METER PROVERS

Abstract: A meter prover for calibrating a flow meter has an inlet chamber (36), an outlet chamber (28) and a cylindrical displacement body (42) movable through an aperture (16) between the chambers. The prover is connected in series with the flow meter such that the linear movement of the displacement body is indicative of flow rate. Apertures (98, 100) enable flow through the displacement body before and after the proving run. Compensating rods (78) ensure that there is equal fluid displacement upstream and downstream of the displacement body, despite the necessary presence of a communicating shaft (60).

Description: - - METER PROVERS This invention relates to meter provers for use, for example, in proving rotary flow meters 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 k factor and it is the purpose of the prover to enable calibration of the meter k 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 0.02#. The conventional form of meter prover utilises 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 metres in length were not uncommon. Provers of this size are clearly impractical on oil rigs and in other confined spaces and efforts have therefore been made to develop so-called compact provers. By increasing the inherent accuracy of the proving operation with the use of electrical pulse handling techniques, it has been possible to reduce the displaced volume which is necessary for the desired 0.02% accuracy and compact meter provers have been produced in the form of a piston and cylinder having a stroke in the order of one metre. Compact piston and cylinder provers have been produced in uni-directional and bi-directional forms. In both cases, the prover cylinder has a bore which is accurately cylindrical to within very close limits and a highly polished surface. A piston moves within the bore and is provided with suitable annular sealing means. In a bi-directional meter prover, there may be two rigid piston rods extending in opposite axial directions from the piston and there are associated with these piston rods the displacement sensors and "nudge" piston arrangements well known in the art. It is of course critical that there should be no leakage passed the annular piston sealing means and a practice has developed of providing two spaced sealing rings with means for detecting encroachment of fluid within the annular chamber defined by the sealing rings. This detection means usually requires the presence of an axial bore along at least one piston rod communicating with a leak detector. Bi-directional meter provers are useful in that they maximise the number of proving runs that can be made in any time interval but do involve the duplication of certain components and also require valve arrangements for reversing flow through the prover. Such valve arrangments must of course be shown to be leak-proof and at larger prover sizes this may become prohibitively expensive. Uni-directional meter provers are provided with auxiliary means for returning the piston to a start position. This may comprise a slave piston and cylinder or, in larger sizes, a simple winch. Both uni and bi-directional provers require a very accurately machined prover bore of a significant length. This - as will be evident - is both expensive to manufacture and demanding of the support structure, if dimensional stability is to be assured over a range of working temperatures and pressures. There is a further difficulty, not previously appreciated, that will arise with many uni-directional provers. If a piston rod, or some other means of communication with the piston, is provided at one side only of the piston, it will usually be the case that on movement of the piston 10 different volumes of fluid are displaced upstream and downstream of the piston. This may introduce errors into the flow measurement, as will be described. In a typical arrangement, the meter to be proved is connected in parallel with a section of the working fluid flow path but normally isolated through valves. On initiation of a proving run, the valves are so operated that the entire flow passes through the prover with the piston of course moving in synchronism with that flow, over the length of its stroke. If unequal volumes of fluid are displaced upstream and downstream of the piston, there will be an instantaneous change - and usually an increase - in the flow rate out of the prover. In a typical installation, the flow downstream of the prover may have a substantial mass and such an instantaneous flow change may have significant shock effects. Moreover, it is well known that flow meters of the type which are to be proved in way generall have characteristics which are not totally independent of the flow rate. Accordingly, efforts are made to prove the meter under conditions which are as close as possible to the normal working conditions. For the reasons set out above, it will be appreciated that in some cases the actual flow rate during a piston run is different from that which would be expected and, depending upon the position of the operating point within the characteristic of the meter, this flow rate discrepancy may introduce a significant error. It is an object of one form of this invention to provide a meter prover which may be constructed in a simple and economic manner. Accordingly the present invention consists, in one aspect, in a meter prover for proving a flow meter in a fluid path, comprising an inlet chamber having an inlet; an outlet chamber having an outlet and communicating with said inlet chamber through an aperture, the prover being adapted for connection with the inlet and outlet in series with the meter to be proved, and a displacement body movable in sealing engagement through said aperture on entry of fluid into said inlet chamber to displace a corresponding volume of fluid from the outlet chamber, the amount of movement of the displacement body being an accurate measure of flow through said meter; a flow path through said displacement body to enable a flow to be established for the inlet to the outlet chamber prior to movement of the displacement body and means for selectively blocking said flow path. In order to sense externally the position of the displacement body and to apply forces, where necessary, to the displacement body, it is necessary to have communication means are required which are analogous with the piston rod or rods of conventional compact provers. It is an object of a preferred form of the invention to provide a meter prover with improved such communication means which avoid the difficulty described above of unequal fluid displacement. Accordingly, in a preferred form of the invention communication means are connected with the displacement body and extend through the inlet or outlet chamber to enable exterior communication with the displacement body, the fluid volume displaced by that portion of the communication means contained within the said chamber increasing on movement in one sense of the displacement body, there being provided compensation means extending from the displacement body through the same said chamber such that the fluid volume displaced by that portion of the compensation means contained within the said chamber decreases as the displacement body moves in said one sense, whereby communication is made at one side only of the displacement body whilst maintaining equal flow displacements upstream and downstream of the displacement body. Advantageously, the communication means comprise first shaft means extending from the displacement body and wherein the compensation means comprise second shaft means extending from the displacement body. It will be understood that the dimensions of the first and second shaft means are so selected to satisfy the condition of equal flow displacement upstream and downstream of the displacement body. In one example, the first shaft means comprises a single shaft extending along the axis of the displacement body and the second shaft means comprises two shafts equal in aggregate cross sectional area with the first shaft and both extending parallel to the first shaft. In known compact provers, whether uni-directional or bi-directional, means are provided for launching the piston into the bore, usually after steady flow through the bore has been established. In some cases a nudge piston is provided which acts over a short distance only, with the piston being propelled thereafter by the fluid. A small fluid pressure differential developed across the piston serves to overcome friction. By keeping friction as low as possible, it is hoped to minimise the perturbation of fluid flow and it is presumed that flow will continue through the prover at essentially the same rate as immediately prior to launching of the piston. In other known arrangements, positive steps are taken to overcome friction and the auxiliary piston and cylinder arrangement that serves to launch the piston maintains engagement with the piston throughout the proving stroke, applying a force calculated to balance the frictional force. On the basis of the calculations, it is presumed that the flow through the prover during the proving stroke will be essentially the same as the flow immediately prior to launching. There is believed by the present Applicants to be significant advantage in ensuring that flow through the prover is within very close limits unaffected by the presence of the piston in piston and cylinder provers or of the displacement body in a prover according to this invention. Moreover, the equality of flow before and during proving should be verifiable. I Accordingly, the present invention consists in a further aspect in a meter prover for proving a fluid flow meter, comprising chamber means arranged for fluid flow therethrough in series with the meter; a body movable in sealed engagement within the chamber means in synchronism with fluid flow therethrough; means for launching the body into said flow and means for continuously monitoring the position of the body within the chamber means; wherein said means *for ' launching the body is so controlled in response to the meter output and the instantaneous velocity of the body as derived from said position monitoring means as to bring the body to the velocity associated with that flow through the chamber existing prior to launching of the body. Preferably, said launching means are controlled in response to a delayed meter output signal. Advantageously, said launching means comprises a hydraulic linear actuator. This invention will now be described by way of example with reference to the accompanying drawings in which:- Figure 1 is a section on split radial planes (shown as AA in Figures 2 and 3) through a meter prover according to this invention, Figure 2 is an axial section on line B-B of Figure 1, Figure 3 is an axial section on line C-C of Figure 1, and Figure 4 is a block diagram illustrating the instrumentation which is associated with the meter prover of Figure 1. The meter prover shown generally at 10 comprises a base plate 12 to which the remaining components are bolted or otherwise suitably secured. An aperture plate 14 is arranged generally centrally of thebase plate and 12 has an aperture 16 aligned coaxially with the axis of the prover shown at 18. A cylindrical outlet sleeve 20 is secured through welds 22 to the aperture plate and, as seen best in Figure 3. is positioned eccentrically of the prover axis 18. An outlet port 2k extends radially from the sleeve 20 in the region of maximum offset from the prover axis. The outlet sleeve 20 is , closed at the end remote from the aperture plate by an end plate 26 so defining a cylindrical outlet chamber shown at 28. To the opposite face of the aperture plate 14 there is connected an inlet sleeve 30 of similar length to but of larger diameter than the outlet sleeve 20. The inlet sleeve 30 " is arranged coaxially wj.th the prover axis and is secured to the aperture plate 14 through a substantial flange 32. The inlet sleeve 30 is closed at its free end by an end plate 34 to form an inlet chamber shown at 36- An inlet port 38 communicates with this chamber, there being provided inwardly of the chamber an arcuate baffle plate 4θ which opposes the inlet port 38. A displacement body shown generally at 42 is arranged for sliding movement through the aperture 16. The displacement body comprises a displacement tube 44, the outer cylindrical surface of which is in complementary engagement with the aperture 16. To reduce friction and to prevent leakage, the aperture 16 is provided with two bearing collars 46 and with two annular sealing arrangements 48. A channel 50 in the aperture plate 14 enables exterior sensing of the fluid pressure in the annular chamber defined between the two sealing arrangements 48. Within the displacement tube 44, there is positioned a cylindrical buoyancy member 52. This buoyancy member is sealed and is aligned coaxially with the displacement tube. A radially outwardly directed flange 54 on the buoyancy member 5 is bolted or otherwise suitably secured to an inwardly directed flange 6 on the displacement tube. At the end of the buoyancy member remote from the aperture plate, two ears 8 provide a mounting for one end of a communication shaft 60. This shaft 60 extends through an aperture 62 in the end plate 34 and forms part of a hydraulic linear actuator shown schemmatically at 64. A twin-lobed plate 66 is secured to the end of the displacement tube remote from the aperture plate 14. The plate 66 has a central collar 68 which supports the intermediate region of the shaft 60. Four quadrant shaped apertures 70 are provided in the plate 66 to allow full fluid access to the interior of the displacement tube 44. Diammetrically opposed lobes 7 and 74 on the plate 66 carry respective collars 7 which support corresponding ends of respective compensation shafts 78 (only one of which is seen in Figure 1) . The compensation shafts 78 extend through corresponding apertures 80 in the aperture plate, each aperture 82 having a suitable bearing and seal arrangement shown generally at 82. The uppermost compensation shaft 78 communicates at its free end with a rider 84 which slides along a track 86. This track is supported at one end on the aperture plate 16 through anchorage 88 and at the other end from the end plate 26 through cranked support member 90. The rider 84 and track 86 together form a length encoder which may take a variety of known forms. The function of the length encoder is to provide an electrical signal indicative of the position of the displacement body. To avoid any tendency of the displacement body to twist within the meter prover, angled guide rails 9 are provided on the inner surface of the inlet sleeve 30 so as to engage opposite edges of lobe 7 on the plate 66. In order to support the opposite end of the displacement body when it enters the outlet chamber 28, a series of three support rings 94 are mounted within the outlet sleeve 20. As seen best in Figure 3 t each guide ring 9 is mounted on twin posts 96 and is arranged eccentrically within the outlet sleeve 20 so as to be centred on the axis 18 of the prover. For a purpose to be described, the displacement tube 44 is provided with an array of apertures 98 in the region between the free end of the displacement tube and the flange 54. The diameters of the apertures 98 decrease in the array in the axial direction away from the aperture plate 14. A similar ring of apertures 100 is provided towards the opposite end of the displacement tube. As illustrated in diagrammatic form in Figure 4, a processor receives inputs both from the length encoder comprising rider 84 and track 86 and from the meter to be proved. A controller for the linear actuator 64 also receives the meter output together with a signal from the length encoder which represents the instaneous velocity of the displacement body. The manner of operation of the described meter prover can now be understood. The prover inlet 38 and outlet 24 are connected through suitalbe valves with the fluid path containing the meter, such that when it is desired to initiate a proving operation, the prover may be connected in series with the meter. In this state, and with the displacement body in the position shown in Figure 1, fluid will pass through inlet port 38 into the chamber 36, escaping through apertures 98 to the interior of the displacement tube 44 downstream of flange 54 and thereby to the outlet chamber 28 and outlet port 24. To initiate a proving operation, linear actuator 64.is caused to move the displacement body to the right as shown in Figure 1. In this movement it will be appreciated that apertures 8 are gradually closed with the fluid pressure acting upon the displacement body increasing until a point at which all of the apertures 98 have passed into the aperture 16. During this first phase, the controller for the linear actuator is arranged so that a fixed and predetermined force is applied to the displacement body. In a second phase, which commences immediately the apertures 98 have been closed, a sufficient force is applied to the displacement body to bring it into synchronism with the true fluid flow, that is to say the flow that would pass through the prover in the absence of the displacement body. This is achieved by the controller comparing a signal from the meter to be proved with a signal from the length encoder which represents the velocity of the displacement body. Since the displacement body may initially be responsible for a slowing down of the meter, the signal which is taken is representative of the flow rate immediately before the proving run is initiated. At the beginning of this second phase the velocity of the displacement body will ordinarily be less than that associated with true fluid flow rate. The controller therefore ensures that a sufficient force is applied to the displacement body to bring it to the correct velocity. This done, the third phase is entered in which the displacement body moves in synchronism with the unperturbed fluid flow and a corresponding volume of fluid will be displaced from the outlet chamber 28 through the port 24. During this third phase, the hydraulic pressure in the actuator is maintained constant and the processor is arranged to carry out the proving function by comparison of the known movement of the displacement body with the meter output. In case where the time required for the proving run is long compared with the period of variations in flow rate through the meter, it will be advantageous to control the velocity of the displacement body throughout the run. Since the aggregate cross sectional area of the shafts 78 equals that of the shaft 60, the area of the displacement body upstream upon which fluid pressure acts is exactly equal to the downstream area. There is accordingly no unbalanced force applied to the piston through fluid pressure, as might occur with a conventional arrangement of a piston and single piston rod. In movement of the displacement body, it will be recognised that the portion of shaft 68 contained within the inlet chamber 36 will increase whilst the portions of shafts 78 within the inlet chamber will decrease. The volume of fluid entering the inlet chamber 36 will therefore be precisely the volume of that portion of the displacement body leaving the inlet chamber which is in turn precisely that volume of fluid displaced from the outlet chamber. The rate at which fluid leaves the port 24 is therefore equal to the rate at which fluid enters the port 38 and the difficulties of instantaneous changes in flow rate which are discussed in the introduction to this specification are neatly avoided. Towards the end of the proving stroke, the apertures 100 gradually open to the outlet chamber. Fluid pressure on the displacement body is therefore reduced gradually so avoiding shocks associated with sudden deceleration. The presence of the baffle plate 14 opposite the inlet port 38 reduces the risk of a side loading on the displacement body through the velocity head of the inward fluid flow. The baffle plate dissipates the energy of the incoming flow and helps to direct the flow equally around the circumference of the displacement body. In the outlet chamber, the above-described eccentricity of the displacement body relative to the chamber serves a useful purpose. Towards the end of the proving run, when the apertures 100 begin to pass through the aperture 16, there will be a flow radially outwardly of the displacement body. That flow will then pass circumferentially around the displacement body towards the outlet port 24. Starting from a position diametrically opposed to the outlet port, the circumferential flow will increase in a direction toward the inlet port as the contributions are added from more and more of the apertures 100. The eccentricity of the displacement body provides that the transverse dimension of the flow path for this circumferential flow slowly increases as the flow itself increases. In this way, any side loading on the displacement body is significantly reduced. It will be understood that because of the steps taken to ensure that the displacement body is injected in true synchronism with fluid flow and to avoid differential fluid displacement upstream and downstream,the meter is proved under flow conditions which are virtually identical with conditions existing in the absence of the prover. In the described meter prover, the critical engagement is between the exterior of the displacement tube 44 and the aperture 16 with its bearing collars 46 and seals 48. The outer surface of the displacement tube 44 must be accurately cylindrical to within the same limits as the bore of a conventional compact prover but the skilled man will appreciate that the operation of forming an external cylindrical surface to within close tolerances is markedly simpler than is the case with an internal cylindrical surface or bore. Moreover, the bore of a conventional compact prover is usually subject to the pressure and temperature of the fluid on its interior surface but to ambient, or at least some other pressure and temperature on its external surface. In the case of the displacement tube 44, this is totally immersed within the fluid and is not subject to distortion through differential pressure or temperature effects. It should further be noted that the critical seals 48 are fixed so that the detection of leaks is much simplified. In addition it is to be expected that a meter prover according to this invention will be shorter than many conventional provers of equivalent capacity. This is likely to be particularly the case with larger bore sizes where the flanges required at both ends of the prover cylinder may add considerably to the overall length. It will be appreciated that in the described prover, the size of flange 3 is not a critical factor in determining overall length. In a further embodiment of this invention, the linear acuator 64 and length encoder are effectively combined in one device. The hydraulic actuator then takes the form of a generally conventional hydraulic cylinder having a linear displacement transducer mounted within the piston rod. The transducer comprises fixed and movable coils and provides an output signal through mutual inductance. In this modification, rider 84 and track 86 are no longer required. It will be understood that with suitable feedback control, the one device can not only drive the displacement body at the correct velocity but also provide the positional information required by the processor. If fixed volume proving is used, the device is required merely to indicate when the displacement body passes the start and finish points; the displaced volume during this travel being an accurately known constant. If desired, the device can also be used to check the velocity of the displacement body during the proving run to ensure that any flow variations are within acceptable tolerance. If variable volume proving is required, the device will be arranged to provide an output signal indicative of the current displacement body position at instants of time dictated by the processor and effectively by_the pulse output of the meter to be proved'. The skilled man will appreciate the advantages, in appropriate circumstances, of the variable volume mode as opposed to the fixed mode of proving. In the variable volume mode, further accuracy can be achieved by determining in a calibration process the displacement volumes associated with incremental changes in position of the displacement body over the entire stroke. The output of the displacement transducer can then be used to select the start and finish points for an effective integration of the incremental displacement volumes. It should be understood that this, invention has been described by way of example only and a variety of modifications are possible without departing from the scope of the invention. Thus, whilst the described displacement body and aperture are cylindrical, an alternative construction would be possible of, say, elliptical form. This may be more expensive to produce but will of itself prevent twisting of the displacement body. The described construction is formed largely as a fabrication from tubes and shafts of standard dimensions. This enables substantial economies to be made in manufacture though it should be understood that cast and machined or other constructions are possible. The advantages offered by the compensating rods 78 have been fully described. It should.be noted that a single compensating shaft of. appropriate cross-sectional area or indeed other forms of compensating shaft means could alternatively be used. In a more radical modification, the compensation meas may take forms other than shaft means. The important feature is that the changing volume displaced by the rod 60 (or indeed other means of communication with the displacement body) should be compensated for by a change in volume displaced by the compensation means. If the communication means are more conveniently positioned in the outlet rather than in the inlet chamber, the position of the compensation means will similarly be reversed. The manner in which side loading of the displacement body in the outlet chamber is reduced by eccentric mounting of the outlet chamber should be regarded as one example only of a technique of radial balancing. A similar effect can be achieved by the suitable positioning of baffles within an otherwise concentric outlet chamber or the required circumferential variations in flow path could alternatively be achieved by varying the diameters of the apertures 100. It may, in appropriate circumstances, be desirable to employ a similar technique in the inlet chamber although the effects of side loading are here not so powerful, having a regard to the support of the displacement body at both ends. In certain circumstances the circumferential variation in flow rate into or out of the displacement body may be useful in disturbing sediment settling under gravity. It is important to establish a flow through the displacement body and then have the ability to block that flow in the proving run. The apertures 98 and 200 provide a simple method of achieving that aim but more complicated alternatives such as valves within the displacement body will suggest themselves to the skilled man. The use of guide rings 94 should again be regarded as one example, in an alternative, a spiral cage could be used or a rider positioned on the free end of the displacement body for engagement within a suitable track fixed relatively to the outlet chamber.

Claims: CLAIMS 1. A meter prover for proving a flow meter in a fluid path, comprising an inlet chamber having an inlet; an outlet chamber having an outlet and communicating with said inlet chamber through an aperture, the prover being adapted for connection with the inlet and outlet in series with the meter to be proved; and a displacement body movable in sealing engagement through said aperture on entry of fluid into said inlet chamber to displace a corresponding volume of fluid from the outlet chamber, the amount of movement of the displacement body being an accurate measure of flow through said meter; a flow path through said displacement body to enable a flow to be established ». from the inlet to the outlet chamber prior to movement of the displacement body and means for selectively blocking said flow "path. 2. A meter prover according to claim 1, wherein the displacement body comprises a cylindrical surface in complementary and sealed engagement with said aperture. 3. A meter prover according to claim 1 or claim 2, wherein communication means are connected with the displacement body and extend through the inlet or outlet chamber to enable exterior communication with the displacement body, the fluid volume displaced by that portion of the communication means contained within the said chamber increasing on movement in one sense of the displacement body, there being provided compensation means extending from the displacement body through the same said chamber such that the fluid volume displaced by that portion of the shaft means contained within the said chamber decreases as the displacement body moves in said one sense, whereby communication is made at one side only of the displacement body whilst maintaining equal flow displacements upstream and downstream of the displacement body. 4. A meter prover according to claim 3 wherein the communication means comprise first shaft means extending from the displacement body and whereing the compensation means comprise second shaft means extending from the displacement body. 5. A meter prover according to claim 4, wherein said first and second shaft means extend in opposite directions from respective points of connection with the displacement body. 6. A meter prover according to any one of claims 2 to 5, wherein said flow path through the displacement body includes apertures in said cylindrical surface. 7. A meter prover according to claim 6, arranged and adapted such that the radial gap between the displacement body at said apertures and the opposing surface of the relevant chamber varies with the distance circumferentially from the corresponding inlet or outlet port. 8. A meter prover for proving a fluid flow meter, comprising chamber means arranged for fluid flow therethrough in series with the meter; a body movable in sealed engagement within the chamber means in synchronism with fluid flow therethrough; means for launching the body into said flow and means for continuously monitoring the position of the body within the chamber means; wherein said means for launching the body is so controlled in response to the meter output and the instantaneous velocity of the body as derived from said position monitoring means as to bring the body to the velocity associated with that flow through the chamber existing prior to launching of the body. 9. A meter prover according to claim 8 wherein the meter output is delayed. 10. A meter prover according to claim 8 or claim 9 wherein said means for launching incorporates said means for monitoring position.

Inventors: HALL ANTHONY, PIX KEITH ANTHONY, SPENCER COLIN JOHN, HALL, ANTHONY, PIX, KEITH, ANTHONY, SPENCER, COLIN, JOHN
Applicants: SKELTONHALL LTD, SKELTONHALL LIMITED
Application year: 1986
Main classification: G01F25
IPC codes: G01F25