US3575204A

US3575204A - Fail-safe assembly for gas-lift production systems

Info

Publication number: US3575204A
Authority: US
Inventors: Everett D Mcmurry
Original assignee: McMurry Oil Tools Inc
Current assignee: Baker Hughes Holdings LLC
Priority date: 1969-07-22
Filing date: 1969-07-22
Publication date: 1971-04-20
Anticipated expiration: 1988-04-20

Abstract

A fail-safe system is provided for use with gas-lift production of oil from a well, including a novel blocking valve actuable to interrupt gas injection in response to an excessive pressure differential in the injection line. In particular, the valve stem in the blocking valve is spring-loaded open and is also arranged to be urged open by the low pressure side of a differential upstream in the flow line. A pneumatic piston is arranged to urge the stem into the valve seat by the high pressure side of the differential, but the spring is selected to exceed the normally expected differential between the low and high pressures.

Description

United States Patent 1,478,320 12/1923 Baxter 2,027,762

Everett D. McMurry Houston, Tex.

July 22, 1969 Apr. 20, 1971 McMurry Oil Tools, Inc. Houston, Tex.

lnventor Appl. No. Filed Patented Assignee References Cited UNITED STATES PATENTS 1/1936 Becker 137/498 2,418,743 4/1947 Baker l37/498X 2,674,261 4/1954 Wilson 137/498 2,834,371 5/1958 Liljestrand 137/46 1 X ABSTRACT: A fail-safe system is provided for use with gaslift production of oil from a well, including a novel blocking valve actuable to interrupt gas injection in response to an excessive pressure differential in the injection line. In particular, the valve stem in the blocking valve is springloaded open and is also arranged to be urged open by the low pressure side of a differential upstream in the flow line. A pneumatic piston is arranged to urge the stem into the valve seat by the high pressure side of the differential, but the spring is selected to exceed the normally expected differential between the low and high pressures.

PATENTEUAPRZUIQH 3575204 SHEET 1 UF 2 FIG. I

Everett D. McMurry INVENTOR A TTORNEYS ATENTEDA 3,575,204

' sum 2 UF 2 I, i I

i 74 8 1 Hi FIG. 3

E ver ett D. Mc Murry /N VE N TOR BY flruwgcl, Mu,

WXIUMM ATTORNEYS FAIL-SAFE EMIBIJY FOR GAS-LIFT PRODUCTION SYSTEMS BACKGROUND OF INVENTION This invention relates to methods and apparatus for fluid flow control, and more particularly relates to methods and apparatus for interrupting the flow of injection gas used for gas-lift operations in oil wells and the like in cases of increases in the delivery volume of such injection gas above a preselected maximum volume. The invention is specifically directed to a novel blocking valve assembly especially suited to gas flow control.

There are many electricallyactuated devices for interrupting fluid flow upon the occurrence of certain predetermined circumstances. However, there are also many situations where it is either impractical or undesirable to utilize such electrically-actuated equipment.

For example, natural gas is often injected into oil wells for the purpose of lifting the oil to the surface. In most such cases, however, it is necessary to utilize such injection gas only at pressures within a relatively narrow range. For example, if the injection pressure falls below a certain minimum, the gas pressure will not open the gas-lift valve and enter the tubing or producing conduit, and oil production will stop. If the differential pressure is too great between the injection conduit (casing) and the production conduit (tubing), however, more gas will be injected than is actually required to lift the oil through the production conduit to the surface.

Gas-lift operations are generally unattended, and are merely checked by inspection personnel at occasional intervals. Thus, abnormal pressure fluctuations in either the injection conduit or the production conduit can produce a condition which may continue unabated or uncorrected for an extended period of time.

If a leak occurs in the tubing (production conduit) in the well, or if one of the gas-lift valves fails, this will cause an abnormal pressure drop to occur in the casing (injection conduit) in the well which, in turn, will create an abnormal increase in the pressure differential between the casing and the gas injection line leading to the wellhead. Accordingly, the injection rate of gas into the casing will rise proportionately which, in turn, will proportionately increase the flow of injection gas into the tubing and out of the well without increasing the flow of oil to any significant extent.

Heretofore, this did not create a serious problem for most well operators, since the field price of gas has always been quite low in relation to the field price of oil. The present day value of field gas in now many times greater than its former price, however. Furthermore, it is conventional to operate most wells with only periodic inspection. Accordingly, an abnormal increase in the pressure differential between the casing and the gas injection line may cause the loss of hundreds of thousands of cubic feet of injection gas during only a relatively short time interval, and may cause an otherwise profitable oil well to operate at a substantial loss.

Accordingly, there is now a substantial need for blocking means which will automatically interrupt the flow of injection gas into the well whenever an abnormal increase occurs in the pressure differential between the injection gasline and the casing or other injection conduit in the well. Electrically operated equipment which is responsive to a pressure difierential has long been available. However, most oil wells are remotely located relative to conventional sources of electrical power. Furthermore, it is undesirable for reasons of tire and explosions to utilize electrically operated equipment in the immediate vicinity of pipelines or other equipment handling oil or gas under high pressures. Thus, the particular need is for mechanically actuated blocking equipment for automatically interrupting gas flow into the well in such a contingency.

SUMMARY OF INVENTION In a larger sense, the preferred apparatus embodiment of the present invention includes a closure means operable in combination with means to create a pressure differential and responsive to any abnormal increase in such pressure drop or differential to close and block fluid flow in the flow line sought to be controlled. As will hereinafter be explained in greater detail, the blocking valve assembly is therefore preferably installed adjacent a component such as an orifice, which provides a normal pressure drop at a selected location in the gas injection line leading to the wellhead. The pressure drop may be established solely and specifically for the purpose of actuating the blocking valve assembly, or the valve assembly may be located adjacent some component (such as a needle valve), which creates a pressure drop in the line but which is installed primarily for some other reason.

As will also hereinafter be apparent, the subject blocking valve assembly includes a closure assembly, which is springloaded so as to be normally open. A pressure-operated driving means, such as a piston, is arranged and connected with the upstream side of the orifice or other component producing the subject pressure drop, whereby the piston tends to be driven against the closing member or valve stem to drive it into shutoff engagement with an appropriate valve seat. The pressure downstream of the orifice or like component, however, is routed against the valve stem to urge it out of engagement with the valve seat.

The compression force in the valve spring is normally greater than the pressure differential across the injection orifice. Accordingly, the valve spring will keep the blocking valve assembly in its normally-open condition provided the pressure differential does not exceed the opposing force of the valve spring.

An abnormal increase in pressure upstream of the orifice will, in the absence of a corresponding increase in downstream pressure, provide an increase in the pressure differential across the orifice or other component used to establish the subject pressure drop in the line. Alternatively, (and more frequently in the case of gas-lift operation) a decrease in pressure downstream will produce the same increase in pressure drop. In either case, the increase in pressure differential will operate to shut the valve and interrupt line flow the instant it exceeds the strength of the valve spring. The valve spring may, of course, be selected to establish the maximum pressure differential to be tolerated.

These and other features and advantages will become apparent from the following detailed description, wherein reference is made to the FIGS. in the accompanying drawings.

IN THE DRAWINGS FIG. it is a simplified pictorial representation of fluid flow line incorporating an orifice for establishing a drop in line pressure, and further incorporating a suitable blocking valve assembly with provision for sensing and responding to such drop or difierential in line pressure.

FIG. 2 is a simplified functional representation of a typical oil well interconnected with a gas-injection line for gas-lift purposes, wherein the aforementioned blocking valve assembly is operably interconnected with a flow rate stabilizer.

FIG. 3 is a more detailed representation, partly in cross section, of the blocking valve assembly depicted in FIGS. 1 and 2.

DETAILED DESCRIPTION Referring now to FIG. 1, there may be seen an ideal embodiment of the present invention, wherein means is provided in a flow line for establishing a pressure drop at a selected line location which is relatively minimal under normal line pressure conditions, and further including a flow interruption means sensible to and actuable by an increase in such pressure differential above a preselected maximum differential. In particular, a conventional orifice 11 may be installed in a pipe or other flow line 7 to create the desired pressure differential, and a suitable blocking valve assembly 8 is preferably interconnected in the flow line 7 immediately downstream of the orifice 11.

As may be seen in FIG. 1, a pressure line 9 is interconnected at one end to the blocking valve assembly 8, and at the other end to the flow line 7 at a location upstream of the orifice 11, whereby upstream line pressure is continually applied to a suitable portion of the blocking valve assembly 8. The valve assembly 8 is also inherently subject to the line pressure downstream of the orifice 11, and thus the valve assembly 8 is always subject to whatever line pressure differential exists in the flow line 7 across the orifice 11. As previously stated, the blocking valve assembly 8 is adapted to be normally open to pass fluid fiow through the flow line 7, and to remain open as long as the pressure drop or differential across the orifice 11 does not exceed a preselected maximum.

Referring now to FIG. 2, there may be seen a representation of an alternative embodiment of the present invention incorporated in the gas-injection line of an oil well operated by a gas-lift system. In particular, there may be seen a simplified representation of an oil well containing a production tubing 3 centrally located in a conventional well casing 2. A functional representation of a well head assembly 4 is suggested, including

suitable gate valves

5 and 6.

In a system of the character depicted, gas is injected into the annulus of the casing 2 from the aforementioned flow line 7. One or more gas-lift valves 3A may be mounted conventionally along the tubing 3, and arranged to open to admit gas into the tubing 3 whenever the pressure in the casing 2 exceeds a preselected magnitude. The pressure in the tubing 3 is lower than the pressure in the casing 2, and thus the gas tends to expand upon entering the tubing 3 to lift any liquids therein to the surface of the earth.

Accordingly, a blocking valve assembly 8 is desirably incorporated in the flow line 7 carrying injection gas into the casing 2, for the purpose of closing in response to an excessive pressure differential. As may be seen in FIG. 2, a suitable flow rate controller 10, of the type depicted in the copending US. Pat. application Ser. No. 794,189, which was filed .Ian. 27, 1969, by Everett D. McMurry and Bolling A, Abercrombie, may be included for the purpose of maintaining a preselected injection rate notwithstanding fluctuations in line pressure either upstream or downstream of the controller 10. A pressure drop of measurable magnitude will occur across a component such as the flow rate controller 10. Accordingly, the blocking valve assembly 8 is preferably located downstream of the flow rate controller 10, and the pressure line 9 is preferably connected at a location upstream of the controller 10, as indicated in FIG. 2.

Referring now to FIG. 3, there may be seen a more detailed representation of the blocking valve assembly 8 depicted in FIGS. 1 and 2. In particular, a valve body 12 is provided, which has a generally T-shaped configuration, and which includes threaded intake and

outlet ports

24 and 25 for interconnection into the flow line 7. A generally cylindrical and hollow barrel member 13 may be seen to be threadedly interconnected with the valve body 12, with a suitable O-ring 28 included therebetween to insure a gastight connection. An annular valve seat member 22 may be seen to be slidably inserted in the valve body 12, and provided with a shoulder portion for engaging the internal portions of the valve body 12, whereby it is prevented from being dislocated during closure of the valve assembly 8. An O-ring 26 may be provided as indicated, to insure a gastight fit between the seat member 22 and the adjacent surfaces of the valve body 12 and to provide for gas flow only through the orifice in the valve seat member 22.

A suitable ringlike member, hereinafter called a seat cage 23, is also positioned in the valve body 12 to lock the valve seat member 22 into location when the barrel 13 is properly interconnected with the valve body 12. In addition, the seat cage 23 includes a suitable aperture to provide communication between the intake port 24 and the valve seat member 22. As may be further seen, the upper end of the barrel 13 is closed or stoppered by a suitable barrel plug 15 threadedly connected therein, the barrel plug 15 having a threaded input aperture for receiving the threaded downstream end of the pressure line 9 depicted in FIGS. 1 and 2. The barrel plug 15 is also provided with an outlet aperture located opposite of the pressure line 9, and interconnected with a suitable reset valve 17 by means of a suitably threaded nipple 16, or the like.

The shutoff components of the blocking valve assembly 8, may be seen to include a suitable valve stem 20, having a shutoff end portion at one end formed for shutoff engagement with the confronting portions of the seat member 22, and threadably connected at its opposite upper end to a stem holder 19. As may also be seen in FIG. 3, the stem holder 19 is provided at its upper end with an enlarged shoulder or springretaining portion 19A. The barrel 13 is provided with a cylindrical internal configuration, narrowed along its lower end, to slidably support the stem 20, whereby the stem 20 is guided properly to and from the valve seat member 22 along a vertical axis of travel. A suitable O-ring 27 may also be included for the purpose of preventing gas leakage along the abutting surfaces of the valve stem 20 and barrel 13. A suitable valve spring 21 may be disposed in the larger upper cylindrical internal portion of the barrel member 13, for compression between the lower or undersurface of the springretaining shoulder portion 19A of the valve stem holder 19, and the upper surface of the internal shoulder defined by the smaller internal portion of the barrel 13. Thus, the compressed valve spring 21 will tend to expand upwardly to hold the valve stem 20 out of blocking engagement with the valve seat member 22.

A suitable power piston 18 may be slidably disposed within the upper end of the barrel plug 15, and may be provided with a narrower upper cylindrical end portion, and a larger lower cylindrical end portion, whereby pressure in the barrel plug 15 about the narrower portion of the piston 18 tends to act against the larger end portion. The larger lower end portion of the power piston 18 is slidably disposed between the adjacent inside surfaces of the barrel plug 15 to function as a gasoperated piston and a suitable O-ring 29 may be provided to prevent gas seepage therebetween. The narrower upper end portion of the power piston 18 is also slidably disposed between adjacent inside surfaces of the barrel plug 15, for the purpose of trapping pressure within the barrel plug 15, and a suitable O-ring 30 may also be provided to obviate gas seepage therebetween.

As hereinbefore stated, the object of the blocking valve assembly 8 is to close automatically in response to an abnormal pressure differential across a selected location in the pipeline 7, to prevent waste of injection gas because of either an abnormal increase in upstream pressure, or an unusual or abnormal decrease in downstream pressure. Pressure entering through the intake port 24 will be lower than the gas pressure entering the barrel plug 15 through the pressure line 9, for the reasons hereinbefore given. Thus, gas pressure in the barrel plug 15 tends to act against the larger lower portion of the power piston 18, and to urge the power piston 18 down into abutting engagement with the spring-retaining portion of the stem holder 19, to thereby drive the valve stem 20 into shutoff engagement with the valve seat member 22. However, the force being applied to drive the power piston 18 downward is only equal to the pressure differential between the upstream pressure in the pressure line 9 and barrel plug 15, and the downstream pressure in the intake port 24 and across the valve seat member 22. The opposing force of the compressed valve spring 21 is preselected to be great enough to overcome or exceed the pressure differential, and thus the valve spring 21 will normally maintain the valve stem 20 in a raised position notwithstanding the upstream pressure being applied to the power piston 18. If an abnormal increase in upstream pressure occurs, however, the differential between the pressure across the power piston 18 and the pressure across the valve stem will be correspondingly increased to exceed the opposing force of the compressed valve spring 21. Alternatively, if the downstream pressure decreases abnormally, the same increase in pressure differential will result. in either case, this increase in pressure difi'erential will tend to urge the power piston l8 downward to, in turn, urge the valve stem 20 into shutoff engagement with the valve seat member 22.

It will be noted that if either of these abnormal pressure conditions is removed, and if the pressure differential returns to normal, the compressed valve spring 21 will again draw the valve stem 20 out of shutoff engagement with the valve seat member 22. It sometimes occurs, however, that the aforementioned abnormal pressure conditions cannot be conveniently corrected. If it is nevertheless desired to continue to inject gas into the casing 2, the reset valve 17 may be opened to bypass or release pressure from the power piston 118, whereupon the power piston 18 will be urged upwardly in the barrel plug l5 by the valve spring 21, and the valve stem 2'0 will be drawn upward.

it will be noted that the lower portion of the barrel plug l5 is provided with an enlarged cylindrical cavity for slidably receiving the spring-retaining portion 19A of the stem holder 119 in order to maintain the stem 20 in coaxial alignment with the valve seat body 22. Since the O-

rings

27 and 29 tend to create a gastight compartment within the barrel 13, one or more notches or the like may be desirably provided in the rim of the retaining portion 19A of the stern holder 19 to facilitate movement of the stem 20. In addition, it may be desirable to provide a vent R4 in the barrel 13 as depicted in FIG. 3 in order to provide a fixed pressure within the barrel l3 and against the spring-retaining portion 19A of the stem holder 19 irrespective of the position of the stem holder 19 within the barrel 13.

It will be apparent from the foregoing that various modifications and variations may be made in the structures and procedures described herein without substantial departure from the essential concept of the present invention. Accordingly, it should be clearly understood that the forms of the invention described herein and depicted in the accompanying drawings, are exemplary only and are not intended as limitations on the scope of the present invention.

lclaim: l. A flow control system for interconnection with a flow line to control the supply of injection gas into an oil well or the like, said system comprising:

differential means for establishing a pressure differential at a first location along said flow line, and

normally open blocking means at a second location along said flow line for interrupting said supply of injected gas in response to an increase in said pressure differential at said first location above a preselected differential.

2. The control system described in claim 1, wherein said blocking means comprises:

a valve seat member disposed adjacent said second location,

a shutoff member urged away from said seat member by pressure in said flow line on one side of said differential means, driving means for urging said shutoff member into engagement with said seat member by pressure in said flow line on the other side of said differential means, and

spring loading means urging said shutoff member away from said seat member by a preselected force not greater than said preselected differential.

3. The control system described in claim 2, wherein said second location is downstream of said first location and said pressure on said other side of said differential means is greater than said pressure between said first and second locations.

6. The control system described in claim 3, wherein said differential means includes:

throttling means located in said flow line upstream of said valve seat member and shutoff member and providing a flow restriction at said first location.

5. The control system described in claim 4, wherein said throttling means is adapted to stabilize the flow rate of fluid flow between said first and second locations in said flow line.

6. A blocking valve assembly for interrupting fluid flow in response to an excessive pressure differential, said assembly comprising:

a hollow body member for interconnection in a flow line and having an intake port and an outlet port,

an annular valve seat member interconnected in said body member between said intake and outlet ports and across the flow path of said fluid,

means for establishing a pressure differential at a location upstream of said seat member,

a normally open valve shutoff member for engaging said seat member when the pressure differential at said location is greater than a preselected differential,

a cylindrical valve stem having one end connected to said shutoff member,

guiding means slidably and gas-tightly supporting said valve stem,

spring means having a preselected compression strength and interconnected with said valve stem to urge said shutoff member out of shutoff engagement with said valve seat member, and

driving means adjacent the opposite end of said valve stem for urging said stem against said spring means in response to the pressure upstream of said location.

7. The blocking valve assembly described in claim 6,

wherein said driving means includes:

cylinder means having a pressure chamber communicating with said upstream pressure, I

a piston member slidably disposed in said cylinder means and arranged to be urged into abutting engagement with said opposite end of said valve stem in response to said upstream pressure in said pressure chamber.

8. A blocking valve assembly responsive to an excessive pressure differential for interrupting fluid flow, comprising:

a body member adapted for interconnection in a flow line,

an annular valve seat member mounted in said body member,

a hollow barrel member interconnected with said body member and having a cylindrical passageway in one end adjacent to and axially aligned with said seat member,

a cylindrical valve stem having a shutoff surface at one end confronting said seat member and slidably disposed in said cylindrical passageway,

a spring member having a preselected compression strength and disposed in said barrel member for urging said stem from said valve seat member,

pressure cylinder means interconnected to stopper the other end of said barrel member and having a cylindrical borehole opening at one end toward said valve stem and a gas port communicating with said cylindrical borehole adjacent its other end,

means for establishing a pressure differential at a location upstream of said valve seat member and for communicating the upstream pressure at said location to said gas port, and

piston means slidably disposed in said cylindrical borehole and adapted to be urged against said valve stem in response to gas pressure received from said gas port for urging said valve stem into shutoff engagement with said valve seat member.

9. The valve assembly described in claim 8, wherein said valve stem is further provided with a spring-engaging portion, and

wherein said spring member comprises a helical spring disposed coaxially about said valve stem and interconnected with said spring-engaging portion thereof.

10. The valve assembly described in claim 9, wherein said barrel member is threadedly interconnected generally transversely of said body member and is further provided with an internal shoulder portion for supporting and maintaining said helical spring in a compressed condition, and

wherein said spring-engaging portion of said valve stem is a flangelike portion located at the opposite end of said valve stem from said shutoff surface and adapted for abutting engagement.

11. A blocking valve assembly responsive to an excessive pressure differential for interrupting fluid flow, comprising:

a body member adapted for interconnection in a flow line,

an annular valve seat member mounted in said body member,

a cylindrical valve stem having a shutoff surface at one end confronting said seat member and slidably disposed in said cylindrical passageway to be urged from said seat member by gas pressure in said body member.

a spring member having a preselected compression spring and disposed in said barrel member for urging said stem from said valve seat member.

pressure cylinder means interconnected to stopper the other end of said barrel member and having a cylindrical borehole opening at one end toward said valve stem and at least two gas ports each communicating with said cylindrical borehole adjacent its other end,

piston means slidably disposed in said cylindrical borehole and adapted to be urged against said valve stem in response to gas pressure received from one of said gas ports for urging said valve stem into shutoff engagement with said valve seat member and closure means stoppen'ng the other of said gas ports and openable for releasing gas pressure from said piston means.

POMS I UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. qyw sgoq Dated Avril 20. 1970 lnvent fl i Everett D. McMurrv It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

line 67, delete "loading".

Signed and sealed this 10th day of August 1971.

(SEAL) Attest:

WILLIAM E. SCHUYLER, J'R.

EDWARD M.FLETCHER,JR.

Commissioner of Patents Attesting Officer

Claims

1. A flow control system for interconnection with a flow line to control the supply of injection gas into an oil well or the like, said system comprising: differential means for establishing a pressure differential at a first location along said flow line, and normally open blocking means at a second location along said flow line for interrupting said supply of injected gas in response to an increase in said pressure differential at said first location above a preselected differential.

2. The control system described in claim 1, wherein said blocking means comprises: a valve seat member disposed adjacent said second location, a shutoff member urged away from said seat member by pressure in said flow line on one side of said differential means, driving means for urging said shutoff member into engagement with said seat member by pressure in said flow line on the other side of said differential means, and spring loading means urging said shutoff member away from said seat member by a preselected force not greater than said preselected differential.

4. The control system described in claim 3, wherein said differential means includes: throttling means located in said flow line upstream of said valve seat member and shutoff member and providing a flow restriction at said first location.

6. A blocking valve assembly for interrupting fluid flow in response to an excessive pressure differential, said assembly comprising: a hollow body member for interconnection in a flow line and having an intake port and an outlet port, an annular valve seat member interconnected in said body member between said intake and outlet ports and across the flow path of said fluid, means for establishing a pressure differential at a location upstream of said seat member, a normally open valve shutoff member for engaging said seat member when the pressure differential at said location is greater than a preselected differential, a cylindrical valve stem having one end connected to said shutoff member, guiding means slidably and gas-tightly supporting said valve stem, spring means having a preselected compression strength and interconnected with said valve stem to urge said shutoff member out of shutoff engagement with said valve seat member, and driving means adjacent the opposite end of said valve stem for urging said stem against said spring means in response to the pressure upstream of said location.

7. The blocking valve assembly described in claim 6, wherein said driving means includes: cylinder means having a pressure chamber communicating with said upstream pressure, a piston member slidably disposed in said cylinder means and arranged to be urged into abutting engagement with said opposite end of said valve stem in response to said upstream pressure in said pressure chamber.

8. A blocking valve assembly responsive to an excessive pressure differential for interrupting fluid flow, comprising: a body member adapted for interconnection in a flow line, an annular valve seat member mounted in said body member, a hollow barrel member interconnected with said body member and having a cylindrical passageway in one end adjacent to and axially aligned with said seat member, a cylindrical valve stem having a shutoff surface at one end confronting said seat member and slidably disposed in said cylindrical passageway, a Spring member having a preselected compression strength and disposed in said barrel member for urging said stem from said valve seat member, pressure cylinder means interconnected to stopper the other end of said barrel member and having a cylindrical borehole opening at one end toward said valve stem and a gas port communicating with said cylindrical borehole adjacent its other end, means for establishing a pressure differential at a location upstream of said valve seat member and for communicating the upstream pressure at said location to said gas port, and piston means slidably disposed in said cylindrical borehole and adapted to be urged against said valve stem in response to gas pressure received from said gas port for urging said valve stem into shutoff engagement with said valve seat member.

9. The valve assembly described in claim 8, wherein said valve stem is further provided with a spring-engaging portion, and wherein said spring member comprises a helical spring disposed coaxially about said valve stem and interconnected with said spring-engaging portion thereof.

10. The valve assembly described in claim 9, wherein said barrel member is threadedly interconnected generally transversely of said body member and is further provided with an internal shoulder portion for supporting and maintaining said helical spring in a compressed condition, and wherein said spring-engaging portion of said valve stem is a flangelike portion located at the opposite end of said valve stem from said shutoff surface and adapted for abutting engagement.

11. A blocking valve assembly responsive to an excessive pressure differential for interrupting fluid flow, comprising: a body member adapted for interconnection in a flow line, an annular valve seat member mounted in said body member, a hollow barrel member interconnected with said body member and having a cylindrical passageway in one end adjacent to and axially aligned with said seat member, a cylindrical valve stem having a shutoff surface at one end confronting said seat member and slidably disposed in said cylindrical passageway to be urged from said seat member by gas pressure in said body member. a spring member having a preselected compression spring and disposed in said barrel member for urging said stem from said valve seat member. pressure cylinder means interconnected to stopper the other end of said barrel member and having a cylindrical borehole opening at one end toward said valve stem and at least two gas ports each communicating with said cylindrical borehole adjacent its other end, piston means slidably disposed in said cylindrical borehole and adapted to be urged against said valve stem in response to gas pressure received from one of said gas ports for urging said valve stem into shutoff engagement with said valve seat member and closure means stoppering the other of said gas ports and openable for releasing gas pressure from said piston means.