Saturated fluid pumping apparatus

A reciprocating pump which substantially eliminates cavitation problems without imposing capacity and flow limitations to maintain a net positive suction head. The pump comprises a cylinder having a liquid inlet and a liquid outlet, a piston reciprocably movable within the cylinder and generally intermediate the liquid inlet and liquid outlet, the piston having a liqud flow conduit therethrough generally co-axial with the cylinder, the liquid flow conduit having an inlet end in liquid communication with the cylinder liquid inlet and an outlet end in liquid communication with the cylinder liquid outlet, a piston rod reciprocably movable generally along the axis of the cylinder for reciprocably moving the piston within the cylinder in a direction toward the cylinder liquid outlet and in a reciprocal direction and a valve operatively associated with an intermediate the piston rod and the piston liquid flow conduit inlet end for alternatively opening and closing the inlet to liquid flow, the valve being closed when the piston rod and piston are moved in the direction toward the cylinder liquid outlet and being open when the piston rod and piston are moved in the reciprocal direction. In one embodiment, the valve comprises a valve disk freely movable on one end of the piston rod.

TECHNICAL FIELD 
The present invention relates to the technical field of mechanical 
apparatus for pumping liquids and, in particular, to pumps for saturated 
or near saturated liquids. 
BACKGROUND ART 
The term "saturated liquid" is used in this application to describe a 
liquid at its exact boiling point for a corresponding vapor pressure. A 
common situation which involves pumping saturated liquids occurs where the 
gas is the vapor phase of the liquid substance. Either a decrease of the 
pressure or an increase in the temperature will vaporize some of the 
liquid until a new boiling point equilibrium at a new vapor pressure is 
reached. This situation is found in the storage and transportation of 
cryogenic liquids, which exist in gaseous phase at normal ambient 
temperatures and pressures, but can be stored and transported in 
well-insulated low temperature containers in their liquid state, thus 
drastically reducing their volume without need for increasing the 
pressure. Liquid elements such as hydrogen, oxygen or nitrogen and 
liquified hydrocarbons such as methane or natural gas are common examples 
of cryogenic liquids. 
Reciprocating pumps are frequently used to move such cryogenic fluids 
between containers or to any other point of use. With the conventional 
pumps of the prior art, it has been found necessary to maintain a net 
positive suction head (NPSH) to prevent cavitation within the pump. Net 
positive suction head (NPSH) is the required additional pressure above the 
saturation vapor pressure of the liquid at any given temperature. 
Cavitation is the formation of vapor-filled cavities within the liquid 
caused in areas of the pump where the pressure of the moving liquid drops 
below the saturation vapor pressure. It typically occurs at the inlet 
valve of the prior art pumps, as the initial opening of the valve allows 
liquid to be drawn into an evacuated chamber. The pressure drop associated 
with the accelerated liquid produces vapor cavities, which become trapped 
in the cylinder below the piston. As the piston moves down on the 
compression stroke, the liquid pressure is increased above its saturation 
vapor pressure and the vapor cavities instantly condense back into liquid. 
The collapsing cavities generally cause significant shock, vibration, 
noise, and rapid erosion of metal surfaces, all of which can damage the 
pump. 
For brevity purposes, the remainder of this disclosure will refer only to 
saturated liquid or to a particular saturated liquid such as liquified 
natural gas. It should be understood that the invention likewise applies 
to near saturated liquids, which in the context of this application means 
a liquid sufficiently near saturation that cavitation is a potential 
problem when it is being pumped. 
DISCLOSURE OF THE INVENTION 
The present invention is a reciprocating pump which substantially 
eliminates cavitation problems without imposing capacity and flow 
limitations to maintain a net positive suction head. The pump comprises 
cylinder means having liquid inlet means and liquid outlet means, piston 
means reciprocably movable within the cylinder means and generally 
intermediate the liquid inlet means and liquid outlet means, the piston 
means having a liquid flow conduit therethrough generally co-axial with 
the cylinder, the liquid flow conduit having an inlet end in liquid 
communication with the cylinder means liquid inlet means and an outlet end 
in liquid communication with the cylinder means liquid outlet means, 
piston rod means reciprocably movable generally along the axis of the 
cylinder means for reciprocably moving the piston means within the 
cylinder means in a direction toward the cylinder means liquid outlet 
means and in a reciprocal direction, and valve means operatively 
associated with and intermediate the piston rod means and the piston means 
liquid flow conduit inlet end for alternately opening and closing the 
inlet to liquid flow, the valve means being closed when the piston rod 
means and piston means are moved in the direction toward the cylinder 
means liquid outlet means and being open when the piston rod means and 
piston means are moved in the reciprocal direction. 
In a particular embodiment of the invention, the novel pump comprises a 
piston having a hollow conical bore which makes a close conforming fit 
over a conical discharge valve housing extending from the bottom of the 
cylinder when the piston reaches the bottom of its stroke. The top rim of 
the piston bore also serves as an inlet valve seat for an inlet valve disk 
which is completely opened by the piston rod before the piston begins to 
rise away from the bottom conical housing. As the piston begins to rise, 
it opens a pumping chamber between the piston bore and the bottom housing 
into which the saturated liquid smoothly flows under the gravitational 
force of the overlying fluid column. Any cavitation is thus largely 
eliminated, since the hollow piston is moving upward through the 
essentially static liquid, and if any minor liquid vaporization occurs, 
the resulting bubbles are allowed to migrate upward along the conical 
walls of the bore and out the still-open inlet valve, rather than to 
collapse in the pumping chamber. 
The complete opening of the inlet valve prior to upward movement of the 
piston is accomplished by coupling the piston to the push rod by a cage 
means. Flanged posts extend upward from the piston in a cage arrangement 
surrounding the top end of the conical bore, and the post flanges are 
adapted to engage a circumferential flange on a valve disk which is 
attached to the push rod. Thus, the initial upward movement of the push 
rod lifts the valve disk to its fully open position while the piston 
itself remains seated over the conical housing in the cylinder. No 
evacuated chamber is created for the liquid to fill until the valve disk 
engages the cage and begins to raise the piston, at which time the inlet 
valve is completely open, thus eliminating the restrictive nozzle effect 
of cracking open an inlet valve to an evacuated chamber. 
The valve disk is attached to the push rod by a ball joint which allows the 
disk to swivel into flat alignment over the top rim of the conical bore to 
effect a tight seal and eliminate wear caused by misalignment.

BEST MODE FOR CARRYING OUT THE INVENTION 
FIG. 1 depicts a partial cross section of a preferred embodiment of a pump 
10 submerged in a saturated liquid such as liquified natural gas (LNG). 
Those familiar with the storage and transport of LNG will understand, 
without drawings or detailed description, that the pump 10 will be located 
within a high pressure cryogenic tank needed to keep the material in a 
liquid state. The familiar reader will also understand that the pump 10 
will be located within a cryogenic container needed to keep the material 
in a liquid state and with the cylinder submerged under the level of the 
liquid. The familiar reader will also understand that the pump 10 
discharge port, described in greater detail below, will be associated with 
a transport line or distribution manifold to move the LNG to other tanks 
or to any other point of use. Since the configuration of such associated 
elements may vary, and no specific configuration is required, they are not 
depicted in the drawings. 
Pump 10 is a positive displacement reciprocating pump comprising a cylinder 
12 and reciprocating piston 14. The piston 14 is depicted at the top of 
its stroke in FIG. 1. A plurality of inlet openings 16 are evenly spaced 
around the upper circumference of the cylinder 12 to allow the liquid to 
flow into the cylinder above the piston 14. 
The piston 14 is a cylindrical block 18 having a hollow frusto-conical bore 
20 (hereafter "conical bore" for brevity) oriented with its base at the 
bottom of the block 18 and tapering to open at the top where the block 18 
is raised in a circular boss 22 to provide a sealing seat for an inlet 
valve means 24. 
The valve means 24 comprises a cylindrical stem 26 with a circumferential 
disk 28 at its base to provide a flat sealing face 30 which, when seated 
against the boss 22, closes the conical bore 20 to liquid passage. The 
stem 26 is coupled to a push rod 32 by a ball coupling comprising a short 
rod extension 34 terminating in a ball 38. The stem 26 has a hollow 
chamber 40 with hemispherical bottom sides conforming to the dimensions of 
the ball 38, and an opening through which the rod extension 34 passes to 
connect to the push rod 32. The stem 26 and disk 28 are thus permitted to 
swivel around the ball 38 to seek a flat seating of its sealing face 30 
against the boss 22. The reader will understand that push rod 32 will be 
reciprocated by an appropriate engine and gearing to provide pumping 
power. 
The absence of a rigid connection between the long push rod and the piston 
eliminates stresses which exist in other pumps due to machining, welding 
and assembly tolerances combined with distortions (uneven shrinkage) when 
cooled to cryogenic temperatures, which result in side loads on and an 
early failure of piston rings and seals. This "free floating" piston, 
however, centers itself within the cylinder bore without causing uneven 
side forces on the piston rings and seals. 
The valve means 24 is operatively connected with the piston 14 by a cage 42 
comprising multiple posts 44, each extending upward from the piston block 
18 and having an inward facing flange 46 extending over the 
circumferential disk 28 of the valve stem 26. The cage 42 allows the valve 
disk 28 to open on the push rod upstroke with the piston remaining unmoved 
until the disk 28 engages the cage flanges 46; thereafter the piston is 
pulled up by the push rod to the top-of-stroke position depicted in FIG. 
1. In this position, liquid flowing into the cylinder through the inlet 
openings 16 passes around the cage posts 44 and through the conical bore 
20 to fill the portion of the cylinder below the piston, as indicated by 
the directional arrows in FIG. 1. 
The base of the cylinder 12 includes a frusto-conical valve housing 50 
(hereafter "conical housing" for brevity) dimensioned to conform to the 
conical bore 20 such that, when the piston is at the bottom of its stroke, 
the conical housing 50 substantially fills the bore 20, as depicted in 
FIG. 2. Within the conical housing 50 is a discharge valve, comprising an 
outlet channel 52 opening to a wider channel 56 by a tapered intermediate 
section 54 which acts as a seat for a ball valve 60. A spring 62 pushes 
the ball valve 60 against seat to close the outlet channel. When the 
descending piston compresses the liquid trapped between the conical bore 
20 and housing 50, the increased fluid pressure is transmitted to the ball 
valve 60 and compresses the spring 62 sufficiently to allow the trapped 
liquid to discharge through the outlet channel. A removable appropriate 
fitting 64 is provided for connecting the outlet channel to an appropriate 
transfer pipe. 
The piston 14 also has conventional guide rings 66 and seal rings 68. If 
desired, a resilient "friction ring" 70, compatible with cryogenic 
service, can be provided which prevents the piston from sliding downward 
under its own weight before the valve disk "catches up" with the valve 
seat on the piston and builds up pressure in the pumping chamber. 
The operating sequence of the pump 10 is apparent from the above 
description, but the reader should particularly consider the advantages 
this apparatus provides in pumping a saturated liquid without creating 
cavitation bubbles. Note first that the liquid flows into the top of the 
cylinder solely by the gravity induced pressure of the liquid column. When 
the piston is at the bottom of its stroke, the inlet valve is closed and a 
column of liquid sits on top of the piston, as shown in FIG. 2. The 
initial upward movement of the push rod opens the inlet valve but does not 
move the piston. When the piston begins to move upward, the only pressure 
differential is that created by the increased fluid velocity as the liquid 
falls through the conical bore into the cavity formed below. If any vapor 
bubbles do form, due to the heat of the piston seal friction, they will be 
allowed to escape up along the conical bore walls and out the inlet valve, 
since there is no place for bubbles to be trapped between the conical 
surfaces. 
INDUSTRIAL APPLICABILITY 
The described pump will have primary application in the cryogenic industry 
where the end use requires cryogenic liquids in low or moderate quantities 
but at a medium to very high pressures, and particularly where a net 
positive suction head is not available or difficult to attain, or where 
the pump is required to operate intermittently. 
The present invention may be embodied in other specific forms without 
departing from the spirit or essential attributes thereof and, 
accordingly, reference should be made to the appended claims, rather than 
to the foregoing specification, as indicating the scope of the invention.