GB2125552A - Fluid pressure transducers - Google Patents

Abstract

A system for indicating the pressure of a process fluid includes an upstream impact probe 4 and a downstream static probe 8 mounted in a conduit 2. These probes 4 and 8 are connected to high pressure and low pressure compartments 14, 16 respectively of a differential pressure transducer 12 having a central diaphragm 18. Process fluid is passed along the conduit 2, and the diaphragm 18 takes up a position dependent on the pressure difference between the probes 4 and 8, and hence dependent on the fluid flow in the conduit 2. Purging gas, air and nitrogen being examples, or liquid is passed from a source 20 through a pressure regulator 22 and restrictors 24 and 26 to the compartments 14 and 16 and passes over the diaphragm surfaces thereby preventing contact between the diaphragm 18 and the process fluid. <IMAGE>

Description

SPECIFICATION A pressure transducer The present invention relates to a pressuretrans- ducerfor indicating the pressure of a fluid, and to a pressure detecting system comprising such a trans ducer. The invention is particularly concerned with a pressure transducer having a movable diaphragm in which movement ofthe diaphragm in response to fluid pressurethereon changes the inductance or capacitance of part ofthe transducer circuit. Two examples oftransducers of the invention are the balanced variable reluctance type and the balanced capacitive type.

In certain circumstances it is desirable thatthe fluid undergoing the pressure measurement, which will hereinafter be referred to as the process fluid, does not come into contact with the movable diaphragm of the transducer. In a previously proposed arrangement the measuring diaphragm of the transducer is protected from the process fluid by isolating diaphragms which themselves come into contact with the process fluid.

Each isolating diaphragm is constructed from a material which is compatible with the process fluid. A liquid such as silicon oil is employed as a bufferfluid in the space between the measuring diaphragm and each isolating diaphragm. In this way, movement of the isolating diaphragms in response to pressure of the process fluid is transmitted by the bufferfluid to the measuring diaphragm which moves to change the appropriate electrical parameter ofthe transducer circuit.

It is an aim ofthe invention to obviate the need for isolating diaphragms and a pressure transmitting liquid.

According to one aspect of the invention there is provided a transducerto indicate the pressure of a process fluid, said transducercomprising a diaphragm movable in response to the pressure thereon of a process fluid, a supply system for directing a purging fluid across the surface ofthe diaphragm to maintain separate the diaphragm and the process fluid, and regulating means to control the purging fluid velocity to prevent a significant error in the process fluid pressure indication.

According to another aspect ofthe invention there is provided a method of indicating the pressure of a process fluid using a transducer comprising a diaphragm movable in response to the pressure thereon of a process fluid, said method comprising directing a purging fluid across the surface ofthe diaphragm to maintain separate the diaphragm and the process fluid, and controlling the purging fluid velocity to prevent significant errors in the processfluid pressure indication.

The purging fluid is selected to have the appropriate characteristics so as to be compatible with the process fluid.

In a presenttechnique of indicating fluid flow a probe having an opening therein is inserted in a conduitto interruptthefluid flow with the opening exposed to the impact ofthefluid stream. Because of the moving stream of the processfluidtheso-called "impact" or "H.P." fluid pressure in the vicinity of the probe opening will differ from the so-called "static" or "L.P." fluid pressure immediately downstream ofthe opening, and this pressure difference is a function of the fluid velocity. This "static" or "L.P." fluid pressure referred to throughout the Specification is not necessarilythe same as "true static pressure".The pressure difference is used to provide a suitable indication of the fluid flow in the conduit.

According to a further aspect of the invention there is provided a system for indicating the pressure of a process fluid, said system comprising a transducer having a diaphragm movable in response to the pressure thereon of a process fluid, an upstream sensor to sense the impact pressure of the process fluid, a downstream sensor to sense the static pressure of the process fluid, conduit means to transmit the said impact pressure and static pressure to respective opposite surfaces ofthe diaphragm, a supply system to direct a purging fluid across the said surfaces ofthe diaphragm to maintain separate the diaphragmandtheprocessfluid,and regulating means to control the purging fluid velocity to prevent significant errors in the process fluid pressure indication According to a further aspect ofthe invention there is provided a method of indicating the pressure of a process fluid by means of a system comprising a transducer having a diaphragm movable in response to the pressure thereon of a process fluid, an upstream sensorto sense the impact pressure ofthe process fluid, and a downstream sensorto sense the static pressure ofthe process fluid, said method comprising transmitting the said impact pressure and static pressure to respective opposite surfaces of the diaphragm, directing purging fluid across the said surfaces of the diaphragm to maintain separate the diaphragm andthe processfluid, and controlling the purging fluid velocityto preventsignificanterrors in the process fluid pressure indication. The purging fluid may be either a liquid or a gas.

One embodiment of the invention will now be described byway of example with reference to the accompanying drawing which is a diagrammatic side elevation of a pressure indicating system.

The illustrated pressure indicating system is located in a fluid flow conduit 2 in which the process fluid is flowing from left to right. A pitottube 4 constituting an impact probe is mounted in an opening in the wall of the conduit 2 so as to extend into the conduit. The pitot tube 4 has an opening 6 therein facing upstream of the conduit 2 so as to enable the pitot tube 4to sense the impact pressure ofthe process fluid. A second pitot tube 8, ofthe same construction as the pitottube 4 is mounted in an opening in the wall oftheconduit2so as to extend into the conduit downstream ofthe pitot tube 4. This pitot tube 8 constitutes a static probe and has an opening 10 therein facing downstream ofthe conduit 2 so as to sense the static pressure of the process fluid.

The drawings originally filed were informal and the print here reproduced is taken from a later filed formal copy.

Atransducer 12 is divided into high pressure compartment 14 and a low pressure compartment 16 by a diaphragm 18 which is movable in response to a pressure difference between the two compartments 14 and 16. Asource 20 of purging gas is connected by a conduit 21 to a pressure regulator 22 which is adjustabletosetthe pressureofthe purging gastoa suitable level. The downstream end of the regulator 22 is connected to two parallel restrictors 24 and 26 which are adjustable to control the flow ofthe purging gas to a suitable rate. The downstream ends ofthe restrictors 24 and 26 are connected to the upstream ends of the transducer compartments 14 and 16 respectively.The downstream ends of these compartments 14 and 16 are connected by conduits 28 and 30 to the openings 6 and 10 of the pitottubes 4 and 8 respectively. The purging gas is chosen so as to be compatible with the transducer diaphragm 18 and its associated components and the process fluid flowing along the conduit 2. Examples oftypical suitable purging gases are nitrogen and air. It is not essential to the invention for the piton tube 8 to be similar to the piton tube 4.

In orderto obtain a satisfactory reading of the static pressure i.e. the reference pressure, it is sometimes preferable to locate the static probe, such as the static probe 32 in a recess 34 in the wall of the fluid flow conduit 2. A preferred construction of this static probe 32 is a cylinder containing baffles, and which is connected to the bore of the flow conduit 2 by a hole 36 of small cross-section.Afurther hole in the opposite end ofthe probe 32 to the hole 36 is connected to a conduit 38 which in operation will replace the conduit 30. It will be appreciated that when the static probe 32 is used it replaces the pitottube 8. An electrical signal proportional to the pressure difference between the transducer compartments 14 and 16 is passed along a cable 40 to a suitable indicating device.

In operation, the process fluid is passed along the fluid flow conduit 2 from left to right, and the opening 6 of the pitottube 4 is exposed to the impact ofthe fluid stream so as to provide an indication of the impact pressure atthe opening 6 in the transducer compartment 14. The opening 10 of the pitottube 8 provides an indication of the static pressure at the opening 10 in the transducer compartment 16, and the diaphragm 18 takes up a position in the transducer 12 dependent upon this pressure difference. The trans- ducer 12transmits an electrical signal along the cable 40 dependent upon this position ofthe diaphragm 18 thereby providing an indication ofthe pressure difference between the pitottube openings 6 and 10.

This pressure difference is proportional to the fluid flow in the conduit 2, and consequently this embodiment provide an indication of this fluid flow.

The purging gas is passed from the source 20 through the pressure regulator 22 and restrictors 24 and26tothetransducercompartments 14and 16.

This gas passes overthetwo surfaces of the diaphragm 18to form an interface between the diaphragm 18 and the process fluid thereby maintaining separate the diaphragm 18 and the process fluid. The purging gas then passes along the conduits 28 and 30 and through the openings 6 and 10 into the conduit 2.

The pressure ofthe purging gas can be set by the pressure regulator 22, and the individual flow rates of the gas in the transducer compartments 14 and 16 can be set by the restrictors 24 and 26. The gas pressure and flow rates are adjusted so that gas passes out of the openings 6 and 10 at a very low flow rate thereby minimising any inaccuracy in the fluid pressure readings of the system. If the pitottube 8 is replaced by the static probe 32 then of course the gas from the transducer compartment 16 passes through the conduit 38 and static probe 32 and out through the hole 36 into the conduit 2.

The restrictors24and 26maytaketheform of simple regulators or they may constitute complex devices utilising pneumatic techniques. The transduc- er 12 may be of any type suitable for differential pressure measurement provided that it has a very low volumecontainmentand displacement. In preffered embodiments ofthe invention this transducer 12 will be ofthevariable reluctance or variable capacitance type.

The purging gas constitutes an interface or buffer fluid, and in consequence it is not necessary to provide an isolating diaphragm as in previously proposed systems. An advantage of this embodiment is that it avoids the loss in accuracy inherent with previously proposed liquid and metallic isolating diaphragm systems. The purging gas ofthis invention has almost no mass compared with the previously proposed liquid and isolating diaphragm systems. Also when carrying out pressure and fluid flow measurements of gases such as steam, condensate of steam would not form in the conduits leading from the gas source 20 to the pitottubes 4and 8 provided thatthe conduits of the gas supply system are suitably located.

Another advantage of this embodiment is that it reduces an electrical danger caused by the presence of electrically charged surfaces, in the transducer and associated components.

In this invention,the purging fluid can be either a gas or a liquid.

Claims (19)

CLAIMS Filed on 20.7.83

1. Atransducerto indicatethe pressure of a process fluid, said transducer comprising a diaphragm movable in response to the pressure thereon of a process fluid, a supply system for directing a purging fluid across the surface of the diaphragm to maintain separate the diaphragm and the process fluid, and regulating means to control the purging fluid velocityto prevent a significant error in the process fluid pressure indication.

2. A transducer as claimed in Claim 1 in which the regulating means comprises a regulatorto control the pressure of the purging gas.

3. Atransduceras claimed in Claim 1 or Claim 2, in which the regulating means comprises restricting meansto control the flow rate ofthegas acrossthe transducer diaphragm.

4. Atransducer as claimed in Claim 3 in which the restricting means comprises two parallel restrictors to control the flow rate ofthe gas across respective surfaces ofthetransducer diaphragm.

5. Atransducer as claimed in any preceding Claim in which the supply system comprises a source of purging gas and the regulating means of any preceding Claim.

6. Asystemforindicatingthe pressure of a process fluid, said system comprising a transducer having a diaphragm movable in response to the pressure thereon of a process fluid, an upstream sensorto sensethe impact pressure ofthe process fluid, a downstream sensor to sense the static pressure ofthe process fluid, conduit means to transmitthe said impact pressure and static pressure to respective opposite surfaces ofthe diaphragm, a supply system to direct a purging fluid across the said surfaces ofthe diaphragm to maintain separate the diaphragm and the processfluid,and regulating means to control the purging fluid velocity to prevent significant errors in the process fluid pressure indication.

7. A system as claimed in Claim 6 in which the regulating meanscomprisesa regulatorto control the pressure of the purging gas.

8. A system as claimed in Claim 6 or Claim 7, in which the regulating means comprises restricting means to control the flow rate ofthe gas across the transducer diaphragm.

9. A system as claimed in Claim 8 in which the restricting means comprises two parallel restrictors to control the flow rate of the gas across respective surfaces ofthe transducer diaphragm.

10. A system as claimed in any one of Claims 6to 9 in which the supply system comprises a source of purging gas and the regulating means of any one of Claims 6 to 9.

11. A system as claimed in any one of Claims 6 to 10 in which the downstream sensor is located in a recess in the wall of a conduit for the process fluid.

12. A system as claimed in Claim 11 in which the static probe is a cylinder containing baffles.

13. Amethod of indicating the pressure of a process fluid using atransducercomprising a diaphragm movable in response to the pressure thereon of a process fluid, said method comprising directing a purging fluid across the surface of the diaphragm to maintain separate the diaphragm and the process fluid, and controlling the purging fluid velocity to prevent significant errors in the process fluid pressure indication.

14. A method of indicating the pressure of a process fluid by means of a system comprising a transducer having a diaphragm movable in response to the pressure thereon of a process fluid, an upstream sensor to sense the impact pressure of the process fluid, and a downstream sensor,to sense the static pressure ofthe process fluid said method comprising transmitting the said impact pressure and static pressure to respective opposite surfaces of the diaphragm, directing purging fluid across the said surfaces of the diaphragm to maintain separate the diaphragm and the process fluid, and controlling the purging fluid velocity to prevent signigicant errors in the process fluid pressure indication.

15. A method as claimed in Claim 13 or Claim 14, including controlling the individual flow rates of the purging fluid across each surface ofthe diaphragm.

16. A method as claimed in any one of Claims 13 to 15 including using nitrogen or air as a purging fluid.

17. Atransducerto indicate the pressureofa process fluid, substantially as herein described and shown in the accompanying drawing.

18. A system for indicating the pressure of a process fluid, substantially as herein described and shown in the accompanying drawing.

19. A method of indicating the pressure of a process fluid substantially as herein described with reference to the accompanying drawing.