US3568706A

US3568706A - Pressure relieving system

Info

Publication number: US3568706A
Authority: US
Inventors: Irvin B Weise
Original assignee: Anderson Greenwood and Co
Current assignee: Anderson Greenwood and Co
Priority date: 1969-02-06
Filing date: 1969-02-06
Publication date: 1971-03-09
Anticipated expiration: 1988-03-09
Also published as: DE1927704B2; DE1927704C3; JPS506055B1; DE1927704A1; GB1266511A

Abstract

A pressure-relieving system including a main relief valve, a pilot valve which controls the pressure setting for relief and blowdown through the main valve and a back flow preventer which prevents reverse flow through the main valve. The pilot valve assures that there is no reverse flow through the fluid pressure supply line and thereby allows field testing of the valve to determine if it is functioning properly while the pilot valve is installed in the system controlling the operation of the main valve.

Description

United States Patent lrvin B. Weise [72] Inventor Bellaire, Tex. [211 App]. No. 796,961 [22] Filed Feb. 6, 1969 [45] Patented Mar. 9, 1971 [73] Assignee Anderson Greenwood 8: Co.

Houston, Tex'.

[S4] PRESSURE RELIEVING SYSTEM 4 Claims, 5 Drawing Figs.

[52] U.S. Cl 137/112, 137/492 [51] Int. Cl. Fl6k 17/10 [50] Field ofSearch 137/488, 489.5, 496, 489, 492, 112; 251/34, 26

[56] References Cited UNITED STATES PATENTS 3,294,1 11 12/1966 Abercrombie 137/492 2,609,832 9/1952 Smith 137/489 2,661,023 12/1953 Griswold l37/488X 3,010,691 11/1961 Canfield 251/34X 3,373,763 3/1968 Smilges et al 137/489X FOREIGN PATENTS 286,903 3/1928 Great Britain 137/488 Primary Examiner-M. Cary Nelson Assistant Examiner-R. B. Rothman Att0rneysJ. Vincent Martin, Joe E. Edwards and Jack R.

Springgate FRESSURIE RELIEVWG SYSTEM The present invention relates to an improved relieving system, to an improved pilot valve for controlling a relief system and to a pressure responsive device preventing reverse flow of fluid through the relief valve. This invention is an improvement on the system and structure disclosedin my prior copentling application Ser. No. 7 l l ,821 filed Feb. 19, 1968.

An object of the present invention is to provide an improved pressurerelieving system in which the outlet of the main valve may be connected into a header with assurance that the main valve will not open to allow reverse flow therethrough when the header pressure exceeds the main valve inlet pressure.

Another object is to provide an improved pilot valve for a pressure-relieving system whose setting may be tested with a testing fluid after it has been installed in controlling position on a relief valve without having the testing fluid flow through the pilot fluid supply line.

A further object is to provide an improved pilot valve suitable for use on liquids and gases which includes a check valve to prevent back flow through .the fluid supply line and eliminates the possibility of the inlet valve and check valve both closing and being held closed by pressure of fluid trapped therebetween..

Still another object of the present invention is to provide an improved pressure responsive device adapted to be used with a pressure-relieving valve to prevent the valve from opening when the valve is subjected to higher pressures at its outlet than at its inlet.

A still further object is to provide an improved pressure-relieving system which may be set for a preselected relieving pressure and also for a preselected blowdown independent of the relieving pressure setting.

Another object is to provide as a subcombination in an improved pressure-relieving system a pressure responsive device to be used with the pressure-relieving system which applies the higher of two fluid pressures to a pressure responsive member in the system.

BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and advantages of the present invention are hereinafter set forth and described with reference to the structure of the present invention illustrated in the drawings wherein:

FIG. I is a schematic diagram of the improved relieving system of the present invention.

FIG. 2 is a detailed sectional view of the improved pilot valve of the present invention and schematically illustrating the equipment for testing the pilot .valve relief setting.

H6. 3 is an enlarged detailed sectional view of the improved'pilot valve showing the inlet valve member in seated position.

FIG. 4 is a partial sectional view of the improved back flow preventer of the present invention.

FIG. 5 is a sectional view taken along line 5-5 in FIG. 4 showing the cross-sectional configuration of the pressure responsive member of the improved back flow preventer of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in FIG. I, the improved pressure-relieving system of the present invention is installed to protect tank T from over pressure conditions. The main relief valve R is connected to have its inlet Iii in communication with the interior of the tanlr T and has its outlet 12 connected to a suitable disposal line L which may be a manifold or header of outlets from several pressure-relieving systems (not shown). The main valve R includes a valve member l4 controlling the flow therethrough responsive to the pressure delivered to the pressure responsive member 116. Main valve R may be of any suitable type which includes a pressure responsive means actuating the valve member tocontrol the flow through the main valve. With valve R pressure on the pressure responsive member 16 is used to hold valve member I4 in closed position and when the pressure is vented from member I6, the pressure in inlet 10 causes valve member M to open. 7

The improved pilot valve is supplied with fluid under pressure from the tank T by the line 18 which connects from inlet 10 to the pilot valve P. Control pressure is supplied by pilot valve P through the line 20 and the back flow preventer 22 to the pressure responsive member 16 of main valve R. By controlling the pressure supplied to the pressure responsive member 16, the pilot valve. P controls the venting of tank T through the main valve R. Also, line 24 connects from outlet 12 to the back fiow preventer 22 to cause the main valve R to close or remain closed whenever the pressure in the disposal line L rises to a pressure above the pressure at the inlet 10 to thereby prevent flow of fluids through the main valve R in the reverse direction. The pilot valve I is also provided with a vent port 26 through which fluid pressure in pressure responsive member 16 is vented to allow the main valve to open and relieve pressure within tank T.

The pilot valve P is shown in greater detail in FIGS. 2 and 3. Pilot valve P includes the body 28 in which a central bore 30 is formed with a plurality of ports communicating with bore 30 as hereinafter described. The shoulder 32 projecting into the bore 30 receives the valve seat assembly 34 which is held on shoulder 32 by the cage 36 and the bonnet 38. The cage 40 supports the seat. insert 42 within the central bore 30 and is adjustable axially of bore 30 as hereinafter described. Communication through the central bore 30 is first through the back check seat 44 defined within the interior of cage 40, the second seat 46 defined by seat insert 42 and the outlet seat 48 defined by the seat assembly 34. The cage 40 is sealed against the walls. of bore 30 and provided with ports 50 communicating from the inlet chamber 52 to the inlet seat 44 so that all flow through the pilot valve P is directed through the inlet seat 44 and the second seat 46. The

inlet ports

54 and 56 both communicate with the inlet chamber 52, inlet port 54 being provided with a filter screen 58 and inlet port 56 being an alternate communication into inlet chamber 52 and normally closed by the plug 60. Control ports 62 and 64 both communicate with intermediate chamber 66 between

seats

46 and 68. Port 64 is normally closed but is provided to allow testing of the pilot valve I after it has been installed in position controlling the relief valve R as hereinafter explained. Outlet or vent port 68 communicates with outlet chamber 70.

Ports

72 and 74 through cage 36 probide communication between the interior of cage 36 and outlet chamber 70. The port 76 extends through cage 36 to provide communication between the interior of bonnet 38 and the interior of cage 36.

The valve member 78 which is slidably mounted within cage 36 controls the flow through valve seat 413 responsive to the pressure forces exerted thereon and the force of the adjustable biasing means (spring 86). Spring 86 is adjustable by the threading of screw 82 through bonnet 38. Lock nut 86 is provided to retain screw 82 in its-preselected position and cap 36 engages bonnet 36 and covers the outer end of screw 82 and lock nut 84. When valve member 78 is open allowing flow through valve seat 48 a portion of this flow flows through each of port '74 and port 72. The flow to port 72 is restricted by the flange 88 extending outwardly from valve member 7 3 and terminating in close spaced relationship to the interior of cage 36. This restricts the flow to cause the valve member 72% to snap to full open position when it first opens by exposing the entire lower side of valve member '78 to the fluid pressure from the intermediate chamber 66.

Flow from the inlet chamber 52 to the intermediate chamber 66 is controlled by valve member 90 which is slidably mounted within bore 92 of cage 40 and movable between seats 44 and d6 responsive to the pressure differential across valve member 90. Valve member 96 is preferably closely spaced with respectto the wall of bore 92 to provide a restriction to the flow of fluid through bore 92 around valve member 96. It as been found that if valve member fill has a polygonal cross-sectional shape such as a hexagon, the points of the polygon engage the walls of bore 92 to guide the movement of valve member 90 in the bore 92 and the restricted flow area is provided between the flats of the hexagon and the wall of the bore 92.

The valve member 90 includes the body 91, the sealing ring 94 which is adapted to engage seat 44 when valve member 90 is in its lower position and the sealing ring 96 held in place by the tubular retainer 98 which sealing ring 96 is adapted to engage the seat 46 as best shown in FIG. 3 when valve member 90 is in its upper position. When valve member 90 is not required to function as a check valve then seal ring 94 may be omitted so that valve member 90 does not seal on seat 44. Tubular retainer 98 is positioned within the bore 100 defined in body 91, includes the annular flange 102 which projects radially outwardly and is adapted to engage the sealing ring 96 to support it in position for sealing engagement with valve seat 46. This support configuration for sealing ring 96 allows pressure to be equalized across ring 96 through tubular retainer 98 and the space around retainer 98 within the bore 100 in body 91. By equalizing pressure on opposite sides of ring 96 it remains in position to engage seat 46 at all times and is not extruded or pulled out of its support position because of pressure differentials to which it might be exposed resulting from the flow of fluids through seat 46. Valve member 90 thus is the inlet valve when seated on seat 46 and the back check valve when seated on seat 44. Valve member 90 when seated on seat 46 is held by pressure differential to assure that it also cannot engage seat 44. When lower pressures are encountered, the two piece construction of valve member 90 may be eliminated in favor of a single piece and a relatively firm O-ring 96 which would not require pressure equalization during flow.

In order to insure that valve member 90 is disengaged from seat 46 when valve member 78 is in engagement with seat 48 and that valve member 78 is open when valve member 90 is in engagement with seat 46, the spacer rod 104 is positioned between the two

valve members

78 and 90. Rod 104 extends through both

valve seats

46 and 48 and is closely spaced with respect to the valve seats to provide a flow restriction to the fluids flowirg through these two annular passages. The engagement of spacer rod 104 with valve member 78 is within the bore 106 defined in member 78 to allow some relative movement between the

valve members

78 and 90 while assuring that both valve members are not seated at the same time. The length of rod 104 is preselected with respect to the distance between the seated position of valve member 78 on seat 48 and the seated position of valve member 90 on seat 46 to assure that both are not closed at the same time.

The effective areas of

valve members

78 and 90 are each larger than the area of valve seat 46. The cross-sectional area of rod 106 is significantly smaller than the area of either seat 46 or seat 48 so that it is ineffective as a force developing member.

In addition, the position of valve seat 46 is adjustable with respect to valve seat 48 to provide a blowdown control adjustment. This adjustment is provided by the threaded engagement of the lower or projecting end of cage 40 in bushing 108. Lock nut 110 is provided to secure the cage 40 in its preselected position. Also, shim 112 is positioned between the shoulder 114 on cage 40 and the inner end of bushing 108. The thickness of the shim 112 is preselected to assure that

seats

46 and 48 are not spaced-apart a sufficient distance so that both may be closed at the same time. For convenience in adjusting, the outer end of cage 40 is provided with flats to allow cage 40 to be turned with a wrench or a suitable handle.

The testing system shown in FIG. 2 includes a test pressure source or tank 116 and a pressure gauge 118. Valve 120 connects into alternate port 64 and flow lines connect between valve 120 and pressure gauge 118 and also through valve 122 to tank 116 as shown. With the testing system connected as shown, valve 120 is opened first and valve 122 is then opened gradually to pressurize intermediate chamber 66. This pres sure is registered on gauge 118. By increasing the pressure until valve member 78 lifts off valve seat 48 to vent chamber 66, the opening setting of pilot valve P may be checked and reset if needed. During the time that the pressure in intermediate chamber 66 exceeds the pressure in inlet chamber 52, valve member Is held in seated position on seat 44 to prevent test fluid from flowing back into the supply system. This allows air to be used in testing even though it could not be tolerated in tank T. The pressure fluid delivered to intermediate chamber 66 only leaves the system through valve seat 48 and vent port 68 and is not conducted to the tank T or the disposal line L. As previously mentioned, the seal ring 94 may be omitted if this testing system is not to be used. With the testing system valve remains connected in port 64 and when the testing system is not to be used, a plug may be used to close port 64 in place of valve 120.

The control pressure maintained in intermediate chamber 66 is delivered through port 62, line 20 to back flow preventer 22. If this control pressure is greater than the pressure in outlet 12, it is conducted through back flow preventer 22 into the main valve R where it is exerted against the pressure responsive member 16 to control the position of valve member 14. Thus, back flow preventer 22 functions as a switching valve to direct the greater of the two fluid pressures to the pressure responsive means 16. Back flow preventer 22 has a T-shaped body 124 and includes a double-acting valve member 126. The branch leg of body 124 is threaded for connection into main valve R.

Connection of line 20 to back flow preventer 22 is provided into one of the aligned legs of body 124 by the bushing 128 which also defines the inwardly facing valve seat 130. Line 24 is connected into the other of the aligned legs of body 124 by bushing 132 which defines inwardly facing seat 134. As shown, valve member 126 is positioned within body 124, responsive to the pressures exerted on opposite ends thereof and coacts with seats and 134 to provide communication to main valve R from line 20 or line 24 whichever has the higher pressure.

The valve member 126 includes a cylindrical body 136 with

heads

138 and 140 at each end of body 136. Head 138 supports the seal ring 142 which in one position of member 126 engages seat 130 to seal off flow from line 20. Head 140 supports seal ring 144 which in the other position of valve member 126 engages seat 134.

Heads

138 and 140 are preferred to be generally polygonal shaped in cross section. As shown, the

heads

138 and 140 are square shaped with rounded comers and terminate in a frustoconical portion supporting the seal rings 142 and 144. As best seen in FIG. 5, the rounded corners of the heads guide the valve member 126 in its axial movement by engagement with the walls of the bores in

bushings

128 and 132. Further, the heads provide sufficient restriction to flow of fluids through the spaces between the head flats and the walls of the bushing bores to cause movement of the valve member 126 to the other seat responsive to changes in the pressures delivered to

bushings

128 and 132. Thus, the back flow preventer 22 functions to assure that the pressure responsive member 16 in main valve R is exposed to the fluid pressure in line 20 or line 24 whichever is higher.

In operation, the pressure fluid from inlet 10 is supplied to inlet port 54 and inlet chamber 52 of pilot valve P. When the valve member 90 is disengaged from both

seats

44 and 46 fluid flows around valve member 90 into intermediate chamber 66. Port 62 and line 20 communicate from chamber 66 to back flow preventer 22 and supply fluid to the pressure responsive member 16 of valve R to hold valve member 14 in closed position. Member 16 has a greater effective pressure area than valve member 14 to assure that valve R is closed when inlet pressure is delivered through pilot valve P and back flow preventer 22 to pressure responsive member 16.

The fluid pressure in intermediate chamber 66 is exerted on the exposed portion of valve member 78 through seat 48. The force exerted by spring 80 is adjusted to allow valve member 78 to lift off seat 48 at the maximum pressure which is desired in tank T. As previously explained when valve member 78 cracks open, it snaps to its full open position because of the increased area of valve member exposed to the pressure in intermediate chamber 66 and the flow through valve seat 48 causes a flow past valve member 94} tourge valve member 90 toward closed position and exert a force on valve member 7% by the connection of rod 164 urging valve member 78 toward full open position. When valve member 78 is fully open, valve member 9% engages seat 46 to shut off the inlet supply of fluid. The pressure control fluid in valve R is vented through valve seat and out vent port 6%. With the reduced control, pressure valve member l4 opens to relieve the pressure in tank T.

Control of blowdown or closing pressure is controlled by the adjustment of the position of seat 46 in relation to seat 4L8. The effectiveness of such control is explained in my aforementioned patent application. Such control is accomplished by loosening lock nut lllfl and rotating cage 40 within bushing Thus, as cage 30 is threaded inwardly, seat 46 is moved closer to seat 48 providing a greater lift of valve member 78 to cause valve member 96) to engage seat 46 thereby compressing spring 80 so that the force exerted by spring 80 toward closing is greater. With

valve seats

46 and 48 closer together, the closing pressure of valve member 7 8 is higher than the closing pressure when valve seats 46 and 48 are spaced farther apart. The closing pressure of valve member 78 is preselected to provide the desired blowdown of tank T before valve R closes. This adjusting of blowdown has no effect on the opening setting of pilot valve Psince the force exerted by spring 80 on valve member 78 when valve member 78 is seated remains the same regardless of the position of seat 46.

The closing of seat 48 and the opening of flow through seat 46 is responsive to, the pressure forceon valve member 90 and the spring force exerted by spring 80. The restriction to flow in the areas around rod 106 through the

seats

46 and 48 creates a restriction to flow which causes valve 90 to snap away from seat 46 once it cracks open and also causes valve member 78 to snap to closed position on seat 48. This restriction creates a slight increasein back pressure on valve member 96 as it first opens to provide some additional opening force which is suffrcient to snap valve member so open.

The restriction between valve 90 and bore 92 is particularly important to the operation of pilot valve P when it is set to handle gas and liquid is supplied through line 18. Such condi-' tions can occur in propane or butane systems and it is important that the pilot valve P function properly to cause the system to be relieved at the preselected relieving pressure when exposed to either gas or liquid. With liquid in pilot valve l, as valve member 73 cracks open, flow around valve member 90 creates a small pressure drop to produce an added force-lifting valve member 78 to snap it to full open position.

From the foregoing, it can be seen that the improved pressure-relieving system is responsive-to pressure in the pressure vessel and in the discharge line so that the system vents the pressure vessel to the discharge line when the pressure in the vessel reaches a preselected pressure but the system functions to assure that there is no communication between the vessel and the discharge line when the pressure in the discharge line increases to a level above the pressure in the vessel. Also, the improved pilot valve may be tested after it is installed in a pressure-relieving system without danger of the testing fluid leading back into the pressure system. The pressure-relieving system of the present invention is adaptable for relieving pressure of a gas or a liquid in a pressure vessel The foregoing disclosure and description of the invention are illustrative and explanatory thereof, and various changes in the size, shape and materials, as well as inthe details of the illustrated construction, may be made within the scope of the appended claims without departing from the spirit of the invcntion.

lclaim: l. A pressure relieving system for a fluid pressure container comprising:

a main relieving valve; said main valve having an inlet connected to the fluid pressure container to be protected, an outlet and pressure responsive means for controlling flow between said inlet and said outlet;

a pilot valve;

means supplying control pressure fluid to said pilot valve from said fluid pressure container;

said pilot valve having an output of control pressure fluid (pressure) responsive to the pressure in said fluid pressure containers;

a fluid flow switching device connected (to) between said pressure responsive means of said main valve and said outlet of said main valve and between said main valve pressure responsive means and said pilot valve control pressure fluid outlet to deliver the greater of fluid pressure to said pressure responsive means from said pilot valve output and from the outlet of said main valve; and

said pilot valve being preset to relieve control pressure fluid from the pressure responsive means of said main valve when the container fluid pressure reaches a preselected desired maximum whereby the fluid pressure in said container is relieved 2. A pressure relieving system according to claim 1, including;

a check valve in said pilot valve to prevent reverse flow of fluid through said pressure fluid supplying means. 3. A pressure relieving system for a fluid pressure container comprising;

a main relieving valve;

said main valve having an inlet connected to a fluid pressure container, an outlet and pressure responsive means for controlling flow between said inlet and said outlet;

a pilot valve;

said pilot valve having an output of control pressure fluid (pressure) responsive to the pressure in said fluid pressure container;

means connecting said pilot valve to said pressure responsive means of said main valve to control the flow through said main valve responsive to the control pressure fluid from (output of )said pilot valve;

means connecting said outlet of said main valve to said pressure responsive means of said main valve whereby said main valve is held in closed position by fluid pressure in said outlet in excess of the fluid pressure at said inlet;

a back flow preventer connected between said pressure responsive means and each of said pilot valve connecting means and said outlet connecting means; and

said back flow preventer having a pressure responsive valve member being actuated to move responsive to the pressure in each of said connecting means to close communication between said pressure responsive means and. the one of said connecting means having the lowest fluid pressure whereby said pressure responsive means is maintained in communication with the one of said connecting means having the highest fluid pressure.

4. A pressure relieving system for a fluid pressure container comprising:

a main relieving valve;

said main valve having an inlet connected to the fluid pressure container to be protected, an outlet and pressure responsive means for controlling flow between a said inlet and said outlet;

(a pilot valve having an inlet valve and a vent valve, means supplying pressure fluid to said pilot valve from said fluid pressure container;)

a pilot valve having an inlet, a control port, a vent outlet, a vent valve controlling flow from said control port to said vent outlet, and an inlet valve seating in response to flow from said inlet to said control port upon opening of said vent valve;

means for supplying pressure fluid to said pilot valve inlet from said fluid pressure container;

said pilot valve having (an) a control port output of fluid pressure responsive to the pressure in said fluid pressure container;

said pilot valve also having a connection means for the connection of a test line therein;

said pilot valve inlet having a back flow valve seat coacting with said inlet valve whereby pressure fluid from said test pressure source is prevented from flowing through said supply means to said fluid pressure container.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3 558,706 Dated March 9 91] Inventor(s) Irvin B. Weise It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 6, line 5, cancel (pressure) line 7, cancel (to) line 33, cancel (pressure) line 38, cancel (output of) cancel lines 61, 62 and 63; line 71, cancel II II.

Signed and sealed this 31st day of October 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patent FORM PC1-1050 HILGQ)

Claims

1. A pressure relieving system for a fluid pressure container comprising: a main relieving valve; said main valve having an inlet connected to the fluid pressure container to be protected, an outlet and pressure responsive means for controlling flow between said inlet and said outlet; a pilot valve; means supplying control pressure fluid to said pilot valve from said fluid pressure container; said pilot valve having an output of control pressure fluid (pressure) responsive to the pressure in said fluid pressure containers; a fluid flow switching device connected (to) between said pressure responsive means of said main valve and said outlet of said main valve and between said main valve pressure responsive means and said pilot valve control pressure fluid outlet to deliver the greater of fluid pressure to said pressure responsive means from said pilot valve output and from the outlet of said main valve; and said pilot valve being preset to relieve control pressure fluid from the pressure responsive means of said main valve when the container fluid pressure reaches a preselected desired maximum whereby the fluid pressure in said container is relieved

2. A pressure relieving system according to claim 1, including; a check valve in said pilot valve to prevent reverse flow of fluid through said pressure fluid supplying means.

3. A pressure relieving system for a fluid pressure container comprising; a main relieving valve; said main valve having an inlet connected to a fluid pressure container, an outlet and pressure responsive means for controlling flow between said inlet and said outlet; a pilot valve; means supplying control pressure fluid to said pilot valve from said fluid pressure container; said pilot valve having an output of control pressure fluid (pressure) responsive to the pressure in said fluid pressure container; means connecting said pilot valve to said pressure responsive means of said main Valve to control the flow through said main valve responsive to the control pressure fluid from (output of )said pilot valve; means connecting said outlet of said main valve to said pressure responsive means of said main valve whereby said main valve is held in closed position by fluid pressure in said outlet in excess of the fluid pressure at said inlet; a back flow preventer connected between said pressure responsive means and each of said pilot valve connecting means and said outlet connecting means; and said back flow preventer having a pressure responsive valve member being actuated to move responsive to the pressure in each of said connecting means to close communication between said pressure responsive means and the one of said connecting means having the lowest fluid pressure whereby said pressure responsive means is maintained in communication with the one of said connecting means having the highest fluid pressure.

4. A pressure relieving system for a fluid pressure container comprising: a main relieving valve; said main valve having an inlet connected to the fluid pressure container to be protected, an outlet and pressure responsive means for controlling flow between a said inlet and said outlet; (a pilot valve having an inlet valve and a vent valve, means supplying pressure fluid to said pilot valve from said fluid pressure container;) a pilot valve having an inlet, a control port, a vent outlet, a vent valve controlling flow from said control port to said vent outlet, and an inlet valve seating in response to flow from said inlet to said control port upon opening of said vent valve; means for supplying pressure fluid to said pilot valve inlet from said fluid pressure container; said pilot valve having (an) a control port output of fluid pressure responsive to the pressure in said fluid pressure container; said pilot valve also having a connection means for the connection of a test line therein; a valve connected to said pilot valve in said test line; a test pressure source; opening of said valve-delivering pressure fluid from said test pressure source to said pilot valve to test the setting of said vent valve; and said pilot valve inlet having a back flow valve seat coacting with said inlet valve whereby pressure fluid from said test pressure source is prevented from flowing through said supply means to said fluid pressure container.