WO1982003437A1

WO1982003437A1 - Improved automatic stop valve

Info

Publication number: WO1982003437A1
Application number: PCT/AU1982/000041
Authority: WO
Inventors: Eng Co Pty Ltd Cash
Original assignee: Kitchener Anthony John
Priority date: 1981-03-27
Filing date: 1982-03-26
Publication date: 1982-10-14
Also published as: EP0074975A1

Abstract

A stop valve has a piston (12) slidable within a housing (10). Co-operating valve element (20) and seat (14a) regulate fluid flow between inlet port (13) and outlet port (15) in response to movement of piston (12). Piston (12) moves in response to control fluid pressure applied to control port (16). When fully open, piston (12) and second valve element (21) seal the control fluid from the regulated fluid. Dynamic seals are not required between piston (12) and housing (10). The valve is suitable for both normally open and normally closed constructions.

Description

IMPROVED AUTOMATIC STOP VALVE

This invention relates to a stop valve that operates to regulate the flow of a fluid in accordance with a predetermined condition in a machine. The invention is particularly applicable to a stop valve for incorporation in an oil feed line wherein the oil feed will be automatically initiated and terminated on the starting and stopping of a machine.

One important application of such stop valves is in the feeding of oil to a screw type compressor wherein the oil is injected into the. gas being compressed. In such screw compressors the oil is normally injected into the air entering the compressor and the discharge from the compressor is passed through a gas/oil separator and the subsequent oil returned for re-use in the compressor, normally with associated cooling and filtering of the oil.

This cycle continues as long as the compressor is running and thus the separator pressure is greater than the injection pressure. When the compressor is shut down, and before the separator is blown down to equalize all system pressures, the situation arises where the separator is at full pressure and the compression space at a relatively low pressure. Under these conditions the contents of the separator are forced back through the compressor, and thus the compressor is driven in a direction reverse to normal and the normal inlet and outlet ports are also reversed in function. To prevent this a non return valve is provided in the discharge line and an oil stop valve is provided in the oil injection line of the compressor. The oil stop valve must have the following operating characteristics.

1. It must close immediately the compressor shuts down.

2. It must open immediately on compressor start up, ensuring flow of cooling oil.

3. It must be extremely reliable even after long periods of in-operation, i.e. long operating runs or long shut down periods.

Failure of the stop valve may result in seizure of the screws at start up or discharge of gallons of hot oil at shut down. Furthermore reverse rotation of the screws alters the internal thrust forces and may result in bearing or screw damage.

Contemporary practice has seen three main types of oil stop valves. 1. Solenoid operated valves. These are easily controlled in conjunction with the electric driving motor, quickly sensin the state of rotation of the compressor, and open and close quickly.

The problem with solenoid valves is the life of the driving coil. The temperature of the injected oil combined wit the internally generated heat of the coil leads to low life spans and attendant failure. The solenoid type valve is also relatively expensive. 2. Pneumatically operated diaphragm valves. In this type of valve compressor rotation is sensed by the pressure in the compressor discharge line upstream of the non return valve. On start up and shut down this pressure changes very quickly due to the small enclosed volume.

The diaphragm is spring loaded to normally close the oil stop valve and upon start up of the compressor the pressur increase in the discharge line moves the diaphragm to open the valve. At shut down this pressure drops and the spring closes off the flow of oil.

To ensure early opening of the valve at start up the diaphragm's dimensions must be generous, and to ensure low pressure drop for the oil across the valve the movement must be large. These requirements lead to high stresses in the diaphragm material, and coupled with the high operating temperature and aggressive oils involved, lead to diaphragm failure. 3. Piston operated valves. The principle of operation is much the same except the diaphragm is replaced by a piston in which the seal between the compressed fluid and the injected oil is made by an "O" ring or similar seal mounted in the pist on and sliding in the valve body. The main failing here is tha the friction between piston and valve body is high, causing sticking of the valve. The ratio of driving to friction forces is not high enough to ensure positive operation. The common failing of all three types mentioned is th they all have a finite life in a situation where finite lives are intolerable. Eventually all solenoid coils will burn out, all diaphragms harden and break, and all dynamic seals will wear out (if they do not stick first).

It is the object of the present invention to provide a stop valve which will largely reduce or eliminate the problems existing in the currently used valves.

With this object in view there is provided a stop valvee comprising a chamber having regulated fluid inlet and outlet ports, a regulated fluid valve having a stationary valve element and a movable valve element co-operating to control the flow of the regulated fluid between said inlet and outlet ports, a piston within the chamber carrying said movable valve element arranged to slidably move within the chamber to select- ively open and close the regulated fluid valve and having no dynamic seals interposed between the piston and chamber, a cont- rol fluid port in said chamber for admission of a control flui to the chamber to effect said movement of the piston to open or close the regulated fluid valve, said piston and chamber being adapted to form a seal between the regulated fluid and control fluid when the regulated fluid valve is open. Preferably the piston member is arranged to move in a direction to open the regulated fluid valve when there is a sufficient pressure difference across the piston. Conveniently the piston member may be spring loaded so as to be urged to move in a direction opposite to that induced by the application of the pressure of the control fluid to the piston.

Conveniently the movable and stationary valve elements have respective faces that sealably engage with the regulated fluid valve, said faces being in respective planes substantially transverse to the direction of movement of the piston.

Similarly the piston and chamber may have second movable and stationary valve elements that sealably engage when the regulated fluid valve is full open, to provide the seal between the regulated and control fluids. These second movable and stationary valve elements may have faces in respective planes substantially transverse to the direction of movement of the piston. The piston is a free sliding fit in the chamber, however, the clearance between the piston and the walls of the chamber, and the length of the flow path therebetween, is such that although some control fluid may escape therebetween the is a substantial pressure drop and accordingly the pressure of the control fluid at the point of admission to the chamber is sufficient to effect the required sliding movement of the piston in the chamber. This eliminates the necessity for a dynamic seal to be provided between the piston and the chamber wall which would be subject to wear and possible sticking. The function of the dynamic seal which would normally prevent the mixing of the control fluid with the regulated fluid is perf ed by the seal formed between the piston and chamber when the regulated fluid valve is fully open but this seal does not restrict the free movement of the piston necessary to open and close the regulated fluid valve.

One practical arrangement of a stop valve in accordance with the present invention is disclosed in the accompanying drawings and will now be described by way of example with reference to the accompanying drawings, in which:

Figs. 1 to 4 schematically represent the stop valve of the invention in various working positions;

Fig. 5 schematically represents a further embodiment of the stop valve; and Fig. 6 shows the assembly of the stop valve of the invention.

In the drawings the stop valve comprises a body 10 defining the cavity 11 and a piston 12 freely slidable in the chamber. The conduit 13 extends co-axially into the chamber 11 and affixed to the body 10, and is provided with a flange 14 at the inner edge. The conduit 13 is adapted for connection to the source of regulated fluid. The port 15 in the body 10 is adapted for connection to the receiver of the regulated fluid such as to the chamber of a screw compressor. The internal wall of the cavity 11 and the external wall of the piston 12 are conveniently both cylindrical so that the piston may freely move axially thereon. The clearance between the wall of the piston and the wall of the cavity is sufficient to ermit this free sliding movement of the iston but is dimensioned to provide a restriction to the flow of fluid therebetween so that fluid passing between the piston and the cavity will be subjected to a pressure drop from one axial end to the other of the piston. Typically in the valve shown in Figure 6, the clearance cross-sectional area is 1-2% of the bore cross-sectional area, and the flow path therebetween is about 0.5 to 0.75 times the bore diameter.

The port 16 in body 10 is adapted for connection to a source of control fluid such as the fluid at or in the immedidate vicinity of the delivery port of a screw compressor. Accordingly the pressure of this fluid will tend to move the piston in a downward direction viewed in Figure 1 when the pressure difference is sufficient to overcome the upward force applie to the piston by the spring 17 disposed within the piston be ween the flange 14 and the underside of the top of the piston. The piston is provided with an enlarged port 18 in the lower end surrounded on its margin by a seal element or valve seat 20 which co-operates with the underface 14a of the flange 14 to provide a valve between the conduit 13 and the outlet port 15 to control the flow of the regulated fluid to the outlet port 15. The annular sealing element or piston seat 21 is provided in the lower end of the cylindrical portion of the cavity 11 to co-operate with the marginal portion of the underside of the piston 12 when the piston is in its lowermost position to provide a seal between the portion of the cavity above and below the piston respectively.

The positions of the stop valve illustrated in the accompanying drawings will now be described as applied to the controlling of the supply of oil to the compression chamber of a screw compressor.

The conduit 13 is coupled to a source delivering oil under pressure. The outlet port 15 is coupled to a compression chamber of the screw compressor at or near the inlet end thereof. The control port 16 is coupled to the discharge line of the compressor at a location upstream of a non-return valve provided in the discharge line to prevent the compressed fluid returning from the oil separator when the compressor is switched off. When the compressor is shut down and consequently the delivery pressure applied to port 16 has dropped, the valve is in the condition as shown in Figure 1 so that there is effectively reduced pressure applied to the piston through the control port 16 and the resultant pressure difference will force the piston upwards to bring the sealing element 20 into engagement with the underface 14a of the flange 14 so that the outlet port 15 is isolated from the conduit 13 and thus no oil can flow to the compression chamber of the compressor. Upon starting up of the compressor, the delivery pressure will build up, resulting in a pressure difference across the piston, which will commence to move downwardly as shown in Figure 2 of the drawings. In this position the seal element 20 has moved clear of the underface 14a of the flange 14 and thus oil will commence to flow from the conduit 13 through the opening 18 in the lower face of the piston to the outlet port 15. Some compressed gas admitted through control port 16 will commence to flow down the restricted gap between the piston and the wall of the chamber 11, however, the restriction offered to the flow through this gap will prevent the full delivery pressure of the compressor being applied to the underside of the piston, and a sufficient pressure difference will exist to ensure the piston moves downwardly in the chamber against the restraining force of the spring 17. The downward movement of the piston is terminated upon the lower end of the piston contacting the seal element 21, and the delivery pressure of the fluid from the compessor will force the piston against the sealing element 21 so that a seal is formed at this point between the piston and the chamber, thus preventing mixing of the fluid from the compressor with the oil passing from the conduit 13 to the outlet port 15. Also with the piston in this position there is an unrestricted flow of the oil from the conduit to the opening 18 of the piston to the outlet port 15 to thus provide a full flow of oil to the compression space of the compressor.

The respective elements of the valve will remain in this position as shown in Figure 3 so long as a sufficient pressure difference exists across the piston. Upon the closing down of the compressor, the delivery pressure drops and thus there is a drop in pressure on the top of the piston so that the pressure difference takes over and moves the piston upwardly in the chamber as shown in Figure 4, perhaps with the aid of a spring 17. In this position oil under pressure will still exist in the lower part of the chamber but the flow thereof upwardly between the piston and the chamber wall is restricted by the clearance therebetween and accordingly there will be no significant flow of oil past the piston. It will also be understood that the pressure of the oil in the hollow interior of the piston adds to the spring force in moving the piston upwardly in the chamber and if desired the spring may be omitted. This upward movement continues until the seal element 20 again engages the underface of the flange 14 thus isolating the chamber 11 and the outlet port 15 from the oil supplied thus the supply of oil to the compression space of the compressor is terminated.

The alternative construction of the stop valve is illustrated in Figure 5 of the drawings wherein elements corresponding to elements incorporated in the embodiments shown in Figure 1 to 4 have the same reference numerals with the suffix "a". The housing 10a has a control fluid port 16a communicating with a chamber 11a in which there is disposed a piston 12a. The port 30 is adapted for connection to a source of regulated fluid. Port 15a is adapted for connection to the receiver of the regulated fluid such as a compression chamber of a screw compressor. As a variation on this construction the port 30 may be located in the side of the body 10a as indicated by the dotted lines. The rod 31 is rigidly connected to the piston 12a and carries at its opposite end a flange 32 which co-operates with the seal 20a, these components forming the movable and stationary elements respectively of the regulated fluid valve. The seal 21a is provided to co-operate with the under side of the piston 12a when it is in its lowered position in the same manner and for the same purpose as previously described in respect of the embodiment shown in Figure 1 to 4. The spring 17a urges the piston 12a, rod 31 and flange 32 in a direction to normally close the regulated fluid valve. It should be understood that in both of the embodiments described the spring 17 or 17a may be omitted because when the control pressure drops the pressure of the regulated fluid will cause the piston 12, 12a to rise to close the regulated valve. This construction has the further advantage that when the stop valve is used in association with a screw type compressor after the compressor has been shut down and the oil supply terminated by the closing of the regulated fluid valve, the pressure of the oil supply will subsequently decrease upon release of the pressure in the separator and thus the regulated fluid valve can open under gravity. This will allow an immediate supply of

011 to the compressor upon restarting.

Fig. 6 illustrates the construction of a stop valve in accordance with the invention. Elements of the valve correspond- ing with those of Figs. 1 to 4 have been given like reference numerals. The cup-shaped housing 10 is of cast iron and has apertures 13' , 15 forming oil inlet and outlet ports, and threaded aperture 40 to receive valve stem or conduit 13.

Piston 12 is also cast iron and is generally cup-shaped with an aperture in its base forming port 18. The piston is sealed at its top by piston cover 42 fastened by bolts 43.

To assemble the valve, sealing element 21, a teflon ring is fixed within the lower part of the housing 10. Piston

12 to which teflon sealing element 20 has been fixed, is then inserted in the housing. Conduit 13 is secured into aperture 40 and secured to the housing with lock nut 41. Spring 17 is then placed on flange 14 of conduit 13, and the piston 12 is then closed by cover 42. The open end of the valve housing is then sealed by cover 44 screwed thereto by bolts 46 with sealing gasket 45 therebetween. Cover 44 includes aperture 16 forming the port for the control fluid. In the embodiment shown, an oil filtering medium 48 is attached to the lower end of housing 10, enabling filtration of oil entering port 13' and passing to conduit 13. Typically, the cross-sectional clearance area between piston 12 and housing 10 is up to 5% of the cross-sectional area of the piston bore, and the height of the piston, hence the flow path, is about 50 mm. Typical fluid pressures at start up are 0.5 bar at control port 16, 0.0 at inlet port 13', and 3.0 to -0.5 bar at outlet port 15. Pressures in operation are about 7.0 bar at 16, and about 5.0 bar at 13' and 15.

It will be appreciated from the above description that this particular stop valve can be applied to many applications other than the particular application described. The valve is suitable for both normally open or normally closed constructions.

Claims

CLAIMS :

1. A stop valve comprising a chamber having regulated fluid inlet and outlet ports, a regulated fluid valve having a stationary valve element and a movable valve element co-operating to control the flow of the regulated fluid between said inlet and outlet ports, a piston within the chamber carrying said movable valve element arranged to slidably move within the chamber to selectively open and close the regulated fluid valve and having no dynamic seals interposed between the piston and chamber, a control fluid port in said chamber for admission of a control fluid to the chamber to effect said movement of the piston to open or close the regulated fluid valve, said piston and chamber being adapted to form a seal between the regulated fluid and control fluid when the regulated fluid valve is open

2. A stop valve as defined in claim 1, wherein the piston is arranged to move in a direction to open the regulated fluid valve when there is a sufficient pressure difference across the piston.

3. A stop valve as defined in claim 2, wherein the piston is biased in a direction opposite to that induced by the application of the pressure of the control fluid on the piston.

4. A stop valve as defined in claim 3, wherein the piston is biased by a spring.

5. A stop valve as defined in claim 1, wherein the crosssectional clearance area between the piston and chamber is up to 5% of the cross-sectional area of the chamber bore.

6. A stop valve as defined in claim 1, wherein the movabl and stationary valve elements have respective faces that sealably engage when the regulated fluid valve is closed, said faces being in respective planes substantially transverse to the direction of movement of the piston.

7. A stop valve as defined in claim 1 or 6, wherein the piston and chamber are provided with second movable and station- ary valve elements that sealably engage when the regulated fluid valve is fully open, to provide a seal between the regul- ated and control fluids.

8. A stop valve as defined in claim 7, wherein the second movable and stationary valve elements have faces in respective planes substantially transverse to the direction of movement of the piston.