Method and apparatus for improving pump net positive suction head

A method and apparatus are disclosed for improving the available pump net positive suction head in a system for pumping liquid from a storage tank. When the liquid in the system is at or near its boiling point, a pump suction vessel is used to store and cool the liquid. The liquid in the pump suction vessel is kept at a temperature lower than its boiling point by surrounding the vessel with a jacketed space containing liquid from the storage tank. As the liquid in the jacketed space evaporates, the vapor is vented to the storage tank and more liquid is fed into the jacketed space.

The present invention relates generally to pumps for liquids and more 
particularly to a method and apparatus for improving pump available net 
positive suction head in a system for pumping liquid that is near its 
boiling point. 
BACKGROUND OF THE INVENTION 
A pump operates by drawing fluid at a low pressure from a suction line into 
a pump inlet and propelling fluid out of a pump outlet at a higher 
pressure or velocity. For proper operation each pump requires a net 
positive suction head (NPSH-R) which is the equivalent total head of 
liquid at the pump centerline less the vapor pressure of the liquid at the 
pump centerline. The pump manufacturer establishes the NPSH-R required for 
each pump. In an installation, the available net positive suction head 
(NPSH-A) must be equal to or greater than the NPSH-R of the pump. If the 
NPSH-A is not adequate, the pump may cavitate. Cavitation at start up may 
prevent the pump from pumping and may cause damage to the pump parts. 
Fluids near their boiling point have lower NSPH-A, making them difficult 
to pump. When the fluid in the suction line is colder than the ambient 
temperature, the heat leak into the suction line will warm the fluid and 
further reduce the NPSH. 
Various methods are used to increase the NPSH-A in a pumping system where 
the fluid is at or near its boiling point. Insulating the suction line 
decreases the rate of heat leak to the fluid, but if the pump is not 
operating, the heat leak will eventually cause the fluid to boil and the 
NPSH available to the pump at start up will be near zero. Unless the pump 
NPSH-R requirement is zero, the pump at start up will cavitate and cause a 
pumping failure, regardless of the head in a reservoir from which the 
fluid is pumped. Thus, after inactive periods, the initial pump start-up 
may be difficult or impossible. 
SUMMARY OF THE INVENTION 
The present invention provides a method and apparatus for improving the 
NPSH-A to a pump, particularly at initial start-up. 
Apparatus for improving available NPSH in accordance with the present 
invention includes a storage tank for supplying liquid, a pump suction 
vessel for receiving and storing liquid from the storage tank; a jacketed 
space at least partially surrounding the pump suction vessel and for 
receiving and storing liquid from the storage tank, a liquid level 
controller for the jacketed space, and a pump having an inlet, the inlet 
having means for receiving liquid from the pump suction vessel. 
The apparatus may include a hose for receiving liquid from a pump outlet, a 
liquid-vapor separator for receiving liquid from the storage tank and 
separating vapor from the liquid and feeding liquid to the pump suction 
vessel, conduits for returning vapor to the storage tank from the 
liquid-vapor separator, the jacketed space, or the pump. The liquid-vapor 
separator may be a float valve having means for venting vapor from the 
pump suction vessel. Further, there may be a drain for the jacketed space 
to drain liquid from the space. 
One way to maintain the level of liquid in the jacketed space is to provide 
a level sensor such as a differential pressure gauge that opens a valve to 
admit liquid from the storage tank into the jacketed space until the 
desired level is reached. 
A method in accordance with the present invention includes storing liquid 
in a storage tank, draining liquid from the storage tank into a pump 
suction vessel, surrounding at least part of the pump suction vessel with 
liquid from the storage tank to maintain the temperature of the liquid in 
the pump suction vessel at or lower than its boiling point, and pumping 
liquid from the pump suction vessel. 
The method may include separating vapor from the liquid before it enters 
the pump suction vessel. 
The step of surrounding at least part of the pump suction vessel with 
liquid from the storage tank may include draining liquid from the storage 
tank into a jacketed space that surrounds the pump suction vessel. The 
liquid in the jacketed space will be at its boiling point and colder than 
the liquid in the pump suction vessel. Heat transfer from the pump suction 
vessel liquid to the jacketed space provides the pump suction vessel 
cooling. Vapor may be recovered from the jacketed space, the pump, the 
liquid-vapor separator and returned to the storage tank.

DETAILED DESCRIPTION OF THE DRAWING 
FIG. 1 illustrates apparatus 10 for improving available pump net positive 
suction head (NPSH) where the liquid being pumped is at or near its 
boiling point. A storage tank 12 is constructed of a suitable material for 
safely storing a liquid 14 such as liquefied natural gas, propane, liquid 
nitrogen, and liquid carbon dioxide. The illustrated apparatus 10 is 
suitable for use as part of a liquefied natural gas (LNG) fueling station 
in which the tank 12 stores LNG at about 15 psig to 45 psig and 
corresponding saturation temperature of about -242.degree. F. to 
-222.degree. F. 
The tank 12 is preferably insulated in some suitable manner to reduce the 
rate of heat leak into the tank 12. Here the insulating feature is 
illustrated as a vacuum jacket 15 that defines a vacuum space 16. A vapor 
space 18 is inside the upper part of tank 12. Fill conduit 19 communicates 
with the interior of the tank 10 to provide a means for filling the tank 
with liquid 14. 
Tank drain conduit 20 communicates with the lower interior space of the 
tank 12 to provide a means for draining the tank 12 and includes tank 
drain valve 22. 
A conduit 26 downstream of the tank drain valve 22 transfers liquid 14 from 
the tank drain valve 22 to a liquid-vapor separator 30. The liquid-vapor 
separator 30 is optionally, but not necessarily, provided to draw off any 
vapor that may accumulate as a result of heat leak into conduits 20 and 
26. The vapor is returned to the vapor space 18 in the tank 12 via vapor 
return conduit 32. The liquid-vapor separator 30 can be any suitable 
mechanism and is preferably a float valve. 
A conduit 36 communicates with the liquid-vapor separator 30 to deliver 
liquid 14 downstream to a pump suction vessel 40 that is constructed of 
any suitable material for storing liquid 14 in much the same manner as the 
tank 12. The pump suction vessel 40 is at least partially surrounded by a 
jacket 42 to define a jacketed space 44. The pump suction vessel 40 
illustrated in FIG. 1 is completely surrounded by the jacket 42 and 
jacketed space 44, less of course, any openings necessary to make conduit 
or sensor connections. Outside of the jacketed space 44 there is 
insulation 46 to reduce the rate of heat leak to the vessel 40. 
Once liquid 14 is received into the pump suction vessel 40 it is maintained 
at a temperature less than its boiling point by filling the jacketed space 
44 with liquid 14 from the tank 12 via conduit 52 which communicates with 
conduit 26 to deliver liquid to a control valve 54 that controls the 
amount of liquid 14 that is supplied to the jacketed space 44. 
The control valve 54 is activated by a level sensor 60 so that liquid 14 
within the jacketed space 44 is maintained at a predetermined level. Level 
sensor 60 may be any suitable mechanism including, but not limited to, a 
differential pressure gauge, capacitance probes, sonic probes, optical 
sensors, or float switches, and includes means for generating a signal to 
open or close the control valve 54 as needed. 
Vapor generated in jacketed space 44, vapor is vented through conduit 66 
and vapor vent control valve 68. Vapor passes through the vapor vent valve 
68 to conduit 72 which communicates with vapor return conduit 32 to return 
vapor to the vapor space 18 in the tank 12. 
From time to time it may become necessary to drain liquid 14 from the 
jacketed space 44. This draining may be necessary when, for example, 
liquid 14 is liquefied natural gas, which includes methane, propane, 
ethane and other constituents. As liquid 14 absorbs heat, the "lightest" 
constituent, methane, will boil first, leaving the "heavier" constituents 
behind. Consequently, the boiling point of the remaining "heavier" liquid 
will be warmer than liquid 14. 
Thus, when pump 94 is not operating, the liquid in the jacketed space 44 
will need to be drained through conduit 78 which communicates with a lower 
part of the jacketed space 44. Valve 68 is closed causing the pressure in 
the jacketed space 44 to increase. Liquid 14 is drained through conduit 78 
and flows through a check valve 80 into conduit 92. After the liquid has 
been drained from space 44, valves 68 and 54 are opened and fresh liquid 
is added to space 44. 
A suction conduit 92 communicates with the lower right side of the pump 
suction vessel 40 to transfer liquid 14 downstream to the pump 94 which 
has an intake 96 and an outlet 98. As the pump 94 is activated, liquid is 
drawn from the pump suction vessel 40, through the suction conduit 92, and 
out of the outlet 98 where it flows under pressure into conduit 102. 
If the pump 94 is required to be in a continual operating stand-by mode 
such that it must constantly pump liquid 14, a by-pass conduit 134 is 
provided which communicates with conduit 102 upstream from valve 110. 
Valve 110 is shut-off while in stand-by mode to direct liquid 14 into 
conduit 134, through a by-pass valve 136 or other suitable flow 
restriction device which is open in stand-by mode, and through conduit 138 
so that pumped liquid 14 can be recycled back to the tank 12. 
In operation, the apparatus 10 can be used in LNG fueling service. The tank 
12 is filled with liquefied natural gas 14 or other appropriate liquid 
through fill conduit 19 using any conventional means. Opening the drain 
valve 22 allows liquefied natural gas 14 to flow through tank drain 
conduit 20, conduit 26, and into the liquid-vapor separator 30. The 
liquid-vapor separator 30 vents vapor that is generated by heat gain into 
conduits 20 and 26 to the vapor space 18 of the tank 12 and passes liquid 
14 to flow through conduit 36 and into the pump suction vessel 40. 
To cool liquid in the pump suction vessel 40, the control valve 54 is 
opened by the level sensor 60 and the jacketed space 44 is filled with 
liquid 14 from the tank 12. Liquid 14 slowly boils and vaporizes, the 
resulting vapor is vented through conduit 66, vapor vent control valve 68, 
conduit 72, and vapor return conduit 32, to be returned to the tank vapor 
space 18. In this way liquid 14 in the pump suction vessel 40 is cooled 
and maintained below its boiling point and the available net positive 
suction head is increased. 
As stated above, this higher net positive suction head prevents cavitation 
of the pump 94 particularly at start-up. With the apparatus just 
described, the pump suction vessel 40 and the pump 94 can be located 
remotely from the tank 12 so long as the pump 94 is near the pump suction 
vessel 40. 
If the liquid 14 in the tank 12 is liquefied natural gas then the liquid 14 
in the jacketed space 44 will require draining periodically because 
methane will boil off and leave behind a heavier mixture that will have a 
warmer boiling point. As the temperature in the pump suction vessel 40 
rises the NPSH available to the pump 94 is reduced. To avoid that, the 
jacketed space 44 is drained by closing valve 68 to allow the "heavy" 
liquid to drain through conduits 78 and 82 for delivery into suction line 
92. 
When liquid in the pump suction vessel 40 is cooled down, the pump 94 is 
activated to pump liquid. As stated above, the pump 94 may need to be on 
stand-by and circulate liquid 14 at all times. Under these circumstances, 
the valve 110 is closed and by-pass valve 136 is opened so that pumped 
liquid will simply recirculate through the apparatus 10. The valve 110 and 
the by-pass valve 136 may be opened and closed either manually or 
automatically using any suitable mechanism. 
The foregoing detailed description has been given for clearness of 
understanding only, and no unnecessary limitations should be understood 
therefrom, as modifications will be apparent to those skilled in the art.