GB2091883A - Differential pressure transducer - Google Patents

Abstract

A differential pressure transducer comprises a pressure-sensitive solid state electronic device 11 of hybrid form, to which are applied in opposition the pressures whose difference is to be measured. The device is located within a chamber 36 of a block 30 having two pressure inlets 31, 32. Each inlet 31, 32 has a communicating bore 40, 47 respectively via a pressure relief valve 46, 48 respectively in an end-plugged counter- bore 45,49 respectively. The relief valves 46, 48 open interconnection between the inlets 31, 32 when more than a predetermined overload pressure is reached, thus protecting the device from undue pressure from either of its inlets. <IMAGE>

Description

SPECIFICATION Differential pressure transducer The invention relates to differential pressure trans ducers such as are applicable in industrial process plant for measuring fluid flow in a pipe, conduit or the like, by measuring the pressure drop across an orifice plate or the like in that pipe, conduit or the like.

For the purpose just mentioned, a differential pressure transducer will be connected to fluid on both sides of the orifice plate or the like, and it is common practice for typical measured differential pressures to be very small in relation to the static pressure in the pipe, conduit or the like, often by a factor of about 100.

Hitherto, conventional types of differential press ure transducers have incorporated diaphragms (or bellows) with a space between them filled with a fluid whose pressure is sensed. Given the relative smallness of the actual difference of pressure to be measured, and measured as accurately as possible over its full design range, it is clearly undesirable, if not impossible, to use diaphragms (or bellows) that can withstand themselves the very much higher static pressure if, for any reason, that is applied only to one side of the transducer.Protection is, however, required against such an event in case of operator error, for example in operating isolating valves in feeds to both sides of the transducer and an equalising valve across those feeds on the transduc ear sides of the isolating valves, or in the case of a sudden loss of pressure in one feed as can occur if the pipe, conduit or the like, or something connected thereto, bursts or otherwise malfunctions.Such protection has, conventionally, been afforded by taking advantage of the operational displacement of transducer parts (i.e. volume displacement on which those transducers depend), so that the latter is limited safely in overload conditions, say by a rigid diaphragm backplate onto which it is pressed in overload conditions, or a rod for internal connection of bellows to close a valve that will produce hydraulic lock and prevent further transfer of fluid from one bellows to another.

The factor of significant physical movement of transducer parts, and accompanying volume dis placement, does, of course, affect adversely the sensitivity and speed of response of such conven tional differential pressure transducers, and it is an object of this invention to provide a more sensitive and generally advantageous differential pressure transducer.

What we envisage is reduction of parts movement, and thus volume displacement to or near a mini mum by taking advantage of the availability of so-called pressure-sensitive solid state electronic devices that exploit piezoelectric or other effects whereby electromagnetic, specifically electrical, activity is induced by pressure or changes of press ure applied to such materials as quartz crystals or semiconductors such as silicon crystal bodies.

One such pressure-sensitive device using a hybrid integrated circuit with a silicon crystal on a ceramic wafer is available from National Semiconductor Corporation.

In application of such devices to industrial plant, where a typical static pressure could be of the order of 500 p.s.i., with a differential pressure of up to about 5 p.s.i., to be measured to an accuracy of 0.5% or better, we find that it is impractical to contemplate transducer structures that will tolerate without damage overload pressures more than a few times, typically about eight times, the full scale of differential pressure to be measured.

Accordingly, we now propose herein that a differential pressure transducer comprises a pressuresensitive solid state electronic device, to which are applied in opposition the pressures whose difference is to be measured, and, extending between inlets for such pressures, interconnection means including pressure sensitive relief means that opens interconnection between those inlets when more than a predetermined overload pressure is reached, thus protecting the device from undue pressure from either of its inlets.

Such relief means may conveniently take the form of normally closed valves controlling connection between inlet fluid pipes, conduits or the like, preferably passageways in a block accommodating the valves and adapted for connection to a housing for the device itself.

We also prefer that the device be in communication with inlet pressure via a suitable incompressible pressure transmission medium, advantageously at capillary interfaces for a liquid medium so that the fluid cannot be lost from filled chamber means therefore during handling, transportation or fitting.

Only short capillaries are required as the volumetric displacement of the fluid in operation of the device is negligible, typically 0.001 cc or less, which gives very high sensitivity and fast response. Such capillaries are conveniently provided within the aforementioned block. Alternatively, a suitable semiconductorgel may be used.

In using a hydrid device as mentioned before, we find that, in its hitherto normal configuration with pressure inlets through spaced apertures in its substrate to opposite sides of its pressure sensitive crystal, differential pressure measurements can be seriously affected according to the actual values of static pressures applied. That may be due to forces exerted on the substrate. However, whatever the reason, we find that such affects, are substantially eliminated if substantially all of the device except for one inlet aperture is subjected to the pressure applied via the other inlet aperture.That is conveniently achieved by ensuring that all but a region immediately about that one inlet aperture is contacted by pressure transmission fluid in a chamber therefor that a Iso houses the device with a communication to its other aperture that is effectively isolated from that chamber.

Practical implementation of the invention will now be described, by way of example, with reference to the accompanying drawing, in which: Figure 1 is a schematic diagram; Figure 2 is a plan view of one embodiment; Figure 3 is a part sectional side view thereof; and Figure 4 is a schematic diagram of an automatic overload pressure protection system.

In Figure 1, a differential pressure transducer 10 is shown comprising a pressure-sensitive solid state electronic device 11 in a chamber 12 having two capillary type inlets 13 and 14 one inlet 13 is taken direct to one side of the sensor crystal of the device 11 in a manner isolated (15) from the rest of the interior of the chamber 12. All of the device except for the direct connection 15 is exposed to pressure via the other inlet 14. The chamber 12 and connection 15 are both filled with silicone as a suitable pressure transmission medium that will not be lost through the capillaries 13, 14 once filled, preferably by vacuum pump means.

Two pressure feed lines 16 and 17, one from each side of an orifice plate in a feed pipe, conduit or the like of inductrial plant, are connected to the capillary inlets 13 and 14, respectively. Each of the feed lines 16 and 17 includes an isolating valve, see 18 and 19, respectively. Between those isolating valves 18, 19 and the differential pressure transducer 10 is an interconnection 20 via an equalising valve 21 for use at start up, maintenance, etc., and usually of a manual type.

Two other connections 22 and 23 are shown between the lines 16 and 17 and include pressure relief valves 24 and 25, respectively, that are poled oppositely one to the other in order to respond to different inequalities of pressure in the feed lines 16 and 17. The relief valves 24,25 are normally closed but open at a predetermined pressure difference across them. We find it advantageous for that pressure difference to be substantially less than, say about half, what would damage the transducer 10, and to be several times, say 4 or 5 times, the maximum pressure difference actually to be measured bythetransducer 10.

Turning to Figures 2 and 3, a block 30 has bores 31, 32 for inlets of pressure from lines such as 16, 17 of Figure 1. Bores 31,32 extend from the same face of the block 30 of which the opposite face carries a dished plate 33 suitably secured thereto, as by bolts at 34, and sealed, as by ring seal 35. The dished cavity of the plate 33 forms a chamber 36 equivalent to the chamber 12 of Figure 1. There are two pressure inlets to the chamber 36 from the block, one (37) a short capillary terminating the bore 31, and the otherfrom a short capillary 39 terminating a bore 38 at the latter's communication with a passage 40 from the bore 32. Enlarged open end 41 of the bore 38 carries a seal 42 for an inlet 43 affixed to substrate 44 of the device 11 and serving, in effect, as the connection 15 of Figure 1.

The device 11 is of hybrid form with components thereof on its ceramic substrate 44, in particular a silicon crystal to both sides of which access is gained, usually via a conformal coating at least on one side, from spaced apertures in its substrate, one of which communicates with the inlet 43 and the other of which is open to the chamber 36.

All of the bore 38 and inlet 43 on the one hand, and the otherwise free space of the chamber 36 on the other hand, are filled with pressure transmission liquid.

The passageway 40 from inlet bore 32 is actually the relatively reduced end of an end-plugged counter bore 45 that is intersected by the inlet bore 31 and houses a pressure relief valve 46. In a generally similar manner the bore 31 also has a communicating bore 47 via a pressure relief valve 48 in end-plugged counterbore 49 that is intersected by the inlet bore 32. It will be apparent that the relief valves 46,48 are equivalent to those 24,25 of Figure 1.

Instead of a liquid pressure transmission medium, the device can be in communication with the inlet pressures via a suitable semiconduter gel. The use of the gel may avoid having to provide capillaries.

An automatic overload pressure protection system omitting individual pressure relief valves 24,25 can be provided if the equalising valve 21 of Figure 1 is operated to open when the transducer senses an excessive pressure.

Figure 4 shows a solenoid operated equalising valve 21A operated electrically. The pressure transducer output signal outline 26 is connected to two detectors such as threshold or detector electronic circuits each with a reference potential input (50,51) corresponding to full range or overload for the system. Outputs from the detectors (52,53) are connected in common to the solenoid valve so as to open same. One detector 54 will open the solenoid valve when the signal is excessively high; the other 55 would open the valve when the signal corresponds to an excessively large reverse pressure.

Claims (14)

1. A differential pressure transducer comprising a pressure-sensitive solid state electronic device, to which are applied in opposition the pressures whose difference is to be measured, and, extending between inlets for such pressures, interconnection means including pressure sensitive relief means that opens interconnection between those inlets when more than a predetermined overload pressure is reached, thus protecting the device from undue pressure from either of its inlets.

2. A differential pressure transducer as claimed in claim 1,wherein said relief means conveniently take the form of normally closed valves controlling connection between inlet fluid pipes, conduits or the like.

3. A differential pressure transducer as claimed in claim 2, wherein the inlet fluid pipes, conduits or the like are passageways in a block accommodating the valves and adapted for connection to a housing for the devive itself.

4. A differential pressure transducer as claimed in claim 1, wherein the device is in communication with inlet pressure via a suitable incompressible pressure transmission medium.

5. A differential pressure transducer as claimed in claim 4, wherein the medium is a liquid and the communication is at capillary interfaces so that the fluid cannot be lost from filled chamber means therefor during handling, transportation or fitting.

6. A differential pressure transducer as claimed in claim 5, wherein the capillaries have a longitudinal dimension such that the volumetric displacement of the fluid in operation of the device is negligible, typically 0.001 cc or less.

7. A differential pressure transducer as claimed in claim 3,4, 5 or 6, wherein the capillaries are provided within the block.

8. A differential pressure transducer as claimed in any one of the preceding claims, wherein the device is in communication with inlet pressures via a suitable semiconductor gel.

9. A differential pressure transducer as claimed in claim 8, wherein the semi-conductor gel is a silicone gel.

10. A differential pressure transducer as claimed in claim 1,4 or 5, wherein the device is of hybrid form with components thereof on its ceramic substrate, in particular a silicon crystal to both sides of which access is gained, usually via a conformal coating at least on one side, from spaced apertures in its substrate, one of which communicates with an inlet and the other of which is open to the chamber means.

11. A differential pressure transducer as claimed in claim 10, wherein substantially all of the device except for one inlet aperture is subjected to the pressure applied via the other inlet aperture.

12. A differential pressure transducer as claimed in claim 10 or 11,wherein all but a region immediately about that one inlet aperture is contacted by pressure transmission medium in the chamber means therefor that also houses the device with a communication to its other aperture that is effectively isolated from that chamber.

13. A differential pressure transducer as claimed in claim 12, wherein the pressure transmission medium is silicone.

14. A differential pressure transducer substan- tially as herein described with reference to and as illustrated in the accompanying drawings.