Abstract:
An object is to provide a gate valve capable of preventing operational deficiencies caused by particulate materials accumulating in a valve box, thus providing superior sealing properties and high durability, having a simple structure, and allowing inspection and maintenance to be performed easily. A gate valve installed in a transport line for transporting fluid containing particulate material at a coal gasification plant includes a valve box having an inlet portion and an outlet portion in which a channel constituting a part of the transport line is formed; a valve unit, provided between the inlet portion and the outlet portion, which extends in an opening-and-closing direction orthogonal to an axial direction of the channel, and which is movable in the opening-and-closing direction; a wedge plate provided between the valve unit and the inlet portion so as to extend in the opening-and-closing direction; and a guide plate provided between the valve unit and the outlet portion so as to extend in the opening-and-closing direction. Areas between the valve unit and the wedge plate and between and the valve unit and guide plate are sealed by means of surface contact.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to gate valves disposed in transport lines for transporting fluids containing mainly particulate materials, at coal gasification plants where coal is gasified and used as fuel for gas turbines and other devices. 
         [0003]    2. Description of Related Art 
         [0004]    Conventionally, coal gasification plants have been drawing attention because they make efficient use of abundant coal reserves. These types of coal gasification plants have transport lines for transporting fluids containing particulate materials, such as coal used as a fuel for a gasification furnace or unburned compounds generated from at the gasification furnace, between each constituent device. Valves for shutting off and sealing the fluids are provided to ensure operational control and safety of the plants. 
         [0005]    With valves operated in such a particulate material environment, particulate materials enter the valve box and accumulate therein, which possibly obstructs the operation of a valve unit. Accordingly, measures for preventing the particulate materials from entering the valve box are required. 
         [0006]    In addition, in some cases, sealing characteristics at high temperature and high pressure and durability in high-frequency operations are required at the same time, depending on the operating conditions at the plant. 
         [0007]    Because the lines used for transporting fluids containing particulate materials usually have a large valve bore, it is not easy to remove the valve from the system. Accordingly, it is desirable that the valves have a structure that allows parts to be easily changed while remaining installed during inspection or maintenance of the valve. 
         [0008]    In the related art, a through-conduit gate valve disclosed, for example, in Japanese Unexamined Patent Application, Publication No. HEI 5-141548 has been proposed as a valve for particulate materials used for preventing the particulate materials from entering gaps in the valve box and accumulating therein. This type of valve has a secondary seal, outside a primary seal, that seals the channel and the valve box of the known through-conduit gate valve to prevent the particulate materials from entering the valve box. 
         [0009]    A known gate valve in which a seal is subjected to lower wear by abrasive fluids is a slurry gate valve disclosed, for example, in Japanese Unexamined Patent Application, Publication No. HEI 5-196153. 
         [0010]    This type of gate valve has multiple gate plates that sequentially open; first, it shuts off fluids with an upstream gate plate and then operates a downstream gate plate to prevent a sealing portion on the downstream side from being damaged while the gate plate is partially opened. 
         [0011]    In addition, a known gate valve that is less complex and externally adjustable without disassembling the valve is disclosed in, for example, Japanese Unexamined Patent Application, Publication No. SHO-54-142625.  FIG. 11  shows the structure of a gate valve  100  disclosed in Japanese Unexamined Patent Application, Publication No. SHO-54-142625. 
         [0012]    With this gate valve, the front and rear of a valve element  105  that has an opening portion  103  and a closing portion  101  are flanked by a main valve seat  107  and a sub valve seat  109 , and a seal around a channel  111  is provided in front of and behind the main valve seat  107  and the sub valve seat  109 , respectively. It is constructed so that the main valve seat  107  is moved back and forth using a bolt  113  to maintain surface pressure on a contact surface by the effect of a wedge. The gate valve  100  is opened and closed by the valve element  105  moving upward and downward. 
         [0013]    However, the gate valve disclosed in Japanese Unexamined Patent Application, Publication No. HEI-5-141548 has a complex double sealing structure, and a seal is disposed on a moving portion; therefore, there is a problem in that the seal has low durability and is easily damaged. 
         [0014]    The gate valve disclosed in Japanese Unexamined Patent Application, Publication No. HEI-5-196153 cannot prevent fluids from entering a valve box in a partially opened state. Accordingly, when the valve is used for fluids containing particulate materials, there is a problem in that motion of the gate valve is prevented due to the particulate materials accumulating in the valve box. 
         [0015]    The gate valve disclosed in Japanese Unexamined Patent Application, Publication No. SHO-54-142625, similar to Japanese Unexamined Patent Application, Publication No. HEI-5-196153, also cannot prevent a fluid from entering a valve box in a partially opened state; therefore, there is a problem in that the operation of the gate valve  100  is disturbed by the particulate materials accumulating in the valve box when the fluid contains particulate materials. 
         [0016]    When the gate valve  100  is used at high temperatures, a contact surface pressure exerted on the valve element  105  by the main valve seat  107  is adjusted by screwing the bolt  113 ; therefore, it is not possible to provide a seal with an accurate gap size for absorbing thermal expansion of the valve box body  115  generated at high temperatures. In addition, when loosening the main valve seat  107 , the main valve seat  107  does not follow the bolt  113  even when it is loosened; therefore, there is a problem in that the main valve seat  107  and the valve element  105  cannot be secured. 
         [0017]    In addition, because the sealing gasket  117  is provided on a sliding surface of the valve element  105 , particulate materials easily enter the sealing gasket  117  and adhere thereto. When sliding in this state, the sealing gasket  117  is liable to be damaged and has low durability. Accordingly, it is difficult to maintain the sealing function. 
         [0018]    Furthermore, there is a problem in that a complicated procedure is required to disassemble the valve in inspection and maintenance, namely, removing a cover  119 , pulling out the main valve seat  107 , removing an upper flange  121 , and taking out the valve element  105  and the sub valve seat  109 . 
       BRIEF SUMMARY OF THE INVENTION 
       [0019]    The present invention has been conceived in light of the problems of the related art described above, and an object thereof is to provide a gate valve capable of preventing operational deficiencies caused by accumulating particulate materials in a valve box, thus providing superior sealing properties and high durability, having a simple structure, and allowing inspection and maintenance to be performed easily. 
         [0020]    In order to solve the problems described above, the present invention employs the following solutions. 
         [0021]    A first aspect of the present invention provides a gate valve installed in a transport line for transporting fluid containing particulate material at a coal gasification plant, comprising a valve box having an inlet portion and an outlet portion in which a channel constituting a part of the transport line is formed; a valve unit, provided between the inlet portion and the outlet portion, which extends in an opening-and-closing direction orthogonal to an axial direction of the channel and which is movable in the opening-and-closing direction; a wedge plate provided between the valve unit and the inlet portion so as to extend in the opening-and-closing direction; a guide plate provided between the valve unit and the outlet portion so as to extend in the opening-and-closing direction; and a moving device engaged with the valve unit and configured to move the valve unit in the opening-and-closing direction, wherein the valve unit is provided with a first sliding surface at the inlet portion side and a second sliding surface at the outlet portion side, which are substantially orthogonal to the channel and substantially parallel to each other, and a valve unit channel passing through so as to form a part of the channel is provided closer to a first end than a midpoint in the opening-and-closing direction; the wedge plate is provided with a third sliding surface that is in surface contact with the first sliding surface at the valve unit side and an inclined supporting surface which is provided closer to a second end than substantially the midpoint in the opening-and-closing direction at the inlet portion side and whose height gradually reduces toward the second end, and a wedge-plate channel that passes from the inclined supporting surface toward the third sliding surface and that forms a part of the channel is provided so that the third sliding surface has a length equal to or greater than the diameter of at least the valve unit channel at the first end in the opening-and-closing direction; the guide plate is provided with a fourth sliding surface that is in surface contact with the second sliding surface at the valve unit side, and a guide-plate channel that passes through so as to form a part of the channel is provided so that the fourth sliding surface has a length equal to or greater than the diameter of at least the valve unit channel at the first end in the opening-and-closing direction; an inclined surface opposing the inclined supporting surface is formed at an inner end portion of the inlet portion; a first sealing member having elasticity is mounted between the inclined surface and a wedge-plate annular groove provided around the wedge-plate channel of the inclined supporting surface; a second sealing member having elasticity is mounted between the guide plate and the outlet portion so as to surround the channel; and the valve box is provided with a first urging member capable of urging the wedge plate to the second end in the opening-and-closing direction, and a second urging member that is disposed at the second end, in the opening-and-closing direction, of the first urging member and that is capable of urging the wedge plate to the first end in the opening-and-closing direction. 
         [0022]    According to this aspect, the fluid channel is formed by the channels of the inlet portion and the outlet portion, the wedge-plate channel, and the guide-plate channel. When the valve unit is slid by the moving device, and when the valve unit channel overlaps with the wedge-plate channel of the wedge plate and the guide-plate channel of the guide plate, the channel is formed so as to pass through; that is, the gate valve is opened. In this state, when the valve unit is slid, by the moving device, to the first end in the opening-and-closing direction along the wedge plate and the guide plate, the valve unit channel moves to the first end, and the overlapping portion of the wedge-plate channel of the wedge plate and the guide-plate channel of the guide plate gradually decreases in size, and finally they do not overlap. In other words, the gate valve is closed. 
         [0023]    At this time, the channel formed by the inlet portion and the wedge-plate channel of the wedge plate is sealed by the first sealing member, and the channel formed by the outlet portion and the guide-plate channel of the guide plate is sealed by the second sealing member. 
         [0024]    The wedge plate is provided with a third sliding surface having a length equal to or greater than the diameter of at least the valve unit channel at the first end of the wedge-plate channel in the opening-and-closing direction. The guide plate is provided with a fourth sliding surface having a length equal to or greater than the diameter of at least the valve unit channel at the first end of the guide-plate channel in the opening-and-closing direction. Accordingly, the valve unit channel is covered by the third sliding surface and the fourth sliding surface until the gate valve is closed. In other words, the opening plane of the valve unit channel is covered by the third sliding surface and the fourth sliding surface at all times during the opening and closing operation of the gate valve. 
         [0025]    Because the valve unit channel is provided closer to the first end than the midpoint of the valve unit in the opening-and-closing direction, the first sliding surface and the second sliding surface, which are longer than the diameter of the valve unit channel, are provided at the second end of the valve unit channel. Accordingly, the wedge-plate channel is covered and sealed by the first sliding surface of the valve unit, and the guide-plate channel is covered by the second sliding surface of the valve unit during the time when the gate valve is closed by sliding the valve unit. 
         [0026]    In this way, because a portion between the channel and inside the valve box is sealed at all degrees of opening, it is possible to prevent the fluid containing particulate materials transmitted through the channel from entering the valve box and accumulating therein. 
         [0027]    The movable portions, i.e., the portions between the valve unit and the wedge plate and between the valve unit and the guide plate, are sealed by making surface contact with each of the sliding surfaces, thus allowing high durability and improved sealing efficiency. 
         [0028]    According to this aspect, when the wedge plate is urged to the second end in the opening-and-closing direction by the first urging member, the first sealing member is compressed because the inclined supporting surface of the wedge plate approaches the inclined surface of the inlet portion. When the first sealing member is compressed, the wedge plate is pressed against the valve unit by its restoring force. Because this pressing force is transmitted from the valve plate to the guide plate, and further transmitted from the guide plate to the second sealing member, it is possible to increase the contact pressure between the first sealing member and the second sealing member and the contact surface force between the valve unit and the wedge plate and between the valve unit and the guide plate. This increased-force state can be maintained by urging the wedge plate to the second end using the second urging member. 
         [0029]    On the other hand, by loosening the first sealing member and urging the wedge plate to the first end in the opening-and-closing direction using the second sealing member, the first sealing member expands because the inclined supporting surface of the wedge plate is separated from the inclined surface of the inlet portion. 
         [0030]    When the first sealing member expands, the pressing force of the first sealing member decreases; therefore, it is possible to decrease the contact pressure of the first sealing member and the second sealing member, and the contact surface force between the valve unit and the wedge plate and between the valve unit and the guide plate. This reduced-force state can be maintained by operating the first urging member to urge the wedge plate to the first end. 
         [0031]    In this way, by operating the first urging member and the second urging member to adjust the position of the wedge plate as necessary, the contact pressure of the first sealing member and the second sealing member, and the contact surface force between the valve unit and the wedge plate and between the valve unit and the guide plate can be adjusted, and that state be maintained. 
         [0032]    In the aspect of the invention described above, the moving device may be disposed at one of the first end and the second end of the valve unit and is detachably engaged with the valve unit; and an opening portion capable of opening and closing may be provided at an opposite side, in the opening-and-closing direction, from the moving device of the valve box. 
         [0033]    Accordingly, by opening the opening portion, and by visually inspecting the sliding paths of the wedge plate and the guide plate, and determining the degree of wear and tear to the sliding surfaces, including the valve unit or other parts, it is possible to determine in advance whether or not inspection is required. In addition, by checking for particulate materials left inside the valve box, it is possible to determine whether or not it is necessary to perform adjustment and inspection of the contact pressure of the first sealing member and the second sealing member, and the contact surface force between the valve unit and the wedge plate and between the valve unit and the guide plate. 
         [0034]    By opening the opening portion, and by separating the moving device from the valve, the valve unit can be removed from the opening portion, and then the wedge plate and the guide plate can be removed from the opening portion. Accordingly, maintenance, such as inspection, replacement, and assembly, can be performed inside the valve box without removing the valve box from the pipe in which it is installed. 
         [0035]    In the aspect of the invention described above, coating layers may be provided on the first sliding surface, the second sliding surface, the third sliding surface, and the fourth sliding surface. 
         [0036]    Because coating layers are provided on the first sliding surface, the second sliding surface, the third sliding surface, and the fourth sliding surface, the sealing properties of these sliding surfaces can be maintained for a long time even when they slide in high-temperature, corrosive environments, as well as in particulate environments. 
         [0037]    Materials having excellent wear-resistance, corrosion-resistance, and mechanical sliding characteristics is suitable for the coating layers, for example, chromium carbide (Cr 3 C 2 ). 
         [0038]    In the aspect of the invention described above, the first sealing member and the second sealing member may be formed of spiral wound gaskets having a plurality of elasticities in a channel axial direction and a metal sealing plate interposed between each spiral wound gasket. 
         [0039]    Because the first sealing member and the second sealing member are formed of the spiral wound gaskets having a plurality of elasticities in the channel axial direction and the metal sealing plate interposed between each spiral wound gasket, the thermal expansion of the valve box is distributed and absorbed by the plurality of the spiral wound gaskets. Accordingly, the thermal expansion applied to each of the spiral wound gaskets is within an elastic limit. It is possible to prevent a large contact surface force from being applied to the first sliding surface, the second sliding surface, the third sliding surface, and the fourth sliding surface, allowing operability to be improved. 
         [0040]    Air-tightness can be maintained by the sealing plate even when the spiral wound gasket is deformed. 
         [0041]    In the aspect of the invention described above, the channel may be formed in substantially the vertical direction such that the outlet portion is disposed at a lower end thereof; the second-end portion of the guide plate may be disposed so as to extend closer to the second end than the guide-plate channel; the valve box may be provided with a holding portion, between the valve box and the guide plate, substantially below the end at the first end of the guide plate, with a gap therebetween to allow thermal expansion of the valve box; and a spring having a spring force for supporting weights of the guide plate, the valve unit, and the wedge plate may be disposed between the holding portion and the guide plate. 
         [0042]    Because the second-end portion of the guide plate extends closer to the second end than the guide-plate channel, even when the valve unit has a center of gravity closer to the second end than the valve unit channel, the valve unit is always guided by the guide plate and stably slides. 
         [0043]    Because the wedge plate and the guide plate are disposed closer to the first end, the center of gravity of the wedge plate, the valve unit, and the guide plate is positioned closer to the first end than the guide-plate channel. A downward moment is thus always exerted at the first-end portion of the guide plate while the valve unit is sliding. 
         [0044]    Because this downward moment is supported by the spring provided between the holding portion and the guide plate, the guide plate, the valve unit, and the wedge plate are always kept horizontal; thus, the valve plate operates stably. 
         [0045]    Because the holding portion is provided, substantially below the end at the first end of the guide plate, with a gap between the guide plate and the holding portion to allow thermal expansion of the valve box, the holding portion does not abut against and press the guide plate upward, even when the valve box thermally expands. 
         [0046]    In the aspect of the invention described above, a leading plate may be removably provided on an inner surface of the valve box opposing the valve unit, with a small gap therebetween. 
         [0047]    Because the leading plate is provided on an inner surface of the valve box that the valve unit faces, with a small gap therebetween, even when the valve unit rocks due to the presence of a small amount of particulate materials, it abuts against only the leading plate, which prevents the problem of abutting against the valve box. 
         [0048]    Because the leading plate is provided on the valve box in a detachable manner, when the valve plate becomes worn due to contact and sliding, maintenance can be easily carried out by changing only the leading plate. 
         [0049]    According to the present invention, between the channel and inside the valve box, it is possible to prevent the fluid containing particulate materials transmitted through the channel from entering the valve box and accumulating therein at all levels of opening. 
         [0050]    The movable portions, i.e., the portions between the valve unit and the wedge plate and between the valve unit and the guide plate, are sealed by making surface contact, thus allowing high durability and improved sealing efficiency. 
         [0051]    By operating the first urging member and the second urging member to adjust the position of the wedge plate, the contact pressure of the first sealing member and the second sealing member, and the contact surface force between the valve unit and the wedge plate and between the valve unit and the guide plate can be adjusted, and that state be maintained. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0052]      FIG. 1  is a sectional view showing the front of a gate valve according to an embodiment of the present invention. 
           [0053]      FIG. 2  is a plan view in  FIG. 1 . 
           [0054]      FIG. 3A  is a front sectional view showing a fully opened state of the gate valve according to an embodiment of the present invention. 
           [0055]      FIG. 3B  is a sectional view taken along line X-X of  FIG. 3A . 
           [0056]      FIG. 3C  is a sectional view taken along line Y-Y of  FIG. 3A . 
           [0057]      FIG. 4A  is a front sectional view showing a partially opened state of the gate valve according to an embodiment of the present invention. 
           [0058]      FIG. 4B  is a sectional view taken along Z-Z of  FIG. 4A . 
           [0059]      FIG. 4C  is a sectional view taken along U-U of  FIG. 4A . 
           [0060]      FIG. 5A  is a front sectional view showing a closed state of the gate valve according to an embodiment of the present invention. 
           [0061]      FIG. 5B  is a sectional view taken along V-V of  FIG. 5A . 
           [0062]      FIG. 5C  is a sectional view taken along W-W of  FIG. 5A . 
           [0063]      FIG. 6  is a plan view showing a second-end portion of a valve unit according to an embodiment of the present invention. 
           [0064]      FIG. 7  is a sectional view showing a sealing gasket according to an embodiment of the present invention. 
           [0065]      FIG. 8  is a partial sectional view showing a guide-plate sealing gasket according to an embodiment of the present invention. 
           [0066]      FIG. 9  is a sectional view showing a spiral wound gasket according to an embodiment of the present invention. 
           [0067]      FIG. 10  is a partially cutaway front view showing a holding portion according to an embodiment of the present invention. 
           [0068]      FIG. 11  is a longitudinal sectional view showing a known gate valve. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0069]    A gate valve  1  according to an embodiment of the present invention will be described below using  FIGS. 1 to 10 . The gate valve  1  according to this embodiment is used for transport lines for transporting high-temperature particulate materials. 
         [0070]      FIG. 1  is a front view showing a cross section of the gate valve  1 .  FIG. 2  shows a top view of the gate valve  1 , and a cross section thereof is shown in the lower half of  FIG. 2 . 
         [0071]    A valve box  3  includes a substantially cylindrical valve box body  5 , a lid (opening portion)  7  at a first end for opening and closing a first end (A) of the valve box body  5 , and a lid  9  at a second end for opening and closing a second end (B) of the valve box body  5 . 
         [0072]    The valve box body  5  is disposed so that the axial direction thereof is horizontal. An inlet portion  13  with an inlet channel  11  and an outlet portion  17  with an outlet channel  15  are provided at substantially the midpoint in the axial direction (opening-and-closing direction)  26  of the valve box body  5 . The axial centers of the inlet channel  11  and the outlet channel  15  extend in the vertical direction (channel axis direction)  28  on substantially the same line. 
         [0073]    The inner end of the inlet portion  13  has an inclined surface  19  opening towards the first end. 
         [0074]    A wedge plate  21 , a valve unit  23 , and a guide plate  25  are provided between the inlet portion  13  and the outlet portion  17  in this order from the inlet portion  13  side, each horizontally extending in the axial direction (opening-and-closing direction)  26  of the valve box body  5 . 
         [0075]    The valve unit  23  is a substantially rectangular plate member having a semicircular protrusion at the first-end portion and includes an upper sliding surface (first sliding surface)  27  at the top and a lower sliding surface (second sliding surface)  29  at the bottom. In addition, the valve unit  23  has a valve unit channel  31  that is disposed closer to the first end than the midpoint in the opening-and-closing direction  26  and passes from the upper sliding surface  27  to the lower sliding surface  29 . 
         [0076]    The wedge plate  21  is a substantially oval plate member and has a wedge-plate sliding surface (third sliding surface)  33  that is in surface contact with the upper sliding surface  27  of the valve unit  23 . The wedge plate  21  has an inclined supporting surface  35  protruding in the form of a cylinder on the inlet portion  13  side at a position closer to the second end than substantially the midpoint in the opening-and-closing direction  26 . The height of the upper end of the inclined supporting surface  35  gradually reduces toward the second end. A wedge-plate channel  37  passing from the inclined supporting surface  35  to the wedge-plate sliding surface  33  is provided. An annular groove  39  around the circumference of the wedge-plate channel  37  is provided on the inclined supporting surface  35 . In addition, the wedge-plate sliding surface  33  extends from the first-end portion of the wedge plate  21  to the first-end portion of the wedge-plate channel  37  so as to have a length equal to or greater than the diameter of at least the valve unit channel  31 . 
         [0077]    The guide plate  25  is a substantially rectangular plate member having a semicircular protrusion at the first-end portion, and has a guide-plate sliding surface (the fourth sliding surface)  41  that is in surface contact with the lower sliding surface  29  at the valve unit  23  side. 
         [0078]    A guide-plate channel  43  passing through in a vertical direction  28  is provided in the guide plate  25  at a position closer to the second end than substantially the midpoint in the opening-and-closing direction  26 . The guide-plate sliding surface  41  extends from the first-end portion of the guide plate  25  to the first-end portion of the guide-plate channel  43  so as to have a length equal to or greater than the diameter of at least the valve unit channel  31 . In addition, the second-end portion of the guide plate  25  is disposed closer to the second end than a center of gravity of the combined guide plate  25 , valve unit  23 , and wedge plate  21  when the valve unit channel  31  overlaps with the guide-plate channel  43 . 
         [0079]    A guide-plate annular groove  45  around the circumference of the guide-plate channel  43  is provided at the outlet portion  17  side of the guide plate  25 . 
         [0080]    A protrusion  47  (see  FIG. 8 ) around the circumference of the outlet channel  15  is provided on the upper surface of the outlet portion  17  so as to fit in the guide-plate annular groove  45 . 
         [0081]    Next, a seal structure between the wedge plate  21  and the inlet portion  13 , and between the guide plate  25  and the outlet portion  17  will be described using  FIGS. 7 to 9 . 
         [0082]    As shown in  FIG. 7 , an inlet-portion seal-gasket (first sealing member)  57  is provided in the annular groove  39  of the wedge plate  21  so as to protrude from the inclined supporting surface  35 . The inlet-portion seal-gasket  57  is formed of three spiral wound gaskets  59  and two sealing plates  61 . 
         [0083]    As shown in  FIG. 8 , an outlet-portion sealing gaskets  63  having the same structure as the inlet-portion seal-gasket  57  is provided between the guide-plate annular groove  45  of the guide plate  25  and the protrusion  47  of the outlet portion  17 . 
         [0084]    As shown in  FIG. 9 , the spiral wound gasket  59  has a plurality of metal hoop members  65  disposed in a filler  64  with predetermined spaces. The hoop members  65  are bent substantially in the shape of a letter V and are configured so that they can elastically deform in the vertical direction. 
         [0085]    The sealing plates  6 i are made of metal, are disposed in close contact with the walls of the annular groove  39  and the guide-plate annular groove  45 , and have a function of maintaining air-tightness. 
         [0086]    When the hoop members  65  are used in an elastic deformation range, the hoop members  65  behave like a spring to ensure resiliency. Accordingly, the number of spiral wound gasket  59  is, determined so that the level of compression per gasket is within the elastic deformation range of the hoop members  65 . 
         [0087]    In addition, because the restoring forces of the inlet-portion sealing gasket  57  and the outlet-portion sealing gasket  63  are contact surface forces between the metal sliding surfaces of the valve unit  23  and the wedge plate  21 , and the valve unit  23  and the guide plate  25 , when the contact surface force of the metal sliding surfaces is too strong due to the increased reaction force of the gasket generated during absorption of the thermal expansion, the metal sliding surfaces are damaged by seizing and so forth. 
         [0088]    Accordingly, by stacking the plurality of spiral wound gaskets  59 , an increase of the contact surface force of the metal sliding surfaces during absorption of the thermal expansion is reduced. Therefore, the number of spiral wound gaskets  59  is determined considering also the limit contact surface force of the metal sliding surfaces. 
         [0089]    The spiral wound gasket  59  is usually compressed to be 1.2 mm thick per sheet; in this embodiment, the compression is limited to ¼ to ⅕ (0.24 mm to 0.3 mm), considering the increased reaction forces of the inlet-portion sealing gasket  57  and the outlet-portion sealing gasket  63  due to restoration and thermal expansion. 
         [0090]    A protruding portion  49  is provided on an upper surface of the first end of the wedge plate  21 . An adjusting bolt (first urging member)  51  screwed into the valve box body  5  at the first end of the protruding portion  49  and a bolt (second urging member)  53  screwed into the valve box body  5  at the second end are provided as to be capable of being screwed in. 
         [0091]    In addition, chromium carbide (Cr 3 C 2 ) is coated on the upper sliding surface  27  and the lower sliding surface  29  of the valve unit  23 , on the wedge-plate sliding surface  33  of the wedge plate  21 , and on the guide-plate sliding surface  41  of the guide plate  25 . Cr 3 C 2  has excellent sliding characteristics at high temperature, resulting in much less damage to the object materials, and is wear-resistant and corrosion-resistant in a particulate environment. 
         [0092]    Leading plates  67 , which are parallel to both surfaces of the valve unit  23 , are removably attached to an inner circumferential surface of the valve box body  5  using bolts  69  with a small gap between the leading plates  67  and the valve unit  23 . Guards  71  fitted to the valve box body  5  are provided at both ends of the leading plate  67  in the opening-and-closing direction  26 . A material exhibiting excellent metal sliding and corrosion-resistance properties, for example, a cobalt-base alloy, is used on the sliding surfaces of the leading plates  67 . 
         [0093]    Next, a moving device  73  will be described. The moving device  73  is disposed at the second end of the valve box  3 . When a rotational driving force is applied to a rotating body by an operating mechanism  77  or a handle, a screw stem  79  threaded with the rotating body moves in the opening-and-closing direction  26 . A valve stem  75  attached to the first-end portion of the screw stem  79  moves in the opening-and-closing direction  26  together with the movement of the screw stem  79 . 
         [0094]    An engaging portion  55  that is formed in an inverted T-shape and whose inner side is wider in plan view is provided at the second-end portion of the valve unit  23 . A hanger portion  81  is provided at the first-end portion of the valve stem  75 . 
         [0095]    The hanger portion  81  is a block having an oval shape in cross section and is configured so that the length of the minor axis is shorter than the width D of an entrance of the engaging portion  55 , and the length of the major axis B is longer than the width D of the entrance of the engaging portion  55  and shorter than the width A of the wide portion of the engaging portion  55 . Accordingly, as shown in  FIG. 6 , when the major axis of the hanger portion  81  is engaged with the wide portion of the engaging portion  55 , the valve unit  23  moves in the opening-and-closing direction  26  together with the movement of the valve stem  75  in the opening-and-closing direction. On the other hand, when the valve stem  75  is rotated by 90° from the state shown in  FIG. 6 , it is not engaged with the engaging portion  55  because the minor axis of the hanger portion  81  is located in an engaging positional relationship. 
         [0096]    Next, holding portions  83  will be explained also with reference to  FIG. 10 . 
         [0097]    A pair of holding portions  83  is provided at either side substantially below the end at the first end of the guide plate  25 . Holding shafts  85  that are attached so as to be capable of moving in the vertical direction with respect to the valve box body are provided on each holding portion  83 . The upper portion of the holding shaft  85  is configured to be expanded, and the lower portion of the large-diameter portion thereof is configured to be supported by a holding-shaft mounting plate  87  which is fixed to the bottom of the guide plate  25 . 
         [0098]    An appropriate gap a to allow thermal expansion of the valve box  3  is provided between the guide plate  25  and the upper end of the holding shaft  85 . A spring  89  that balances the weights of the guide plate  25 , the valve unit  23 , and the wedge plate  21  is mounted between the holding shaft  85  and the guide plate  25 . 
         [0099]    The operation of the gate valve  1  according to this embodiment will be described. 
         [0100]    First, adjustment of a contact surface force of a seal will be described. 
         [0101]    When the protruding portion  49  is urged to the second end in the opening-and-closing direction  26  using the adjusting bolt  51 , the wedge plate  21  is urged in the same direction. By doing so, the spiral wound gasket  59  of the sealing gasket  57  is compressed because the inclined supporting surface  35  of the wedge plate  21  approaches the inclined surface  19  of the inlet portion  13 . When the spiral wound gasket  59  is compressed, the wedge plate  21  is pressed against the valve unit  23  by its restoring force. Because this pressing force is transmitted from the valve unit  23  to the guide plate  25 , and further transmitted from the guide plate  25  to the guide-plate sealing gasket  63 , it is possible to increase the contact pressure and the contact surface force between the sealing gasket  57  and the inclined surface  19 , between the wedge-plate sliding surface  33  and the upper sliding surface  27 , between the lower sliding surface  29  and the guide-plate sliding surface  41 , and between the guide-plate sealing gasket  63  and the protrusion  47 . By urging the protruding portion  49  to the second end using the bolt  53 , the wedge plate  21  can be fixed in this increased-force state. 
         [0102]    In contrast, by loosening the adjusting bolt  51  and urging the protrusion  49  to the first end in the opening-and-closing direction  26  using the bolt  53 , the sealing gasket  57  expands because the inclined supporting surface  35  of the wedge plate  21  is separated from the inclined surface  19  of the inlet portion  13 . When the sealing gasket  57  expands, the reaction force of the sealing gasket  57  decreases, the contact pressure of the sealing gasket  57  and the guide-plate sealing gasket  63 , and the contact surface force between the valve unit  23  and the wedge plate  21  and between the valve unit  23  and the guide plate  25  can be decreased. By operating the adjusting bolt  51  to urge the protruding portion  49  to the first end, this reduced-force state can be maintained. 
         [0103]    In this way, by operating the adjusting bolt  51  and the bolt  53  to finely adjust the position of the wedge plate  21  in the opening-and-closing direction  26  as necessary, the contact pressure of the sealing gasket  57  and the guide-plate sealing gasket  63 , and the contact surface force between the valve unit  23  and the wedge plate  21  and between the valve unit  23  and the guide plate  25  can be adjusted, and that state be maintained. 
         [0104]    Next, the opening and closing operation of the gate valve  1  will be described with reference to  FIGS. 3A to 5C . 
         [0105]      FIGS. 3A to 3C  show a state in which the gate valve  1  is fully opened,  FIGS. 4A to 4C  show a state in which the gate valve  1  is opened, and  FIGS. 5A to 5C  show a state in which the gate valve  1  is fully closed. 
         [0106]    A fluid channel is formed in the gate valve  1  by the inlet channel  11 , the wedge-plate channel  37 , the guide-plate channel  43 , and the outlet channel  15 . When the valve unit  23  is slid by the moving device  73  in the opening-and-closing direction  26 , and when the valve unit channel  31  overlaps with the wedge-plate channel  37  of the wedge plate  21  and the guide-plate channel  43  of the guide plate  25 , a channel passing in the vertical direction is formed; that is, the gate valve is opened. 
         [0107]      FIGS. 3A to 3C  show the state in which the valve unit  23  moves to the extremity at the second end, and the valve unit channel  31  substantially completely overlaps with the wedge-plate channel  37  and the guide-plate channel  43 ; that is, the gate valve  1  is fully opened. 
         [0108]    In this state, when the valve unit  23  is slid by the moving device  73  to the first end in the opening-and-closing direction  26  along the wedge-plate sliding surface  33  of the wedge plate  21  and the guide-plate sliding surface  41  of the guide plate  25 , the valve unit channel  31  moves to the first end, and the overlapping portion of the wedge-plate channel  37  of the wedge plate  21  and the guide-plate channel  43  of the guide plate  25  gradually decreases in size (see  FIGS. 4A to 4C ), and finally they do not overlap (see  FIGS. 5A to 5C ). In other words, the gate valve  1  is closed. 
         [0109]    At this time, the channel formed by the inlet channel  11  and the wedge-plate channel  37  of the wedge plate  21  is sealed by the sealing gasket  57 , and the channel formed by the outlet channel  15  and the guide-plate channel  43  of the guide plate  25  is sealed by the guide-plate sealing gasket  63 . 
         [0110]    The wedge plate  21  is provided with the wedge-plate sliding surface  33  having a length equal to or greater than the diameter of at least the valve unit channel  31  at the first end of the wedge-plate channel  37  in the opening-and-closing direction. The guide plate  25  is provided with the guide-plate sliding surface  41  having a length equal to or greater than the diameter of at least the valve unit channel  31  at the first end of the guide-plate channel  43  in the opening-and-closing direction. Accordingly, the valve unit channel  31  is covered by the wedge-plate sliding surface  33  and the guide-plate sliding surface  41  while the valve unit  23  slides from fully open to fully closed. In other words, the opening plane of the valve unit channel  31  is covered by the wedge-plate sliding surface  33  and the guide-plate sliding surface  41  at all times during the opening and closing operation of the gate valve  1 . 
         [0111]    Because the valve unit channel  31  is provided closer to the first end than the midpoint of the valve unit  23  in the opening-and-closing direction  26 , the upper sliding surface  27  and the lower sliding surface  29 , which are longer than the diameter of the valve unit channel  31 , are provided at the second end of the valve unit channel  31 . Accordingly, the wedge-plate channel  37  is covered and sealed by the upper sliding surface  27  of the valve unit  23 , and the guide-plate channel  43  is covered and sealed by the lower sliding surface  29  of the valve unit  23  during the time from when the valve unit  23  is in the fully opened state (see  FIGS. 3A to 3C ) to when the gate valve  1  is closed by sliding to the first end (see  FIGS. 5A to 5C ). 
         [0112]    In this way, because a portion between the fluid channel and a space inside the valve box  3  is sealed at all levels of opening, it is possible to prevent the fluid containing particulate materials transmitted through the channel from entering the valve box  3  and accumulating therein. 
         [0113]    The movable portions, i.e., the portions between the valve unit  23  and the wedge plate  21  and between the valve unit  23  and the guide plate  25 , are sealed by making surface contact with each of the sliding surfaces, thus allowing high durability and improved sealing efficiency. 
         [0114]    Because a layer of chromium carbide is coated on the upper sliding surface  27 , the lower sliding surface  29 , the wedge-plate sliding surface  33 , and the guide-plate sliding surface  41 , the sealing characteristics of these sliding surfaces can be maintained for a long time even when they slide in high-temperature, corrosive environments, as well as in particulate environments. 
         [0115]    The wedge plate  21 , valve unit  23 , and the guide plate  25  are supported by the outlet portion  17  and the holding portions  83 . 
         [0116]    The center of gravity of the wedge plate  21  and the guide plate  25  is towards the first end. Accordingly, the center of gravity of the wedge plate  21 , the valve unit  23 , and the guide plate  25  is positioned closer to the first end than the guide-plate channel  43 , regardless of the position of the valve unit  23 . A downward moment is thus always exerted at the first-end portion of the guide plate  25 . 
         [0117]    Because this downward moment is supported by the spring  89  provided between the holding shaft  85  and the guide plate  25 , the guide plate  25 , the valve unit  23 , and the wedge plate  21  are always kept horizontal. 
         [0118]    Because the second-end portion of the guide plate  25  extends closer to the second end than the guide-plate channel  43 , the valve unit  23  is guided by the guide plate  25 , even when the valve unit  23  having the center of gravity closer to the second end than the valve unit channel  31  moves to the second end. 
         [0119]    Therefore, the valve unit  23  is supported by the guide plate  25  and slides substantially horizontally; thus, a stable motion can be realized. 
         [0120]    In addition, adjustment of the horizontal angle of the guide plate  25  is performed by raising and lowering the holding shaft  85 . 
         [0121]    Because an appropriate gap a to allow thermal expansion of the valve box  3  is provided between the guide plate  25  and the holding shaft  85 , the holding shaft  85  does not abut against and press the guide plate  25  upward, even when the valve box  3  thermally expands. 
         [0122]    Because the valve unit  23  is coupled to, for example, the valve stem  75  with a gap therebetween, there is a possibility that it may shift from side to side during operation. In addition, the valve unit  23  may also rock due to the presence of a small amount of particulate materials adhering to the sliding surfaces. 
         [0123]    When the valve unit  23  rocks from side to side during sliding, it slides while abutting against the leading plate  67 , which prevents the problem of the valve unit  23  abutting against the valve box body  5 . Because the leading plate  67  is made of a material exhibiting excellent mechanical sliding properties, seizure and scoring do not occur even during contact and sliding. The sliding motion of the leading plate  67  is limited by the guard  71 , even when the valve unit  23  slides in contact with the leading plate  67 . 
         [0124]    Because the leading plate  67  is provided on the valve box body  5  in a detachable manner, when the valve unit  23  becomes worn due to contact and sliding, maintenance can be easily carried out by changing only the leading plate  67 . 
         [0125]    By providing the guard  71 , play of the guide caused by a load in the stroke direction can be prevented. 
         [0126]    Next, disassembly and inspection of the gate valve  1  will be described. 
         [0127]    When the interior is checked, the lid at the first end  7  is removed, thereby widely opening the first end. By visually examining the sliding paths of the wedge plate  21  and the guide plate  25  via this opening, and determining the degree of wear and tear to the sliding surfaces, including the valve unit  23  or other parts, it is possible to determine in advance whether or not inspection is required. 
         [0128]    In addition, by checking for particulate materials left inside the valve box  3 , it is possible to determine whether or not it is necessary to perform adjustment and inspection of the contact pressure of the sealing gasket  57  and the guide-plate sealing gasket  63 , and the contact surface force between the upper sliding surface  27  and the wedge-plate sliding surface  33 , and between the guide-plate sliding surface  41  and the lower sliding surface  29 . 
         [0129]    The valve unit  23  is removed from the opening portion by rotating the valve stem  75  by 90° and disengaging the valve stem  75  from the valve unit  23 . In this state, a working space is provided, allowing the wedge plate  21  and the guide plate  25  to be removed. Accordingly, maintenance, such as inspection, replacement, and assembly, can be performed inside the valve box  3  without removing the valve box  3  from the pipe in which it is installed. 
         [0130]    New parts can be easily assembled by following this procedure in reverse. A valve-rod rotation lock  91  is fixed to the valve stem  75  with a tapered pin  93 , thus preventing it from disengaging from the valve unit  23  during operation.