GB2088566A - Meter provers and methods of proving flow meters - Google Patents

Abstract

A meter prover has a linearly displaceable piston 15 which is associated with a position encoder 24 adapted to provide signals denoting increments of movement of the piston along a cylinder during a proving run. The beginning and end of the proving run are defined by pulses from the meter 2 and in particular pulses which are spaced apart by an integral number of cycles of metering by the meter. Thus the actual displacement of the piston is measured during an interval defined by the meter. The piston is retracted by a piston and cylinder arrangement 30,31. <IMAGE>

Description

SPECIFICATION Meter provers and methods of proving flow meters This invention relates to the proving of flow meters and to various improvements in the design, construction and operation of meter provers.

It is ordinary practice to prove a flow meter, usually a rotary meter adapted to provide a plurality of output electrical pulses for each revolution.

Theoretically, each pulse, which represents an increment of revolution of the meter, should represent a predetermined volume of the liquid which is metered. In realitythe theoretical relationship between the number of pulses and the volume which actually flows through the meter is not precise, not necessarily linear, and it is necessary to calibrate the meter.

For this purpose it is customary to use a meter prover, which in a typical form comprises a proving loop which may be connected in a flow path that includes the meter and a sphere which can be launched into the loop. The sphere is a close fit within the walls of the loop and is adapted to cooperate with two spaced apart detector switches the actuations of which define the beginning and end of a proving run during which the meter pulses are counted. In this manner there is a comparison of the number of pulses obtained during the proving run with a volume swept out by the sphere in its passage between the positions in which it cooperates with the detector switches. That volume is, or should be, accurately known so that the volumetric increment represented by each pulse of the meter can be ascertained.

The ordinary practice of proving meters is subject to various inconsistencies which make current practice inconvenient. In particular, although the cooperation of the sphere with the detector switches should be consistent, so that the sphere always sweeps out the same volume as it moves between the positions causing actuation of the switches, the cooperation is not particularly consistent. Afurther disadvantage in practice is the susceptibility of the prover to thermal error, that is to say changes in dimension, particularly length, of the proving run. Furthermore, the use of a sphere moving round a curved path is a further source of error. Error can also arise because the meter can suffer from "intra-rotational nonlinearity", that is to say a lack of consistency between the angle through which, for example, the rotor of the flow meter rotates between successive pulses.Thus a given number of pulses does not always represent the same volume of flow through the meter.

In order to lessen the significance of the various errors, it has been customary to employ meter provers which are physically quite large, it being supposed that the inconsistencies within the meter and the prover are of lesser significance as the swept volume associated with a proving run becomes larger. Nevertheless, the size of meter provers is inconveniently large for many purposes.

It has been proposed to provide a meter prover in a form including a piston which is linearly displaced in a cylinder during a proving run; the piston, or an extension thereof, cooperates with two spaced apart position detector switches so as to provide a definition of the beginning and end of the proving run during which pulses of the flow meter are counted, the swept volume being that associated with the movement of the piston between the two positions in which it cooperates, directly or indirectly, with the position detector switches.

Although provers which have linearly displaceable pistons can in practice be smaller than provers using spheres, quite a few of the errors inherent in the customary method of proving are not avoided.

One aspect of the present invention concerns a method of proving in which the beginning and end of a proving run are defined not by the prover but by the meter, the actual distance moved by a linearly displaceable piston in the prover being measured to provide an indication of swept volume which is compared with a predetermined number of pulses from the meter to provide calibration of the meter.

This requires, as will be explained, the use of a position encoder or other means which can measure increments of movement of the piston to a sufficient accuracy. As will be explained, various errors to which a meter prover was liable, and particularly thermal changes in the length of the proving run, can be substantially reduced or eliminated. In the performance of the method now devised, it is preferable to measure the distance moved by the piston in an interval corresponding to and defined by an integral number of cycles of revolution of the flow meter; in this manner, error arising from intra-rotational nonlinearity of the flow meter can be eliminated.

A further aspect of the invention concerns the design of a flow meter which is particularly though not exclusively intended for use in a method of proving in which the beginning and end of the proving run is defined by the meter rather than the prover. The present invention provides, in this aspect, for the provision of a position encoder or other means for measuring increments of movement of a linearly displaceable piston along part of the movement thereof, so that a measure of the volume swept by the piston in response to the flow of liquid is actually measured rather that predetermined.

The present invention further provides various other improvements in the design and operation of a meter prover; these improvements will be made apparent from the following descriptions of an exemplary embodiment which is schematically illustrated in the accompanying drawing.

The embodiment of prover shown in the drawing is illustrated in connection with an inlet pipe 1 which is connected to a turbine flow meter 2 which is to be proved by means of the prover. The meter 2 is of a well-known kind adapted to provide a predetermined multiplicity of output electrical pulses for each revolution, that is to say each cycle of operation of the meter. The meter is connected to a short pipe 3 leading to a valve 4 which is at the inlet side of the prover.

The prover has an inlet pipe 5 which has a branch pipe 6 connected to a valve 7, which has preferably a double seal and, in accordance with known practice, means for detecting any leak between the seals. The valve7 is connected to an outlet branch 8 which is in communication with an outlet pipe 9.

The main part of the prover is indicated by the reference 10; it has an outlet pipe 11 to which the branch 8 is joined and which itself is connected to the pipe 9. If the prover is not in use or not executing a proving run, the valve 7 is open so as to provide a by-pass for the prover. In preparation for a proving run the valve 7 is closed and a check for leakage between the seals of the valve 7 is made to ensure that there is no by-pass of the flow path through the prover.

The cylinder 44 has enlargements in its diameter at each end so as to form terminal chambers 13 and 14. The prover is illustrated in a rest state before a proving run and, as shown, the piston 15, which constitutes the displaceable element of the prover, is disposed in the chamber 13 clear of the main central part 16 of the cylinder 12. The enlargement of the terminal chambers ensures that there is a path for the flow of liquid from the pipe 1 to the pipe 9 after the valve 7 has been closed but before and after a proving run.

Enlargement of the terminal parts of the cylinder is not necessary; the cylinder 12 could be of uniform diameter provided that it had appropriate ports in the terminal parts, the ports communicating with the pipe 5 and the pipe 11 respectively. However, care must be taken to ensure that the ports are not sharp so as to damage seals on the piston. Moreover, it may be preferable to ensure that the piston is always engaged with the wall of the cylinder 12.

The piston has two sets 17 and 18 of annular seals so that the piston is slidingly but sealingly movable along the main central part of the cylinder 12.

Extending from the rear of the piston is a rod 19 which extends out through a gland 20 at the rear end of the cylinder 12. This gland is on the outlet side of the prover and accordingly leakage from this gland is of no consequence. Disposed on the rod 19 is a spider 21 which is a loose fit within the central part 16 of the cylinder 12. The spider provides a means for supporting the piston 15 is approximate alignment with the cylinder 12 before the piston enters the central part 16; in the later part of the proving run the piston 15 is maintained in adequate alignment with the piston 12 firstly by virtue of the fit of the piston 15 in the part 16 and secondly by the cooperation of the rod 19 and the gland 20.

The inside surface of the part 16 may be made "benign", that is to say provided with slight undulations in the troughs of which oil may be retained in order that the piston can the more easily move along the length of the chamber 12.

It is, as mentioned earlier, desirable to provide a means of measuring increments of movement of the piston along the chamber 12 and in particular along the central part 16 so that an indication of the actual distance moved during the proving run may be determined. Very many forms of position encoder are in common useforthe measurement of, for example, a member in a machine tool and in general any of the encoders, suitably adapted, which can be used in numerically controlled machine tools for providing an indication of the position of a member can be used in conjunction with the piston 15.

Suitable encoders may be in the form which provide a digitally coded indication of position, the least significant bit in the encoded signal corresponding to the smallest distance which can be resolved by the encoder along the direction of movement of the piston, but other suitable encoders include those which merely provide, for each increment of movement, a single pulse, so that the measurement of the distance moved by the piston requires the accumulation of pulses. Various well-known expedients are available for providing, for example, two phase displaced trains of pulses which according to the phase relationship between the trains of pulses denote the direction of movement; such encoders are well-known and need not be described in detail.

Whateverform of encoder is used, it is preferably capable of providing an indication of the distance moved by the piston along part of the central part 16 during an interval defined by two pulses from the meter. In this particular embodiment, the rod 19 has on its part which is disposed outside the chamber 12 a crosshead 2 bearing an index 23 cooperating with a grating 24. This grating may be a "Heidenheime" grating which can provide an indication of position to very great accuracy. The grating 24 is associated with means not shown for providing signals denoting position or increments of change of position to a logic circuit 25 having an input terminal 26 to which meter pulses are fed.The logic circuit 25 preferably includes a divider which divides the output pulses of the meter by a number which is an integral multiple of the number of pulses that the meter provides for each revolution.

A proving run is preferably defined to begin on the leading edge of a pulse from the meter and to finish on the corresponding edge of a pulse produced n revolutions later; n being preferably an integer. The displacement of the piston during the interval is measured by means of the encoder, in this embodiment the index 23 and the grating 24.

The divider responsive to the meter pulses may be held in a reset state until the encoder indicates that the piston is properly launched into the central part 16 of the cylinder 12. Thereupon the logic can release the divider to begin counting meter pulses, the counting of the first pulse defining the start of the proving run and the counting of the last pulse of the predetermined number of pulses that are to be counted defining the end of the proving run. It will be apparent that this technique avoids any susceptibility to intra-rotational non-linearity and avoids any necessity to wait for any particular meter pulse.

Various advantages accrue from the incremental measurement of the movement of the piston. In particular, it is quite feasible to obtain very high resolution in the measurement of the piston's position; and the accuracy is not significantly affected if parts are replaced. The absolute accuracy of the system depends almost wholly on the accuracy of machining of the cylinder. It is probably unnecessary to calibrate the prover, because it fundamentally has an accuracy determined by the mean diameter of the piston, on the assumption that the encoder is properly designed and arrangedKbeing preferably corrected for variations in temperature.

In particular, the temperature and pressure coefficients of change in the "proof volume" are smaller than in ordinary provers, because changes in the length of the cylinder are of no consequence.

It will be apparent that one significant advantage of the described arrangement is its ability for cooperation with a multiplicity of meters. It is often found in practice that a flow rate which is to be expected for a meter cannot be accomodated conveniently in a single prover and it would be desirable to operate a plurality of provers in parallel. Such operation is readily feasible with the present system because a single meter can provide the reference signals to define a proving interval for a multiplicity of provers at the same time.

Because the prover can be made quite small it is now feasible to provide a more adaptable arrangement of meters and prover on a "skid" which is normally used for the accomodation of meters and provers. For example, instead of using a meter which is large enough in itself to accomodate a comparatively high flow rate, it is feasible to arrange for the normal flow to proceed by way of a multiplicity of meters in parallel, each meter being of a smaller standard size than would otherwise have to be used. Each of the meters can, of course, be proved by the single prove.

In order to provide retraction of the piston 15 from the terminal chamber 14 back to the terminal chamber 13, any suitable means may be provided. In the particular embodiment shown, thereis a double acting cylinder 27 having end ports 28 and 29. A piston 30 is disposed within the cylinder 27; it has a connecting rod 31 extending out of one end of the cylinder and carrying externally thereof a crosshead 32 from which extends back along and outside the cylinder 12 a rod 33, the rod 33 extending through an aperture 34 in the crosshead 22 and being terminated by an end stop 35. The piston 30 has, in effect, a lost motion connection with the crosshead 22 and thereby with the piston 15. Pressure may be applied to the cylinder 27 to move the crosshead 32 in the leftwards direction so as to engage the crosshead 22 and move the piston 15 back to the chamber 13.In order to urge the piston 15 into the part 16, pressure may be applied to the cylinder 27 to move the crosshead 32 and rod 33 rightwards so that the stop 35 engages the crosshead 32; thereafter the crosshead 22 travels rightwards relative to the stop 35 so that the piston 15 moves during the proving run only in response to the pressure of the liquid coming from the meter 2.

Since the rod 19 which extends from the piston 15 is of a significant diameter, the swept volume on the output side of the piston is less than that on the inlet side. Consequently there is a step change in the flow rate at the instant the piston enters the close fitting part 16 of the cylinder 12. This step change is not necessarily important. It could be eliminated by providing a rod extending from the piston in the opposite direction to that of the rod 19, but such another rod would require a good sliding seal with the end plate of the cylinder 12 to void leakage on the inlet side of the prover.

In this embodiment, the rod 19 is hollow to provide communication from the space between the seals 17 and 18 to a pressure sensor 36 carried on the end of the rod 19, so as to provide a means of monitoring the efficacy of the seals while the piston is in the portion 16.

Claims (8)

1. A method of proving a flow meter comprising defining by means of signals from the meter the beginning and end of a proving run and measuring the linear displacement which is executed during the run by a piston which is disposed to respond to the same flow as the meter.

2. A method according to claim 1 in which the flow meter is of a kind adapted to produce a multiplicity of pulses for each cycle of revolution, the method comprising using two temporally spaced apart pulses from the meter to define the beginning and end of the proving run respectively.

3. A method according to claim 1 or claim 2 in which the proving run is defined by the execution of the flow meter of an integral number of cycles of metering.

4. A method according to claim 2 and claim 3 in which pulses from the meter are counted subsequentto the occurrence of a pulse defining the commencement of the proving run and the occurrence of the nth meter pulse thereafter defines the end of the proving run, wherein n is an integral muitiple of the number of pulses which the meter produces in each cycle of metering.

5. A method according to any foregoing claim, in which the displacement of the piston is measured by an encoder which produces signals denoting increments of movement of the piston along its path of displacement.

6. A meter prover which has a linearly displaceable piston disposed in a cylinder adapted for connection into a flow path, and means for producing signals denoting increments of movement of the piston along the said cylinder.

7. A meter prover according to claim 6 in which a part extends from the piston on the downstream side thereof externally of the cylinder and the said part carries means cooperating with an encoder to provide the signals denoting increments of movement of the piston.

8. A prover according to claim 6 or claim 7 in combination with a meter and arranged to receive the same flow of liquid as the meter, the combination further comprising electrical logic arranged to respond to pulses from the meter to define an interval of measurement and to signals from the encoder to ascertain the displacement of the piston during said interval.