METER PROVERS

Abstract: A meter prover comprises a cylinder (16) having a proving bore (22) and enlarged end chambers (20). A piston (12) is mounted within the cylider for movement along the bore in sealed engagement therewith, the axial length of the piston exceeding that of the enlarged end chamber. When the piston is at rest in the end chamber, fluid can pass through a continuously open passage in the piston. This passage preferably takes the form of apertures (24) in an annular wall portion (36). When the piston is moved out of the end chamber, flow through the passage is no longer possible and the piston continues in synchronism with the fluid flow. At the end of the proving run, the progressive closing of the apertures provides a fluid damping effect to arrest the piston.

Description: DIE ER PROYERS This invention relates to meter provers for use, for example, in proving rotary flowmeters of the type producing an electrical output pulse per increment of rotation. The number of pulses per unit volume is a characteristic of the meter which is defined as the factor and it is the purpose of the prover to enable calibration of the meter factor. A principal application is the measurement of oil flow rates and it is here a requirement for the meter k factor to be determined to an accuracy of at least 0.02J&. The conventional form of meter prover utilizes the passage of a sphere along an accurately dimensioned pipe between fixed detectors to displace a known volume of fluid. The volume of fluid displaced by the prover passes in series through the meter to be proved and the number of pulses generated in the meter during the passage of the sphere between the detectors is counted to enable determination of the k factor. To achieve the necessary high accuracy, a large displaced volume is essential and provers of 20 meters in length are not uncommon. Proposals have been made to reduce the size of meter provers by using piston and cylinder arrangements and by reducing inherent inaccuracies in the prover to the extent that large displaced fluid volumes are no longer necessary. Significant improvements in inherent accuracy have been achieved through the use of electronic pulse handling techniques. Thus, in one example, pulse interpolation, techniques are used to "inject" a train, of uniformally spaced pulses between each pulse generated ' O in. the rotary meter undergoing proving. In this way, it is possible to compare the known, volume displaced by the meter prover not merely with the number of whole pulses provided by the rotary meter but also with the number of interpolated pulses representing "fractions" of meter pulses. A second known technique for improving accuracy is concerned to deal with the fact that through irregularities in the meter turbine, equal increments of rotation may not necessarily be associated with equal displaced fluid volumes. To remove this possible source of inaccuracy, the proving run is defined not by fixed points in the cylinder but by the receipt of meter pulses. In this way it can. be arranged that the proving run covers an integral number of rotations of the meter turbine. With this technique, the piston displacement becomes a variable and must be measured with a suitable length encoder. To enable advantage to be taken of the theoretical improvements in accuracy offered by such techniques, great care must be taken to ensure that the volume measured as displaced bj the prover (whether a constant volume or variable) is identical with that flowing through the meter being proved. It is essential that there should be no leakage past the piston in the proving run and also that the proving run should be initiated under steady flow conditions and in such a manner as admits of complete reproduceability of results, within of course the constraints of unavoidable random errors. Because of the need to have steady flow conditions, it is desirable for flow to be established through the cylinder of the prover before the piston is launched on a proving run. Various proposals have been made to this end and attention is drawn in particular to the following prior art. United States patent specification No. 3492856 French published application No. 2471590 British patent specification No. 2023295B French published application No. 2481449 British published application No. 2088566 The above documents evince the improvements in theoretical accuracy achievable through pulse techniques as described above but are of primary relevance here in connection with their disclosure of different methods for launching a prover piston. In US 3492856, the piston is provided with a through aperture on the axis of the piston. A valve is provided in this aperture which is normally open to enable a flow to be established through the piston when the piston is at rest; the valve being closed to initiate a proving run. A similar approach is adopted in il 2 471 590 and GB 2 023 295B. Whilst this approach can be made to work satisfactorily, the complexity of the piston construction and the presence of moving parts within the piston increases the difficulty of ensuring that there is no leakage past the piston in the proving run. Moreover, any leakage that does occur is likely to remain undetected. In FR 2481449, a solid piston is employed with the cylinder having at each end a ring of axially extending slots. In the rest positions, the piston is disposed among the slots with the result that flow can pass around the piston into the bore. Initial movement of the piston takes it away from the slots and movement of the piston thereafter is in synchronism with flow through the bore. By the use of a symmetrical piston and the provision of slots at both ends of the cylinder bore, a bi-directional prover is obtained. To ensure that there is no leakage past the piston in the proving run, the piston carries a pair of 0-ring seals which are under compression in the radial direction when the piston is in the main portion of the cylinder bore. With the piston in either of its rest positions, the 0-ring seals will contact the slots in the cylinder wall. There will be local -^0R OM deformation of each O-ring as it relaxes outwardly at the slots. This local deformation will on repeated use of the prover lead to degradation of the seals and to increased risks of leakage. Furthermore, the piston is necessarily of small axial dimension with the result that great care must be taken to prevent twisting of the piston relatively to the cylinder in a proving run. GB 2008 566 discloses a cylinder having a proving bore with an end chamber at each end, the diameter of the end chamber exceeding that of the bore. At the beginning and end of a proving run, the piston is received in the end chamber enabling flow around the piston. When the piston lies in one of the enlarged end chambers, it is no longer aligned through its engagement with the cylinder bore and separate means must be provided to maintain alignment. This introduces a further perturbation into the flow through the cylinder and unless great care is taken, some slight re-alignment will still occur each time the piston enters the bore. Such re-alignment will impose unwanted strains on the prover and hasten the loss of seal integrity. It is an object of one form of this invention to provide an improved meter prover in which the above described disadvantages can be avoided. Accordingly, the present invention consists in one aspect in a meter prover comprising a cylinder having a proving bore and an end chamber of a diameter exceeding that of the bore, the end chamber communicating with a fluid port for connection to a meter to be proved; a piston mounted in the cylinder for movement along the bore in sealed engagement therewith and being receivable in said end chamber to permit fluid flow past the piston in a rest position, characterised in that the piston has an axial dimension exceeding that of the end chamber and that continuously open passage means are provided in the piston enabling flow through the piston between the end chamber and the bore, when the piston is in said rest position. OMP Advantageously, the passage means comprises at least one aperture disposed in the circumference of the piston communicating with an opening in the piston end face adjacent the proving bore. Suitably, annular sealing means are provided circumferentially of the piston, said aperture lying between the sealing means and the proving bore. Preferably, the piston comprises an annular wall portion coaxial with the bore and defining said opening in the piston end face adjacent the bore, said aperture being formed in the wall portion. It will be appreciated that as the piston according to this aspect of the invention is of an axial dimension exceeding that of the end chamber, the piston is continually aligned within the bore. Strictly coaxial movement of the piston relatively to the cylinder is therefore assured. As the piston begins to move into the proving bore wall of the cylinder, the proportion of each aperture remaining in the end chamber and thus available for flow through the piston, is progressively reduced. In this way, there is a smooth acceleration of the piston into the proving run. Similarly, at the end of the proving run the proportion of each aperture which is open to the end chamber will increase as the piston enters the end chamber. The pressure exerted by the fluid upon the piston will therefore reduce smoothly. In a preferred form of the invention, fluid trapped in a cylinder end portion in advance of the piston is exhausted through the passage means in the piston to the end chamber. As the piston moves towards the rest position, the proportion of each aperture available for such exhaust flow will reduce. The apertures can thus together be regarded as an orifice through which fluid trapped in the end portion is throttled, the effective orifice area being steadily reduced. In this way there is a steadily increasing force bringing the piston to rest. In GB 2088566 the described piston is provided with two circumferentially extending seals between which and the cylinder bore is defined an annular chamber. A channel in the piston communicates between this annular chamber and an external pressure sensor for the purpose of checking the integrity of the seals. It will be understood that when the piston is in the enlarged end chamber, the space between the piston seals is exposed to the fluid pressure in the bore. Accordingly, when the piston enters the bore the fluid pressure in the annular chamber between the seals will be the same or closely similar to that in the cylinder bore. Under these circumstances, any leakage past the seals which is not extreme is. unlikely to result in a measurable pressure change. It is an object of a further form of this invention to provide an improved meter prover in which any fluid leakage past the piston can be detected. Accordingly, a further form of this invention is characterised in that means are provided for establishing, after initial movement of the piston from its rest position, a fluid pressure differential between the bore and the annular chamber defined between the two piston seals. In this way, any leakage past one or other of the seals will _e detected as a loss of pressure differential. The invention will now be described by way of example with reference to the accompanying drawings in which:- Figure 1 is a diagrammatic sectional view of a meter prover according to this Invention, Figure 2 is a perspective view of the piston shown in Figure 1, Figure 3 is a diagrammatic section view of a detail of the prover shown in Figure 1, Figure 4 is a diagram illustrating the length encoding system of the prover shown in Figure 1, and Figure 5 is a diagram showing pulse outputs from certain parts of the system illustrated in Figure 4- Referring first to Figure 1, the meter prover shown generally at 10 comprises a piston 12 rigidly mounted on a piston rod 14 for movement within a cylinder 16. The cylinder is formed with a relatively thick wall 18 having towards each end thereof an annular groove in its interior surface defining a chamber 20 of enlarged diameter. The groove is bounded by faces 22 inclined at an angle © * £. to the cylindrical axis. Between the enlarged chambers, the cylinder is of constant internal diameter providing a proving bore 22. Outwardly of each enlarged chamber, an arresting portion 24 of the cylinder has the same diameter as the proving bore, these end portions being closed with cylinder caps 26. The two enlarged chambers 20 communicate with respective fluid ports 28 through which the cylinder is connected with a four way valve 30. In known manner, the four way valve can be used to connect the cylinder in series with the flowmeter (shown schemmatically at 32) for movement of the piston 12 in either direction relatively to the cylinder. Calibration take-off ports 33 are provided to enable the volume displaced by the piston to be collected and measured for calibration of the prover. Referring additionally to Figure 2, the piston 12 comprises a central disc portion 34 from which integral and annular wall portions 36 extend in opposite directions. Within the central portion 34 the piston is provided with circumferential grooves 38 which receive 0-ring seals 40. Each .annular wall portion 36 is formed with a series of three rectangular apertures 42 which are equiangularly spaced about the circumference of the piston. Externally of the cylinder, the piston rod 14 carries two sliders 44, one each side of the cylinder. These sliders cooperate with fixed alignment rods 46 which extend in opposite directions from the cylinder and have there free ends anchored in plates 48. The left hand slider 44 carries a head 50 which cooperates with a pair of linear encoders 52 extending from the cylinder to the left hand plate 48, parallel with the piston rod. The arrangement of linear encoders will be described more fully hereinafter. When the piston is in the rest position shown in Figure 1, fluid can pass radially of the piston from the end chamber through the apertures 42 and then axially outwardly through the opening defined by the end of the wall portion 36. At the start of the proving run, the piston is moved to the right as shown in Figure 1. During this movement, the proportion of apertures 42 available for flow through the piston is progressively reduced until seals 40 engage the bore, whereupon further flow past the piston is prevented and the piston is carried forward in synchronism with the fluid flow through the cylinder. By this means, a steady flow can be established in the cylinder, with the piston in a rest position yet still axially aligned within the cylinder. As the piston is moved away from its rest position to initiate a proving run, the proportion of the flow that "by-passes" the piston seals is progressively reduced so that a steadily increasing fluid pressure is applied to the piston. As the piston is continually aligned within the cylinder, engagement of the seals within the proving bore causes the minimal perturbation to the flow. At the end of the proving run, the piston will enter the enlarged chamber 20 at the opposite end of the cylinder. The two sets of apertures in the piston then serve differing functions. As soon as the seals 40 enter the enlarged chamber, some flow is possible through the open end of the left hand annular wall portion and radially outward of the apertures 42. Initially, the proportion of each aperture available for such flow is small but this increases as the piston moves to the right until the full area of the apertures Is used. In this way, the fraction of the flow which is permitted to pass through the piston increases steadily from nil, when the piston is still in the proving bore, to unity, when the piston is in the rest position. It follows that the fluid pressure acting upon the piston is reduced steadily from a maximum to a minimum. Considering now the fluid trapped to the right of the piston in arresting portion 24, this will be forced out through the right hand apertures 42 as the piston moves to its rest position. Initially, the full area of the apertures is available for this exhaust flow but the effective area is steadily decreased as the piston moves to the right. The effect is that the fluid trapped in the end portion of the cylinder is throttled through an orifice, the effective area of which is steadily decreased. As a result, the trapped fluid has a progressively increasing damping affect upon the piston causing it to be brought smoothly to rest without imparting a fluid shock wave to pipes associated with the prover or damaging itself on the end cap 26. The rate of deceleration will be dependent upon the geometry of the apertures 42; the angle -^^at which the cylinder bore opens to the enlarged end chamber 20 and the fluid viscosity. The first two factors will be chosen in the design of the prover to produce the desired deceleration characteristic. In a modification (not illustrated) the apertures in the piston wall take the form of slots opening to the circular wall edge. In another modification, the described three apertures are replaced by a larger number of apertures having a smaller circumferential dimension. The aggregate aperture area remains preferably equal to or greater than the cross sectional area of the fluid port 28-. It is highly desirable that each aperture should be of significant axial extent in order to achieve the discussed smooth launching and arresting. Each aperture should preferably occupy 30 or more of the length of the associated piston half, or 15 of the total piston length. It should be recognised that whilst the use of an annular piston wall portion with circumferential apertures is preferred, other forms of passage means could be provided communicating between an aperture in the piston circumference and an opening in the piston end face; for example a drilled bore in a solid piston. The enlarged chambers 20 in the cylinder can be formed otherwise than by cutting an annular groove in a thick walled cylindrical tube. The cylinder could for example be fabricated from relatively thin section material with a central portion of a first diameter, chamber portions of a second, increased diameter and intermediate connecting sections of truncated conical form. The Interior of such a fabricated cylinder could be coated with phenolic resin or the like to provide the necessary smooth bore. Reference is now made to Figure 3 which shows certain details of the prover of Figure 1. Between the two piston 0-ring seals 40 there is formed a channel 0 extending radially of the piston and connecting with a first axial bore 52 in the piston rod 14» Remotely from the piston, the piston rod carries a housing 54 supporting a plunger 56 which cooperates with a cavity 58 in the piston rod, this cavity opening to the bore 52. A compression spring 60 acts between the housing and a flange 62 formed on the plunger urging a finger 64 of the plunger into engagement with a fixed track 66 extending in parallel with the piston rod. The fixed track includes a raised portion 68 bounded by shoulders 70 and 72. A second bore 74 in the piston rod communicates at the inner end with a passage 76 opening to the interior of the cylinder. Outwardly of the plunger 56 , the piston rod carries a differential pressure transducer 78 having ports connected respectively with the bores 52 and 74. A launching piston 80 is mounted at the side of the cylinder and has a piston rod 82 extending through an aperture 84 in the cylinder end cap 26 so as to engage the central disc portion 34 of the main piston. The launching piston 80 is mounted within a pneumatic cylinder 86 of the double acting type. In a modification, the launching piston is arranged to act upon the piston rod, externally of the cylinder. With the piston in the rest position shown in Figure 3, the pressure will be equal at both ports of the pressure transducer 78. As the launching piston 80 is actuated to initiate a proving run, the finger 64 of the plunger will ride over track 66 with the plunger remaining initially in its outer position. After both seals 40 have entered the proving bore, finger 64 will encounter the shoulder 70 of track 66 urging the plunger upwards and increasing the fluid pressure within bore 52 and within the annular space between the two seals. The pressure transducer 78 will then measure the difference between this increased pressure and the cylinder pressure as present in bore 22. Provided that there is no leakage past either of the seals 40, this pressure differential will remain throughout the proving run, disappearing only as finger 64 passes the second shoulder 72 of the track and the seals 40 enter the enlarged chamber at the opposite end of the cylinder. Any leakage past the seals during the proving run will be immediately detectable as a decay in the differential pressure detected by transducer 78. The imposition of an increased pressure difference across the seals, ensures that even slight leakage is detectable. The use of a transducer measuring a differential pressure, rather than the absolute fluid pressure in the space between the two seals, further helps to improve the sensitivity of the check against leaks. In a modification, means other than the described plunger could be employed to establish the desired pressure differential across the seals. A remotely operable solenoid could be employed for example and, moreover, a negative rather than a positive pressure change could be used. Referring now to Figures 1 and 4, piston rod slider 44 carries a head 50 which tracks over two parallel linear encoders 52. The individual encoders are of known form comprising a square-wave winding which is energised from excitation circuit 90. According to this form of the invention, two encoders are provided, the windings of which are 180° out of phase in space. The head carries two head windings 92 associated one with each of the encoders, the arrangement being such that a voltage Is generated in the winding having a substantially sinusoidal dependence upon piston displacement. The outputs of the two windings are fed to corresponding pulse shapers 94 which produce pulse trains having a frequency equal to that of the sinusoidal winding voltage. The pulse trains are fed to a processing unit 96. In a conventional, single length encoder of this type, output pulses are counted to provide an indication of displacement from a datum point. According to this form of the invention, pulses are counted alternately from the two encoders. Referring to Figure 5, graphs a) and b) show respectively the outputs of pulse shapers 94- Graph c) shows the result of leading edge triggered counter operating on the combined pulse trains. The first count is associated with pulse train a), the second count with pulse train b) and so on. It will be seen that in this way the spatial resolution is doubled for a particular encoder winding and also the maximum frequency response. It may be difficult to achieve a single winding with the necessary resolution and frequency response so that this aspect of the invention is of considerable importance. Of course phase differences of other than 180° between the two encoders can be employed and, Indeed, more than two encoders can be applied. With n encoders, a pulse difference of "ff/n would be convenient. In addition to increasing the resolution and frequency response, the use of two or more encoders provides detector security in the sense that an error in any one encoder will be apparent. With reference to graph d), it will be seen that drift in the encoder associated with pulse train b) introduces non-uniformity in the counter output which through suitable logic can be detected. The drift may be due to temperature variations or to damage and errors other than drift can similarly be detected. With regard to temperature variations, two encoders can usefully be arranged to have different thermal characteristics. OMPI The provision of length encoders enables the described meter prover to be used in the variable volume mode described previously, that is to say with the beginning and end of the proving run defined by pulses from the meter. The prover can also be used in a constant volume mode with the number of meter pulses counted (preferably with pulse interpolation) during the passage of the piston between fixed detectors 100. These detectors may be photoelectric in nature or even be incorporated in one of the length encoders in the form of local irregularities of the winding. In the constant volume mode the length encoders provide the useful facility of enabling a continuous comparison to be made between rotational velocity of the meter rotor and linear velocity of the piston. Any short term variation in rotor velocity which is not mirrored by a corresponding variation in piston velocity may indicate a meter error, for example juddering or sticking of the rotor. The continuous comparison also enables compensation to be made for the momentary extra drag imposed on the meter roto -by the launching of the piston. The length encoder or encoders further enable a check to be made against seal leakage by direct measurement of piston displacement for a known fluid volume from, for example, a standard meter. As will be appreciated, it is possible to use forms of encoder other than that specifically described. For example, encoders are available in which the winding excitation is time dependent so that an absolute position signal can be generated, removing the need to count pulses from a datum position. Further, the encoder may take other physical forms, an optical grating being one example. Whilst a parallel arrangement of two linear encoders is to be preferred since it enables mounting of the encoder head upon the piston rod, other arrangements are possible; examples being an arrangement of one linear encoder at each end of the cylinder or coaxially mounted rotary encoders.

Claims: CLAIMS 1. A meter prover comprising a cylinder having a proving bore and an end chamber of a diameter exceeding that of the bore, the end chamber communicating with a fluid port for connection to a meter to be proved; a piston mounted in the cylinder for movement along the bore in sealed engagement therewith and being receivable in said end chamber to permit fluid flow past the piston in a rest position, characterised in that the piston has an axial dimension exceeding that of the end chamber and that continuously open passage means are provided in the piston - enabling flow through the piston between the end chamber and the bore, when the piston is in said rest position. 2. A meter prover according to Claim 1, wherein said passage means comprises at least one aperture disposed in the circumference of the piston communicating with an opening in the piston end face adjacent the proving bore. 3« A meter prover according to Claim 2, wherein the or each aperture is of substantial axial extent, such that permitted low through, said passage means decreases steadily to nil over an initial piston movement away from said rest position. 4. A meter prover according to Claim 2, wherein the piston comprises an annular wall portion coaxial with the bore and defining said opening in the piston end face adjacent the bore, said aper-ture being formed in the annular piston wall portion. 5« A meter prover according to Claim 4, wherein a plurality of apertures are provided equiangularly spaced about the circumference of said annular wall. 6. A meter prover according to Claim 1, wherein the cylinder is provided with a further end chamber of a diameter exceeding that of the bore at the opposite end of the proving bore, the piston being provided with like passage means at both ends thereof enabling movement of the piston away from rest positions at either end of the cylinder. OMPI 7. A meter prover according to Claim 6, wherein the cylinder is provided outwardly of each end chamber with an arresting portion of the same diameter as the proving bore such that upon movement of the piston toward one arresting portion, fluid in that arresting portion is expelled through the corresponding passage means. 8. A meter prover according to Claim 6, wherein said piston comprises a central sealing portion provided with annular sealing means and annular wall portions extending from respective ends thereof coaxially of the bore, there being formed apertures in each wall portion. 9« A meter prover according to ' Claim 8, wherein the or each aperture in each wall portion occupies 15$ or more of the axial extent of the piston. 10. A meter prover comprising a cylinder having a proving bore and an end chamber of a diameter exceeding that of the bore, the end chamber communicating with a fluid port for connection to a meter to be proved; a piston mounted in the cylinder for movement along the bore in sealed engagement therewith and being receivable in said end chamber to permit fluid flow past the piston in a rest position, characterised in that the piston has an axial dimension exceeding that of the end chamber and that continuously open passage means are provided in the piston enabling flow through the piston between the end chamber and ' the bore, when the piston is in said rest position; wherein the piston is provided with two circumferentially extending seals between which and the cylinder bore is defined an annular chamber, there being formed channel means in the piston communicating with said annular chamber for monitoring fluid leakage past the seals, characterised in that means are provided for establishing after initial movement of the piston from said rest position a fluid pressure differential between said annular chamber and the bore. _OMP ~ 11. A meter prover according to Claim 10, wherein said means includes mechanism actuated by said initial movement of the piston. 12. A meter prover according to Claim 11, wherein the piston is mounted on a piston rod for movement therewith, the piston rod having an axial bore connecting with said channel means in the piston, characterised In that fluid pressure means are carried on the piston rod communicating with the piston rod bore and actuable to effect a change in the fluid pressure within said bore. 13« A meter prover according to Claim 12, wherein fixed camming means are provided, engagable with said fluid pressure means for actuation thereof after an initial movement of the piston away from a rest position. 14- A meter prover according to Claim 12, wherein the piston rod has a further axial bore communicating with the bore of the cylinder, there being provided a differential pressure sensor having inputs connected respectively with said piston rod bores. 15. A meter prover according to Claim 14, wherein said pressure sensor comprises a differential pressure transducer mounted on the piston rod. 16. _ A meter prover comprising a cylinder having a proving bore, a piston mounted in the cylinder for movement along the bore in sealed engagement therewith and means for continuously indicating the position of the piston relative to the proving bore, characterised in that said means comprises two length encoders providing signals indicative of the position of the piston; processing means for deriving an output position signal from said encoder signals and error detection means serving through comparison of said two signals to detect errors in either encoder. OMPI 17- A meter prover according to Claim 16, wherein said encoders have a constant relative phase displacement. 18. A meter prover according to Claim 16, wherein said encoders comprise parallel linear encoders. OMPI

Inventors: MAURER ROBERT, MAURER, ROBERT
Applicants: SKELTONHALL LTD, SKELTONHALL LIMITED
Application year: 1983
Main classification: G01F25
IPC codes: G01F25