RELIEVING VALVE WITH SURGE CONTROL FOR FLUID STRORAGE TANK

Abstract: A relieving valve (10) for a fluid or liquid storage tank (12) to permit release of pressure from inside the tank as necessary. A piston (42) is movable within the valve (10) to open the same to atmosphere as pressure builds up slowly therewithin. Upon occurrence of a sudden or rapid pressure rise in the tank (12), such as when the fluid therein surges against the valve (10), a snubber assembly (58) in the valve is moved by said pressure rise to a partially closed position to impede the rate at which pressure can build within the valve (10) and thus increase the time before which the valve opens to atmosphere.

Description: This invention relates generally to pressure relief valves for fluid storage tanks and, more particularly, to such a valve which includes a snubber assembly to prevent the valve from opening for a short period of time when the valve is subjected to a sudden or rapid pressure rise, such as occurs in a surge of the fluid retained in such tanks. Tanks for retaining fluid, particularly flammable or corrosive liquids, commonly are provided with a pressure relief valve so as to limit the internal pressure in the tank to a predetermined value for safety. Under normal conditions when the pressure inside the tank rises slowly as in normal storage of flammable liquids, such valves are opened at a predetermined pressure so as to relieve the excess pressure in the tank by venting to atmosphere. Thereafter, when the pressure inside of the tank is reduced to a safe level, the valve is closed to maintain the tank under desired pressure. There are many examples of such prior art pressure relief valves. It is known to provide pressure relief vents, for example in tankers, where normal pressure relief is prevented in the event of a pressure above a predetermined level being present. For example, US-A-4501377 discloses a manhole cover for a mobile tank which includes a fluid-activated piston and cylinder arrangement to cause movement of the cover to vent the tank in the event of over pressure. A valve in a fluid line leading to the piston and cylinder arrangement is closed by pressure above a certain level to prevent opening of the cover if excessive pressure exists. The object of this arrangement in US-A-4501377 is to prevent opening of the cover in the event of the tank rolling over, or a similar accident occurring, but a difficulty with this may arise if the excessive pressure exists for a sustained period as venting would then be prevented. It has been proposed, therefore, in US-A-3886969 to provide a pressure relief valve which permits venting only after a predetermined time delay, the aim being to keep the valve closed for a set, short time typically experienced during a tank roll-over, but then to allow venting if pressure above a predetermined level still exists. The valve of US-A-3886969 is a time-delay vent valve and includes a housing having a vent opening communicating with atmosphere, a chamber and a spool-shaped piston within the chamber. The larger end of the spool-shaped piston is acted upon by pressure within an accumulator and the small end of the piston forms a seal which, in normal conditions, closes the vent opening. The piston is provided with a flow passage to communicate the valve inlet with the accumulator, but includes a flow restrictor in the passage. Thus, in the event of excessive pressure, venting will occur only after a time delay, dependant upon the flow restrictor characteristics, sufficient for pressure to build in the accumulator to unseat the valve. This valve, however, will always vent with a time delay; the time delay will not take effect just where surge pressure exists. It is desirable for a valve to exhibit rapid pressure relief in normal over pressure, but for a time delay to exist in the case of a surge pressure. According to the invention as defined in claim 1, there is provided a valve for relieving pressure from within a tank at a predetermined tank pressure comprising a housing retained on the tank and having an internal portion disposed within the tank and an external portion disposed outside of the tank in atmosphere and at least one vent opening in said external portion being in permanent communication with atmosphere and being capable to communicate via a passageway in said internal portion with the inside of the tank, an expansion chamber in the valve defined by the housing and a pilot piston being movable in the housing between a closed valve position and an open valve position, a poppet affixed to an end of the pilot piston adjacent said internal portion either to close or to open the valve and said vent opening, said pilot piston including a snubber valve chamber between said expansion chamber and the tank separate from and fluidly unconnected with said vent opening, a separate snubber valve assembly for controlling rapid pressure increases within said tank, said snubber valve being movably retained within said snubber valve chamber, being capable of movement independent of said pilot piston and having a plurality of openings therein to permit passage of the contents of the tank therethrough into said expansion chamber to cause movement of said pilot piston from said closed to said open position after a predetermined time delay after a rapid pressure increase within said tank above said predetermined tank pressure, to vent said tank only through said vent opening and including means for moving said pilot piston from said open to said closed position when said tank pressure falls below said predetermined tank pressure. The valve functions as a surge control in the event of sudden pressure rise or surge of liquid in the tank. Liquid surging against the valve causes the snubber assembly to move to a partially closed position in which the passageways are blocked and the only path available for movement of liquid into the valve is through the reduced dimension orifice. Surging liquid thereby moves into the expansion chamber at a slowed rate, providing for extended time for the chamber to fill and move the piston to its open position. The time for movement of the valve is adjustable by variation of the size of the orifice, the volume of the expansion chamber, and the force of springs which are operable against the valve and the snubber assembly. The preferred embodiment of this invention will now be described by way of example, with reference to the drawings accompanying this specification in which: Fig. 1 is a medial sectional view taken through the valve of the invention installed upon a fluid storage tank; Fig. 2 is a sectional view similar to that of Fig. 1, illustrating the open condition of the valve and the fluid and gas flow pattern during pressure release; Fig. 3 is a similar sectional view illustrating the surge control condition of the valve; and Fig. 4 is a side elevational view of said valve installed upon such a tank. The valve 10 of the invention is shown mounted or installed on the shell 12 of a tank by bolts 14 which pass through apertures in the shell and into threaded openings in the body 16 of the valve. A gasket 18 is positioned between the valve body and the tank shell to seal the installation. Thus, an upper or external portion 20 of valve 10 is disposed outside of the tank and above shell 12 in atmosphere and a lower or internal portion 22 of the valve is disposed within the tank and below shell 12. External portion 20 of valve 10 includes vent openings 24 which, when the valve is moved to its opened position described below, permit gases and liquid from within the tank to move to atmosphere. Valve body 16 includes a centrally located hub 26 with radial supports 28 extending to peripheral wall 30. Upper external portion 20 is closed by cap 32 having a peripheral groove 34 for retaining an O-ring 36 to seal the cap within the upper external portion. Cap 32 is retained within portion 20 by retaining ring 38 which seats in groove 40 provided in portion 20. A piston 42 with poppet 44 is positioned within the valve body 16. Piston 42 is formed with a peripheral groove 45 for retaining an O-ring 46 to seal the piston against the inner-facing surface 48 of upper portion 20. Poppet 44 is positioned upon the lower end 49 of piston 42 adjacent gasket 50 which is maintained in contact with poppet 44 by gasket retainer 52. Poppet 44 is rigidly secured to piston 42 against gasket 50 and retainer 52 by retaining nut 54 threadedly engaged upon the lower end 49 of piston 42. Lower end 49 of piston 42 is formed with a chamber 56 within which snubber surge valve assembly 58 is movably positioned. Snubber assembly 58 is retained within chamber 56 by retaining ring 60 which seats in groove 62 formed in the wall of chamber 56. Spring 63 is operable against flange 64 of snubber assembly 58 and shoulder 66 which defines a reduced dimension channel 68 that opens from chamber 56 into piston expansion chamber 70 in the upper portion 20 of valve body 16. Main valve spring 72 is restrained in compression and operable against piston 42 and hub 26 to urge piston 42 into its normally closed position shown in Fig. 1 adjacent cap 32. Snubber surge valve assembly 58 is formed with two chambers 74, 76 disposed cojacent each other with wall 78 therebetween. Open end 79 of chamber 74 is defined by flange 64 and faces the internal confines of tank shell 12. Passageways 80 are formed in the walls 82, 84 of chambers 74, 76, said passageways opening into chamber 56 in which snubber assembly 58 is retained. Open end 86 of chamber 76 is positioned within channel 68 and faces the internal confines of piston expansion chamber 70. Orifice 88 of lesser cross-sectional dimension than each of passageways 80 is formed within wall 78 between chambers 74,76. There is a single orifice 88 and, for purposes of illustration only, there are four passageways 80 in each chamber 74,76. The number of passageways 80 may be varied, as desired. The operation of valve 10 is as follows. As pressure rises in the tank during filling or normal temperature variations, gases will move from the tank through open end 79 of chamber 74 in snubber assembly 58, through passageways 80 and orifice 88, into chamber 76 and piston expansion chamber 70 as shown by arrows A in Fig. 2. Increasing pressure in expansion chamber 70 results in forces illustrated by arrows B exerted against piston 42 to move the same downwardly against the force of main valve spring 72 and the tank pressure action against poppet 44. As a result, poppet 44 is opened to permit the gases and liquid in the tank to escape to atmosphere by the pattern shown by arrows C through vent openings 24. When the pressure in the tank is equalized with that of atmosphere, the force of spring 72 returns piston 42 to its closed position adjacent cap 32 as shown in Fig. 1. The surface area of piston 42 is greater than that of poppet 44 to facilitate the opening or valve 10 as pressure rises in expansion chamber 70. Thus, the poppet 44 will open as shown in Fig. 2 when the force on piston 42 becomes greater than the combined forces exerted against poppet 44 and that of spring 72. The pressure value required to open valve 10 can be predetermined by designing the force of spring 72 in the closed position of piston 42 (Fig. 1) to be equal to the difference in area of piston 42 and poppet 44 multiplied by the design pressure of the tank. Valve 10 functions as a surge control in the following manner to prevent the valve from opening for a short time period when the valve is subjected to sudden or rapid pressure rise, such as occurs in a surge. Referring to Fig. 3, when pressure in the tank rises suddenly, snubber surge valve assembly 58 is moved upwardly against the force of spring 63 as seen in the figure with chamber 76 disposed within channel 68 to close off passageways 80 in said chamber 76. As a result, surging liquid and gases can move into piston expansion chamber 70 only through reduced dimension orifice 88, thus slowing the time during which expansion chamber 70 can receive sufficient pressure to cause piston 42 to move downwardly to open position as seen in Fig. 2. As piston 42 moves downwardly, gases and liquid are permitted to escape from the tank in the manner shown in Fig. 2, but the time for this to occur is less than that which takes place under non-surge conditions. The time for the pressure in expansion chamber 70 to rise is controlled by the size of orifice 88, the volume of the expansion chamber, and the forces of spring 72 and 63. Snubber surge valve assembly 58 returns to its position shown in Figs. 1 and 2 by reason of the force of spring 63 after valve 10 moves to open position (Fig. 2). It will be appreciated from the above specification that when valve 10 is subjected to steady pressure rise such as occurs in normal operation, the valve will open and maintain the pressure in the tank at a predetermined desired level. When valve 10 is subjected to a sudden pressure rise, such as occurs in a tank when it is overturned, the valve will remain closed a sufficient length of time to prevent spillage of the contained liquid.

Claims: A valve for releasing pressure from within a tank at a predetermined tank pressure comprising a housing (16) retained on the tank and having an internal portion (22) disposed within the tank and an external portion (20) disposed outside of the tank in atmosphere and at least one vent opening (24) in said external portion (20) being in permanent communication with atmosphere and being capable of communicating via a passageway in said internal portion (22) with the inside of the tank, an expansion chamber (70) in the valve defined by the housing and a pilot piston (42) being movable in the housing between a closed valve position and an open valve position, a poppet (44) affixed to an end (49) of the pilot piston adjacent said internal portion (22) either to close or to open the valve and said vent opening, said pilot piston including a snubber valve chamber (56) between said expansion chamber (70) and the tank separate from and fluidly unconnected with said vent opening (24), a separate snubber valve assembly (58) for controlling rapid pressure increases within said tank, said snubber valve assembly being movably retained within said snubber valve chamber (56), being capable of movement independent of said pilot piston (42) and having a plurality of openings (80, 86, 88) therein to permit passage of the contents of the tank therethrough into said expansion chamber (70) to cause movement of said pilot piston (42) from said closed to said open position after a predetermined time delay after a rapid pressure increase within said tank above said predetermined tank pressure, to vent said tank only through said vent opening and including means (72) for moving said pilot piston from said open to said closed position when said tank pressure falls below said predetermined tank pressure. A valve as claimed in Claim 1 wherein said snubber valve assembly (58) is formed of two snubber valve assembly chambers (74, 76) disposed cojacent each other with a wall (78) therebetween. A valve as claimed in Claim 2 wherein said openings in said snubber valve assembly include a plurality of passageways (80) formed in the walls (82, 84) of said snubber valve assembly chambers (74, 76), and an orifice (88) of smaller cross-sectional dimension than each of the passageways formed within the wall (78) between said snubber valve assembly chambers. A valve as claimed in Claim 3 including a channel (68) opening between said snubber valve assembly chambers (74, 76) and said expansion chamber (70), one of said snubber valve assembly chambers being movable within said channel. A valve as claimed in Claim 4 wherein said snubber valve assembly (58) is moveable between a first position in which the contents of the tank (12) are permitted to move through said passageways (80) and said orifice (88) into said expansion chamber (70) and a second position in which the passageways in said one snubber valve assembly chamber (76) are blocked to prevent movement of the contents of the tank therethrough and said contents can move only through said orifice into said expansion chamber. A valve as claimed in any one of Claims 1 to 5 wherein said snubber valve assembly (58) is maintained in its first position by a spring (63) engaged against said snubber valve assembly within the snubber valve chamber (56). A valve as claimed in any one of Claims 1 to 6 wherein the pilot piston (42) is maintained in its first position by a valve spring (72) engaged against the pilot piston within the housing (16). A valve as claimed in Claim 7 wherein the surface area of said pilot piston (42) is greater than that of said poppet (44). A valve as claimed in Claim 8 wherein the pressure required to open the valve (10) is determined by equating the force of said valve spring (72) to the difference in the surface area of the pilot piston and poppet (44) multiplied by the design pressure of the tank (12). A valve as claimed in Claim 3, including a channel (68) opening between said snubber valve assembly chambers (74, 76) and said expansion chamber (70) wherein said snubber valve assembly chambers (74, 76) are movable within said channel (68), and said snubber valve assembly (58) is movable between a first position in which the contents of the tank are permitted to move through said passageways (80) and said orifice (88) into said closed expansion chamber (70) and a second position in which the passageways in said one Snubber valve assembly chamber (76) are blocked to prevent movement of the contents of the tank (12) therethrough and said contents can move only through said orifice into said closed expansion chamber, said snubber valve assembly being maintained in its first position by biasing means (63) engaged against said snubber valve assembly within said snubber valve chamber (56) and the pilot piston (42) being maintained in its first position by biasing means (72) engaged against the pilot piston within said housing (16), the surface area of said piston being greater than that of said poppet (44) and the pressure required to open the valve being determined by the force of said pilot piston biasing means being equal to the difference in the surface area of the piston and poppet multiplied by the design pressure of the tank.

Inventors: MORRIS, CARROL VERNON
Applicants: KNAPPCO CORPORATION
Application year: 1987
Main classification: F16K17
IPC codes: F16K17, B65D90