane, is constant for the plurality of reaction tubes disposed side by side along the reference line. A measurement direction of the length measuring instrument is parallel to the axial length direction of the reaction tube when the measuring member is disposed on the base member. The measuring member is disposed on the base member to be able to sequentially move from a position where the distance of one of the reaction tubes is measured to a position where the distance of another of the reaction tubes is to be measured. In the present specification, the “base member” is defined as a member capable of movably disposing the measuring member, and a specific structure is not limited as long as the measuring member can be moved. The “measuring member” is defined as including the length measuring instrument and further including a member used for disposing the measuring member on the base member. In the present specification, the “straight line parallel to a direction in which the measuring member disposed on the base member moves” means a straight line parallel to a direction (vector) in which the measuring member moves on a projection surface when the tube sheet is viewed in plan from above. When two planes perpendicular to the projection surface of the tube sheet in plan view from above are defined as projection surfaces, the “straight line parallel to an axial length direction of the reaction tube” means a straight line parallel to an axial length direction (vector) of the reaction tube on any of the two projection surfaces. 
     The measuring member disposed on the base member is capable of slidably moving. The base member has, for example, a rail shape or a plate shape. In the present specification, “slide movement” means that the measuring member smoothly moves along the reference line in a state where the measuring member is disposed on the base member, and includes a mode in which the measuring member moves by rotation of a bearing, a roller, or the like provided in the measuring member or the base member. 
     Naturally, the base member is not parallel to the axial length direction of the reaction tube, and is oriented in a direction intersecting the axial length direction of the reaction tube. The distance measuring device can further include a support leg that supports the base member and allows the base member to be detachably attached above the tube sheet. The base member can be disposed above the tube sheet using the support leg. Any one of a support leg with a fixed height (length) and a support leg with an adjustable height (length) can be used. The height (length) of the support leg can define a height of the base member from the tube sheet. The base member can be attached to a reaction tube using the support leg inserted into an opening of the reaction tube. 
     As defined above, the “measuring member” includes a member used for disposing the measuring member on the base member. The member used for disposing the measuring member on the base member can be variously changed. For example, the measuring member can include three or more leg members disposed on the base member, and a plate member disposed on the base member or a slider disposed on the base member. Additionally, the measuring member is not limited to a mode in which the measuring member is manually moved, and can include a wheeled platform that is disposed on the base member and is capable of autonomously traveling. 
     In the present specification, the phrase, “dispose”, means “placing a predetermined member in contact with another member”. For example, “a measuring member disposed on the base member” means “a measuring member that can be placed so as to be in contact with the base member”, “a state where the measuring member is disposed on the base member” means “a state where the measuring member is placed in contact with the base member”, and “the base member is disposed on the tube sheet” means “the base member can be placed on the tube sheet so as to be in contact with the tube sheet”. The phrase, “contact”, means that “a predetermined member is actually in contact with another member”. 
     Hereinafter, embodiments and various modifications of the present invention will be described with reference to the accompanying drawings. The following description does not limit the technical scope or meaning of the terms described in the scope of claims. For convenience of description, dimensional ratios in the drawings are exaggerated, and may be different from actual ratios. The present specification describes a range “X to Y” that means “X or more and Y or less”. 
     &lt;Distance Measuring Device  10  (First Embodiment)&gt; 
     First, a distance measuring device  10  according to a first embodiment will be described. 
       FIG. 1(A)  is a perspective view schematically illustrating the distance measuring device  10  of the first embodiment, and  FIG. 1(B)  is a sectional view taken along line  1 B- 1 B of  FIG. 1(A) .  FIG. 2  is a view schematically illustrating a state where a distance from an opening  911  formed at an end of a reaction tube  910  in an axial length direction D 2  to a solid  920  in a granular shape of a catalyst and/or an inert substance filled in the reaction tube  910  is sequentially measured in a non-contact manner by the distance measuring device  10 .  FIG. 3A  is a schematic view for illustrating an angle α formed by a straight line L 1  parallel to the axial length direction of the reaction tube  910  and a reference line L 0 , which are on an identical plane N, when a straight line parallel to a direction in which a measuring member disposed on a base member moves is defined as the reference line L 0 .  FIG. 3B (A) is a perspective view illustrating a relationship between a contact surface  220  of a base member  200  and the axial length direction D 2  of the reaction tube  910 , and  FIG. 3B (B) is an enlarged perspective view illustrating a portion  3 B surrounded by a two-dot chain line in  FIG. 3B (A).  FIGS. 3C (A),  3 C(B),  3 C(C), and  3 C(D) are sectional views schematically illustrating various modes in which a base member and a measuring member are in contact with each other.  FIG. 4(A)  is a sectional view illustrating a tube sheet  916  to which a plurality of reaction tubes  910  is joined, and  FIG. 4(B)  is a top view illustrating the tube sheet  916 .  FIG. 5  is a view schematically illustrating a reactor  900  including the plurality of reaction tubes  910  and schematically illustrating a state where a distance from the opening  911  of the reaction tube  910  to the solid  920  is measured.  FIG. 1(A)  shows a straight line Lm indicated by a two-dot chain line that indicates a row of the reaction tubes  910  for which a measuring member  300  is sequentially moved to measure the space length.  FIG. 2  shows reference numerals P 1 , P 2 , and P 3  that schematically indicate positions at each of which the space length is measured by a length measuring instrument  100 . The axial length direction D 2  of the reaction tube  910  is a vertical direction in  FIGS. 2 and 5 . 
     With reference to  FIGS. 1(A) ,  2 , and  5 , the distance measuring device  10  is generally used for the reactor  900  in which the plurality of reaction tubes  910  arranged parallel to each other is joined to the tube sheet  916 , and used for measuring a distance from the opening  911  formed at an end of a reaction tube  910  in the axial length direction D 2  to the solid  920  in a granular shape of a catalyst and/or an inert substance filled in the reaction tube  910  in a non-contact manner for at least some of the plurality of reaction tubes  910 . The distance measuring device  10  can include the measuring member  300  holding the length measuring instrument  100  and at least one rail member  200  (corresponding to a base member) on which the measuring member  300  is movably disposed. When a straight line parallel to a direction in which the measuring member  300  disposed above the rail member  200  moves is defined as the reference line L 0  as illustrated in  FIG. 3A , the angle α formed by the straight line L 1  parallel to the axial length direction D 2  of the reaction tube  910  and the reference line L 0 , which are on the identical plane N, is constant for the plurality of reaction tubes  910  disposed side by side along the reference line L 0 . As illustrated in  FIG. 2 , a measurement direction D 1  of the length measuring instrument  100  is parallel to the axial length direction D 2  of the reaction tube  910  in a state where the measuring member  300  is disposed above the rail member  200 . Then, the measuring member  300  can be disposed above the rail member  200  to be able to sequentially move from the position P 1  (P 2 ) where a distance of one reaction tube  910  is measured to the position P 2  (P 3 ) where the distance of another reaction tube  910  is to be measured. 
     As illustrated in  FIGS. 2, 4 (A),  4 (B), and  5 , the reactor  900  includes the plurality of reaction tubes  910  arranged parallel to each other. The plurality of reaction tubes  910  in the reactor  900  is usually arranged in the form of a triangular complex array, a quadrangular series, a quadrangular complex array, or the like so that distances between the corresponding adjacent reaction tubes are as equal as possible. For example, when the plurality of reaction tubes  910  is arranged in a triangular complex array, the plurality of reaction tubes  910  is arranged at a predetermined pitch pa (see  FIG. 4(B) ). Upper ends of the plurality of reaction tubes  910  are joined to the tube sheet  916  by welding. A tube sheet surface is uneven due to welding marks such as weld beads and spatter, and thus causing a non-smooth portion. Then, the welding marks are concentrated on an outer peripheral portion of each of the reaction tubes  910 . Thus, there is no welding mark in an intermediate portion between one reaction tube  910  and another reaction tube  910  adjacent to the one reaction tube  910 , so that the tube sheet  916  can have a smooth surface  930 . An angle between the smooth surface  930  of the tube sheet  916  and the axial length direction D 2  of the reaction tube  910  is constant, and usually, the axial length direction D 2  of the reaction tube  910  is perpendicular to the smooth surface  930  of the tube sheet  916 . 
     As illustrated in  FIGS. 1(A)  and  2 , the measuring member  300  can be disposed above the contact surface  220  of the rail member  200 . The contact surface  220  of the rail member  200  can be a smooth continuous plane. In this case, the measuring member  300  is capable of slidably moving while being disposed on the contact surface  220  and maintaining a state where the measurement direction D 1  of the length measuring instrument  100  is parallel to the axial length direction D 2  of the reaction tube  910 . The distance measuring device  10  can include a plurality of rail members  200 . When the tube sheet  916  is viewed in plan from above, a plurality of (e.g., two) rail members  200  can be arranged parallel to each other. 
     As illustrated in  FIG. 1(B) , the rail member  200  can be formed of a member having a rail shape in section. A leg member  303 , which will be described later, of the measuring member  300  can have a pointed (spike-like) tip. The rail member  200  can include a guide groove  221  that guides the tip of the leg member  303  while preventing the tip from coming off. 
     The distance measuring device  10  can further include a support leg  201  that supports the rail member  200  and allow the rail member  200  to be detachably attached above the tube sheet  916 . The support leg  201  can be attached to a lower surface of the rail member  200 . The rail member  200  can be disposed above the smooth surface  930  of the tube sheet  916  using the support leg  201 . The support leg  201  is not necessarily disposed on the smooth surface  930  of the tube sheet  916  as long as the rail member  200  is disposed stably without wobbling. After being adjusted for an attachment position, the rail member  200  can be fixed to an outer peripheral wall of the reactor  900 , the reaction tube  910 , and the like using a clamp jig (not illustrated) or the like. The support leg is not limited in shape and structure as long as the rail member  200  can be supported and the rail member  200  can be detachably attached above the tube sheet  916 . For example, the support leg can be formed in an elongated rod shape, a hollow pipe shape, or a plate shape. 
     A straight line L 0  illustrated in  FIG. 3A  indicates a straight line parallel to the direction in which the measuring member disposed on the base member moves. This straight line is referred to as a “reference line L 0 ”. The straight line L 1  indicates a straight line parallel to the axial length direction of the reaction tube  910 . The reaction tubes  910  are parallel to each other. Thus, the angle α formed by the straight line L 1  and the reference line L 0 , which are on the identical plane N, is constant for the plurality of reaction tubes  910  aligned along the reference line L 0 . When the tube sheet  916  is viewed in plan from above, the reference line L 0  is parallel to a straight line Lm (a row of the reaction tubes  910  for each of which the space length is measured by sequentially moving the measuring member  300 ) indicated in  FIGS. 1(A)  and  3 B(A). Here, the plurality of reaction tubes  910  is aligned along the straight line Lm, so that the plurality of reaction tubes is also referred to as “the plurality of reaction tubes  910  aligned along the reference line L 0 ” for convenience in the present specification. 
       FIGS. 3B (A) and  3 B(B) each show an X axis and a Y axis that indicate two straight lines on the contact surface  220  of the rail member  200 , and the X axis and the Y axis are orthogonal to each other. A Z axis indicates a straight line perpendicular to the two straight lines of the X axis and the Y axis, and is perpendicular to the contact surface  220 .  FIG. 3B (B) shows an origin of XYZ coordinates that coincides with a point at which the axial length direction D 2  of the reaction tube  910  passes through the contact surface  220 . The straight line Lm indicates a row of the reaction tubes  910  for each of which the space length is measured by sequentially moving the measuring member  300 . As illustrated, when the rail member  200  is attached to the reactor  900 , the axial length direction D 2  of the reaction tube  910  may not necessarily be perpendicular to the contact surface  220  of the rail member  200 .  FIG. 3B (B) illustrates a state where the axial length direction D 2  of the reaction tube  910  is inclined to pass through a point (x0, y0, z0) of the XYZ coordinates without passing along the Z axis. As described above, the angle α formed by the straight line L 1  parallel to the axial length direction D 2  of the reaction tube  910  and the reference line L 0 , which are on the identical plane N, is constant for the plurality of reaction tubes  910  aligned along the reference line L 0 . For this reason, every axial length direction D 2  passing through the contact surface  220  is inclined in the same manner. Even when the axial length direction D 2  of the reaction tube  910  is not perpendicular to the contact surface  220  of the rail member  200 , a distance (space length) from the opening  911  of the reaction tube  910  to the solid  920  can be measured by allowing the measurement direction D 1  of the length measuring instrument  100  to be parallel to the axial length direction D 2  of the reaction tube  910  in a state where the measuring member  300  is disposed above the rail member  200 . 
     Various modes in which the base member and the measuring member are in contact with each other will be described. A shape (structure) in which the base member and the measuring member are in contact with each other may be any shape (structure) as long as the measuring member can be slid straight when the measuring member is disposed on the base member. For example, as illustrated in  FIG. 3C (A), when a contact surface  341   a  of the base member  341  is formed into a smooth continuous plane and a contact surface  342   a  of the measuring member  342  is formed into a smooth continuous plane, the base member  341  and the measuring member  342  can be brought into contact with each other in a “plane”. As illustrated in  FIG. 3C (B), when a contact surface  343   a  of the base member  343  is formed into an uneven surface protruding in a triangular shape and a contact surface  344   a  of the measuring member  344  is formed into a smooth continuous plane, the base member  343  and the measuring member  344  can be brought into contact with each other on a “line”. In  FIG. 3C (B), a direction in which the measuring member  344  slidably moves is a direction orthogonal to the paper surface. As illustrated in  FIG. 3C (C), when a contact surface  345   a  of the base member  345  is formed into a smooth continuous plane and a contact surface  346   a  of the measuring member  346  is formed into a spherical surface as in a bearing or the like, the base member  345  and the measuring member  346  can be brought into contact with each other at a “point”. As illustrated in  FIG. 3C (D), when a contact surface  347   a  of the base member  347  is formed into a curved surface and a contact surface  348   a  of the measuring member  348  is formed into a spherical surface as in a bearing or the like, the base member  347  and the measuring member  348  can be brought into contact with each other at a “point”. In  FIG. 3C (D), the contact surface  347   a  of the base member  347  can be formed into a shape like an inner surface obtained by cutting a pipe, the shape having a smooth straight line in a direction in which the measuring member  348  slidably moves (direction orthogonal to the paper surface), and a curved surface protruding downward in a direction orthogonal to the direction in which the measuring member  348  slidably moves (direction of the paper surface). As is apparent from the above description, even when the contact surface of the base member is not a smooth continuous plane, the measuring member is capable of slidably moving while being disposed on the base member. 
     A type of the length measuring instrument  100  is not limited as long as a distance is measured in a non-contact manner. As the length measuring instrument  100 , for example, a known instrument that measures a distance in a non-contact manner using a laser, a sound wave, or a microwave can be used, and a laser-type length measuring instrument is particularly preferable. 
     Although principles of the laser-type length measuring instrument are roughly classified into a triangulation type, a time-of-flight type, and a phase difference type, the principles are not particularly limited. Although a type of laser is not particularly limited, a laser having a wavelength of 635 nm is generally used. As a model of the laser-type length measuring instrument, for example, handy-type models, such as a laser distance meter (model number GLM 50C, model number GLM 150C, and the like) available from Bosch Co., Ltd., and a laser distance meter (Leica DISTO (registered trademark) D1, Leica DISTO (registered trademark) D810, Leica DISTO (registered trademark) X3, and the like) available from Leica Geosystems Co., Ltd., are sold. A laser-type length measuring instrument with an inclination measuring function for displaying an inclination of a main body is also sold, and is preferable because it can be used as an index when an irradiation direction is calibrated. Besides the handy-type models, a module type model such as a laser-type distance sensor (LDS-7A or the like) available from TAKENAKA ELECTRONICS CO., LTD., which is used by being incorporated in a PC or a device, is also sold, and any of these length measuring instruments can be applied to the present invention. 
     As illustrated in  FIGS. 1(A) and 1(B) , the measuring member  300  can include an adaptor  301  that holds the length measuring instrument  100 , and three or more (three in the drawings) leg members  303  to be disposed on the contact surface  220  of the rail member  200 . The three leg members  303  can be formed using a tripod, for example. Each of the leg members  303  has a stretchable structure and can be adjusted in length. The measuring member  300  enables all tips of the leg members  303  to be disposed on the contact surface  220 . Two of the three leg members  303  can be disposed on the contact surface  220  of one rail member  200  of the two rail members arranged parallel to each other, and the remaining one can be disposed on the contact surface  220  of the other rail member  200 . The measuring member  300  may have four or more leg members  303  to be disposed on the contact surface  220  of the rail member  200 . The four leg members  303  can be formed using a tetrapod, for example. 
     The adaptor  301  can have an adjustment mechanism (not illustrated) that adjusts an orientation of the measurement direction D 1  of the length measuring instrument  100 . The adjustment mechanism includes a ball head, a plurality of thumbscrew type fixtures, and the like, and can freely adjust the measurement direction D 1  of the length measuring instrument  100 . The measurement direction D 1  of the length measuring instrument  100  is parallel to the axial length direction D 2  of the reaction tube  910  in a state where all the leg members  303  are in contact with the contact surface  220  (state where the measuring member  300  is disposed above the contact surface  220 ). 
     Adjustment of the measurement direction D 1  of the length measuring instrument  100  is performed, for example, as follows. First, the measuring member  300  is disposed above the contact surface  220  of the rail member  200  such that all the tips of the leg members  303  of the measuring member  300  are brought into contact with the contact surface  220 . First, from a row of the reaction tubes  910  for which measuring member  300  is slidably moved to measure space length (straight line Lm in  FIG. 1(A) ), the space length of any one of the reaction tubes  910  is measured with a measure or the like. Next, a fixing screw of the ball head is loosened to adjust a laser irradiation direction and an angle of the length measuring instrument  100  so that the space length is measured. After the adjustment, the fixing screw of the ball head is tightened to fix the length measuring instrument  100 . When a distance to a tube wall is measured, a short distance is displayed, and thus the angle of the length measuring instrument  100  is readjusted. As a guideline of the adjustment, when the laser distance meter has a measurement value within a range of ±1% of the space length measured by the measure or the like, it can be determined that the adjustment is performed at an appropriate angle. 
     A distance actually measured by the length measuring instrument  100  is a distance from a tip of the length measuring instrument  100  to the solid  920 . As illustrated in  FIG. 1(A) , the tip of the length measuring instrument  100  may be located above an upper end of the reaction tube  910  (on a horizontal plane of the opening  911 ) at the time of measurement depending on a specific structure and shape of the length measuring instrument  100  and the measuring member  300 , so that an offset operation of a length measurement result may be required. In such a case, an offset dimension between the tip of the length measuring instrument  100  and the upper end of the reaction tube  910  can be measured in advance and stored in the length measuring instrument  100  as correction data. At the time of measurement, a distance actually measured by the length measuring instrument  100  can be corrected based on the offset dimension. This enables the distance from the opening  911  of the reaction tube  910  to the solid  920  to be measured. 
     As described above, the angle α formed by the straight line L 1  parallel to the axial length direction D 2  of the reaction tube  910  and the reference line L 0 , which are on the identical plane N, is constant for the plurality of reaction tubes  910  aligned along the reference line L 0  (see  FIG. 3A ). Additionally, the measurement direction D 1  of the length measuring instrument  100  is parallel to the axial length direction D 2  of the reaction tube  910  in a state where the measuring member  300  is disposed above the rail member  200 . Thus, even when the measuring member  300  is slidably moved along the contact surface  220  of the rail member  200 , the measurement direction D 1  of the length measuring instrument  100  is parallel to the axial length direction D 2  of any reaction tube  910  for which the space length is to be measured. Then, to measure the space length of a different reaction tube, the plurality of reaction tubes  910  can be measured for the space length without adjusting the angle of the length measuring instrument  100 . 
     When an attachment position of the rail member  200  is changed, angle adjustment of the length measuring instrument  100  described above needs to be performed again. However, the smooth surface  930  of the tube sheet  916  on which the rail member  200  is placed usually has a constant angle formed with the axial length direction D 2  of the reaction tube  910  in many cases, so that fine adjustment is often sufficient for adjustment work of the length measuring instrument  100  in the measurement direction D 1 . 
     The angle adjustment of the length measuring instrument  100  causes the measurement direction D 1  of the length measuring instrument  100  to orient the axial length direction D 2 . Here, the “orient” means that the measurement direction D 1  for measurement using the length measuring instrument  100 , i.e., an irradiation direction of an irradiation wave for distance measurement (e.g., a laser beam of the laser-type length measuring instrument  100  or the like) is directed downward and substantially parallel to the axial length direction D 2 . 
     In the present specification, “the measurement direction D 1  of the length measuring instrument  100  is parallel to the axial length direction D 2  of the reaction tube  910 ” is to be understood to include not only a case of being strictly parallel, but also a case where the measurement direction D 1  of the length measuring instrument  100  is slightly deviated from the axial length direction D 2  of the reaction tube  910  within a range in which a distance from the opening  911  to the solid  920  can be measured. 
     As illustrated in  FIG. 2 , first, the measuring member  300  is slidably moved to the position P 1 , and the length measuring instrument  100  can measure a distance (space length) from the opening  911  of the reaction tube  910  to the solid  920  at the position P 1 . The measuring member  300  is then slidably moved from the position P 1  to the position P 2 , and the length measuring instrument  100  can measure the distance from the opening  911  of the reaction tube  910  to the solid  920  at the position P 2 . After that, the measuring member  300  is slidably moved from the position P 2  to a position P 3 , and the length measuring instrument  100  can measure the distance from the opening  911  of the reaction tube  910  to the solid  920  at the position P 3 . As described above, the measuring member  300  can be disposed above the contact surface  220  of the rail member  200  to be capable of sequentially slidably moving from the position P 1  (P 2 ) where the distance of one reaction tube  910  is measured to the position P 2  (P 3 ) where the distance of another reaction tube  910  is to be measured. 
     The distance measuring device  10  may have a mechanism that displays whether a measured space length is within an allowable range. The pass/fail display may be in a format displayed on a screen, or can include a mechanism for making a sound as necessary. 
     The distance measuring device  10  may include a mechanism that transfers a measurement result to a personal computer, a mobile terminal, or the like by Bluetooth (registered trademark) or the like and accumulates data. 
     With reference to  FIG. 5  again, the reactor  900  including the plurality of reaction tubes  910  will be described. 
     The reaction tube  910  can be incorporated into, for example, the reactor with multiple tubes  900  installed in a chemical plant in the field of petrochemical industry. Several thousands to several tens of thousands of the reaction tubes  910  can be incorporated into one reactor  900 . The reaction tube  910  can be filled with, for example, a granular catalyst, a granular ceramic (e.g., a spherical body, a ring-shaped body, or the like of silica, alumina, or zirconia), a granular metallic Raschig ring, or the like. The reaction tube  910  has a lower end in its height direction at which a lower end opening  913  communicating with the outside of the reaction tube  910  can be formed. Depending on an intended catalytic reaction, the reaction tube  910  can be formed in a straight tube shape having an inner diameter of 10 mm to 60 mm and a height of 1000 mm to 15000 mm, for example. 
     The reaction tube  910  may be filled inside with only a solid of the same type, or, for example, with first and second layers  914  and  915  that are respectively composed of solids M 1  and M 2  different in type at different positions in the height direction of the reaction tube  910  as illustrated in  FIG. 5 . When the reaction tube  910  is filled inside with solids different in type, the first layer  914  can be composed of the solid M 1  in a granular shape. The second layer  915  can be composed of the solid M 2  in a granular shape. As the solid M 1 , for example, a catalyst in a spherical shape for a catalytic reaction molded to have an outer diameter of 1 mm to 15 mm can be used. As the solid M 2 , for example, a metallic Raschig ring molded in a ring shape (cylindrical shape) can be used. Although not illustrated, the reaction tube  910  may be provided inside with another layer below the second layer  915 , the other layer being formed of a solid in a granular shape that is identical in type to the solid M 1  or the solid M 2 , or different in type from the solid M 1  or the solid M 2 . 
     Each of the solids M 1  and M 2  is not limited to the type exemplified. Each of the solids M 1  and M 2  is not also limited in shape and size. Additionally, each of the solids M 1  and M 2  is not also limited in form (number of layers, height of each layer, and the like) of filling in the reaction tube  910 . 
     &lt;Distance Measuring Device  11  (Second Embodiment)&gt; 
       FIG. 6  is a perspective view schematically illustrating a distance measuring device  11  of a second embodiment, and  FIG. 7  is a sectional view taken along line  7 - 7  of  FIG. 6 . Members common to the embodiment described above are designated by the same reference numerals and description of the members will be partially eliminated.  FIG. 6  shows straight lines Lm 1  and Lm 2  indicated by two-dot chain lines that indicate rows of reaction tubes  910  for which measuring members  305  and  306  are respectively and sequentially moved to measure space lengths.  FIG. 6  shows reference numerals P 1 , P 2 , and P 3  that schematically indicate positions at each of which the space length is measured by a length measuring instrument  100 . 
     A base member is not limited to being positioned near a tube sheet  916  as in the first embodiment, and can be appropriately modified to be positioned above the tube sheet  916 . In this form, a space between the tube sheet  916  and the base member can be used as a movement space of measuring members  305  and  306 . 
     As illustrated in  FIGS. 6 and 7 , the distance measuring device  11  of the second embodiment can include the measuring members  305  and  306  each holding the length measuring instrument  100 , and a rail member  202  (corresponding to the base member) on which the measuring members  305  and  306  are movably disposed. The distance measuring device  11  can include the plurality of (two in the drawing) measuring members  305  and  306 , and can include a plurality of (two in the drawing) length measuring instruments  100 . In the case of the second embodiment, as illustrated in  FIG. 6 , two straight lines parallel to the direction in which the two measuring members  305  and  306  disposed on the rail member  202  move are defined as reference lines L 01  and L 02 . When the tube sheet  916  is viewed in plan from above, the reference lines L 01  and L 02  are parallel to the straight lines Lm 1  and Lm 2  (rows of the reaction tubes  910  for which the two measuring members  305  and  306  are each sequentially moved to measure the space length) indicated in  FIG. 6 . As described in  FIG. 3A , an angle formed by a straight line L 11  parallel to an axial length direction D 2  of a reaction tube  910  and the reference line L 01 , which are on the identical plane, is constant for the plurality of reaction tubes  910  aligned along the reference line L 01 . An angle formed by a straight line L 12  parallel to the axial length direction D 2  of the reaction tube  910  and the reference line L 02 , which are on the identical plane, is constant for the plurality of reaction tubes  910  aligned along the reference line L 02 . A measurement direction D 1  of the length measuring instrument  100  is parallel to the axial length direction D 2  of the reaction tube  910  in a state where the measuring members  305  and  306  are disposed above the rail member  202 . Then, the measuring members  305  and  306  can be disposed on the rail member  202  to be able to sequentially move from the position P 1  (P 2 ) where a distance of one reaction tube  910  is measured to the position P 2  (P 3 ) where the distance of another reaction tube  910  is to be measured. 
     As illustrated in  FIG. 7 , the measuring members  305  and  306  can be disposed on the contact surfaces  222  and  223  of the rail member  202 , respectively. The contact surfaces  222  and  223  of the rail member  202  can be formed into a smooth, continuous plane. In this case, the measuring members  305  and  306  are capable of slidably moving while being disposed on the contact surfaces  222  and  223 , respectively, and maintaining a state where the measurement direction D 1  of the length measuring instrument  100  is parallel to the axial length direction D 2  of the reaction tube  910 . When the rail member  202  is viewed in plan from above, the two contact surfaces  222  and  223  can be arranged parallel to each other. The one measuring member  305  can be disposed on the one contact surface  222  and the other measuring member  306  can be disposed on the other contact surface  223 . 
     As illustrated in  FIG. 7 , the rail member  202  can be formed of a member having a rail shape in section. The rail member  202  of the second embodiment can be formed of, for example, a lower casing  203  and an upper casing  204  covering the lower casing  203 . In this case, the rail member  202  can include slide grooves  205  and  206  into which sliders  307  and  308  described later provided in the measuring members  305  and  306  are fitted, respectively. Bottom surfaces of the slide grooves  205  and  206  serve as contact surfaces  222  and  223 , respectively. The measuring members  305  and  306  can hang down from the rail member  202  when bottom surfaces of the sliders  307  and  308  are in contact with the contact surfaces  222  and  223  of the slide grooves  205  and  206 , respectively. 
     The distance measuring device  11  can further include support legs  401  that support the rail member  202  and allow the rail member  202  to be detachably attached above the tube sheet  916 . The rail member  202  can be disposed above a smooth surface  930  of the tube sheet  916  using the support legs  401 . The support legs  401  are not necessarily disposed on the smooth surface  930  of the tube sheet  916  as long as the rail member  202  is disposed stably without wobbling. 
     The support legs  401  enable the rail member  202  to be positioned above the tube sheet  916 . Although structure of the support legs  401  is not particularly limited as long as movement of the measuring members  305  and  306  is not hindered, the structure can be formed of, for example, a tripod or a tetrapod (a tripod in the illustrated example). Each of the support legs  401  has a stretchable structure, and can adjust a height of the rail member  202  from the tube sheet  916 . A space between the tube sheet  916  and the rail member  202  can be used as a movement space of the measuring members  305  and  306 . 
     As described in  FIGS. 3B (A) and  3 B(B), when the rail member  202  is attached to the reactor  900 , the axial length direction D 2  of the reaction tube  910  is not necessarily perpendicular to the contact surfaces  222  and  223  of the rail member  202 . Even when the axial length direction D 2  of the reaction tube  910  is not perpendicular to the contact surfaces  222  and  223  of the rail member  202 , a distance (space length) from the opening  911  of the reaction tube  910  to a solid  920  can be measured by allowing the measurement direction D 1  of the length measuring instrument  100  to be parallel to the axial length direction D 2  of the reaction tube  910  in a state where the measuring members  305  and  306  are disposed on the rail member  202 . 
     The measuring members  305  and  306  can include respectively adaptors  310  and  311  holding the length measuring instrument  100 , and sliders  307  and  308  disposed on the contact surfaces  222  and  223 , respectively, of the rail member  202 . The sliders  307  and  308  can be provided in upper portions of the adaptors  310  and  311 , respectively. Providing the sliders  307  and  308  enables the measuring members  305  and  306  to be smoothly slidably moved along the rail member  202 . 
     The adaptors  310  and  311  each can have an adjustment mechanism that adjusts an orientation of the measurement direction D 1  of the length measuring instrument  100 . As with the adaptor  301  of the first embodiment described above, the adjustment mechanism includes a ball head, a plurality of thumbscrew type fixtures, and the like, and can freely adjust the measurement direction D 1  of the length measuring instrument  100 . The measurement direction D 1  of the length measuring instrument  100  is parallel to the axial length direction D 2  of the reaction tube  910  in a state where the sliders  307  and  308  are respectively in contact with the contact surfaces  222  and  223  (a state where the measuring members  305  and  306  are disposed above the rail member  202 ). Adjustment of the measurement direction D 1  of the length measuring instrument  100  may be performed by a method similar to that described in the first embodiment. The example of the second embodiment illustrated in  FIGS. 6 and 7  includes two length measuring instruments  100 , so that the measurement direction D 1  of each of the two length measuring instruments  100  is adjusted. 
     Although the example of the second embodiment includes the rail member  202  having the two contact surfaces  222  and  223 , the rail member  202  can be modified to a rail member having only one contact surface or a rail member having three or more contact surfaces. 
     &lt;Distance Measuring Device  12  (Third Embodiment)&gt; 
       FIG. 8  is a perspective view schematically illustrating a distance measuring device  12  of a third embodiment,  FIG. 9(A)  is a sectional view taken along line  9 A- 9 A of  FIG. 8 , and  FIG. 9(B)  is an enlarged sectional view illustrating a portion of  9 B surrounded by a two-dot chain line in  FIG. 9(A) .  FIG. 10  is a sectional view taken along line  10 - 10  in  FIG. 8 . Members common to the embodiment described above are designated by the same reference numerals and description of the members will be partially eliminated.  FIG. 8  shows straight lines Lm 1 , Lm 2 , Lm 3 , and Lm 4  indicated by two-dot chain lines that indicate rows of reaction tubes  910  for which a measuring member  315  is sequentially moved to measure space lengths using a plurality of length measuring instruments  100 .  FIG. 8  shows reference numerals P 1 , P 2 , and P 3  that schematically indicate positions at each of which the space length is measured by one length measuring instrument  100 . 
     The third embodiment is common to the second embodiment in that a base member is positioned above a tube sheet  916 , but is different from the second embodiment in specific shapes of the base member and the measuring member. 
     As illustrated in  FIGS. 8, 9 (A),  9 (B), and  10 , the distance measuring device  12  of the third embodiment can include the measuring member  315  holding the length measuring instrument  100  in a suspended state, and a rail member  207  (corresponding to the base member) on which the measuring member  315  is movably disposed. The measuring member  315  can include a plurality of (four in the drawing) length measuring instruments  100 . The plurality of length measuring instruments  100  can be arranged in accordance with pitches pa of the reaction tubes  910 . In the case of the third embodiment, as illustrated in  FIG. 8 , four straight lines parallel to the direction in which the measuring member  315  disposed on the rail member  207  moves are defined as reference lines L 01 , L 02 , L 03 , and L 04 . When the tube sheet  916  is viewed in plan from above, the reference lines L 01 , L 02 , L 03 , and L 04  are parallel to the straight lines Lm 1 , Lm 2 , Lm 3 , and Lm 4  (rows of the reaction tubes  910  for which measuring members  315  are sequentially moved to measure the space lengths using the plurality of length measuring instruments  100 ) indicated in  FIG. 8 . As described in  FIG. 3A , an angle formed by a straight line L 11  parallel to an axial length direction D 2  of a reaction tube  910  and the reference line L 01 , which are on the identical plane, is constant for the plurality of reaction tubes  910  aligned along the reference line L 01 . An angle formed by a straight line L 12  parallel to the axial length direction D 2  of the reaction tube  910  and the reference line L 02 , which are on the identical plane, is constant for the plurality of reaction tubes  910  aligned along the reference line L 02 . An angle formed by a straight line L 13  parallel to the axial length direction D 2  of the reaction tube  910  and the reference line L 03 , which are on the identical plane, is constant for the plurality of reaction tubes  910  aligned along the reference line L 03 . An angle formed by a straight line L 14  parallel to the axial length direction D 2  of the reaction tube  910  and the reference line L 04 , which are on the identical plane, is constant for the plurality of reaction tubes  910  aligned along the reference line L 04 . A measurement direction D 1  of the length measuring instrument  100  is parallel to the axial length direction D 2  of the reaction tube  910  in a state where the measuring member  315  is disposed above the rail member  207 . Then, the measuring member  315  can be disposed above the rail member  207  to be able to sequentially move from the position P 1  (P 2 ) where a distance of one reaction tube  910  is measured to the position P 2  (P 3 ) where the distance of another reaction tube  910  is to be measured. 
     The measuring member  315  is capable of slidably moving while being disposed above the rail member  207  and maintaining a state where the measurement direction D 1  of the length measuring instrument  100  is parallel to the axial length direction D 2  of the reaction tube  910 . The distance measuring device  12  can include a plurality of rail members  207 . When the tube sheet  916  is viewed in plan from above, a plurality of (e.g., two) rail members  207  can be arranged parallel to each other. 
     As illustrated in  FIGS. 9(A) and 9(B) , the rail member  207  can be formed of a member having a rail shape in section. The rail member  207  of the third embodiment can be formed of, for example, a rail of a linear guide  236 . In this case, a movable block  230  of the linear guide  236  fitted to the rail member  207  can be provided in a slider  317  that is provided in the measuring member  315  and is described later. As is well known, the movable block  230  of the linear guide  236  includes a bearing built in, and can slidably move straight along the rail member  207  by rolling motion of the bearing. The rail and the movable block in the linear guide usually form point contact between a curved surface of the rail and a spherical surface of the bearing. The measuring member  315  can hang down from the rail member  207  when the movable block  230  of the slider  317  is fitted to the rail member  207 . 
     The distance measuring device  12  can further include support legs  406  each of which supports the rail member  207  and allows the rail member  207  to be detachably attached above the tube sheet  916 . The rail member  207  can be disposed above a smooth surface  930  of the tube sheet  916  using a support leg  406 . The support leg  406  is not necessarily disposed on the smooth surface  930  of the tube sheet  916  as long as the rail member  207  is disposed stably without wobbling. 
     The support leg  406  enables the rail member  207  to be positioned above the tube sheet  916 . Structure of the support leg  406  is not particularly limited as long as movement of the measuring member  315  is not hindered. For example, an angle bar can be used as the support leg  406 . Each of the support leg  406  has a stretchable structure, and can adjust a height of the rail member  207  from the tube sheet  916 . A space between the tube sheet  916  and the rail member  207  can be used as a movement space of the measuring member  315 . 
     The support leg  406  can support the rail member  207  using a frame body  231  on which the rail member  207  is installed. The frame body  231  can be made of steel plate. The support leg  406  can directly support the rail member  207  without using the frame body  231  to allow the rail member  207  to be detachably attached above the tube sheet  916 . 
     The distance measuring device  12  can include a drive unit  232  that slidably moves the measuring member  315  along the rail member  207 . Although structure of the drive unit  232  is not limited, for example, as illustrated in  FIGS. 8, 9 (A), and  10 , the drive unit  232  can include a ball screw  233  rotatably supported by the frame body  231 , a motor  234  that rotatably drives the ball screw  233 , and an operation plate  235  through which the ball screw  233  is inserted. The motor  234  can be attached to the frame body  231 . The operation plate  235  can be attached to the slider  317  of the measuring member  315 . The ball screw  233  can fit into a threaded hole in the operation plate  235 . When the drive unit  232  has structure as described above, pressing a start button allows the motor  234  to rotationally drive the ball screw  233 , and then the operation plate  235  moves in an axial direction of the ball screw  233  because the ball screw  233  is fitted therein. This enables the measuring member  315  to move forward in a direction indicated by a white arrow in  FIG. 8 . When the ball screw  233  is rotationally driven in an opposite direction by the motor  234 , the measuring member  315  can move backward in an opposite direction. 
     In the linear guide  236 , the movable block  230  fitted to the rail member  207  slidably moves straight. In the third embodiment in which the linear guide  236  is used as the rail member  207 , an upper surface of the movable block  230  is referred to as a slide surface  225  of the rail member  207  for convenience of description. As described in  FIGS. 3B (A) and  3 B(B), when the rail member  207  is attached to a reactor  900 , the axial length direction D 2  of the reaction tube  910  is not necessarily perpendicular to the slide surface  225  of the rail member  207 . Even when the axial length direction D 2  of the reaction tube  910  is not perpendicular to the slide surface  225  of the rail member  207 , a distance (space length) from an opening  911  of the reaction tube  910  to a solid  920  can be measured by allowing the measurement direction D 1  of the length measuring instrument  100  to be parallel to the axial length direction D 2  of the reaction tube  910  in a state where the measuring member  315  is disposed above the rail member  207 . 
     The measuring member  315  can include an adaptor  316  that holds the length measuring instrument  100  in a suspended state, and the slider  317  disposed above the rail member  207 . A plurality of (e.g., four in the illustrated example) adaptors  316  can be disposed on a lower surface of one slider  317 . Providing the slider  317  enables the measuring member  315  to be smoothly slidably moved along the rail member  207 . A surface of the slider  317  holding the adaptor  316  is parallel to the slide surface  225  of the rail member  207 . 
     The adaptor  316  can have an adjustment mechanism that adjusts an orientation of the length measuring instrument  100  in the measurement direction D 1 . As with the adaptor  301  of the first embodiment described above, the adjustment mechanism includes a ball head, a plurality of thumbscrew type fixtures, and the like, and can freely adjust the measurement direction D 1  of the length measuring instrument  100 . A measurement direction D 1  of the length measuring instrument  100  is parallel to the axial length direction D 2  of the reaction tube  910  in a state where the measuring member  315  is disposed above the rail member  207 . Adjustment of the measurement direction D 1  of the length measuring instrument  100  may be performed by a method similar to that described in the first embodiment. The third embodiment includes four length measuring instruments  100 , so that the measurement direction D 1  of each of the four length measuring instruments  100  is adjusted. 
     Although an example provided with the four length measuring instruments  100  is described in the third embodiment, one to three length measuring instruments  100 , or five or more length measuring instruments  100 , can be provided. 
     &lt;Distance Measuring Device  13  (Fourth Embodiment)&gt; 
       FIG. 11(A)  is a sectional view illustrating a distance measuring device according to a fourth embodiment and corresponding to  FIG. 9(A) , and  FIG. 11(B)  is an enlarged sectional view illustrating a portion  11 B surrounded by a two-dot chain line in  FIG. 11(A) . Members common to the embodiment described above are designated by the same reference numerals and description of the members will be partially eliminated. 
     The fourth embodiment is different from the third embodiment in specific shape of a base member, and is common to the third embodiment in other points. 
     As illustrated in  FIGS. 11(A) and 11(B) , a distance measuring device  13  of the fourth embodiment can include a measuring member  242  holding a length measuring instrument  100  in a suspended state, and a rail member  240  (corresponding to the base member) on which the measuring member  242  is movably disposed. 
     As illustrated in  FIGS. 11(A) and 11(B) , the rail member  240  can be formed of a member having a rail shape in section. The rail member  240  of the fourth embodiment can have a recess  243  having a concave shape in section. In this case, a block  244  fitted in the recess  243  of the rail member  240  can be provided in a slider  246  of the measuring member  242 . The block  244  of the slider  246  is guided along the rail member  240  by sliding motion between a protrusion  245  of the block  244  and the recess  243  of the rail member  240 . A bottom surface of the recess  243  of the rail member  240  serves as a contact surface  241 . 
     The distance measuring device  13  can include a drive unit  232  that slidably moves the measuring member  242  along the rail member  240 . The drive unit  232  can be configured as in the third embodiment. 
     &lt;Distance Measuring Device  14  (Fifth Embodiment)&gt; 
       FIG. 12  is a perspective view schematically illustrating a distance measuring device  14  according to a fifth embodiment. Members common to the embodiment described above are designated by the same reference numerals and description of the members will be partially eliminated.  FIG. 12  shows a straight line Lm indicated by a two-dot chain line that indicates a row of reaction tubes  910  for which a measuring member  320  is sequentially moved to measure a space length. 
     As illustrated in  FIG. 12 , the measuring member  320  of the fifth embodiment can include an adaptor  301  that holds a length measuring instrument  100  and a plate member  321  disposed on a contact surface  220  of a rail member  200 . A pair of plate members  321  is connected to a top plate  322  and can form a substantially box shape. The adaptor  301  is attached to the top plate  322 . The measuring member  320  can be disposed by allowing a lower end surface of the plate member  321  to be in surface contact with the contact surface  220 . Bringing the plate member  321  into surface contact enables the measuring member  320  to be stably disposed above the contact surface  220 . 
     &lt;Distance Measuring Device  15  (Sixth Embodiment)&gt; 
       FIG. 13  is a sectional view illustrating a distance measuring device  15  according to a sixth embodiment and corresponding to  FIG. 1(B) . Members common to the embodiment described above are designated by the same reference numerals and description of the members will be partially eliminated. 
     As illustrated in  FIG. 13 , a measuring member  300  of the sixth embodiment can include a slider  325  disposed on a contact surface  220  of a rail member  200 . The slider  325  can be connected to a tip of a leg member  303  using a ball joint  326 . The slider  325  can be disposed on the contact surface  220  of the rail member  200  regardless of an attitude of the leg member  303 . Providing the slider  325  enables the measuring member  300  to be smoothly slidably moved along the rail member  200 . 
     &lt;Distance Measuring Device  16  (Seventh Embodiment)&gt; 
       FIG. 14  is a sectional view illustrating a distance measuring device  16  according to a seventh embodiment and corresponding to  FIG. 2 . Members common to the embodiment described above are designated by the same reference numerals and description of the members will be partially eliminated. 
     A base member is not limited to being mounted on a tube sheet  916 , and can be appropriately modified. 
     As illustrated in  FIG. 14 , the distance measuring device  16  of the seventh embodiment can include a rail member  200  (corresponding to the base member) on which a measuring member  300  is movably disposed. The distance measuring device  16  can further include a support leg  210  that supports the rail member  200  and allows the rail member  200  to be detachably attached above the tube sheet  916 . The support leg  210  can be attached to a lower surface of the rail member  200 . The support leg  210  of the seventh embodiment can have a tapered shape that is tapered toward a tip of the support leg  210  on a lower end side. The tip of the support leg  210  can enter an interior of a reaction tube  910  beyond an opening  911  of the reaction tube  910 . A base end of the support leg  210  on an upper end side can have an outer diameter dimension larger than an inner diameter dimension of the opening  911  of the reaction tube  910 . Then, the rail member  200  is attached to the reaction tube  910  using the support leg  210  inserted into the opening  911  of the reaction tube  910 . 
     According to the seventh embodiment, even when there is no portion in the tube sheet  916  where the base member can be stably disposed, the rail member  200  as the base member can be laid. In the rail member  200  laid in this manner, as described in  FIGS. 3B (A) and  3 B(B), the contact surface  220  is not necessarily perpendicular to an axial length direction D 2  of the reaction tube  910 . Even when the axial length direction D 2  of the reaction tube  910  is not perpendicular to the contact surface  220 , a distance (space length) from the opening  911  of the reaction tube  910  to a solid  920  can be measured by allowing the measurement direction D 1  of the length measuring instrument  100  to be parallel to the axial length direction D 2  of the reaction tube  910  in a state where the measuring member  300  is disposed above the contact surface  220 . 
     &lt;Distance Measuring Device  17  (Eighth Embodiment)&gt; 
       FIG. 15  is a sectional view illustrating a distance measuring device  17  according to an eighth embodiment and corresponding to  FIG. 2 . Members common to the embodiment described above are designated by the same reference numerals and description of the members will be partially eliminated.  FIG. 15  shows reference numerals P 1 , P 2 , and P 3  that schematically indicate positions at each of which the space length is measured by a length measuring instrument  100 . 
     A base member is not limited to having a rail shape, and can be appropriately modified. Additionally, a measuring member is not limited to a mode of being moved manually by a measurer, and can be appropriately modified. 
     As illustrated in  FIG. 15 , the distance measuring device  17  of the eighth embodiment can include a measuring member  330  holding a length measuring instrument  100  and a plate member  215  (corresponding to the base member) on which the measuring member  330  is movably disposed. The measuring member  330  of the eighth embodiment can include a wheeled platform  333  that is disposed on the plate member  215  and is capable of autonomously traveling. A measurement direction D 1  of the length measuring instrument  100  is parallel to an axial length direction D 2  of a reaction tube  910  in a state where the wheeled platform  333  is disposed on the plate member  215 . Then, the wheeled platform  333  can be disposed on the plate member  215  to be able to sequentially move from the position P 1  (P 2 ) where a distance of one reaction tube  910  is measured to the position P 2  (P 3 ) where the distance of another reaction tube  910  is to be measured. 
     A contact surface  227  of the plate member  215  can be a smooth continuous plane. The wheeled platform  333  is capable of slidably moving while being disposed on the plate member  215  and maintaining a state where the measurement direction D 1  of the length measuring instrument  100  is parallel to the axial length direction D 2  of the reaction tube  910 . 
     The plate member  215  can be formed of a member having a plate shape. The plate member  215  can be formed of a perforated plate in which a plurality of through-holes  217  are formed. A through-hole  217  can have substantially the same size as the opening  911  or a size slightly larger than the opening  911 . The plurality of through-holes  217  can be formed in accordance with pitches pa of reaction tubes  910 . The plate member  215  can be attached to a reactor  900  such that the through-hole  217  and the opening  911  substantially overlap each other. 
     The distance measuring device  17  can further include a support leg  201  that supports the plate member  215  and allows the plate member  215  to be detachably attached above the tube sheet  916 . The support leg  201  can be attached to a lower surface of the plate member  215 . The plate member  215  can be disposed above a smooth surface  930  of the tube sheet  916  using the support leg  201 . The support leg  201  is not necessarily disposed on the smooth surface  930  of the tube sheet  916  as long as the plate member  215  is disposed stably without wobbling. After being adjusted for an attachment position, the plate member  215  can be fixed to an outer peripheral wall of the reactor  900 , the reaction tube  910 , and the like using a clamp jig (not illustrated) or the like. 
     As described in  FIGS. 3B (A) and  3 B(B), when the plate member  215  is attached to the reactor  900 , the axial length direction D 2  of the reaction tube  910  is not necessarily perpendicular to the contact surface  227 . Even when the axial length direction D 2  of the reaction tube  910  is not perpendicular to the contact surface  227 , a distance (space length) from the opening  911  of the reaction tube  910  to a solid  920  can be measured by allowing the measurement direction D 1  of the length measuring instrument  100  to be parallel to the axial length direction D 2  of the reaction tube  910  in a state where the wheeled platform  333  is disposed on the plate member  215 . 
     The wheeled platform  333  includes an adaptor  331  that holds the length measuring instrument  100 , and is capable of autonomously traveling on the contact surface  227  of the plate member  215 . The wheeled platform  333  can include wheels that are rotationally driven by a motor or the like, a controller that controls drive of the motor and measurement operation of the length measuring instrument  100 , a memory that stores a control program and data, a battery, and the like. The wheeled platform  333  can be connected to an external personal computer, a mobile terminal, or the like by Bluetooth (registered trademark) or the like, and can transmit and receive data and a control signal to and from an external device. The wheeled platform  333  can be guided by a guide plate such as an optical reflection plate or a magnetic tape attached to the contact surface  227  of the plate member  215 . The wheeled platform  333  can include a sensor  332  such as an optical sensor or a magnetic sensor that detects the guide plate. The wheeled platform  333  can travel along a predetermined route or stop at a predetermined position. The length measuring instrument  100  can irradiate the inside of the reaction tube  910  with a laser through the through-hole  217  of the plate member  215 . The wheeled platform  333  enables the length measuring instrument  100  to measure the space length of the reaction tube  910  at a stopped position. The respective through-holes  217  of the plate member  215  can be formed corresponding to the respective reaction tubes  910 . Based on a position where the wheeled platform  333  is stopped, a reaction tube  910  present at the position can be identified with the reaction tube  910  on design data. A measurement value of the space length can be stored in association with a number of the reaction tube  910  on the design data. 
     The wheeled platform  333  is not limited to autonomously traveling along a route determined by guided traveling. The wheeled platform  333  is capable of autonomously traveling along an undetermined route by autonomous traveling. For the autonomous traveling, the wheeled platform  333  can include a sensor for measuring a traveling distance, a gyro sensor for detecting a traveling direction, a sensor for detecting the through-hole  217 , and the like. 
     The wheeled platform  333  of the eighth embodiment can also be applied to the distance measuring device  10  of the first embodiment, the distance measuring device  16  of the seventh embodiment, and the like described above. In this case, the wheeled platform  333  is capable of autonomously traveling on the contact surface  220  of the rail member  200 . 
     Distance measurement in the eighth embodiment does not require the measurer to repeat adjustment work of allowing the measurement direction D 1  for measurement using the length measuring instrument  100  to be parallel to the axial length direction D 2  of the reaction tube  910  for the reaction tube  910  for which the space length is to be measured by causing the wheeled platform  333  to autonomously travel. Thus, the distance from the opening  911  of the reaction tube  910  to the solid  920  can be simply and quickly measured in a non-contact manner by causing the wheeled platform  333  to sequentially move along the smooth contact surface  227 . Additionally, the wheeled platform  333  autonomously travels to measure the space length of the reaction tube  910 , so that data collection is further facilitated. 
     The distance measuring device of the present invention as described above is not particularly limited in means for slidably moving the measuring member, and the measuring member may be moved manually, or may be moved such that a drive device is attached separately and the drive device is activated or stopped by remote operation. 
     &lt;Method for Measuring Distance Using Distance Measuring Device  10  (First Embodiment)&gt; 
     Next, a method for measuring a distance from an opening  911  of a reaction tube  910  to a solid  920  in a non-contact manner using the distance measuring device  10  of the first embodiment described above will be described. 
     A method for measuring a distance according to the present invention includes measuring, in a reactor in which a plurality of reaction tubes arranged parallel to each other is joined to a tube sheet, a distance from an opening formed at an end of a reaction tube in an axial length direction to a solid in a granular shape of a catalyst and/or an inert substance filled in the reaction tube in a non-contact manner for at least some of the plurality of reaction tubes, wherein a measuring member holding a length measuring instrument is movably disposed on a base member, a straight line parallel to a direction in which the measuring member disposed on a base member moves is defined as a reference line, an angle formed by a straight line parallel to the axial length direction of the reaction tube and the reference line, which are on an identical plane, is constant for the plurality of reaction tubes disposed side by side along the reference line, a measurement direction of the length measuring instrument is parallel to the axial length direction of the reaction tube in a state where the measuring member is disposed on the base member, and the distance is sequentially measured by sequentially moving the measuring member from a position where the distance of one of the reaction tubes is measured to a position where the distance of another of the reaction tubes is to be measured. 
     That is, a method for measuring a distance according to the present invention includes measuring, in a reactor in which a plurality of reaction tubes arranged parallel to each other is joined to a tube sheet, a distance (space length) from an opening formed at an end of a reaction tube in an axial length direction to a solid in a granular shape of a catalyst and/or an inert substance filled in the reaction tube in a non-contact manner for at least some of the plurality of reaction tubes, wherein the distance is measured by a length measuring instrument  100  in a non-contact manner using the distance measuring device  10  of the first embodiment of the present invention. 
     In the method for measuring a distance using the distance measuring device  10  of the first embodiment, as illustrated in  FIGS. 1(A)  and  2 , first, a rail member  200  (corresponding to the base member) is disposed above a tube sheet  916  using a support leg  201 . The contact surface  220  of the rail member  200  can be a smooth continuous plane. A measuring member  300  holding the length measuring instrument  100  is disposed above the contact surface  220  such that a measurement direction D 1  of the length measuring instrument  100  is parallel to an axial length direction D 2  of the reaction tube  910 . At this time, it is preferable to perform correction based on an offset dimension as described above for the length measuring instrument  100  as necessary. 
     Next, as illustrated in  FIG. 2 , the measurer slidably moves the measuring member  300  to a position P 1 , and measures the distance from the opening  911  of the reaction tube  910  to the solid  920  using the length measuring instrument  100  at the position P 1 . After that, the measurer slidably moves the measuring member  300  from the position P 1  to a position P 2 , and measures the distance from the opening  911  of the reaction tube  910  to the solid  920  using the length measuring instrument  100  at the position P 2 . Subsequently, the measurer slidably moves the measuring member  300  from the position P 2  to a position P 3 , and measures the distance from the opening  911  of the reaction tube  910  to the solid  920  using the length measuring instrument  100  at the position P 3 . As described above, the space length is sequentially measured by sequentially slidably moving the measuring member  300  from the position P 1  (P 2 ) where the space length of one reaction tube  910  is measured to the position P 2  (P 3 ) where the space length of another reaction tube  910  is to be measured. 
     When the distance from the opening  911  of one reaction tube  910  to the solid  920  is measured, for example, only one position is measured for the position P 1 , i.e., only one point is measured for the one reaction tube  910 , and data on the measurement may be used as a measurement result. Alternatively, the measurement may be performed at multiple positions while slightly sliding the measuring member from the position P 1  within a range of the opening  911 , and an arithmetic average of data on the measurement may be acquired to obtain a measurement result. It is preferable to use a result based on data measured at multiple positions for the distance from the opening  911  of one reaction tube  910  to the solid  920  because a measurement error is reduced. 
     An angle α formed by a straight line L 1  parallel to the axial length direction D 2  of the reaction tube  910  and a reference line L 0  parallel to a direction in which the measuring member  300  disposed on the rail member  200  moves, which are on an identical plane N, is constant for the plurality of reaction tubes  910  aligned along the reference line L 0  (see  FIG. 3A ). Additionally, the measurement direction D 1  of the length measuring instrument  100  is parallel to the axial length direction D 2  of the reaction tube  910  in a state where the measuring member  300  is disposed above the rail member  200 . Thus, even when the measuring member  300  is slidably moved along the rail member  200 , the measurement direction D 1  of the length measuring instrument  100  is parallel to the axial length direction D 2  of any reaction tube  910  for which the space length is to be measured. This enables space lengths of the plurality of reaction tubes  910  to be sequentially measured by slidably moving the measuring member  300  along the rail member  200 . For a row of the reaction tubes  910  (the straight line Lm in  FIG. 1(A) ) for each of which the space length is measured by slidably moving the measuring member  300 , the measurer does not need to repeat adjustment work of allowing the measurement direction D 1  measured by the length measuring instrument  100  to be parallel to the axial length direction D 2  of the reaction tube  910 . Thus, the distance from the opening  911  of the reaction tube  910  to the solid  920  can be simply and quickly measured in a non-contact manner by sequentially moving the measuring member  300  along the rail member  200 . 
     Through shortening of measurement time of a filling height of a filling, a construction period at work of filling or replacing the filling can be shortened, so that cost associated with the work can be reduced, and thus this can also contribute to improvement of a plant operation rate. Additionally, variations in the filling height of the catalyst and the like filled in the reaction tube  910  of the reactor with multiple tubes  900  can be efficiently reduced, so that a catalytic reaction can be performed in a preferable state. 
     In the measurement method of the present invention, not only the distance measuring device  10  of the first embodiment but also the distance measuring devices  14 ,  15 ,  16 , and  17  of the fifth embodiment ( FIG. 12 ), the sixth embodiment ( FIG. 13 ), the seventh embodiment ( FIG. 14 ), and the eighth embodiment ( FIG. 15 ) can be used similarly. 
     &lt;Method for Measuring Distance Using Distance Measuring Device  11  (Second Embodiment)&gt; 
     Next, a method for measuring a distance from an opening  911  of a reaction tube  910  to a solid  920  in a non-contact manner using the distance measuring device  11  of the second embodiment described above will be described. 
     That is, a method for measuring a distance according to the present invention includes measuring, in a reactor in which a plurality of reaction tubes arranged parallel to each other is joined to a tube sheet, a distance (space length) from an opening formed at an end of a reaction tube in an axial length direction to a solid in a granular shape of a catalyst and/or an inert substance filled in the reaction tube in a non-contact manner for at least some of the plurality of reaction tubes, wherein the distance is measured by a length measuring instrument  100  in a non-contact manner using the distance measuring device  11  of the second embodiment of the present invention. 
     In the method for measuring a distance of the second embodiment, as illustrated in  FIGS. 6 and 7 , first, a rail member  202  (corresponding to a base member) is disposed above a tube sheet  916  using a support leg  401 . The contact surfaces  222  and  223  of the rail member  202  can be formed into a smooth, continuous plane. The support legs  401  enable the rail member  202  to be positioned above the tube sheet  916 . Measuring members  305  and  306  holding the length measuring instrument  100  are disposed on the rail member  202  such that sliders  307  and  308  are in contact with the contact surfaces  222  and  223 , respectively, and a measurement direction D 1  of the length measuring instrument  100  is parallel to an axial length direction D 2  of the reaction tube  910 . 
     Next, as illustrated in  FIG. 6 , the measurer slidably moves the measuring members  305  and  306  to a position P 1 , and measures the space length of the reaction tube  910  using the length measuring instrument  100  at the position P 1 . After that, the measurer slidably moves the measuring members  305  and  306  from the position P 1  to a position P 2 , and measures the space length of the reaction tube  910  using the length measuring instrument  100  at the position P 2 . Subsequently, the measurer slidably moves the measuring members  305  and  306  from the position P 2  to a position P 3 , and measures the space length of the reaction tube  910  using the length measuring instrument  100  at the position P 3 . As described above, the space length is sequentially measured by sequentially slidably moving the measuring members  305  and  306  from the position P 1  (P 2 ) where the space length of one reaction tube  910  is measured to the position P 2  (P 3 ) where the space length of another reaction tube  910  is to be measured. 
     Next, the measurer sequentially slidably moves another measuring members  305  and  306  from the position P 1  (P 2 ) where the space length of one reaction tube  910  is to be measured to the position P 2  (P 3 ) where the space length of the other reaction tube  910  is to be measured, as in the above procedure, and sequentially measures the space length. As described above, the measurement may be performed at multiple points for one reaction tube  910  while a measurement position is shifted little by little. 
     An angle formed by each of straight lines L 11  and L 12  parallel to the axial length direction D 2  of the reaction tube  910 , and each of reference lines L 01  and L 02  parallel to a direction in which the measuring members  305  and  306  disposed on the rail member  202  moves, which are on an identical plane, is constant for the plurality of reaction tubes  910  aligned along the reference lines L 01  and L 02 . Additionally, the measurement direction D 1  of the length measuring instrument  100  is parallel to the axial length direction D 2  of the reaction tube  910  in a state where the measuring members  305  and  306  are disposed above the rail member  202 . Thus, even when the measuring members  305  and  306  are slidably moved along the rail member  202 , the measurement direction D 1  of the length measuring instrument  100  is parallel to the axial length direction D 2  of any reaction tube  910  for which the space length is to be measured. This enables space lengths of the plurality of reaction tubes  910  to be sequentially measured by slidably moving the measuring members  305  and  306  along the rail member  202 . For rows of the reaction tubes  910  (straight lines Lm 1  and Lm 2  in  FIG. 6 ) for each of which the space length is measured by slidably moving the measuring members  305  and  306 , the measurer does not need to repeat adjustment work of allowing the measurement direction D 1  for measurement using the length measuring instrument  100  to be parallel to the axial length direction D 2  of the reaction tube  910 . Thus, the distance from the opening  911  of the reaction tube  910  to the solid  920  can be simply and quickly measured in a non-contact manner by sequentially moving the measuring members  305  and  306  along the rail member  202 . 
     &lt;Method for Measuring Distance Using Distance Measuring Device  12  (Third Embodiment)&gt; 
     Next, a method for measuring a distance from an opening  911  of a reaction tube  910  to a solid  920  in a non-contact manner using the distance measuring device  12  of the third embodiment described above will be described. 
     That is, a method for measuring a distance according to the present invention includes measuring, in a reactor in which a plurality of reaction tubes arranged parallel to each other is joined to a tube sheet, a distance (space length) from an opening formed at an end of a reaction tube in an axial length direction to a solid in a granular shape of a catalyst and/or an inert substance filled in the reaction tube in a non-contact manner for at least some of the plurality of reaction tubes, wherein the distance is measured by a length measuring instrument  100  in a non-contact manner using the distance measuring device  12  of the third embodiment of the present invention. 
     In the method for measuring a distance of the third embodiment, as illustrated in  FIG. 8 , first, a rail member  207  (corresponding to a base member) is disposed on a tube sheet  916  using a support leg  406 . The support leg  406  enables the rail member  207  to be positioned above the tube sheet  916 . A measuring member  315  holding the length measuring instrument  100  is disposed on the rail member  207  such that a movable block  230  of the slider  317  is fitted to the rail member  207  and a measurement direction D 1  of the length measuring instrument  100  is parallel to an axial length direction D 2  of the reaction tube  910 . 
     Next, as illustrated in  FIG. 8 , the measurer slidably moves the measuring member  315  to a position P 1 , and measures space lengths of four reaction tubes  910  using respective four length measuring instruments  100  at the position P 1 . After that, the measurer slidably moves the measuring member  315  from the position P 1  to a position P 2 , and measures the space lengths of the four reaction tubes  910  using the respective four length measuring instruments  100  at the position P 2 . Subsequently, the measurer slidably moves the measuring member  315  from the position P 2  to a position P 3 , and measures the space lengths of the four reaction tubes  910  using the respective four length measuring instruments  100  at the position P 3 . As described above, the space length is sequentially measured by sequentially slidably moving the measuring member  315  from the position P 1  (P 2 ) where the space length of one reaction tube  910  is measured to the position P 2  (P 3 ) where the distance of another reaction tube  910  is to be measured. As described above, the measurement may be performed at multiple points for one reaction tube  910  while a measurement position is shifted little by little. 
     An angle formed by each of straight lines L 11 , L 12 , L 13 , and L 14  parallel to the axial length direction D 2  of the reaction tube  910 , and each of reference lines L 01 , L 02 , L 03 , and L 04  parallel to a direction in which the measuring member  315  disposed on the rail member  207  moves, which are on an identical plane, is constant for the plurality of reaction tubes  910  aligned along the reference lines L 01 , L 02 , L 03 , and L 04 . Additionally, the measurement direction D 1  of the length measuring instrument  100  is parallel to the axial length direction D 2  of the reaction tube  910  in a state where the measuring member  315  is disposed above the rail member  207 . Thus, even when the measuring member  315  is slidably moved along the rail member  207 , the measurement direction D 1  of the length measuring instrument  100  is parallel to the axial length direction D 2  of any reaction tube  910  for which the space length is to be measured. This enables the space lengths of the plurality of reaction tubes  910  to be measured. For rows of the reaction tubes  910  (straight lines Lm 1 , Lm 2 , Lm 3 , and Lm 4  in  FIG. 8 ) for each of which the space length is measured by slidably moving the measuring member  315 , the measurer does not need to repeat adjustment work of allowing the measurement direction D 1  for measurement using the length measuring instrument  100  to be parallel to the axial length direction D 2  of the reaction tube  910 . Thus, the distance from the opening  911  of the reaction tube  910  to the solid  920  can be simply and quickly measured in a non-contact manner by sequentially moving the measuring member  315  along the rail member  207 . 
     In the measurement method of the present invention, not only the distance measuring device  12  of the third embodiment but also the distance measuring device  13  of the fourth embodiment ( FIGS. 11(A) and 11(B) ) can be used similarly. 
     Reference Example 1 of Method for Measuring Distance 
     Next, as Reference Example 1 of a method for measuring a distance, a method for measuring a distance from an opening  911  of a reaction tube  910  to a solid  920  in a non-contact manner using a tube sheet  916  having a continuous smooth surface  930  will be described. 
       FIG. 16  is a perspective view schematically illustrating a state of embodying Reference Example 1 of the method for measuring a distance. Members common to the embodiment described above are designated by the same reference numerals and description of the members will be partially eliminated. 
     As illustrated in  FIGS. 4(A) and 4(B) , a tube sheet surface is uneven due to welding marks such as weld beads and spatter, and thus causing a non-smooth portion. However, the welding marks are concentrated on an outer peripheral portion of the reaction tube  910 , so that an intermediate portion between one reaction tube  910  and another reaction tube  910  adjacent thereto may have no welding mark, and thus the tube sheet  916  may have a continuous smooth surface  930  without interruption. In such a case, an angle formed by the smooth surface  930  of the tube sheet  916  and an axial length direction D 2  of the reaction tube  910  is usually constant, so that a measuring member  300  can be slidably moved while being in contact with the smooth surface  930  on the tube sheet  916 . 
     The measuring member  300  can be formed as with the measuring member  300  of the first embodiment described above. The measuring member  300  can include an adaptor  301  that holds a length measuring instrument  100  and three or more (three in the drawing) leg members  303  disposed on the smooth surface  930  of the tube sheet  916 . The three leg members  303  can be formed using a tripod, for example. Each of the leg members  303  has a stretchable structure and can be adjusted in length. The measuring member  300  can be disposed above the tube sheet  916  with tips of the leg members  303  in contact with the smooth surface  930 . Only one adaptor  301  holding the length measuring instrument  100  may be connected to one tripod, or a plurality of adapters may be connected to the one tripod. When a plurality of length measuring instruments  100  can be held on the leg members  303  (tripod), space lengths of a plurality of reaction tubes  910  can be simultaneously measured. 
     In Reference Example 1 of the method for measuring a distance, as illustrated in  FIG. 16 , first, the measuring member  300  holding the length measuring instrument  100  is brought into contact with the smooth surface  930  of the tube sheet  916 , and is disposed above the tube sheet  916  such that a measurement direction D 1  of the length measuring instrument  100  is parallel to the axial length direction D 2  of the reaction tube  910 . 
     Next, the measurer sequentially measures a space length by sequentially slidably moving the measuring member  300  in contact with the smooth surface  930  of the tube sheet  916 . 
     The reaction tubes  910  are parallel to each other. Thus, even when the measuring member  300  is slidably moved along the smooth surface  930  of the tube sheet  916 , the tube sheet  916  having the continuous smooth surface  930  allows the measurement direction D 1  of the length measuring instrument  100  to be parallel to the axial length direction D 2  of any reaction tube  910  for which the space length is to be measured. Thus, in such a case, the space lengths of the plurality of reaction tubes  910  can be sequentially measured by slidably moving the measuring member  300  along the smooth surface  930  of the tube sheet  916 . The measurer does not need to repeat adjustment work of allowing the measurement direction D 1  for measurement using the length measuring instrument  100  to be parallel to the axial length direction D 2  of the reaction tube  910 . Thus, when the measuring member  300  is sequentially moved along the smooth surface  930  of the tube sheet  916  to be rearranged at an appropriate position, the distance from the opening  911  of each of the plurality of reaction tubes  910  to the solid  920  can be simply and quickly measured in a non-contact manner. 
     Reference Example 2 of Method for Measuring Distance 
     Next, as Reference Example 2 of a method for measuring a distance, a method for measuring a distance from an opening  911  of a reaction tube  910  to a solid  920  in a non-contact manner using a tube sheet  916  having a discontinuous smooth surface  930  will be described. 
       FIG. 17  is a view schematically illustrating a state of embodying Reference Example 2 of the method for measuring a distance.  FIG. 18(A)  is a sectional view illustrating a tube sheet  916  to which a plurality of reaction tubes  910  is joined, and  FIG. 18(B)  is a top view of  FIG. 18(A)  illustrating the tube sheet  916 . Members common to the embodiments described above and Reference Example 1 of the method for measuring a distance are designated by the same reference numerals and description of the members will be partially eliminated. 
     As illustrated in  FIGS. 18(A) and 18(B) , the tube sheet surface has unevenness due to welding marks such as weld beads and spatter, and thus causing a non-smooth portion. Depending on a pitch pb of the reaction tube  910 , the smooth surface  930  of the tube sheet  916  may be surrounded by a protrusion  931 , and may not be regularly or irregularly continuous. Although the smooth surface  930  of the tube sheet  916  normally forms a constant angle with the axial length direction D 2  of the reaction tube  910 , the smooth surface  930  is not continuous in a case as described above, and thus the measuring member  300  cannot be sequentially slidably moved along the smooth surface  930  of the tube sheet  916  as in Reference Example of the method for measuring a distance. Thus, the measuring member  300  is moved while being separated from the tube sheet  916 , i.e., while being lifted. The protrusion  931  is, for example, a weld bead produced when the reaction tube  910  is welded and joined to the tube sheet  916 . When a relative position of the smooth surface  930  surrounded by the protrusion  931  has almost no deviation, the distance from the opening  911  of each of the plurality of reaction tubes  910  to the solid  920  can be sequentially measured by lifting and moving the measuring member  300  without adjusting a position and length of each of the leg members  303  each time. 
     In Reference Example 2 of the method for measuring a distance, as illustrated in  FIG. 17 , first, the measuring member  300  holding the length measuring instrument  100  is brought into contact with the smooth surface  930  surrounded by the protrusion  931 , and is disposed above the tube sheet  916  such that the measurement direction D 1  of the length measuring instrument  100  is parallel to the axial length direction D 2  of the reaction tube  910 . 
     Next, the measurer sequentially measures the space length by sequentially moving the measuring member  300  in a state of being separated from the smooth surface  930  of the tube sheet  916 , and bringing the measuring member  300  into contact with another smooth surface  930  to dispose the measuring member  300  on the tube sheet  916 . 
     The smooth surface  930  of the tube sheet  916  usually forms a constant angle with the axial length direction D 2  of the reaction tube  910 , and the reaction tubes  910  are parallel to each other. Thus, when a relative position of the smooth surface  930  surrounded by the protrusion  931  has almost no deviation, the measuring member  300  is disposed at an appropriate position even when the measuring member  300  is lifted and moved from the smooth surface  930  of the tube sheet  916 . In this case, the measurement direction D 1  of the length measuring instrument  100  is parallel to the axial length direction D 2  of the plurality of reaction tubes  910 , the measurer does not need to repeat adjustment work of allowing the measurement direction D 1  for measurement using the length measuring instrument  100  to be parallel to the axial length direction D 2  of each of the plurality of reaction tubes  910 . Thus, the distance from the opening  911  of the reaction tube  910  to the solid  920  can be simply and quickly measured in a non-contact manner by sequentially moving the measuring member  300  along the smooth surface  930  of the tube sheet  916 . Only one adaptor  301  holding the length measuring instrument  100  may be connected to one tripod, or a plurality of adaptors may be connected to the one tripod. When a plurality of length measuring instruments  100  can be held on the leg members  303  (tripod), space lengths of a plurality of reaction tubes  910  can be simultaneously measured. 
     Although the distance measuring device and the method for measuring a distance of the present invention have been described above through various embodiments and modifications, the present invention is not limited only to the contents described in the specification, and can be appropriately changed based on the description of the scope of claims. 
     For example, in the first embodiment ( FIG. 1(A)  and  FIG. 2 ), the second embodiment ( FIG. 7 ), the third embodiment ( FIGS. 8 and 9 (A)), the fourth embodiment ( FIG. 11(A) ), the fifth embodiment ( FIG. 12 ), the sixth embodiment ( FIG. 13 ), the seventh embodiment ( FIG. 14 ), and Reference Example 1 of the method for measuring a distance ( FIG. 16 ), the measuring member may be provided at its end in contact with the contact surface with a rollable roller, caster, or the like. Then, the measuring member can move more smoothly on the contact surface. 
     REFERENCE SIGNS LIST 
     
         
           10 ,  11 ,  12 ,  13 ,  14 ,  15 ,  16 ,  17  distance measuring device 
           100  length measuring instrument 
           200  rail member (base member) 
           201  support leg 
           202  rail member (base member) 
           203  lower casing 
           204  upper casing 
           205 ,  206  slide groove 
           207  rail member (base member) 
           210  support leg 
           215  plate member (base member) 
           217  through-hole 
           220  contact surface 
           221  guide groove 
           222 ,  223  contact surface 
           225  slide surface 
           227  contact surface 
           230  movable block 
           231  frame body 
           232  drive unit 
           233  ball screw 
           234  motor 
           235  operation plate 
           236  linear guide 
           240  rail member 
           241  contact surface 
           242  measuring member 
           243  recess 
           244  block 
           245  protrusion 
           246  slider 
           300  measuring member 
           301  adaptor 
           303  leg member 
           305 ,  306  measuring member 
           307 ,  308  slider 
           310 ,  311  adaptor 
           315  measuring member 
           316  adaptor 
           317  slider 
           320  measuring member 
           321  plate member 
           322  top plate 
           325  slider 
           326  ball joint 
           330  measuring member 
           331  adaptor 
           332  sensor 
           333  wheeled platform 
           341  base member 
           341   a  contact surface 
           342  measuring member 
           342   a  contact surface 
           343  base member 
           343   a  contact surface 
           344  measuring member 
           344   a  contact surface 
           345  base member 
           345   a  contact surface 
           346  measuring member 
           346   a  contact surface 
           347  base member 
           347   a  contact surface 
           348  measuring member 
           348   a  contact surface 
           401  support leg 
           406  support leg 
           900  reactor 
           910  reaction tube 
           911  opening 
           913  lower end opening 
           914  first layer 
           915  second layer 
           916  tube sheet 
           920  solid 
           930  smooth surface 
           931  protrusion 
         D 1  measurement direction 
         D 2  axial length direction 
         L 0 , L 01 , L 02 , L 03 , L 04  reference line (straight line parallel to direction in which measuring member disposed on base member moves) 
         L 1 , L 11 , L 12 , L 13 , L 14  straight line parallel to axial length direction of reaction tube 
         Lm, Lm 1 , Lm 2 , Lm 3 , Lm 4  row of reaction tubes for which measuring members are sequentially moved to measure space length 
         M 1  solid 
         M 2  solid 
         N plane 
         P 1 , P 2 , P 3  position where space length is measured by length measuring instrument 
         pa, pb pitch 
         α angle formed by straight line parallel to axial length direction of reaction tube and reference line, which are on identical plane