The present invention relates to a pressure build-up valve.
Pressure build-up valves function to control the build-up of pressure in a controlled environment. The pressure is relieved by allowing the pressurized substance to flow out of the system by an alternative means when the valve is open. A typical pressure build-up valve comprises: at least one gas or liquid containing chamber, a movable valve member, a pressure responsive contractible element and a bonnet.
Prior art pressure build-up valves are known in which a pressure responsive contractible element comprises a set of bellows, for example Howlett (U.S. Pat. No. 2,965,121, 1960). The bellows provides a pressure response valve which combines the functions of a pressure-closing valve with the functions of a pressure-opening valve. However, recent known problems with pressure build up valves include a poor level of sensitivity as identified by McManigal in (U.S. Pat. No. 5,186,209, 1993). Furthermore, in pressure build-up applications a correlation is evident whereby the size of the pressure responsive contractible elements usually determines the sensitivity of the valve to perform pressure control.
The present invention relates to a pressure build-up valve adapted to minimize pressure fall-off and to be mounted to a pressurized vessel storing a gas or a liquid. The valve is particularly, although not exclusively, designed to build and maintain pressure in cryogenic transportation trailer and storage tanks and is primarily for use with a closed loop liquefied natural gas (LNG) fluid control system. As a liquid or gas is drawn from a closed loop fluid control system, the pressure inside the system will fall. To compensate for this, the pressure build-up valve will open to allow liquid to pass through a pressure build-up coil, restoring the system to its normal working pressure.
The flow of fluid is governed by the pressure differential created by the liquid head within a trailer or storage tank. The maximum pressure differential can be achieved in a storage tank which can accommodate LNG to a height of 10 m, giving a corresponding pressure of 6.5 PSI. This low pressure differential accommodates low flow velocities and Low Reynolds number flow with low pressure fluctuations and has allowed the pressure build-up valve to be designed with a unique pressure sensing mechanism allowing for significantly low spring rates to be used for the pressure setting spring.
It is an object of the current invention to provide a pressure build-up valve which is capable of delivering increased sensitivity in combination with a reduced valve size. This is achieved by the inclusion of a preset spring rate which enables the size of the valve to be reduced and heightens the sensitivity of the valve in comparison to the prior art.
According to a first aspect of the present invention, there is provided a pressure build-up valve comprising: a valve body having an inlet and an outlet; a valve bonnet and adjusting means mounted thereon; a pressure chamber housing; a pressure chamber and a relief chamber; a movable valve seat; a piston; a first resilient biasing means having a preset spring rate and located adjacent to said piston and adjacent to the a second resilient biasing means; a second resilient biasing means having one end abutting the valve seat and an opposed end abutting the adjusting means; said first and second resilient biasing means being arranged in series and configured to operate in parallel; whereby, in response to a drop in pressure within the valve, the guide piston is lowered relieving pressure on the first resilient biasing means, causing it to expand; the expanded first resilient biasing means biases the second resilient biasing means to open the valve and divert the flow of fluid from the pressure chamber into the relief chamber. As such, the valve is closed, and will only open once the pressure in the system returns to its normal working pressure.
In response to an increase in pressure within the valve, the guide piston is elevated by the liquid head within the valve compressing the first resilient biasing means; the compressed first resilient biasing means biases the second resilient biasing means to close the valve and prevent the flow of fluid from the pressure chamber into the relief chamber.
Preferably the pressure build-up valve is for use in cryogenic transportation trailer and storage tanks; for use with a closed loop liquefied natural gas (LNG) fluid control system.
Preferably the second resilient biasing means is a bellows with a preset spring rate in the range of 30-41 N/mm.
Preferably, the pressure chamber housing has an upper and lower portion and a plurality of cylindrical apertures arranged circumferentially around said lower portion.
Preferably, said pressure build-up valve inlet and outlet has standard butt weld ends allowing for easy welding onto flanges and directly onto piping.
Other aspects are as set out in the claims herein.
For a better understanding of the invention and to show how the same may be carried into effect, there will now be described by way of example only, specific embodiments, methods and processes according to the present invention with reference to the accompanying drawings in which:
There will now be described by way of example a specific mode contemplated by the inventors. In the following description numerous specific details are set forth in order to provide a thorough understanding. It will be apparent however, to one skilled in the art, that the present invention may be practiced without limitation to these specific details. In other instances, well known methods and structures have not been described in detail so as not to unnecessarily obscure the description.
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An elongate valve member 205 extends through a central aperture 206 of the pressure chamber housing 207 to interact with a guide piston 208 housed within a bottom piston guide 209. Preferably, the pressure chamber housing is stainless steel; the guide piston is hi-tech brass and the bottom piston guide is phosphor bronze. Guide piston 208, through elongate valve member 205 engages with a complementary valve seating 210 carried by one end of a resilient biasing means, in this embodiment a compression spring bellows 211. Guide piston 208 is secured to valve seating 210 by PTFE seals (208a). The opposite end of the bellows 211 is secured to transverse partition 202 via a PTFE seal (202a). A second elongate valve member 212 extends from the valve seating 210, above the pressure chamber housing 207 to engage with the valve seat 213 of a further resilient biasing means in the form of a compression spring 214, which operates as a pressure setting spring, to adjust the pressure at which the valve operates. Preferably the valve seats 210 and 212 are hi-tech brass.
In the preferred embodiment of the invention, bellows 211 has a specifically designed spring rate within a range of 30-41 N/mm and the compression spring 214 and bellows 211 are arranged in series but are configured to operate in parallel. The predetermined spring rate and the parallel interactions of the spring 214 and bellows 211 allow a very low spring rate for the pressure setting spring 214 to be selected, thus allowing a smaller pressure sensing area. For example, in the preferred embodiment the valve diameter is 46 mm compared to a similar marketed product which has a diameter of 286 mm. Thus, the pressure build-up valve delivers increased sensitivity in combination with a reduced valve size.
Preferably, the compression spring 214 is stainless steel and the bellows 211 comprises a stainless steel subassembly.
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The pressure build-up valve can be installed before or after a vaporizer coil such that the pressure sensing element is equally responsive to either liquid natural gas or gas with a similar pressure fall-off/proportional band being maintained. Considering an example in which the pressure build-up valve is installed on a trailer with a gravity driven pressure transfer system and utilizes an ambient air vaporizer located after the pressure build-up valve in the open position, fluid passes from the inlet 104 through the pressure build-up valve into an ambient air vaporizer, the vaporizer heats the liquid natural gas, flowing at a temperature of approximately −165° C., such that the liquid natural gas changes phase into a gas at operating conditions of approximately 65 psi and −139.7° C. The gas then flows into the top of the trailer causing the pressure to increase. When the pressure in the system reaches the pressure build-up valve set pressure, the valve will close, preventing the supply of the liquid natural gas to the vaporizer.
It is understood that the pressure build-up valve may be used with any type of vaporizer including but not limited to: open rack vaporizers, submerged combustion vaporizers and intermediate fluid vaporisers.
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