GB2194311A

GB2194311A - Valves

Info

Publication number: GB2194311A
Application number: GB08620188A
Authority: GB
Inventors: Edward Sagar Fort
Original assignee: Fort Valve Engineering Ltd
Current assignee: Fort Valve Engineering Ltd
Priority date: 1986-08-19
Filing date: 1986-08-19
Publication date: 1988-03-02
Anticipated expiration: 2006-08-19
Also published as: GB2194311B; GB8620188D0

Abstract

A valve (1) has a valve member (4) which is axially movable within a first chamber, having a high pressure inlet (8, 9, 10) and a pressure relief outlet (11), between a first position in which the outlet is isolated from the inlet and a second position in which the outlet communicates with the inlet. Resilient means such as a spring (5) biases the member (4) towards the first, isolating position. The valve also has a piston (23) axially movable in a second chamber (20), the piston being biased towards one end of the second chamber by resilient means such as a spring (24). The piston (23) and the valve member (4) are rigidly connected by means such as a hollow spindle (13), and means are provided to equalise the pressure on the front faces of the valve member (4) and the piston (23). In the preferred embodiment the hollow spindle (13) also serves as the pressure equalising means. Because the piston (23) moves with the valve member (4) and is subjected to the same pressure, the valve (1) has an improved response characteristic, and is sufficiently damped to prevent undue vibration of the valve member. <IMAGE>

Description

SPECIFICATION Improvements in or relating to valves The present invention relates to a valve and more specifically to an improved pressure or vacuum relief valve. The valve of the present invention is especially, although not exclusively suited to use with, or in association with, a positive displacement air blower.

Bulk cargo containers carrying particulate materials, that is to say bulk powdered or granular materials, such as cement, are usually discharged by means of a positive displacement air blower. The outlet of the blower is connected to the interior of the container by means of a pressure tight connector and air is pumped under pressure into the container to discharge the material from the container through an open discharge port or pipe. This method of discharging a bulk cargo container ensures a relatively uniform flow rate of material from the container. It is also quick, simple to implement and discharges most, if not all of the material from the container, in a single operation.

Should a blockage occur in the discharge port which prevents full flow discharge of material from the container a substantial overpressure will quickly develop within the container as the blower continues to pump air into it. This over-pressure can cause severe damage to the blower if it is not relieved quickly and is also potentially hazardous. To prevent such over-pressures from arising a pressure relief valve is provided at some point in the system, usually in the outlet of the blower itself.

It is important that the pressure developed by the blower remains substantially constant throughout each discharge operation and to this end the pressure relief valve is adjusted to operate at a nominal set relief pressure, below the maximum capacity of the blower it is being used with. Ideally, the tolerance of the valve is within 2 p.s.i. of the nominal set relief pressure, so that any rise or fall in pressure within the container as the valve closes and opens should not differ from the set relief pressure by an amount greater than this tolerance.In order to achieve this level of toler- ance the valve must-have very precise relieving and re-seating pressures and must be so designed as to fully rninimise any possibility of "stiction" occuring between the valve member and the valve seat which would reduce the performance of the valve within the aforementioned tolerance. In addition, the valve must have a quick response time, whilst being sufficiently damped to prevent excess vibration of the valve member. This latter point is of special significance in a pressure relief valve intended for use with å positive displacement air blower-or the like, as the blower tends to produce periodic pressure pulses in operation.

The valve member must be able to open and close as these pressure pulses rise and fall about the nominal set relief pressure and yet must be sufficiently damped to prevent it from vibrating excessivley against the valve seat.

Conventional pressure relief valves of the type which find application in a positive displacement air blower generally comprise a valve member which is resiliently biased into sealing engagement with a valve seat by means of a coiled spring engaging against the back face of the valve member. If the tension in the coiled spring is set relatively low to allow the full air flow capacity of the blower to be discharged with relatively little increase in pressure the valve member is prone to excessive vibration. However, if the tension in the coiled spring is simply increased to reduce this vibration the pressure increase at full flow will be unacceptable and the ability of the valve to operate within the aforementioned tolerance is sacrificed.The significance of this problem in conventional pressure relief valves will be more readily appreciated from the fact that it is not unknown for conventional pressure relief valves to shake to pieces. Even where this drastic form of valve failure does not occur, the level of vibration within conventional valves causes unacceptably high noise levels and results in a significant reduction in the useful life expectancy of the valve.

Some conventional pressure relief valves make use of "O" rings and rubber gaskets to afford an efficient seal between the valve member and the valve seat, and whilst these do indeed afford an efficient seal, they also increase the liklihood of "stiction" occuring.

Finally, in addition to the operational problems referred to above, pressure relief valves used in positive displacement air blowers discharging powdered and granular materials from a container have to cope with hostile, and occasionally, chemically aggressive, environments. There will inevitably be high levels of dust in the vicinity of the valve and this may sometimes be accompanied by high moisture levels. The workings of the pressure relief valve must be protected from these environmental conditions, and yet the performance of the valve must not be sacrificed.

It is an object of the present invention to provide a valve and more specifically a pressure or vacuum reiief valve which is especially, although not exclusively, suited to use with, or in association with, a positive displacement air blower, and which obviates or substantially mitigates the problems referred to hereinabove.

According to the present invention there is provided a valve comprising a first chamber defining a high pressure inlet and a pressure relief outlet, a valve member axially moveable within the chamber between a first position in which it isolates the inlet from the outlet and a second position in which it connects the inlet to the outlet, and resilient biasing means acting on the rear face of the valve member to bias it towards said first position, characterised in that the valve further comprises a second chamber, a piston member axially moveable within the second chamber, resilient biasing means acting on the rear face of the piston member to bias it towards one end of the second chamber, pressure equalising means equalising the pressure on the front face of the valve member and the front face of the piston member, and connecting means connecting the piston member to the valve member.

The valve of the present invention is especially suited for use with a positive displacement air blower or the like, preventing overpressures from arising, and to this end the high high pressure inlet is connected in or to the outlet of the blower. The tension in both resilient biasing means is adjusted to give a nominal set relief pressure and when the pressure differential across the valve member exceeds this nominal set relief pressure the valve member moves towards the second position and relieves the excess pressure through the pressure relief outlet. Once the pressure differential across the valve member drops back below the nominal set relief pressure the valve member returns to the first position. In operation therefore, the valve of the present invention is similar to a conventional pressure relief valve.However, by virtue of the piston member moving within the said second chamber in conjunction with the valve member and subject to the same pressure on its front face as the valve member, the valve is found to have a much improved response characteristic, and is sufficiently damped to prevent undue vibration of the valve member.

This improved characteristic is due to the increased working area of the valve provided by the addition of the piston member, the subsequent increased cumulative bias load applicable, and the increased location diameter spread.

In a preferred embodiment of the present invention the first and second chambers are axially aligned and the connecting means between the valve member and the piston member comprises a connecting spindle. The connecting spindle is axially moveable within a tubular extension of the second chamber which guides it as it moves along the axis of the valve and prevents any lateral movement of the assembly within the chambers.

Preferably, the connecting spindle defines a tube which opens at one end onto the front face of the valve member and at the other end onto the front face of the piston member, thereby equalising the pressure on both of these surfaces.

Preferably, the connecting spindle comprises a backing plate which prevents the valve member from pitching unduly on the connecting spindle. Conveniently the resilient biasing means associated with the valve member abuts against the back of this backing plate and the back of the chamber.

Preferably, a resilient pad, of, for example, rubber, is provided between the backing plate and the valve member to allow a degree of resilience to the valve member. In addition, a flexible diaphragm is connected between the valve member and the sides of the chamber to isolate the space behind the valve member from the high pressure inlet and thereby prevent egress of dirt and and contaminates into the workings of the valve. Conveniently, the diaphragm forms an extension of the rubber pad and is in the form of a bellows to accomadate movement of the valve member within the chamber.

Preferably, the enclosed space defined by the diaphragm is vented to atmosphere to prevent a build up of pressure as the valve member moves within the chamber, or if the spindle seals should fail and allow pressure equalisation. Convenientiy, this space is connected to the space in the second chamber behind the piston member, which in turn is vented to atmosphere via a sintered breather.

Preferably, a valve seat is provided within the said first chamber, between the high pressure inlet and the pressure relief outlet, and the valve member sealingly engages against the said valve seat in said first position. Conveniently, the engaging surfaces of the valve member and of the valve seat are comprised of polished stainless steel. This arrangement is not prone to "stiction" to the same extent as a rubber seal would be.

In a preferred embodiment of the present invention the surface area of the piston member is less than that of the valve member.

Moreover, the axially length of the second chamber, in which the piston member is held, is adjustable to allow the tension in the resilient biasing means provided therein to be varied. As a consequence of the smaller surface area afforded by the piston member the load it exerts on its resilient biasing means is less than that exerted by the valve member on its and it is much easier to adjust the tension within this resilient biasing means to set the required nominal set relief pressure of the valve. Convenientiy, the second chamber is comprised of a tubular body and a cap which can be screwed onto the end of the tubular body to increase or decrease its axial length and thereby adjust the tension in the resilient biasing means contained therein.

An embodiment of the present invention will now be described, by way of example with reference to the accompanying drawing which shows a sectional view of a pressure relief valve in accordance with the present invention.

Referring to the accompanying drawing there is shown a valve according to the present invention which is especially, although not exclusively, suited for use with a positive displacement air blower, relieving over-pressures.

For ease of understanding the valve can be divided into two parts, one of which comprises a generally conventional pressure relief valve, designated by the reference numberal 1, and the other part of which comprises a vibration damping assembly, designated by the reference numeral 2.

The pressure relief valve 1 comprises a housing 3 within which is contained a valve member 4, resilient biasing means 5 and a valve seat 6.

The valve member 4 comprises a circular stainless steel plate and the valve seat 5 is also of stainless steel to prevent "stiction", commonly found in rubber sealing faces, from occuring between the two. The resilient biasing means 5 comprises a coiled spring.

The housing 3 comprises a tubular body 7 which is open at one end to define an open neck portion 8 adapted to receive a pipe or connecting tube 9 leading from the outlet of the blower (not shown). The mouth of the connecting tube 9 abuts against an annular lip 10 inside the open neck portion 8 to form a pressure tight seal therewith and is maintained in tight engagement with the lip 10 by connecting means (not shown) which secure the valve as a whole onto the connecting tube 9.

The rear face of the annular lip 10 defines the valve seat 6, and immediately beyond it an aperture 11 is provided in the side of the tubular body 7 which serves as a pressure relief vent opening to atmosphere. The other end of the tubular body 7 is closed by a cap 12.

The valve memer 4 is supported within the housing 3 on a spindle assembly 13 which is itself guided in a tubular extension 14 of the vibration damping assembly 2 which extends into the housing 3 through an aperture in the cap 12. The spindle assembly 13 comprises a backing plate 15 and the valve member 4 is secured against this by a bolt 16. The backing plate 15 prevents the valve member 4 from pitching on the end of the spindle assembly 13, but in order to allow a degree of resilience to the valve member 4 a rubber pad 17 is provided between the valve member 4 and the backing plate 15. The rubber pad 17 forms part of a flexible diaphragm 18 which serves to seperate the space behind the valve member 6 from the high pressure side of the valve member 4 when the valve is open.The edges of the diaphragm 18 are secured against the inside of the tubular body 7 by means of a bush 19 which is in turn clamped in position between the tubular body 7 and the cap 12. The diaphragm 18 prevents material from the high pressure side of the valve member 4 passing through into the workings of the valve when the valve is open. This is important as the valve movement can easily become clogged up by such material, causing complete or partial failure of the valve.

The resilient biasing means 5 is compressed between the cap 12 and the rear face of the backing plate 15, thereby biasing the valve member 4 towards the valve seat 6. However, although biased towards the valve seat 6, the valve member 4 is still axially moveable within the housing 3 against the resilient biasing means 5. As such, when it is biased into engagement with the valve seat 6, the pressure relief vent 11 is isolated from the open neck portion 8 and the connecting tube 9 received therein and there is no pressure relief through the valve. However, when the valve member 4 is pushed off the valve seat 6 the pressure relief vent 11 is connected to the open neck portion 8 and pressure is relieved through the valve.

The vibration damping means 2 is mounted in axial alignment with the pressure relief valve 1 and comprises a chamber 20, defined by a housing 21 and an end cap 22 which is screwed onto an open end of the housing 21.

Within the chamber 20 are provided a piston member 23 and resilient biasing means 24 in the form of a coiled spring. The piston member 23 is supported within the chamber 20 on the opposite end of the piston assembly 13 from the valve member 4, and is secured onto a threaded portion 25 of the piston assembly 13 by a nut 26. As such it is rigidly connected to the valve member 4 and is moveable with it. A rubber O-ring 26 is provided around the piston member 23 and sealingly engages with the inside wall of the chamber 20 to divide it into two seperate compartments 27 and 28, to the front and rear of the piston member 23. The rear compartment 27 is connected by a pipe 29 to the space defined by the diaphragm 18 behind the valve member 4 and to atmosphere via a sintered breather 30.The front compartment is connected to the.high pressure side of the valve member 4 via a hollow tube running through the middle of the spindle assembly 13.

The resilient biasing means 24 is compressed between the back of the piston member 23 and the inside face of the end cap 22, and as such it serves to bias the piston member 23 in the same direction as the valve member 4. Being screwed onto the housing 21, the axial position of the end cap can be adjusted to vary the compression of the resilient biasing means 24 and hence, the tension therein. Locking means 31 are provided between the housing 21 and the end cap 22 to allow the two to be locked in position once the tension in the resilient biasing means 24 has been set to the required level.

It will be readily appreciated that the operation of the pressure relief valve 1 is essentially conventional, in as much as when the pressure differential across the valve member 4 exceeds the tension in the resilient biasing means 5 the valve member is pushed off the valve seat 6 and excess pressure is vented through the pressure relief vent 11. However, this operation is modified by the provision of the vibration damping assembly 2. In this respect, the high pressure acting on the valve member 4 also acts on the piston member 23 which is biased forward by its respect resilient biasing means 24. As such the high pressure acts on a much greater working surface without it being necessary to increase the size of the valve member 4. This gives the valve a much improved damping characteristic. Moreover, because the norminal set relief pressure of the valve is determined by both resilient biasing means 4 and 24, and the load on resilient biasing means 24 is relatively light making it easy to adjust the tension in it accurately, it is possible to set the nominal relief pressure of the valve very accurately.

Whilst the valve of the present invention has been described in terms of a pressure relief valve, that is relieving positive pressure differentials it will be readily appreciated that it may also be used to relieve negative pressure differentials by reversing it.

Claims

1. A valve comprising a first chamber defining a high pressure inlet and a pressure relief outlet, a valve member axially movable within the chamber between a first position in which it isolates the inlet from the outlet, and a second position in which it connects the inlet to the outlet, resilient biasing means acting on the rear face of the valve member to bias it towards said first position, a second chamber, a piston member axially movable within the second chamber, resilient biasing means acting on the rear face of the piston member to bias it towards one end of the second chamber, pressure equalising means equalising the pressure on the front face of the valve member and the front face of the piston member, and connecting means connecting the piston member to the valve member.

2. A valve as claimed in claim 1, wherein the first and second chambers are axially aligned and the connecting means between valve member and the piston member comprises a connecting spindle.

3. A valve as claimed in claim 2, wherein the connecting spindle is axially movable within a tubular extension of the second chamber.

4. A valve as claimed in claim 2 or claim 3, wherein the connecting spindle defines a tube which opens at one end onto the front face of the valve member and at the other end onto the front face of the piston member.

5. A valve as claimed in any of claims 2 to 4, wherein the connecting spindle also comprises a backing plate.

6. A valve as claimed in claim 5, wherein the resilient biasing means associated with the valve member abuts against the back of this backing plate and the back of the chamber.

7. A valve as claimed in claim 5 or claim 6, further comprising a resilient pad between the backing plate and the valve member.

8. A valve as claimed in any of claims 5 to 7, comprising a flexible diaphragm connected between the valve member and the sides of the chamber to isolate the space behind the valve member from the high pressure inlet.

9. A valve as claimed in claim 8, wherein the diaphragm forms an extension of the rubber pad.

10. A valve as claimed in claim 9, wherein the diaphragm is in the form of a bellows to accommodate movement of the valve member within the chamber.

1 1. A valve as claimed in any of claims 8 to 10, wherein the enclosed space defined by the diaphragm is vented to atmosphere.

12. A valve as claimed in claim 11, wherein the enclosed space in connected to the space in the second chamber behind the piston member, which in turn is vented to atmosphere via a sintered breather.

13. A valve as claimed in any of the preceding claims, further comprising within the said first chamber, between the high pressure inlet and the pressure relief outlet, the valve member sealingly engaging against the said valve seat in said first position.

14. A valve as claimed in claim 13, wherein the engaging surfaces of the valve member and of the valve seat comprise polished stainless steel.

15. A valve as claimed in any of the preceding claims, wherein the surface area of the piston member is less than that of the valve member.

16. A valve as claimed in any of the preceding claims, wherein the length of the second chamber, in which the piston member is held, is adjustable to allow the tension in the resilient biasing means provided therein to be varied.

17. A valve as claimed jn claim 16, wherein the second chamber comprises a tubular body and a cap which can be screwed onto the end of the tubular body to increase or decrease its axial length and thereby adjust the tension in the resilient biasing means contained therein.

18. A valve substantially as herein described, with reference to, and as illustrated in, the accompanying drawing.