WO2012141598A1

WO2012141598A1 - Compressibility compensator

Info

Publication number: WO2012141598A1
Application number: PCT/NO2012/050065
Authority: WO
Inventors: Bjørn Erik SEEBERG
Original assignee: Presens As
Priority date: 2011-04-15
Filing date: 2012-04-12
Publication date: 2012-10-18
Also published as: NO20110588A1

Abstract

A compensator device for use with a pressure sensor is disclosed, wherein the pressure sensor includes a separating diaphragm facing a pressure to be measured as well as a cavity of a volume available to a pressure-transferring fluid for transferring pressure between the separating diaphragm and a sensor device. The compensator device comprises a body comprising a rigid structure, and a compressible cavity. In operation, the compensator device is designed for being actuated by the pressure to be measured and for co-operating with the pressure sensor to effect a change of the volume available to the pressure-transferring fluid within the pressure sensor. The compensator device is applicable both with absolute pressure sensors as well as with differential pressure sensors.

Description

Compressibility Compensator

INTRODUCTION

The present invention relates to a compensator device for use with a pressure sensor, wherein the compensator device compensates for a volume change of a pressure-transferring fluid within the pressure sensor itself. The invention also relates to a pressure sensor provided with such a compensator device.

BACKGROUND

The invention is directed to pressure gauges for measuring high pressures in applications with strict requirements for accuracy. The internal transfer of pressure within the pressure sensor occurring between the separation diaphragm sensing the pressure to be measured and the sensor unit itself within the pressure sensor is accomplished by way of a pressure-transferring fluid. The pressure-transferring fluid may be hydraulic oil. Pressure gauges to be used for measuring small pressure differences in oil/gas installations on the seafloor, for example, are subject to strict requirements on robustness, accuracy, and operating life. The sensors must be able to detect small pressure differences in the order of a few millibar. This means that the differential pressure across the separating

diaphragms should not exceed a few millibar.

Prior art pressure gauges commonly contain as much as 1000 mm³ of oil in the long pressure-transfer pipes extending from the separating diaphragm, which faces the process fluid in which the pressure is to be measured, into the sensor itself. The separating diaphragms of such prior art sensors are large (0: 76mm and 59mm as standard) and require a large number of screw bolts for attachment to the sensor so as to remain intact in high pressure conditions. Such large diaphragms are not able to pick up small pressure differences in the process fluid. It is desirable to provide the known pressure gauges with small separating diaphragms against the process fluid to be measured to allow small pressure differences to be read with high accuracy. The pressure-transferring fluid is not an ideal fluid; the volume thereof changes proportionally with the pressure/temperature. In the case of high pressures a significant volume change will occur in the pressure-transferring oil located within the sensor itself. This volume change may be as great as 10% for the hydraulic oil commonly used. This volume change in the pressure-transferring oil internally in the sensor must be accommodated by the separating diaphragm with no associated pressure drop across the separating diaphragm, as a pressure drop across the separating diaphragm causes measurement errors.

SUMMARY OF THE INVENTION

Therefore, in order to be able to use small separating diaphragms and in order to be able to sense small pressure changes in pressure measurements at high pressures, there is a need for a compensator device which compensates for the fact that the process fluid is not an ideal fluid, to allow the true pressure change to be sensed by the pressure sensor.

The compensator device according to the invention is based on a principle in which the compensator is a compressibility compensator occupying volume otherwise available for the pressure-transferring fluid within the pressure sensor itself, so that the separating diaphragm in combination with the pressure-transferring fluid within the pressure sensor correctly senses small changes in the pressure of the process fluid. Hence, the compensator device compensates for the change of volume of the pressure-transferring fluid that occurs when the pressure sensor performs measurements at high pressures and/or temperatures. As mentioned above, the volume change is caused by the non-incompressibility of the pressure- transferring fluid.

According to the invention, this is achieved by a compressibility compensator including a body having a rigid structure and at least one compressible cavity. The compressibility compensator is designed so as to be sufficiently rigid to be able to exert a force on the pressure sensor to cause the volume change, yet sufficiently elastic (resilient) to compensate in accordance with the pressure change. The compressibility compensator is directly influenced by the process fluid pressure. This results in a change in the available volume for the pressure-transferring fluid within the sensor which compensates for the proportional change in the volume of the pressure-transferring fluid with pressure/temperature. The separating diaphragm is then able to transfer the actual pressure.

Thus, in a first aspect, the invention provides a compensator device for use with a pressure sensor, wherein the pressure sensor includes a separating diaphragm facing a pressure to be measured as well as a cavity with an available volume for a pressure-transferring fluid for the transfer of pressure between the separating diaphragm and a sensor device, the compensator device comprising a body comprising a rigid structure, and at least one compressible cavity. In operation, the compensator device is configured so as to be actuated by the pressure to be measured and to co-operate with the pressure sensor to cause a change of the available volume for the pressure-transferring fluid within the pressure sensor.

In an embodiment, the body can be shaped so as to surround and be attached to a part of the cavity containing the pressure-transferring fluid. The cavity is formed inside the body and shaped so as to surround said part of the cavity containing the pressure-transferring fluid. The body may be provided with bellow-shaped external sidewalls. The bellow-shaped external sidewalls impart elastic properties to the body. In one embodiment, the outer sidewalls of the cavity have shapes that are complementary to the external sidewalls of the body. The cavity may be annular with bellow-shaped outer sidewalls. The body can be made of steel.

In another embodiment, the compensator device may further include a first diaphragm m_a facing a part of the volume of pressure-transferring fluid as well as a second diaphragm MA facing the pressure to be measured. The first diaphragm m_a may be disposed so as to oppose a part of the separating diaphragm facing the pressure to be measured. The cavity may be located between the first diaphragm m_a and the second diaphragm MA. An area of the second diaphragm M_A may be significantly larger than an area of the first diaphragm m_a. The cavity may have an approximate torus-shape where the middle of the torus constitutes the rigid structure of the compensator device which transfers an external pressure load from the second diaphragm to the first diaphragm.

The cavity may be a vacuum cavity. Alternatively, the cavity may be filled with a gas or compressible liquid.

In a second aspect, the invention provides a pressure sensor comprising a separating diaphragm facing a pressure to be measured, a cavity containing a pressure-transferring fluid for transferring the pressure between the separating diaphragm and a sensor device, as well as a compensator device as set forth above.

In an embodiment, the compensator device may surround and be attached to a lower part of the cavity containing pressure-transferring fluid, wherein an influence on the separating diaphragm from the pressure to be measured causes the compensator device to contract/expand. The compensator device may be integrated into a lower part of the pressure sensor facing the pressure to be measured. The pressure sensor may include a diaphragm seat. A hydraulic line can be provided to extend into a sensor unit for sensing the pressure. The pressure sensor may be an absolute pressure sensor or a differential pressure sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, exemplary embodiments of the invention will be described with reference to the drawings, in which

Fig. 1 shows a view of a pressure sensor with a compensator device according to an embodiment of the invention,

Fig. 2 shows a view of a pressure sensor with a compensator device according to another embodiment of the invention.

DETAILED DESCRIPTION

The present invention relates to a compensator device for a pressure sensor, as well as a pressure sensor provided with the compensator device. The pressure sensor includes a separating diaphragm facing the pressure to be measured, and a hollow containing a pressure-transferring fluid for transferring the pressure from the separating diaphragm into a sensor unit for measuring the pressure. The pressure sensor may be an absolute pressure sensor or a differential pressure sensor.

In its simplest form, the compensator device is constituted by a body comprising a rigid structure and at least a compressible cavity. When used with a pressure sensor, the compensator device is designed to be influenced by the pressure to be measured and to co-operate with the pressure sensor to effect a change of the volume available to the pressure-transferring fluid within the pressure sensor. The cavity may be a vacuum cavity. Alternatively, the cavity may be filled with a gas or filled with a compressible liquid. The body and the compressible cavity are designed so as to impart elastic properties to the compensator device. The rigid structure will exert a pressure-dependent force that effects a change in the volume available to the pressure sensor.

An embodiment of a pressure sensor with the compensator device is shown in Fig. 1. The embodiment of Fig. 1 shows a pressure sensor having a separating diaphragm against a process fluid whose pressure is to be measured and a volume containing a pressure-transferring fluid for transferring pressure between the separating diaphragm and a sensor device. In Fig. 1 , the volume is defined by the separating diaphragm, a diaphragm seat for the separating diaphragm, and a hydraulic line extending into the actual sensor element. A typical size of such a volume is in the order of 200 mm³. A compensator device is attached to and surrounds the lower part of the pressure sensor. The rigid structure of the compensator device surrounds and is secured to the hydraulic line and the diaphragm seat. The separating diaphragm is fixed to the compensator device. In the embodiment of Fig. 1 , the compensator device is an integral part of the overall pressure sensor structure. The rigid structure of the compensator device has bellow-shaped external sidewalls. The rigid structure may be made of steel. A compressible cavity is formed inside the rigid structure. The cavity surrounds the hydraulic line. The outer sidewalls of the cavity are bellow-shaped and have shapes that are complementary to the external sidewalls of the rigid structure. The cavity may be a vacuum cavity. Alternatively, the cavity may be filled with a gas or compressible liquid.

When the separating diaphragm of the pressure sensor is acted on by the pressure of the process fluid, the bellow-shaped outer walls of the compensator device, together with the internal compressible cavity, including the diaphragm seat but not the hydraulic pipe, will be compressed (or expanded in the case of reduced pressure) to thereby accommodate the pressure in the process fluid. The hydraulic line, together with the rigid surrounding structure, will function as a rigid bar being pushed in against/pulled out relative to the separating diaphragm, and hence the diaphragm seat will also follow this movement. The vacuum cavity absorbs the internal pressure of the compensator device. The positional change of the diaphragm seat will result in a change of the volume available to the pressure- transferring fluid within the pressure sensor.

Thus, the compensator device will co-operate with the pressure sensor to effect a change of the volume available to the pressure-transferring fluid within the pressure sensor. The separating diaphragm may then communicate the correct pressure change with no significant pressure drop.

The separating diaphragm facing the pressure-transferring fluid may be made small (0: 30 mm) even in the case of large internal oil volumes (e.g. in the order of 1000mm³). The rigid structure may be made of Alloy 625. However, the stiffness of the structure, the elastic properties of the bellow-shaped outer walls, and the cavity will be adapted according to the pressure sensor into which the structure is integrated so that the desired volume change may take place over the applicable range of pressure and temperature conditions for the pressure sensor.

An alternative embodiment of a pressure sensor with a compensator device is shown in Fig. 2. Similar to the embodiment of Fig. 1 , the embodiment of Fig. 2 shows a pressure sensor having a separating diaphragm against a process fluid whose pressure is to be measured, and a cavity containing a volume of a pressure-transferring fluid for transferring pressure between the separating diaphragm and a sensor element of the pressure sensor. In Fig. 2, the cavity is defined by the separating diaphragm, a diaphragm seat for the separating diaphragm, a hydraulic line leading into the sensor element itself, as well as part of a compensator device for the pressure sensor. The compensator device is attached to the lower part of the pressure sensor.

The compensator device of Fig. 2 includes a first diaphragm having a small area m_a disposed inside the separating diaphragm. The first diaphragm may be part of a diaphragm seat for the separating diaphragm. The first diaphragm m_a may be pushed inwards into the volume behind the separating diaphragm to reduce the size thereof (dotted line), or alternatively be pushed outwards to increase the volume behind the separating diaphragm. Consequently, the diaphragm m_a is able to move to thereby change the volume available for the pressure-transferring fluid within the pressure sensor.

The body of the compensator device further includes a second diaphragm M_A positioned against the process pressure to be measured and opposite to the first diaphragm. The second diaphragm MA has a larger area compared to the first diaphragm m_a. Preferably, the area of the second diaphragm is significantly larger than the area of the first diaphragm. Between the first diaphragm m_a and the second diaphragm MA a torus-shaped compressible cavity is provided having a rigid bar in the centre of the torus shape. The cavity may be a vacuum cavity. Alternatively, the cavity may be filled with a gas or compressible liquid.

The operation of the compensator device is based on the same main principle as the compensator device of Fig. 1. The second diaphragm and the separating diaphragm are influenced by the pressure to be measured. Due to the large difference in size between the first diaphragm m_a and the second diaphragm MA, the force exerted by the second diaphragm M_A will act on the rigid bar so that the bar is pushed in towards the first diaphragm with a small area. The first diaphragm m_a will move inwards towards the separating diaphragm to thereby change the volume available for the pressure-transferring fluid in the pressure sensor. The torus-shaped cavity is compressed and absorbs the internal pressure in the compensator device. The separating diaphragm is then able to sense the correct pressure without any significant pressure drop. When the pressure to be measu- red decreases, the force acting on the compensator device via the second diaphragm MA will be reduced. This will cause the first diaphragm to move backwards towards the second separating diaphragm, increasing the volume available to the pressure-transferring fluid.

In this embodiment also the separating diaphragm facing the pressure-transferring fluid may be made small (e.g. in the order 0:30 mm). The rigid structure may be made of Alloy 625. However, the stiffness of the structure and the first and second diaphragms, as well as the cavity and the thickness of the compensator device body, will be adapted in view of the pressure sensor with whom the structure is integrated so that the desired volume change may take place over the applicable range of pressure and temperature conditions for the pressure sensor.

The compressibility compensator of the invention may be used in adapting prior art pressure gauges to a maritime environment and for making them capable of measuring small high pressures with only small measurement errors. The pressure gauges may be absolute pressure sensors or differential pressure sensors.

It should be noted that the embodiments disclosed above are only exemplary embodiments, and that a person with skills in the relevant art will be able to devise numerous other modifications and variants of the invention as defined in the accompanying claims.

Claims

C L A I M S

1. A compensator device for use with a pressure sensor, wherein the pressure sensor has a separating diaphragm against a pressure to be measured as well as a cavity for a volume available to a pressure-transferring fluid for transferring pressure between the separating diaphragm and a sensor device, the compensator device comprising:

- a body comprising a rigid structure, and

- at least one compressible cavity,

the compensator device being configured, in operation, for being influenced by the pressure to be measured and for co-operating with the pressure sensor to effect a change of the volume available to the pressure-transferring fluid within the pressure sensor.

2. The compensator device of claim 1 , wherein the body is configured so as to enclose and attach to a part of the cavity with pressure-transferring fluid.

3. The compensator device of claims 1 or 2, wherein the body is provided with bellow-shaped external sidewalls.

4. The compensator device of any one of the previous claims, wherein the compressible cavity is formed inside the body and configured so as to enclose said part of the cavity with pressure-transferring fluid.

5. The compensator device of any one of the previous claims, wherein the outer sidewalls of the compressible cavity have complementary shapes with the external sidewalls of the body.

6. The compensator device of any one of the previous claims, wherein the compressible cavity is annular having bellow-shaped outer sidewalls.

7. The compensator device of any one of the previous claims, wherein the body is made of steel.

8. The compensator device of claim 1 , the compensator device comprising:

- a first diaphragm m_a facing a part of the cavity containing pressure-transferring fluid, and

- a second diaphragm MA facing the pressure to be measured.

9. The compensator device of claim 1 , wherein the first diaphragm m_a is located opposite to a part of the separating diaphragm facing the pressure to be measured.

10. The compensator device of claims 8 or 9, wherein the cavity is formed between the first m_a and second diaphragm MA.

1 1. The compensator device of any one of claims 8-10, wherein an area of the second diaphragm M_A is significantly larger than an area of the first diaphragm m_a.

12. The compensator device of any one of claims 8-1 1 , wherein the compressible cavity is substantially torus shaped and where the middle of the torus constitutes the rigid structure of the compensator device which transfers an external pressure influence from the second diaphragm to the first diaphragm.

13. The compensator device of any one of the previous claims, wherein the compressible cavity is a vacuum cavity.

14. The compensator device of any one of the previous claims, wherein the compressible cavity is filled with gas or a compressible liquid.

15. A pressure sensor, comprising:

- a separating diaphragm facing the pressure to be measured, - a cavity of a volume available to a pressure-transferring fluid for transferring the pressure between the separating diaphragm and a sensor device, and

- a compensator device of any one of claims 1 -14.

16. The pressure sensor of claim 15, wherein the compensator device encloses and is secured to a lower part of the cavity containing pressure-transferring fluid, and wherein an influence on the separating diaphragm from the pressure to be measured effects a compression/expansion of the compensator device.

17. The pressure sensor of claim 15, wherein the compensator device is integrated into a lower part of the pressure sensor facing the pressure to be measured.

18. The pressure sensor of any one of claims 15-17, wherein the pressure sensor comprises a diaphragm seat 9. The pressure sensor of any one of claims 15-18, wherein the cavity comprises a hydraulic line extending into the sensor device for sensing the pressure.

20. The pressure sensor of any one of claims 15-19, wherein the pressure sensor is an absolute pressure sensor or a differential pressure sensor.