Abstract:
A pressure build-up valve is disclosed which has heightened sensitivity and reduced overall valve size and is adapted to minimize pressure fall-off and to be mounted to a pressurized vessel storing a gas or a liquid. The value 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 fluid control system.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a pressure build-up valve. 
       BACKGROUND OF THE INVENTION 
       [0002]    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. 
         [0003]    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. 
       SUMMARY OF THE INVENTION 
       [0004]    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. 
         [0005]    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. 
         [0006]    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. 
         [0007]    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. 
         [0008]    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. 
         [0009]    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. 
         [0010]    Preferably the second resilient biasing means is a bellows with a preset spring rate in the range of 30-41 N/mm. 
         [0011]    Preferably, the pressure chamber housing has an upper and lower portion and a plurality of cylindrical apertures arranged circumferentially around said lower portion. 
         [0012]    Preferably, said pressure build-up valve inlet and outlet has standard butt weld ends allowing for easy welding onto flanges and directly onto piping. 
         [0013]    Other aspects are as set out in the claims herein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    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: 
           [0015]      FIG. 1 : is a perspective view of a pressure build-up valve in accordance with a preferred embodiment of the present invention; 
           [0016]      FIG. 2 : is a cross sectional cutaway view of the pressure build-up valve, in accordance with a preferred embodiment of the present invention; 
           [0017]      FIG. 3 : is an enlarged perspective view of the lower portion of the pressure build-up valve in accordance with a preferred embodiment of the present invention; 
           [0018]      FIG. 4 : is an enlarged view of the pressure chamber housing, attached to the guide piston, in accordance with a preferred embodiment of the present invention in an open position; 
           [0019]      FIG. 5 : is a cross sectional cut away view of the lower portion of the pressure build-up valve, in accordance with a preferred embodiment of the present invention in an open position; 
           [0020]      FIG. 6 : is a cross sectional cut away view of the lower portion of the pressure build-up valve, in accordance with a preferred embodiment of the present invention in a closed position; and 
           [0021]      FIG. 7 : is a view of some of the individual components of the pressure build-up valve. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0022]    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. 
         [0023]    Referring to  FIG. 1  herein there is illustrated a perspective view of a pressure build-up valve  100  in accordance with a preferred embodiment of the present invention. Said pressure build-up valve comprises a main body plate  101  having an upper part  102  and a lower part  103  and having an inlet  104  and an outlet  105  defined therein; a hollow substantially cylindrical bonnet  106 , which is fitted to the upper part  102  of main body  101 ; a set pressure adjusting screw  107 , rotatable about an axis and mounted on an upper part of the bonnet  106 ; said upper part  102  and lower part  103  being connected by four threaded bolts  108  and washers, which extend longitudinally through a plurality of apertures  109  in the respective upper and lower parts  102  and  103 . Preferably, the bonnet  106  is of phosphor bronze, and the pressure adjusting screw  107 , four threaded bolts  108  and washers are of stainless steel. The pressure adjusting screw is secured to the bonnet housing via two stainless steel nuts ( 110 ). 
         [0024]    Referring to  FIG. 2  herein, there is illustrated a cutaway view of the pressure build-up valve, in accordance with a preferred embodiment of the present invention. As illustrated the valve  100  comprises a valve body  201  having its interior divided by a transverse partition  202 , a pressure chamber  203  and a relief chamber  204 . Preferably, the valve body  201  is machine finished stainless steel. 
         [0025]    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 ( 208   a ). The opposite end of the bellows  211  is secured to transverse partition  202  via a PTFE seal ( 202   a ). 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. 
         [0026]    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. 
         [0027]    Preferably, the compression spring  214  is stainless steel and the bellows  211  comprises a stainless steel subassembly. 
         [0028]    Referring to  FIG. 3  herein there is illustrated an enlarged perspective view of the lower portion of the pressure build-up valve  300  in accordance with a preferred embodiment of the present invention, showing internal components of the valve. As illustrated threaded bolts  108 , extend longitudinally through apertures  109 , to connect the upper  102  part and lower part  103  of the valve. The lower part of the valve  103  has a substantially cylindrical shaped main body  301 , with a domed base  302 , said body  301  extending bi-directionally into two substantially symmetrical cylindrical openings, which form the valve inlet  104  and outlet  105 , each having a standard butt weld end, allowing for easy welding on to flanges and direct welding onto pipes. 
         [0029]    Referring to  FIG. 4  herein there is illustrated an enlarged view  400  of pressure chamber housing  207  movably connected to guide piston  208 , in accordance with a preferred embodiment of the present invention in an open position; said pressure chamber housing  207  comprises a substantially cylindrical body  401  having a plurality of cylindrical apertures  402  arranged circumferentially around the lower portion of the pressure chamber housing  207 . 
         [0030]    Referring to  FIG. 5  herein there is illustrated a cross sectional cut away view of the lower portion of the pressure build-up valve, in accordance with a preferred embodiment of the present invention in an open position  500 . As illustrated, the valve has an inlet  104  and an outlet  105 . In response to a fall in pressure in the pressure build-up in the valve, the guide piston  208  is not maintained in a first, elevated position, thus relieving the pressure on the elongate valve member  205 , and the complementary valve seating  210  carried by one end of a resilient biasing means, in this embodiment a pair of bellows  211 . Said bellows  211  therefore no longer compresses spring  214  and the bellows and spring each maintain fully expanded positions and the valve is maintained in an open position. 
         [0031]    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. 
         [0032]    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. 
         [0033]    Referring to  FIG. 6  herein, there is illustrated a cross sectional cut away view of the lower portion of the pressure build-up valve, in accordance with a preferred embodiment of the present invention in a closed position  600 . As illustrated, the valve has an inlet  104  and an outlet  105 . In response to an increase in pressure in the pressure build-up in the valve, the guide piston  208  is elevated in response to the pressure being exerted by fluid in the valve. The elevated guide piston  208  exerts pressure on the elongate valve member  205 , and the complementary valve seating  210  carried by one end of a resilient biasing means, in this embodiment a pair of compression spring bellows  211 . A second elongate valve member  212  extends from the valve seating  210  to engage with the valve seat  213  of a further resilient biasing means in the form of a spring  214 . Both the spring  214  and the bellows  211  are compressed. Subsequently, the valve flow  215  is completely shut off and fluid cannot flow into the pressure chamber  203 . When the pressure in the tank falls, the scenario in  FIG. 5  will be re-established, re-opening the pressure valve. 
         [0034]    Referring to  FIG. 7  herein, there is illustrated a view of some of the individual components of the pressure build up valve  700 ; said components including the pressure chamber housing  207  and guide piston  208 , compression spring  214 , a first  210  and second  213  valve seating and the pressure valve bonnet  106  with pressure adjusting screw  107 .