OVERLOAD PROTECTION OF PNEUMATIC PRESSURE SENSORS WITH A SINGLE DIAPHRAGM

Abstract: Pneumatic pressure sensor of the kind where the pressure to be sensed acts on a diaphragm (50) which moves to control the exhaust of gas from a pressurized control chamber (20). The invention provides means for inhibiting or preventing the diaphragm from becoming overstrained. To support the diaphragm against excessive external pressure the diaphragm is mounted spaced a small distance in front of a backing plate (10); while to protect it from excessive internal pressure, firstly, mechanical stop means (62) are provided for limiting the outward movement permitted to a rod (60) fixed to the diaphragm centre, and secondly, pressure control means are provided for limiting to a predetermined extent, and relative to the pressure acting externally, the maximum gas pressure which can act upon the internal face of the diaphragm.

Description: Overload protection of pneumatic pressure sensors with a single diaphragm. This invention relates to a pneumatic pressure sensor of the type comprising a pneumatically pressurized control chamber whose boundary walls consist in part of a flexible diaphragm, the pressure to be measured acting on the outside of the diaphragm. Compressed air, or other gas, is supplied to the control chamber and exhausted from the chamber through an exhaust valve, with opening and closing of this valve being controlled and effected either directly or indirectly, by movements of the central region of the diaphragm; inwardly of the control chamber to effect closing of the exhaust valve, and outwardly of the control chamber to effect opening of the exhaust valve. The pressure set up in the control chamber is thus substantially equal, at all times with the pressure acting externally on the diaphragm, and this internal control chamber pressure is transmitted to a remotely located indicator device, such as a gauge. The diaphragm should be very flexible so that its resilience contributes as little as possible and hardly affects the pressure changes which take place in the control chamber. Such flexible diaphragms are easily damaged by excessive pressures which may arise either externally or internally. The present invention has for its object to provide such a sensor in which the movements of the diaphragm are controlled so that they are permitted to be just large enough to obtain proper operation of the exhaust valve, and yet not so large that the diaphragm will become distorted and permanently damaged; and it is a particular object of the invention to prevent the diaphragm from being subjected to excessive pressure generated within the control chamber. This invention provides a pressure sensor of the kind with a pneumatically presurized control chamber and an exhaust valve, and in which the control chamber has, as one of its walls, a plate with a diaphragm mounted on its outer side facing the pressure to be sensed, the diaphragm covering a central aperture of the plate, the area of the aperture being small and less than 5% of the area of the inner face of the diaphragm, the diaphragm having its central region connected to a rod which extends through the plate aperture and through the control chamber, axial movement of said rod inwardly of the control chamber, being effective, directly or indirectly, to cause closing of the exhaust valve, while axial movement of said rod outwardly of the control chamber, is effective, directly or indirectly, to cause opening of said exhaust valve; and wherein in order to prevent, or at least inhibit, overloading of the diaphragm, so as to cause distortion or even rupture thereof, under a first excessive pressure condition (i) when pressure externally of the diaphragm excessively exceeds pressure within the control chamber an abutment means is provided for limiting movement of the diaphragm inwardly of the control chamber, by the region of the diaphragm surrounding its central region abutting the plate and being thus supported by the plate under excessive pressure condition (i). According to an essential feature of the invention, in such a sensor, in order to prevent or at least inhibit overloading of the diaphragm so as to cause distortion or even rupture thereof, under a second excessive pressure condition (ii) when pressure internally within the control chamber excessively exceeds pressure externally of the diaphragm, there is formed on the rod within the control chamber, a shoulder which is adapted and arranged to abut against the inside face of the plate in the region surrounding the plate aperture, and this is effective to limit the movement permitted to the central region of the diaphragm in the direction as the rod moves outwardly of the chamber; and wherein pressure control means are provided for limiting to a predetermined extent, relative to the gas pressure acting on the external face of the diaphragm, the maximum gas pressure which can act upon its inner face. Preferably the said shoulder means on the rod, comprises firstly, an annular flange projecting laterally from the rod, and said pressure control means comprise an annular seal carried by the flange, said seal being of resilient sealing material, so that whenever the pressure in the control chamber exceeds the pressure acting externally on the diaphragm and the diaphragm moves outwardly to an extent limited by abutment of the seal with the inside face of the plate, said seal sealingly engages the inside face of the plate in its region surrounding the plate aperture, to prevent any further increase of pressure in the control chamber from being communicated to the inside face of the diaphragm. Preferably also the means for supplying air or gas to the control chamber for pressurizing same, includes a restrictor device which limits, to a predetermined extent, the maximum rate at which air or gas can be supplied to the control chamber. The diaphragm may be a circular disc of metal having concentric circular corrugations, and with a peripheral lip which is bonded, eg welded, to the plate and a central region bonded, eg welded, to the outer end of the rod. Advantageously, the inner end of the rod is adjacent to the exhaust valve and carries a valving member which is movable with the rod to open or close the exhaust valve. The valving member may be a spherical member or ball which is carried at the inner end of the rod and which is brought to seat against a circular port to close the exhaust valve. One embodiment of the pneumatic pressure sensor according to the invention will be described with reference to the accompanying drawings in which:- Figure 1 is frontal end view Figure 2 is a view from the rear, and Figure 3 is a view from the side, taken in cross-section along the lines III - III of Figure 2. Referring now to the drawings, 10 represents a metal plate forming part of the walls of a control chamber 20, whose walls are also, in pact, defined by a rigid metal housing 11 secured to the plate 10 by bolts 12. The plate 10 is in this embodiment circular and has a series of peripheral through holes 14 by means of which it may be bolted, for example to the wall of a tank or reservoir. The plate 10 has a central aperture 15, an annular washer 13 surrounding this aperture and being well spaced therefrom and being interposed between the rigid housing 11 and the inner face of the plate 10. Air or other gas under pressure is supplied to control chamber 20 through an inlet connection generally designated 21, by way of a flow rate restrictor valve 22, which may be adjustable, so that the maximum rate of flow permitted into chamber 20 is regulated. As seen in Figure 2, the pressure in chamber 20 may be communicated to a pressure gauge 30 by way of a connection nipple 31 formed in housing 11 and signal transmission pipe 32. Air is exhausted from chamber 20 by an exhaust valve generally designated 40 leading to an exhaust or venting pipe 41. More details of the exhaust valve 40 will be given below. A pressure sensing diaphragm generally designated 50 is attached to the outer face of the plate 10 and extends to cover the aperture 15. This diaphragm may be of elastomeric rubber like material, but as shown, it is a thin readily flexible sheet of metal, being circular and having annular corrugations 52 surrounding a disc-like flat central region 51. The peripheral edge 53 of the thin metal plate, constituting the diaphragm 50 in the example illustrated, is secured, for example by electron beam welding, to a flange 16 outstanding from the outer face of the plate 10. The diaphragm 50 defines with part of the outer face of the plate 10, a small space 54 which, normally, is in direct communication with the control chamber 20, so as to be effectively a part thereof under normal operating conditions, the communication being by way of the plate aperture 15. The central aperture 15 of the plate 10 has a small area which is less than 5% of the area of the inner face of the diaphragm 50. The central region 51 of the diaphragm 50 is secured to the head 61 of a rod 60 which extends through the plate aperture 15 and through the chamber 20 to terminate in an inner end of the rod which is adjacent to, and usually contiguous with the exhaust valve 40. The exhaust valve 40 comprises a valve body 43 which is of generally tubular configuration and is fitted, with an intervening 'O'-ring seal 44, in a cylindrical bore 17 in the housing 11. The bore 17 terminates at its outer end in the threaded portion receiving the screw-threaded enlarged end portion 45 of the valve body 43. A nipple 42 of the pipe 41 is screw-threadedly connected to the enlarged outer end portion 45 of the valve body 43. The valve body 43 defines a through bore 46 communicating with the pipe 41. The inner end of the valve body 43 is constituted as a valve seat 47 which is adapted to be opened and closed by a valve closure member carried at the inner end of the rod 60. This valve closure member may be a needle, but as shown it is a spherical member 48, which may be a ball, mounted on the inner end of the rod 60. As the diaphragm 50 moves outwardly of the control chamber 20, the rod 60 moves to the left of Figure 3 and the spherical member 48 lifts off the seat 47 to open the exhaust valve 40; and as the diaphragm 50 moves inwardly of the chamber 20, the rod 60 moves to the right in Figure 3, and the spherical member 48 abuts with the seat 47 to close the exhaust valve 40. These movements are very small. Within the chamber 20, and according to an important feature of this invention, the rod 60 has a laterally projecting flange 62 carrying at its periphery an annular seal 64, which may be an O-εectioned sealing ring. When the rod 60 is in its position, to the right of Figure 3, when the exhaust valve 40 is closed, the seal 64 is spaced a small distance from the interior face of the plate 10 surrounding the plate aperture 15. However when the diaphragm 50 moves outwardly of the chamber 50, the rod 60 moves a small way to the left of Figure 3, to open the valve 40. If the rod 60 moves further to the left, Figure3, the seal 64 is moved against the interior face of the plate 10 in the region surrounding the plate aperture 15. Thus the shoulder means constituted by the flange 62, by means of the seal 64 carried thereby, abuts with the plate 10 to limit movement of the rod, to the left in Figure 3, and this prevents the diaphragm from moving outwardly too far and becoming distorted or damaged. Movement of the diaphragm 50 to modulate the aperture of the exhaust valve 40 is very small; so small, in fact, that the diaphragm 40 stays substantially always in its unstrained central position. The elastic resistance of the diaphragm therefore has little effect upon the pressure in the control chamber 20 and this essentially adjusts itself to balance as exactly as possible the pressure to be sensed acting on the outer surface of the diaphragm 50. It is important that the diaphragm 50 should be very flexible and should offer little or no elastic resistance over the small movements required to achieve the required modulation of the exhaust valve 40. A very flexible diaphragm of the kind required has to be protected against accidental damage due to out-of-balance pressure arising on one or other of its faces as a result of some inadvertent event of malfunctioning of the system. To achieve this, firstly, the diaphragm is secured, by being clamped or welded, to the substantial backing plate 10 disposed against the inside face of the diaphragm 50, and spaced away from this by a small distance so that the space 54 is defined. This measure provides firm support to the diaphragm 50 in the event of failure of the air supply when the diaphragm may be exposed to pressure only on its outer side. Excessive deflection inwards into the control chamber 20 will cause the diaphragm to lie up firmly against the support plate 10 and prevent it from being damaged, and only a limited deflection will be permitted since the ball on the inner end of rod 60 will abut with the seat 47 of valve body 43. Secondly, it is also the purpose of this invention to provide a ready and effective means to protect the diaphragm 50 against excessive over-pressure in the opposite direction, that is when the pressure within the control chamber excessively exceeds the pressure acting externally upon the diaphragm. This could for example be caused by a blockage of the valve 40 which exhausts air from the control chamber 20; and any excessive rise in pressure on the inside of the diaphragm might cause excessive outwards deflection of the diaphragm 50, tending to cause damage, distortion, or even rupture. Accordingly, the backing plate 10 to which the diaphragm is fixed is provided with the central aperture 15 through which the exhaust valve control rod 60 passes, and this rod is provided, in the control chamber 20 on the inside of the backing plate 10, with the shoulder or flange 62 which is normally spaced away from and not in contact with the backing plate 10; but which does make contact with the backing plate 10, as soon as the outward deflection of the diaphragm 50 reaches a predetermined extent. As soon as abutment contact is achieved the diaphragm 50 is prevented from bowing out further because its central land to which the rod 60 is attached, is supported. It is a further important feature of the design, however, that the centre of the diaphragm 50 is not only mechanically supported against excessive outward deflection, and that the flange 62 on the rod 60 not only makes contact with the inside face of the backing plate 10, but also achieves an air tight seal with it by means of the ring seal 64 fixed to the support flange 62. This will seal with the inner face of the plate 10 in the region surrounding the aperture 15. This ensures that when excessive pressures arise in the control chamber 20, no air or gas can flow through the aperture 15 and into the space 54 between the diaphragm 50 and the outward face of the support plate 10. If this were not so, the narrow space 54 on the outside of the backing plate 10 could become filled with excessive pneumatic pressure, which could still damage the diaphragm 50 even though its centre is prevented from moving out further. Such excessive internal pneumatic pressure could strain outwards the unsupported areas of the diaphragm 50 around its central supported hub or land, and cause it to be distorted and damaged. The provision of an effective gas seal, however, operating conjointly with the mechanical abutment, in effect, of the shoulder 62 against the inside face of the backing plate 10, prevents any build-up of pneumatic pressure in space 54, beyond the point where the diaphragm 50 has suffered its limiting deflection which is still arranged to be within its safe operating range. The pneumatic seal envisaged here can be of the 'O' ring type, with a *0' ring fitted, as shown, to the flange 62; or it can be of another known and effective gasket type. It will be noted that the sealing-off of the inside face of the diaphragm, when excessive pressures arise in the chamber 20, is in the nature of a one-way or non-return valve in the sense that the seal is arranged to prevent any increase of pressure in the space 54 on the inside of the diaphragm beyond the limiting point, but allows the pressure in the space 54 to leak out into the control chamber 20 as soon as this has dropped below the level beyond which the diaphragm suffers its limiting deflection. This one-way or non-return valving action may be achieved with a simple ring seal which is so fitted that it prevents any flow of air or gas into the space 54 between the backing plate 10 and the diaphragm 50, but does not prevent any outflow of this gas from this small space, as soon as the pressure in the control chamber 20 has dropped sufficiently. The arrangement specified according to this invention, provides for an effective protection of the sensitive regulating diaphragm 50 used in the pressure sensor, against damage due to failure or blockage of the venting system from the control chamber. This will contribute substantially to the safety and robustness of the sensing system which is normally prone to damage by even such inadvertent actions as placing a hand over the venting port from the control chamber 20, which in a very short time, eg. in a matter of one second or so, can cause excessive strain to the operating diaphragm and render it ineffective and inaccurate for further operation. The venting pipe 41 leading from the control chamber 20 may, as shown, be a simple vent tube, or it may have a vent valve of the kind which allows the free passage of air or gas but which seals off under excessive flow conditions, or which will close if flow of liquid is detected which could occur if the the diaphragm were ruptured, and gas or fluid from the reservoir, the pressure of which is to be sensed, were to pass through chamber 20 towards venting pipe 41. As such venting arrangements can be liable to occasional momentary blockages, the present arrangement for the protection of the sensitive diaphragm from pneumatic over pressure substantially adds to the value and effectiveness of the system. By providing an effective one-way sealing action coupled with a mechanical stop action as described, the diaphragm will both be protected against damage and it will be allowed to return to normal functioning as soon as the vent blockage is removed. Additionally, venting of over pressure in the small space 54 behind the diaphragm can be achieved by a separate non-return valving means arranged in conjunction with the exhaust valve control rod 60 attached to the diaphragm 50. For example a passage could be provided in the control rod this passage leading from the space 54 and communicating with the exhaust valve downstream of its closure member; there being provided also a one-way valve permitting flow through this passage only in the direction from the chamber 54 and to the exhaust valve downstream of the exhaust valve closure member. Also venting of the space 54 could be achieved by a separate one-way or non-return valve disposed elsewhere in the backing plate 10 which will allow flow from the space 54 behind the diaphragm into the control chamber 20, but prevent any flow from the control chamber 20 to the space 54, whenever excess pressure occurs in control chamber 20. Hence as above described, the rate of flow into the chamber 20 is regulated and limited by operation of the restrictor 22. Flow from the control chamber 20 to the small space 54 is cut off whenever pressure in the chamber 20 significantly exceeds pressure acting externally on the diaphragm 20; thus preventing excessive pressure from being communicated to the space 54. Cut off is largely, but not absolutely, independent of the resistance to deflection possessed by the diaphragm (which is kept as low as possible) ; and cut off will occur at a range of pressure conditions. Exactly when cut off will occur at any particular conditions of relative pressure, internal and external, and for any particular size and construction of diaphragm, and rate of inlet flow, can be calculated and checked by experiment; and to that extent it can be predetermined.

Claims: CLAIMS 1. A pressure sensor of the kind with a pneumatically presurized control chamber (20) and an exhaust valve (40), and in which the control chamber has, as one of its walls, a plate (10) with a diaphragm (50) mounted on its outer side facing the pressure to be sensed, the diaphragm covering a central aperture (15) of the plate, the area of the aperture (15) being small and less than 5% of the area of the inner face of the diaphragm (50), the diaphragm having its central region connected to a rod (60) which extends through the plate aperture (15) and through the control chamber (20), axial movement of said rod inwardly of the control chamber, being effective, directly or indirectly, to cause closing of the exhaust valve (40), while axial movement of said rod outwardly of the control chamber, is effective, directly or indirectly, to cause opening of said exhaust valve; and wherein in order to prevent, or at least inhibit, overloading of the diaphragm (50), so as to cause distortion or even rupture thereof, under a first excessive pressure condition (i) when pressure externally of the diaphragm excessively exceeds pressure within the control chamber (20), a support means is provided for limiting movement of the diaphragm inwardly of the control chamber, said support means being achieved by the outer region of the diaphragm abutting the plate (10) and being thus supported by the plate under excessive pressure condition (i); and wherein in order to prevent or at least inhibit overloading of the diaphragm so as to cause distortion or even rupture thereof, under a second excessive pressure condition (ii) when pressure internally within the control chamber excessively exceeds pressure externally of the diaphragm, firstly, there is formed on the rod (60), within the control chamber (20), a shoulder (62) which is adapted and arranged to abut against the inside face of the plate (10) in the region surrounding the plate aperture (15), and this is effective to limit the movement permitted to the central region of the diaphragm (50), in the direction as the rod moves outwardly of the chamber; and secondly, pressure control means are provided for limiting to a predetermined extent, relative to the gas pressure acting on the external face of the diaphragm, the maximum gas pressure which can act upon the internal face of the diaphragm. 2. A pressure sensor according to claim 1, and in which the said shoulder means on the rod (60), comprises firstly, an annular flange (62) projecting laterally from the rod, and wherein said maximum gas pressure control means comprise an annular seal (64) carried by the flange, said seal being of resilient sealing material, so that whenever the pressure in the control chamber (20) exceeds the pressure acting externally on the diaphragm (50) and the diaphragm moves outwardly to an extent limited by abutment of the shoulder (62) with the inside face of the plate (10), said seal (64) sealingly engages the inside face of the plate (10) in its region surrounding the plate aperture (15), to prevent any further increase of pressure in the control chamber (20) from being communicated to the inside face of the diaphragm (50). 3. A pressure sensor according to claim 1, and with means (21) for supplying air or gas to the control chamber (20) for pressurizing same, and including a restrictor device (22) in the gas supply means which limits to a predetermined extent, the maximum rate at which air or gas can be supplied to the control chamber. 4. A pressure sensor according to claim 1, and in which the inner end of the rod (60) is adjacent to the exhaust valve and carries a valving member (48) which is movable with the rod to open or close the exhaust valve (40) . 5. A pressure sensor according to claim 1, and in which the annular seal (64) is an 'O'-ring. 6. A pressure sensor according to claim 1, and in which the diaphragm (50) is a circular disc of metal having concentric circular corrugations (52), and with a peripheral lip (53) which is secured to the plate (10), and a central region (51) which is secured to the outer end of the rod (60). 7. A pressure sensor according to claim 4, and in which the valving member (43) .is a ball which is brought to seat against a circular port (47) to close the exhaust valve (40). 8. A pressure sensor according to claim 1, and in which there are additionally provided one-way valve means permitting controlled flow of gas to the control chamber (20) from a small space (54) defined between the diaphragm (50) and the plate (10). 9. A pressure sensor according to claim 1, and in which there are additionally provided one-way valve means permitting cυntrolled flow of gas to the downstream side of the exhaust valve closure member (48) from a small space (54) defined between the diaphragm (50) and the plate (10) .

Inventors: GODDARD ROBERT WILLIAM, VARGA OTTO HERMAN, GODDARD, ROBERT, WILLIAM, VARGA, OTTO, HERMAN
Applicants: SEETRU LTD, SEETRU LIMITED
Application year: 1987
Main classification: G01L7
IPC codes: G01L7, G01L11