US3568705A

US3568705A - Hydraulic system circuit breaker

Info

Publication number: US3568705A
Authority: US
Inventors: George I Boyadjieff; Robert K Van Ausdal; Ralph L Vick
Original assignee: Bendix Corp
Current assignee: Bendix Corp
Priority date: 1969-08-19
Filing date: 1969-08-19
Publication date: 1971-03-09
Anticipated expiration: 1988-03-09
Also published as: FR2058370B1; GB1267310A; FR2058370A1; DE2041213A1; JPS4914806B1

Abstract

A hydraulic circuit breaker or fail-safe device is disclosed for sensing leaks in lines to and from hydraulic actuators or the like. Flow through the line is sensed across a restriction on a piston valve member movable to close the line, the size of the restriction being such that flow in excess of normal requirements causes a significantly increased pressure drop, which drop is sensed across a quantity measuring piston movable in a chamber within the piston-valve, and causing the latter piston to move within its chamber at a rate controlled by the area of a control orifice. As this latter piston moves, it carries a shaft which is spring biased to resist movement until the pressure drop across the restriction reaches a value indicative of a leak. Two spool valve members are carried on the shaft whose function in the two embodiments shown is to direct either supply pressure or lower pressure against operating areas of the piston-valve to control its movement to open or close the line. A check valve is placed in a return line and an additional piston in contact with the biasing spring senses a leak in the return line as indicated by a changed pressure drop across the check valve and moves to either remove the effective spring bias on the shaft or to move the shaft directly such that the spool valve members direct flow to cause the piston-valve to close the line.

Description

United States Patent [72] lnventors George I. Boyadjieff Woodland Hills; Robert K. Van Ausdal, La Crescenta; Ralph L. Vick, Granada Hills, Calif. [21] Appl. No. 851,288 [22] Filed Aug. 19, 1969 [45] Patented Mar. 9, 1971 [73] Assignee The Bendix Corporation [54] HYDRAULIC SYSTEM CIRCUIT BREAKER 9 Claims, 5 Drawing Figs.

[52] U.S. Cl 137/87, 91/468, 137/460 [51] Int. Cl. Fl6k 17/22 [50] Field of Search 137/87, 460, 487; 91/446, 447, 468

[5 6] References Cited UNITED STATES PATENTS 2,649,773 8/1953 Griswold 137/460 Primary ExaminerRobert G. Nilson Attorneys- Robert C. Smith and Plante, Arens, Hartz, Hix &

Smith ABSTRACT: A hydraulic circuit breaker or fail-safe device is disclosed for sensing leaks in lines to and from hydraulic actuators or the like. Flow through the line is sensed across a restriction on a piston valve member movable to close the line, the size of the restriction being such that flow in excess of normal requirements causes a significantly increased pressure drop, which drop is sensed across a quantity measuring piston movable in a chamber within the piston-valve, and causing the latter piston to move within its chamber at a rate controlled by the area of a control orifice. As this latter piston moves, it carries a shaft which is spring biased to resist movement until the pressure drop across the restriction reaches a value indicative of a leak. Two spool valve members are carried on the shaft whose function in the two embodiments shown is to direct either supply pressure or lower pressure against operating areas of the piston-valve to control its movement to open or close the line. A check valve is placed in a return line and an additional piston in contact with the biasing spring senses a leak in the return line as indicated by a changed pressure drop across the check valve and moves to either remove the effective spring bias on the shaft or to move the shaft directly such that the spool valve members direct flow to cause the pistonvalve to close the line.

t n as y 23 r \\u 62 T if" 1:; A r 2 1Z6 7 4 f V 49 i 64 74 W 7 r '72 HYDRAULIIC SYSTEM CIRCUTT BREAKER BACKGROUND OF THE INVENTlON in large aircraft currently available and under development many separate hydraulic actuators are used to operate control surfaces, landing gear, and other hydraulically operated devices. In the case of actuators for control surfaces, it has become a rather common feature of design to include paralleled actuators arranged for redundant control such that if one actuator fails or sticks, another will make possible either partial or complete operation of the control surface. This large number of hydraulic actuators, of course, requires that a source of a good quantity of hydraulic fluid under pressure be available, and it is apparent that the loss of a substantial proportion of the available hydraulic fluid may cause impairment of a number of the actuator functions, even though redundancy is provided. There is, therefore, a need for means which will sense a loss or leakage of fluid and will act to block certain passages to eliminate this fluid loss so that a sufficient supply will remain to operate other control surfaces even though one or more may be inoperative. Such a sensing and control device should not introduce substantial problems of its own, either as to operation or as to safety.

SUMMARY OF THE INVENTION Two embodiments of our hydraulic circuit breaker design are shown which include three basic elements: (1) a pressure line shutoff valve; 2) a flow rate/quantity sensor; and (3) a return line check valve. The units operate to trip and block flow from a source to a downstream utilization device upon the occurrence of either excess supply flow or a loss of return line pressure. Either case requires a fixed quantity of flow to pass through the circuit breaker after sensing a loss before the unit trips.

The flow rate and quantity sensor, built into the shutoff valve spool, operates by sensing supply line pressure drop across the shutoff valve lands. This pressure drop, a square root function of the supply flow rate, drives a small piston inside the spool against a larger piston with a trapped volume behind it. The trapped fluid flows through a restricting orifice, proportional to the square root of the developed pressure. This developed pressure is related to the supply flow pressure drop by the area ratio of the pistons, and hence, cancels out, and the bleed flow through the restriction will be directly proportional to the supply flow rate. Thus the movement of the pistons is a measure of the supply flow quantity and is used as a quantity sensor. Another embodiment makes use of a bleed in a small conduit communicating with a chamber containing the sensing piston without utilizing the larger piston and bleed arrangement.

The pistons are preloaded by a caged spring which determines the threshold or tripping flow rate. For pressure drops below the spring preload, no piston motion occurs. For pressure drops above the preload, this piston moves at a rate proportional to the supply flow rate and hence reaches the end of its stroke at a given quantity of flow. The piston rod also carries small shuttle valves that port supply pressure to the ends of the supply shutoff valve. At the end of the quantity-measuring stroke, supply pressure is shuttled from holding the shutoff valve open to forcing it closed. Finally, as the shutoff valve closes, it locks the pistons in the tripped position, preventing recycling. Resetting of the circuit breaker occurs automatically when the supply pressure is reduced to zero by shutting off the engine-driven pump. The preloaded threshold spring will drive the pistons and shutoff spool to their initial position. Also, if the overload flow quantity is insufficient to fully stroke the pistons and the supply line pressure drop decreases below the threshold pressure, the spring will reset the pistons. This feature prevents a series of transient overloads such as start-up flow in excess of rated amounts, due to air in the lines, from tripping the circuit breaker.

Return line failure is sensed by comparing the pressure drop across the return line check valve. When the check valve closes and the load side of the return line pressure drops a given amount below the reservoir pressure, the return failure sensor decoupled the threshold spring from the supply flow sensor. When the supply flow quantity accumulates to the tripping quantity under this condition, the circuit breaker trips. The flow sensor can no longer reset; hence, it trips on accumulated quantity when return failure occurs. This accumulated supply flow trip in necessary for return failures because excess flow does not occur due to return line failure.

From the foregoing, it will be apparent that our hydraulic circuit breaker either senses excess supply flow indicating a loss downstream from itself and trips to close off the line or it responds to return line failure to close off the line. It resets in normal operation when the system is shut down because the supply pressure is reduced to zero, but so long as there is any substantial supply pressure it does not reset and there is, therefore, no inadvertent reset in operation. Reset occurs normally without specific crew action, and the device does not react to short term pressure fluctuations and cause a shutoff prema:

turely since a significant amount of flow must occur simultaneously with a lower pressure downstream of the actuator before shutoff will occur.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view of an embodiment of our hydraulic circuit breaker;

FIG. 2 is a partial sectional view somewhat enlarged, of the device of FIG. 1 showing the circuit breaker in tripped position;

FIG. 3 is a sectional view of another embodiment of our hydraulic circuit breaker.

FIG. 4 is a partial sectional view of the device of FIG. 3 wherein the device is shown tripped in response to pressure line failure.

FIG. 5 is a partial sectional view of the device of FIG. 3 shown in tripped position in response to a return line pressure failure.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, a circuit breaker is depicted in section in connection with a housing 10. Operating fluid from a high pressure source enters an inlet passage l2 where it communicates with a cylindrical chamber 114. An outlet passage 16 communicating with an actuator or other utilization device downstream from the housing 10 also communicates with cylindrical chamber 14. Axially movable within the chamber 14 is a piston member 18 carrying on its surface a land 20 which operates in conjunction with the surface of cylindrical chamber 14; to provide a metering passageway. A pair of

ports

22 and 24 provide access to the interior of piston 18 which contains a cylindrical chamber 26 and a piston 28 reciprocates within chamber 26. Piston 28 is carried on a shaft 30 which also carries

pistons

32 and 34, a spool member 36 at the left end containing a spring retainer 39 and at the opposite end a larger piston 38 positioned within a cylinder 40 formed in an enlarged extension of piston member 18, the piston 38 containing a bleed 42. Also positioned within piston 18 is a cylindrical chamber 44 which communicates with a chamber 46 and a port 48 communicating with a larger chamber 50 exterior of piston member 18. Alsopositioned within piston 18 is an additional small cylindrical chamber 52 which communicates with the cylindrical chamber 26, as well as with a larger diameter chamber 54 through a passageway 56. A. spring 58 is positioned within a chamber 60 in housing and urges piston 18 to the right away from retainer member 39. Also positioned in cylindrical chamber 60 is a slide valve member 62 which communicates through a passage 64 with return fluid pressure in a return passage 66. A, check valve 6% in the return line 66 to a reservoir (not shown) is biased against pressure from the return side of the actuating device in conduit 69 by means of a spring 70. Chambers 50 and 60 communicate with the load side of the return line through conduits 72 and 7 3, respective- Under normal operating conditions the flow from inlet passage 12 to outlet passage 16 across land 20 will be limited to that required by the necessary displacement of the associated actuator or other device. The pressure drop across land 20, which is sensed by means of piston 28, will be insuffcient to cause a significant movement of piston 28 and, hence, shaft 30. In the event of a substantial leak downstream of land 20, the pressure drop across this orifice will increase substantially, causing piston 28 to begin moving toward the right. As piston 28 carries shaft 30 toward the right, it also carries piston 38 in cylindrical chamber 40 at a rate controlled by the area of bleed 42. Bleed 42 is sized such that the flow through this restriction will be directly proportional to-the supply flow rate, and thus the movement of the piston is a measure of the supply flow quantity which is passing orifice 20. As shaft 30 moves toward the right, it carries with it

pistons

32 and 34 which, as they approach the end of their stroke, open certain passageways and close others to cause changes in the pressures operating on the various effective areas of piston 18. As specifically shown in FIG. 2, as piston 32 moves toward the right it blocks communication between inlet passage 12 and chamber 52 and at the same time opens communication between chamber 52 and the return pressure conduit 74. Similarly, as piston 34 moves to the right it interrupts communication between

chambers

44 and 46 and passageway 48 communicating with the low pressure in passage 50 and opens communication between

chambers

44 and 46 and a higher pressure in passageway 16. This permits the higher pressure in passageway 16 to be communicated into chamber 46 while at the same time the pressure in chamber 54 is reduced to return line pressure, and this pressure differential forces piston 18 to the left such that it reaches a position where it completely blocks flow out of passageway 16 as shown in FIG. 2.

A substantial loss of fluid can also occur from a break in the return line 69. When the pressure sensed in line 69 drops to a low value, this pressure loss is communicated through lines 72 and 74 to chambers 59 and 60. Since piston 62 has a substantial area in communication with a low, but still significant pressure in line 66, piston 62 will be forced to the right carrying retainer 39 to remove the preload on piston 28 caused by the caged spring 58. The piston 28 acting in conjunction with bleed 42 will again effectively measure the flow across land 20 as it moves, and when a given flow has occurred, it will reach a position such that the higher pressure from conduit 16 is communicated to

chambers

44 and 46 while the pressure in chamber 54 is exhausted through conduit 74. This pressure differential forces the piston 18 to the left such that it blocks communication between passages 12 and 16 as shown in FIG. 2.

Another embodiment ofour invention appears in FIGS. 3, 4 and 5. FIG. 3 is a sectional view of our device shown in normal position. FIG. 4 is a partial sectional view of this device with the parts in tripped position in response to a failure in a pres sure line, and FIG. is a similar partial sectional view with the parts shown after the device has been tripped in response to a return line pressure failure.

In FIG. 3 an inlet conduit 80 provides actuating fiuid under high pressure to a housing 82 having a port 84 communicating the fluid with a cylindrical chamber 86. Also communicating with chamber 86 is a port 88 connected to a pressure outlet conduit 90 to provide supply flow to an associated actuator. A conduit 92 is connected to receive return flow from the associated actuator or other utilization device which flow is connected through a line 93 to a reservoir (not shown) through a check valve 94 biased against return pressure by means of a spring 96.

A cylindrical piston 98 is arranged to be reciprocable in chamber 86 and includes a land 100 on its surface which cooperates with the wall of chamber 86 to form a restriction of a desired area. Within the piston 98 is a cylindrical chamber 102 and a timing piston 104, having the same function as piston 28 of FIG. 1, which is movable within chamber 102. The left side of piston 104 is in communication with inlet pressure in port 84 through a passageway 106 which communicates with a chamber formed by a groove 108 on the surface of piston 08 and this chamber, in turn, communicates with the left side of piston 104 through ports 110. On the right side of piston 104 the chamber 102 communicates with the pressure outlet port 88 through a passage 112, a chamber formed by a groove 118 in the surface of a piston 120, a port 122, and a passageway with a timing bleed 124.

Piston 104 is carried on a rod 126 which also carries an elongated smaller diameter piston 128, a smaller piston 130 and a spring retainer 132 which operates to retain a spring 134 between itself and an extension 136 of piston 98, slidable within a small diameter chamber 138 in housing 82. Movable in chamber 138 is a piston 140 which communicates at its left end with reservoir pressure through a conduit 142. A conduit 144 communicates chamber 138 with return flow conduit 92 through a short passage 146. Also connected to passage 146 through a conduit 148 is an annular chamber 150 positioned between the end of piston 98 and a flange on piston 120. A plurality of small ports 152 provide communication between chamber 150 and a large chamber 154 at the right end of piston 120.

With the system of FIG. 3 operating normally, the assembly will be in the position shown with a limited amount of flow permitted across the restriction between the inside wall of chamber 86 and the land 100. High pressure from inlet port 84 is communicated through

passages

106 and 110 and groove 108 to the left side of piston 104 from whence it flows through an annular passageway 156 and a port 158 to the left end of piston 98. Since chamber 150 on the right end of piston 98 is connected to return pressure, a substantial pressure differential holds piston 98 toward the right, as shown. In the event of a break or substantial leak downstream of port 88, a reduced pressure will appear on the downstream side of land 100 which is transmitted through passage 112, annulus 118, radial passage 122, and bleed 124 into chamber 102 on the right side of piston 104, causing increased flow across land 100, with accompanying increased pressure drop thereacross, which increased pressure drop causespiston 104 to begin moving toward the right. The return pressure is maintained in chambers 138 and 150 through

passageways

144, 146 and 148; and through ports 152 to chamber 154. As piston 104 moves'to the right it carries shaft 126, piston 130, and retainer 132 compressing spring 134. As it approaches the end of its travel piston 130 passes port 158 permitting the high inlet pressure at the left end of piston 98 to be ven ed into chamber 138 which is effectively at return pressure. Piston 128 also moves toward the right, finally approaching the end wall of chamber 154, where, as shown in FIG. 5, it opens an annular passageway connecting with radial passage 122 and which bypasses bleed 124, thus exhausting the pressure on the right side of piston 104 to the outlet port 88. With a large leak downstream of port 88, the pressure on the right side of piston 104 drops to a very low value while pressure on the left side is maintained at a higher value by land 100. With piston 128 bottomed against the end of chamber 154, the high fluid pressure on the left side of piston 104 acts to force piston 98 to the left to its maximum travel, .thereby blocking port 88. Piston 120 will also move to the left because it has return pressure on its right face and a very low opposing pressure in chamber 102. With port 88 blocked, pressure on the right side of piston 104 will rise to a value approaching supply pressure because of communication through port 112, annulus 118, port 122 and orifice 124. This increasing pressure will then force piston 120 to the right with the ports finally assuming the positions shown in FIG. 4.

The device of FIG. 3 also blocks flow when a substantial leak or rupture occurs in the line 92 between the associated actuator or other device and the check valve 94. In this condition there will be a substantial reduction in the pressure in line 92 while the pressure in the return line to the reservoir 93 remains at a significant value. This pressure differential will cause check valve 94 to be held closed and the lowered pressure in line 92 will be communicated to chambers 138 and 150 through

lines

144, 11,46 and Md. This pressure differential also appears across piston i 10, causing it to move toward the right, compressing spring 134 and moving shaft 126 with pistons ltld, i128 and 130 to the right. Again, the rate of movement of piston ltl t is limited by the area of orifice 124 and the pressure drop thereacross so that a given significant amount of flow across land lfitl will occur before piston 12% is bottomed against the right end of chamber 154. As piston l30moves to the right it passes orifice l53 thus interrupting communication between the inlet pressure on the left side of piston 104 and the left end of piston 98, and moving further, opening communication between the left end of piston 98 and chamber 138. This exhausts the high pressure acting on the left end of piston 98 into chamber 138 which is at a very low pressure reflecting the leak in line 2. With piston 128 anchored, piston 104 cannot move further to the right and inlet pressure will then force piston 98 to its limit of travel toward the left, blocking outlet port 38. Because inlet pressure will be communicated through port 112, annulus i118, passage 122 and orifice 124, piston 120 will be retained in the position shown in FIG. 5. The movable parts will all remain in this position until the inlet pressure is removed, thus avoiding a possible inadvertent reset.

From the foregoing it will be appreciated that applicants device, in both embodiments shown, operates to block the flow of operating fluid when leaks of significant size occur while at the same time providing assurance that operation of the associated hydraulic system is not interrupted or impaired from normal startup flows or normal leakage such as would be expected from servo valves, etc. Once'a substantial leak has been sensed, the devices effectively measure the flow quantity past the metering orifice before closing the outlet and will not reset until supply pressure is removed.

We claim:

l. A hydraulic safety device for connection between a source of hydraulic fluid under pressure and a device to be controlled comprising:

a housing having an inlet port connected to said source, a

high pressure outlet port connected to said supply controlled device, and a return port for receiving return flow from said device;

a chamber in said housing connecting said inlet port and said high pressure outlet port;

a first piston movable in said chamber between a first position in which flow passes between said inlet port and said high pressure outlet port and a second position where said high pressure outlet port is blocked; said piston including ridge means cooperating with the walls of said chamber to form a restricted passageway, and a second chamber within said piston;

resilient means in said housing urging said piston toward said first position;

a shaft and a second piston in said second chamber carried on said shaft and first and second passages communicating opposite sides of said second piston with opposite sides of said ridge means;

a third piston on said shaft positioned in normal operation to permit communication between the inlet pressure side of said second piston and one end of said first piston, and movable with said second piston to a second position to interrupt said communication and to open communication between said one end of said first piston and a source of hydraulic fluid at return pressure;

a fluid chamber adjacent said shaft and an orifice positioned to control the rate of flow out of said chamber and, therefore, the movement of said shaft; whereby when a leak occurs downstream of said high pressure outlet port the resulting fluid pressure differential across said restricted passageway is sensed across said second piston causing said second and'third pistons to move against the force of said resilientrneans and at a rate controlled by said orifice until said third piston reaches its second position, causing said first piston to be moved by the force of inlet pressure from said first position to said second position.

2. A hydraulic safety device as set forth in claim l wherein a check valve is located between said return port and the return side of said source, a fourth piston is positioned coaxially with said shaft and fluid conduits are provided connecting opposite sides of said fourth piston across said check valve, such that upon the occurrence of a substantial loss of pressure upstream of said return port, said check valve will close and said fourth piston will sense the-differential across said check valve and will move to compress said spring, moving said shaft until said third piston blocks communication between said inlet port and said one end of said first piston thus releasing the high inlet pressure acting against said one end and causing said first piston to move to said seco'nd position.

3. A device for blocking a flow of fluid from a high pressure source to a utilization device in response to excessive flow indicating a leakage comprising:

a housing;

an inlet conduit connecting said source with said housing;

an outlet conduit connecting said housing with said utilization device;

a return conduit connecting said utilization device with said housing;

first valve means in said housing movable from a first position permitting flow from said inlet conduit to said outlet conduit to a second position in which flow from said inlet conduit to said outlet conduit is blocked, and flow restriction means associated with said first valve means;

second valve means including pressure responsive means for sensing the pressure drop across said flow restriction means and flow directing means movable with said pressure responsive means to a first position to connect one side of said first valve means with fluid pressure in said inlet conduit to maintain said first valve means in its said first position or to a second position to direct fluid pressure acting against said one side of said first valve means to said return conduit which permits said first valve means to be moved to said second .position;

a chamber containing fluid in contact with said second valve means and orifice means restricting flow from said chamber to control the rate of movement of said second valve means; and

resilient means urging said second valve means toward a position where inlet conduit pressure is directed against said one side of said first valve means.

4. A device for blocking a flow of fluid as set forth in claim 4 including a check valve connected between said return conduitand the return side of said source; pressure responsive means connected across said check valve to sense the pressure drop across said check valve and movable in response to a substantial drop in pressure in said return conduit in a direction to oppose said resilient means, thereby moving said second valve means such that said flow directing means connects said one side of said first valve means with said return conduit, thus causing said first valve means to be moved to its second position.

5. A device for blocking a flow of fluid as set forth in claim 3 wherein said first valve means includes a piston having said one side, an opposite side and in inner chamber, and said second valve means includes a second piston movable within said inner chamber and communicating with opposite sides of said flow restriction means.

6. A device for sensing a flow of fluid as set forth in claim 5 wherein said piston is fastened to a shaft movable therewith, said flow directing means constitutes a spool valve member on said shaft, a chamber communicating with the opposite side of said piston is provided containing fluid communicating with said return conduit, and an additional spool valve member is carried on said shaft such that when said second valve means is in its first position, it'blocks flow between said outlet conduit and said chamber, and when said second valve means is in its second position it permits flow between said outlet conduit and said chamber.

7. A device for sensing a flow of fluid as set forth in claim 6 wherein said first valve means encloses said first named chamber which communicates with said return conduit, and a second piston is carried on said shaft and movable within said first named chamber, said piston including said orifice means.

8. A device for blocking a flow of fluid as set forth in claim wherein said piston is fastened to a shaft movable therewith, said flow directing means constitutes a spool valve member on said shaft, said first named chamber forms part of said inner chamber, a third piston forms one end of said inner chamber, a passageway is provided in said third piston communicating said first named chamber with said return conduit and said orifice means is positioned in said passageway.

9. A device for blocking a flow of fluid as set forth in claim 8 including a check valve connected between said return conduit and the return side of said source, a fourth piston is connected across said check valve to sense the pressure drop across said check valve and movable in response to a substantial drop in pressure in said return conduit in a direction to oppose said resilient means, thereby moving said second valve means such that said spool valve member connects said one side of said first named piston with said return conduit thus causing said first piston to be moved to its second position.

Claims

1. A hydraulic safety device for connection between a source of hydraulic fluid under pressure and a device to be controlled comprising: a housing having an inlet port connected to said source, a high pressure outlet port connected to said supply controlled device, and a return port for receiving return flow from said device; a chamber in said housing connecting said inlet port and said high pressure outlet port; a first piston movable in said chamber between a first position in which flow passes between said inlet port and said high pressure outlet port and a second position where said high pressure outlet port is blocked; said piston including ridge means cooperating with the walls of said chamber to form a restricted passageway, and a second chamber within said piston; resilient means in said housing urging said piston toward said first position; a shaft and a second piston in said second chamber carried on said shaft and first and second passages communicating opposite sides of said second piston with opposite sides of said ridge means; a third piston on said shaft positioned in normal operation to permit communication between the inlet pressure side of said second piston and one end of said first piston, and movable with said second piston to a second position to interrupt said communication and to open communication between said one end of said first piston and a source of hydraulic fluid at return pressure; a fluid chamber adjacent said shaft and an orifice positioned to control the rate of flow out of said chamber and, therefore, the movement of said shaft; whereby when a leak occurs downstream of said high pressure outlet port the resulting fluid pressure differential across said restricted passageway is sensed across said second piston causing said second and third pistons to move against the force of said resilient means and at a rate controlled by said orifice until said third piston reaches its second position, causing said first piston to be moved by the force of inlet pressure from said first position to said second position.

2. A hydraulic safety device as set forth in claim 1 wherein a check valve is located between said return port and the return side of said source, a fourth piston is positioned coaxially with said shaft and fluid conduits are provided connecting opposite sides of said fourth piston across said check valve, such that upon the occurrence of a substantial loss of pressure upstream of said return port, said check valve will close and said fourth piston will sense the differential across said check valve and will move to compress said spring, moving said shaft until said third piston blocks communication between said inlet port and said one end of said first piston thus releasing the high inlet pressure acting against said one end and causing said first piston to move to said second position.

3. A device for blocking a flow of fluid from a high pressure source to a utilization device in response to excessive flow indicating a leakage comprising: a housing; an inlet conduit connecting said source with said housing; an outlet conduit connecting said housing with said utilization device; a return conduit connecting said utilization device with said housing; first valve means in said housing movable from a First position permitting flow from said inlet conduit to said outlet conduit to a second position in which flow from said inlet conduit to said outlet conduit is blocked, and flow restriction means associated with said first valve means; second valve means including pressure responsive means for sensing the pressure drop across said flow restriction means and flow directing means movable with said pressure responsive means to a first position to connect one side of said first valve means with fluid pressure in said inlet conduit to maintain said first valve means in its said first position or to a second position to direct fluid pressure acting against said one side of said first valve means to said return conduit which permits said first valve means to be moved to said second position; a chamber containing fluid in contact with said second valve means and orifice means restricting flow from said chamber to control the rate of movement of said second valve means; and resilient means urging said second valve means toward a position where inlet conduit pressure is directed against said one side of said first valve means.

4. A device for blocking a flow of fluid as set forth in claim 4 including a check valve connected between said return conduit and the return side of said source; pressure responsive means connected across said check valve to sense the pressure drop across said check valve and movable in response to a substantial drop in pressure in said return conduit in a direction to oppose said resilient means, thereby moving said second valve means such that said flow directing means connects said one side of said first valve means with said return conduit, thus causing said first valve means to be moved to its second position.

6. A device for sensing a flow of fluid as set forth in claim 5 wherein said piston is fastened to a shaft movable therewith, said flow directing means constitutes a spool valve member on said shaft, a chamber communicating with the opposite side of said piston is provided containing fluid communicating with said return conduit, and an additional spool valve member is carried on said shaft such that when said second valve means is in its first position, it blocks flow between said outlet conduit and said chamber, and when said second valve means is in its second position it permits flow between said outlet conduit and said chamber.

8. A device for blocking a flow of fluid as set forth in claim 5 wherein said piston is fastened to a shaft movable therewith, said flow directing means constitutes a spool valve member on said shaft, said first named chamber forms part of said inner chamber, a third piston forms one end of said inner chamber, a passageway is provided in said third piston communicating said first named chamber with said return conduit and said orifice means is positioned in said passageway.