Roy
Charles
Roy
Charles
| 1. | A mass flow prover comprising: volume proving means for receiving a defined volume of fluid defined by the positions of first and second signal means from a continuously flowing stream of fluid, said first and second signal means for generating start measurement and stop measurement signals corresponding to the fluid reaching first one and then the other measurement locations respectively as it flows into the volume proving means, means within the volume proving means for measuring the density of said fluid. |
| 2. | A flow prover according to claim 1 which also comprises means within the volume proving means for taking a sample of a fraction of the defined volume of fluid, means for isolating the sample from the flow of fluid, said density measuring means being within said sampling means. |
| 3. | A flow prover according to claim 1 or 2 wherein said density measuring means comprises means for determining the hydrostatic head between defined measurement locations in the isolated sample. |
| 4. | A flow prover according to any one of claims 13, wherein said volume proving means comprises a chamber with an inlet at one end and an outlet at the other end, and a first piston, adapted to be driven by fluid fed into said inlet, from a position near the inlet to a position which exposes the outlet. |
| 5. | A flow prover according to claim 4 wherein said sampling means comprises a sampling chamber provided with a second piston capable of movement at least in part in synchronization with said first piston. |
| 6. | A flow prover according to claim 5 wherein said first piston is hollow and has an opening therein, through which said sampling chamber extends. |
| 7. | A flow prover according to any one of the preceding claims, which comprises (a) a first measuring chamber, said chamber having an inlet and outlet for fluid at upper and lower ends of the chamber, and a first piston slidable vertically within the chamber, (b) a second measurement chamber fixed within the first chamber, and opening into the first chamber at the upper end of the second chamber, said second chamber having (i) a second piston vertically slidable within the second chamber, and connected to said first piston by a lost motion device, so as to move in synchronization with the first piston during a part of the downward movement of the first piston, and (ϋ) a sealing means which may be actuated to seal the second chamber when the second chamber is full of fluid, and (iii) means for determining the pressure differential between upper and lower calibration positions within the second measurement cylinder when it is filled with fluid. (c) means for generating start measurement and stop measurement signals corresponding to the pistons passing the upper and lower calibration positions. |
| 8. | A flow prover according to claim 6, which comprises a third chamber having an upper opening and said lower end of said first chamber having an opening, said openings being adapted to allow said first piston to move into said third chamber, and said third chamber containing a displacement measurement system comprising at least one sensor, which interacts with at least one sensor associated with said first piston to generate start and stop measurement signals. |
| 9. | A flow prover according to claim 8, wherein said second chamber comprises an upper and a lower location for measuring the hydrostatic head and said measurement system comprises at least one start sensor and at least one stop sensor, the positions of said sensors being such that the upper face of said second piston coincides with the positions of the upper and lower locations when said sensors produce start and stop signals respectively. |
| 10. | A method of measuring the mass flow of a fluid, which comprises passing said fluid continuously into a flow prover comprising volume proving means causing generation of first and second signals corresponding to start and stop measurements as said fluid passes first and second measurement locations defining a specific volume of fluid in said volume proving means, and during and/or after said passing measuring in said volume proving means the density of fluid therein. |
| 11. | A method according to claim 10 which comprises passing said fluid into a flow prover as claimed in any one of claims 29. |
The present invention relates to improvements relating to flow measurement.
It is known to use devices known as positive displacement provers to calibrate volumetric fluid flowmeters when installed and operating in their final locations in for example oil fields. By connecting the flowmeter and prover in series the volume of liquid passing through the flowmeter for a given number of measurement signals, e.g. in a given time may be precisely determined, so enabling the meter to be precisely calibrated under the process conditions of use.
Known positive displacement flow provers comprise a section of pipe of precise reference volume through which a tightly fitting dispiacer is caused to move under the influence of the fluid flow. Detector switches at the beginning and end of the calibrated volume pipe section are operated by the dispiacer to start and stop integration of the signals, proportional to the amount of flow, produced by the flowmeter thereby allowing the flowmeter to be calibrated.
Known voiume provers may be of conventional size with a spherical dispiacer or of compact design with a piston dispiacer. Neither type is directly suitable for calibrating flowmeters which measure mass flow. However, mass flowmeters may be calibrated bv a combination of a voiume prover and a separate densitometer to determine fluid density. This method has the disadvantage, especially for nor.-homogeneous fluids, that the instantaneous
reference volume and density measurements are made on separate portions of fluid which may be different in constitution and under slightly different process conditions. The reference measurement of mass rlow may thus be imprecise. An alternative method used for calibrating mass flowmeters installed in the field is by transfer standard mass flow meter. The transfer standard meter is normally calibrated against a gravimetric standard laboratory method. This method is dependent upon the precision of the original calibration of the standard transfer meter remaining constant during transit and site installation, and during operation under different process conditions.
The uncertainties associated with the available proving methods make it desirable to be able to calibrate mass flow meters in the field by a single piece of apparatus with constant traceable calibration which is not affected by or subject to the conditions of use.
According to the present invention there is provided a mass flow prover comprising: volume proving means for receiving a defined volume of fluid defined by the positions of first and second signal means from a continuously flowing stream of fluid, said first and second signal means for generating start measurement and stop measurement signals corresponding to the fluid reaching first one and then the other measurement locations respectively as it flows into the volume proving means, means within the volume proving means for measuring the density of said fluid. Preferably the flow prover also comprises means within the volume proving means for taking a representative sample of a fraction of the defined volume of fluid, means for isolating the sample from the flow of fluid, said density measuring means being within said sampling means. Especially said density measuring means comprises means for determining the hydrostatic head between defined measurement locations in the isolated sample.
The present invention also provides a method of measuring the mass flow of a fluid, which comprises passing said fluid continuously into a flow prover comprising volume proving means causing generation of first and second signals corresponding to start and stop measurements as said fluid passes first and second measurement locations defining a specific volume of fluid in said volume proving means , and during and/or after said passing measuring in said volume proving means the density of said fluid therein. Preferably the method comprises, during said passing, also passing said fluid into sampling means inside said volume proving means, isolating said fluid in said sampling means and during or preferably after said former passing determining the density of said isolated fluid. Especially the density is determined by determining the hydrostatic head between defined measurement locations in the isolated sample.
The volume proving means is preferably a first chamber with an inlet at one end and an outlet at the other end and a first piston which can be driven, by fluid fed into the inlet, from a position near the inlet (when the piston seals the outlet from the incoming fluid) to a position which exposes the outlet to that fluid. The dimensions of the chamber are determined with precision to enable an accurate value for the volume of fluid introduced into it to be determined.
The sampling means is preferably a sampling chamber provided with a second piston which moves at least in part in synchronization with the piston already mentioned (i.e. the first piston) .
In order to have a relatively large hydrostatic head it is desirable for the sampling chamber to be relatively high, and in practice the sampling chamber will generally be a vertically disposed elongated chamber within a vertically disposed volume proving chamber.
The arrangement with vertical downflow has the advantage that any segregation of the sample into separate phases does not affect the accuracy of sampling, or prevent the hydrostatic head
measurement corresponding to that for a representative sample.
Providing a means for sealing or isolating the sampling chamber enables the hydrostatic head (which can be used to determine density) to be determined with the fluid in the quiescent state, without disturbance from the flow of fluid through the device.
Preferably the first piston is hollow and has an opening therein, through which the sampling chamber extends. Thus the sampling chamber can be fixed to the first chamber, e.g. at its upper end, and the sampling chamber has an opening in its wall at the upper end to allow fluid to enter it. Sealing means for isolating the fluid in the sampling chamber may comprise means to close said opening, e.g. a shutter, but preferably comprises a valve mounted on the upper end of the first chamber moveable between an open position allowing fluid to enter the chamber and a closed position, in which at least part of the sampling chamber is isolated.
A specific mass flow prover in accordance with the present invention comprises (a) a first measuring chamber, said chamber having an inlet and outlet for fluid at upper and lower ends of the chamber, and a first piston slidable vertically within the chamber, (b) a second measurement chamber fixed within the first chamber, and opening into the first chamber at the upper end of the second chamber, said second chamber having
(i) a second piston vertically slidable within the second chamber, and connected to said first piston by a lost motion device, so as to move in synchronization with the first piston during a part of the downward movement of the first piston, and (ii) a sealing means which may be actuated to seal the second chamber when the second chamber is full of fluid, and
(iii) means for determining the pressure differential between upper and lower calibration positions within the second measurement chamber, e.g. cylinder when it is filled with fluid. (c) means for generating start measurement and stop measurement signals corresponding to the pistons passing the upper and lower calibration positions Preferably the flow prover also comprises a third chamber, usually situated below the first chamber. The third chamber has an opening in its face towards the first chamber, e.g. its upper wall, and the first chamber also has an opening in its face towards the third chamber, the two openings being adapted to allow part of said first piston to move into said third chamber; the two openings may be separate with one in each chamber, but preferably the chambers share a common wall so there is one combined opening. The third chamber can contain a displacement measurement system comprising at least one sensor, which interacts with at least one sensor associated with said first piston to generate start and stop measurement signals. Thus the first piston may have at least one sensor, e.g. a pair, which interacts with at least one first sensor, e.g. a pair in the third chamber generating a start signal and at least one second sensor, e.g. a pair in the third chamber generating a stop signal. Alternatively the first piston may have the first and second sensors and the third chamber the interacting sensor.
The second chamber can comprise an upper and a lower location for measuring the hydrostatic head. The second piston passes through the second chamber and as it passes one of these locations, e.g. its upper face passes the upper location, the other end of the second piston moves with the first piston, the sensor of which is arranged to pass the start sensor in the third chamber, and similarly when the second piston passes the other location, e.g. its upper face passes the lower location, the sensor on the first piston is arranged to pass the stop sensor in the third chamber.
The apparatus of the present invention is used for the proving of meters used in the oil industry. The fluids whose mass flow is measured are often two phase mixtures of oil and water. If a two phase liquid is being used then a mixing system may be provided at the inlet to ensure homogeneity within the chambers. The mass flow prover of the present invention enables the mass flow meter to be calibrated in dynamic mode without interruption to the flow of fluid through the meter. The calibration can be carried out under the same operating conditions as those at which the meter normally operates, and with the same fluid. The hydrostatic head (and thus density) is measured under quiescent conditions in the measurement tube. The contents of the measurement tube are isolated from but under the same process conditions as the fluid subjected to volumetric measurement in the main chamber.
The method of sampling from the incoming main flow helps to give a representative sample. Any subsequent separation of the isolated fluid constituents, e.g. water from oil, does not affect the validity of the hydrostatic head measurement. When the isolated sample is discharged after each hydrostatic head measurement the pressure difference system may be automatically zeroed before the next pass.
Instead of the hydrostatic head measurement for the density in the sampling means, there may be used an internal densitometer, such as of the vibrating element type, which would also have the benefit of operating under the quiescent conditions in the sampling tube, and would provide the density of the fluid passing through the volume proving means at that time, but may be affected by any separation of the fluid. Less preferred but still advantageous is an internal densitometer, such as one described above, inside the volume proving means, without the need for any sampling or isolation means, as there can still be determined the density of the fluid passing through the volume proving means at that time and hence representative with respect to time, but the density may be affected by the turbulence of fluid passing through
the volume proving means and/or any separation of the fluid.
Flow provers in accordance with the invention can be constructed with a small "footprint" facilitating installation in confined spaces . A specific embodiment of the invention will now be illustrated by reference to the drawings in which
Figure 1 is a diagrammatic cross sectional view of a mass flow prover of the invention at the start of a flow proving step, and Figure 2 is a view corresponding to Figure 1, but at the end of a flow proving step.
The apparatus comprises a first measurement cylinder (1) , having an inlet (2) and outlet (3) at its upper and lower ends respectively. A first piston (4) is slidably mounted within cylinder (1) , and has a downwardly extending hollow cylindrical piston rod (5) , which extends through the base of cylinder (1) . A second measurement cylinder (6) is fixed to the upper end of cylinder (1) by perforated tube (7) . The second measurement cylinder extends into the hollow piston rod (5) through a circular opening in the first piston. The first piston seals against the second cylinder as well as against the inner side of the first cylinder.
A second piston (8) is slidably mounted within cylinder (6).It is connected to piston (4) through a lost motion device (9) . Lost motion devices are conventional and the device is not shown in detail. The two pistons will normally move together, but the lost motion device allows the first piston to continue its downward motion after the second piston has reached the lower end of the second cylinder. The inner cylinder (6) is provided with ports (10) and (11) at its upper and lower end respectively. These ports communicate with a pressure measurement cells (12a and 12b) to enable the difference between the pressure at the upper and lower ports to be determined. Suitable measurement cells can be readily selected by persons skilled in the art.
The piston rod (5) as it moves downwards moves into a downwardly extending cylinder (13) at the base of cylinder (1). within this extension (13) is located a displacement measurement system indicated generally at (14) . This comprises pairs of sensors (15,16) mounted at the upper and lower ends of either rods (17) or the inside face of a hollow cylinder (not shown) . These sensors interact with sensors (18) fixed at the base of piston rod (5) , such that a signal is produced when the moving sensor passes the fixed sensor. Suitable sensing devices are well-known. The position of the start sensors (15) and the stop sensors (16) are selected such that the start signal is produced as the position of the upper face of the second piston (8) coincides with the position of the upper port (10) and a stop signal is produced when upper face of the piston coincides with the position of the lower port (11) .
Sealing valve (19) may be moved from a first position in which fluid can enter the upper end of cylinder (5) (as shown in. figure 1) to a second position in which the upper end of cylinder (5) is sealed. In use fluid which has passed through a flow meter to be calibrated (not shown) is fed through inlet (2) and displaces piston (4) downwardly within cylinder (1) . Piston (8) moves down in step with piston (4) . A signal is produced by the start count sensors. On receipt of this signal the measurement signals produced by the meter to be proved are recorded by proving measurement circuitry (not shown) . This circuitry is conventional and suitable arrangements can be readily designed by those skilled in meter proving technology. The pistons continue moving down together until piston (8) is stopped from further movement within cylinder (6) at a position at which its upper face coincides with the lower port (11) .
When this point is reached the stop count sensors produce a signal which is used to stop the recording of the measurement signal from the flow meter. The signal from the flow meter is generally in the form of pulses, and the proving measurement
circuitry will then give a count of the number of pulses received as piston (8) moves between start and stop positions.
The piston (4) will be free to continue moving down because of the action of the lost motion device, and will expose the outlet (3) allowing fluid to leave the cylinder (1) . The effect of this is that flow through the prover is continuous while a sample is being collected in the cylinder (6) .
When the cylinder (6) is full of liquid the valve (19) is closed. The contents of cylinder (6) are now protected against disturbance resulting from flow of fluid through the prover. The hydrostatic head (pressure difference) between the top and bottom of the cylinder is determined using the pressure cells (12a and 12b).
The density of the fluid in the cylinder (6) can be determined from the hydrostatic head, and from a knowledge of the dimensions of the cylinder (6) the weight of fluid trapped in cylinder (6) can be determined. A knowledge of the relative diameters of the cylinders (1) and (6) allows the mass passed through the prover during the sampling period required to fill cylinder (6) to be calculated. A knowledge of the number of measurement pulses produced by the flowmeter during the sampling period corresponding to the mass calculated to have been passed into the prover enables the mass flow meter to be calibrated. An alternative mathematical route is to multiply the free volume contained in the two cylinders (1 and 6) between the start and stop detectors by the density established by measurement of the hydrostatic head in the measurement tube.
The height of the cylinder (6) determines the size of the hydrostatic head. A pressure difference of about 200 mbar (20 KPa) can be used, and this corresponds to a height of about 2m.
The dimensions of the pistons are desirably selected so as to avoid too rapid travel of the pistons at the maximum flow rate to be tested, which could impose excessive shock when the inner piston reaches the end of its travel in the inner cylinder. A speed of under 250 mm/sec is desirable.
The time taken for the pistons to move between the calibration positions is desirably sufficient for sufficient number of signals to be received from the meter being proved, and may be for example be about 10 seconds.
As an example of dimensions which could be used a prover to calibrate a mass flow meter for use in a 50 mm line with a maximum throughput of 2.5 tonnes/min could have an internal diameter of about 0.5m, and a height for the calibrated section of about 2m.
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