IMPROVED AUTOMATIC STOP VALVE

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 pre¬ determined 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 out¬ let 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, ensur¬ ing 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 dam¬ age. 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 dia¬ phragm material, and coupled with the high operating temperat¬ ure 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 "0" 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 probl¬ ems existing in the currently used valves. With this object in view there is provided a stop valv comprising a chamber having regulated fluid inlet and outlet ports, a regulated fluid valve having a stationary valve elem¬ ent 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 con 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. Prefer¬ ably 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 pisto 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 substant¬ ially transverse to the direction of movement of the piston. Similarly the piston and chamber may have second mov¬ able 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, how ever, the clearance between the piston and the walls of the chamber, and the length of the flow path therebetween, is su 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 chambe is sufficient to effect the required sliding movement of the piston in the chamber. This eliminates the necessity for a d amic seal to be provided between the piston and the chamber wall which would be subject to wear and possible sticking. T function of the dynamic seal which would normally prevent th mixing of the control fluid with the regulated fluid is perf ed by the seal formed between the piston and chamber when th regulated fluid valve is fully open but this seal does not r trict the free movement of the piston necessary to open and close the regulated fluid valve. One practical arrangement of a stop valve in accorda with the present invention is disclosed in the accompanying drawings and will now be described by way of example with re ence to the accompanying drawings, in which: Figs. 1 to 4 schematically represent the stop valve the invention in various working positions; Fig. 5 schematically represents a further embodiment the stop valve; and Fig. 6 shows the assembly of the stop valve of the i vention. In the drawings the stop valve comprises a body 10 d fining 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 at the inner edge. The conduit 13 is adapted for connection the source of regulated fluid. The port 15 in the body 10 is adapted for connection to the receiver of the regulated flui 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 t the piston may freely move axially thereon. The clearance be ween the wall of the piston and the wall of the cavity is su icient to ermit this free slidin movement of the iston 3 τ>u is dimensioned to provide a restriction to the flow of flui therebetween so that fluid passing between the piston and t cavity will be subjected to a pressure drop from one axial e 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 im idate vicinity of the delivery port of a screw compressor. A ordingly the pressure of this fluid will tend to move the pi on in a downward direction viewed in Figure 1 when the press 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 pisto The piston is provided with an enlarged port 18 in t lower end surrounded on its margin by a seal element or valv seat 20 which co-operates with the underface 14a of the flan 14 to provide a valve between the conduit 13 and the outlet port 15 to control the flow of the regulated fluid to the ou let port 15. The annular sealing element or piston seat 21 i provided in the lower end of the cylindrical portion of the cavity 11 to co-operate with the marginal portion of the und side of the piston 12 when the piston is in its lowermost po ion to provide a seal between the portion of the cavity abov and below the piston respectively. The positions of the stop valve illustrated in the a ompanying drawings will now be described as applied to the c rolling 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 compressi 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 val provided in the discharge line to prevent the compressed flui 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 th 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 th 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 press ure 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 elemen 20 has moved clear of the underface 14a of the flange 14 and thus oil will commence to flow from the conduit 13 through th 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 a the wall of the chamber 11, however, the restriction offered 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 t ensure the piston moves downwardly in the chamber against the restraining force of the spring 17. The downward movement of the piston is terminated upo the lower end of the piston contacting the seal element 21, a the delivery pressure of the fluid from the compessor will force the piston against the sealing element 21 so that a sea 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. Al with the piston in this position there is an unrestricted flo of the oil from the conduit to the opening 18 of the piston t 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 unde 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 oi in the hollow interior of the piston adds to the spring force in moving the piston upwardly in the chamber and if desired th spring may be omitted. This upward movement continues until th 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 o the compressor is terminated. The alternative construction of the stop valve is illus trated in Figure 5 of the drawings wherein elements correspond ing 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 regul ated 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 elem¬ ents 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 em¬ bodiment shown in Figure 1 to 4. The spring 17a urges the pist¬ on 12a, rod 31 and flange 32 in a direction to normally close the regulated fluid valve. us Ul to to P* * Ui o Ul o Ui O Ui the flow path, is about 50 mm. Typical fluid pressures at s up are 0.5 bar at control port 16, 0.0 at inlet port 13', a 3.0 to -0.5 bar at outlet port 15. Pressures in operation a about 7.0 bar at 16, and about 5.0 bar at 13' and 15. It will be appreciated from the above description t this particular stop valve can be applied to many .applicati other than the particular application described. The valve 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-operat ing 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 th chamber to selectively open and close the regulated fluid valv 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 th piston. 3. A stop valve as defined in claim 2, wherein the piston is biased in a direction opposite to that induced by the appli ation 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 cross- sectional clearance area between the piston and chamber is up to 57o 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 seal¬ ably 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 statio 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.

Inventors: KITCHENER ANTHONY JOHN
Applicants: CASH ENG CO PTY LTD, KITCHENER ANTHONY JOHN
Application year: 1982
Main classification: F16K31
IPC codes: F16K31