U.S. patent number 3,697,199 [Application Number 04/850,201] was granted by the patent office on 1972-10-10 for slide valve pump.
Invention is credited to Harry L. Spears.
United States Patent |
3,697,199 |
Spears |
October 10, 1972 |
SLIDE VALVE PUMP
Abstract
A cylindrical piston fixed to the lower end of a cylindrical rod
and a second tubular piston slideably carried over the rod. The
pistons and a portion of the rod are disposed within the bore of a
tubular housing. The housing includes inlet and outlet ports
adjacent to its lower and upper ends, respectively, which extend
from the bore through the housing wall. Reciprocating movement of
the rod through the bore causes the fixed and tubular pistons to
cyclically seal and unseal the inlet and outlet ports respectively
whereby fluid is drawn into the inlet ports and expelled through
the outlet ports.
Inventors: |
Spears; Harry L. (Houston,
TX) |
Family
ID: |
25307538 |
Appl.
No.: |
04/850,201 |
Filed: |
August 14, 1969 |
Current U.S.
Class: |
417/498; 417/501;
417/430 |
Current CPC
Class: |
F04B
53/14 (20130101) |
Current International
Class: |
F04B
53/00 (20060101); F04B 53/14 (20060101); F04b
007/04 (); F04b 039/10 () |
Field of
Search: |
;417/430,478,488,489,490,455,228,501 ;103/155,129 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Croyle; Carlton R.
Assistant Examiner: Gluck; Richard E.
Claims
I claim:
1. A fluid pump comprising:
a. housing means having first and second axially spaced ends;
b. axially spaced outlet and inlet means opening into said housing
means adjacent said first and second housing means ends
respectively;
c. piston lifting means disposed within said housing means and
movable axially through a piston bore in said housing means to form
a sliding seal with a portion of said piston bore for moving fluid
into said housing means through said inlet means and out of said
housing means through said outlet means;
d. drive means secured to said lifting means for moving said
lifting means axially through said housing means;
e. valving means having first and second axially spaced ends
disposed within said housing means and movable axially within a
valve bore in said housing means to form a sliding seal with a
portion of said valve bore to open and close said outlet means;
f. a closed bore section extending axially between the axially
uppermost opening in said inlet means and the axially lowermost
sealing point of said valve bore and valving means when said
valving means is at the lowest axial point in said valve bore;
g. an axially extending opening formed between said first and
second ends of said valving means;
h. an elongate rod means included in said driving means, extending
into said first housing means end and through said valving means
opening and being axially movable with respect to said valving
means;
i. sealing means included between said valving means and rod means
for forming a sliding seal between said valving means and said rod
means;
j. said valving means including a substantially tubular valve body
having an axially extending cylindrical outer surface formed along
at least a portion of said valve body;
k. first and second axially spaced restricting means included in
said housing means for limiting the axial travel of said valve body
in said housing means;
l. said lifting means including a piston body having first and
second axially spaced ends and a substantially cylindrical outer
surface formed along at least a portion of said piston body;
m. third and fourth restricting means included in said housing
means for limiting the axial travel of said piston body in said
piston bore;
n. said inlet means including inlet port means extending through
said housing means between said third and fourth restricting
means;
o. said third restricting means disposed axially between said inlet
port means and said first housing means end; and
p. said cylindrical piston surface movable axially between said
inlet port means and said fourth restricting means to an axial
location where said inlet port means is non-coincident with said
cylindrical piston surface for permitting fluid to flow into said
piston bore through said inlet port means.
2. The fluid pump as defined in claim 1 wherein:
a. said outlet means include outlet port means extending through
said housing means axially between said first and second
restricting means;
b. said second restricting means is positioned axially between said
outlet port means and said second housing end; and
c. said cylindrical outer surface on said valve body is closely
surrounded by a valve bore in said housing means and is moveable
axially in said valve bore between said outlet port means and said
first restricting means to an axial location where said outlet port
means is non-coincident with said cylindrical valve body surface
for permitting fluid to flow out of said housing means through said
outlet port means.
3. A fluid pump comprising:
a. housing means having first and second axially spaced ends;
b. axially spaced outlet and inlet means opening into said housing
means adjacent said first and second housing means ends
respectively;
c. piston lifting means disposed within said housing means and
movable axially through a piston bore in said housing means to form
a sliding seal with a portion of said piston bore for moving fluid
into said housing means through said inlet means and out of said
housing means through said outlet means;
d. drive means secured to said lifting means for moving said
lifting means axially through said housing means;
e. valving means having first and second axially spaced ends
disposed within said housing means and movable axially within a
valve bore in said housing means to form a sliding seal with a
portion of said bore to open and close said outlet means;
f. a closed bore section extending axially between the axially
uppermost opening in said inlet means and the axially lowermost
sealing point of said valve bore and valving means when said
valving means is at the lowest axial point in said valve bore;
g. first circulating means adjacent said first housing means end
for moving fluid into and out of said housing means as said valving
means is moved; and
h. lower openings included in said first circulating means
extending through said housing means into said valve bore adjacent
and above the uppermost sealing point of said valve bore and
valving means when said valving means is at its lowermost point in
said valve bore whereby fluid and solid particles may flow into and
out of said valve bore at axially spaced locations for preventing
particle buildup in said valve bore.
4. The fluid pump as defined in claim 3 further including second
circulating means adjacent said second housing means end for moving
fluid into and out of said housing means as said lifting means is
moved with said inlet means being adjacent and below the lowermost
sealing point between said piston means and said piston bore when
said piston means is at its uppermost point in said piston bore
whereby fluid and solid particles may flow into and out of said
piston bore at axially spaced housing locations to prevent particle
buildup in said piston bore.
5. A fluid pump comprising:
a. housing means having first and second axially spaced ends;
b. axially spaced outlet and inlet means opening into said housing
means adjacent said first and second housing means ends
respectively;
c. piston lifting means disposed within said housing means and
movable axially through a piston bore in said housing means to form
a sliding seal with a portion of said piston bore for moving fluid
into said housing means through said inlet means and out of said
housing means through said outlet means;
d. drive means secured to said lifting means for moving said
lifting means axially through said housing means;
e. valving means having first and second axially spaced ends
disposed within said housing means and movable axially within a
valve bore in said housing means to form a sliding seal with a
portion of said valve bore to open and close said outlet means;
f. a closed bore section extending axially between the axially
uppermost opening in said inlet means and the axially lowermost
sealing point of said valve bore and valving means when said
valving means is at the lowest axial point in said valve bore;
and
g. second circulating means adjacent said second housing means end
for moving fluid into and out of said housing means as said lifting
means is moved with said inlet means being adjacent and below the
lowermost sealing point between said piston means and said piston
bore when said piston means is at its uppermost point in said
piston bore whereby fluid and solid particles may flow into and out
of said piston bore at axially spaced housing locations to prevent
particle buildup in said piston bore.
6. A fluid pump comprising:
a. housing means having first and second axially spaced ends;
b. axially spaced outlet and inlet means opening into said housing
means adjacent said first and second housing means ends
respectively;
c. piston lifting means disposed within said housing means and
moveable axially through a piston bore in said housing means to
form a sliding seal with a portion of said piston bore for moving
fluid into said housing means through said inlet means and out of
said housing means through said outlet means;
d. drive means secured to said lifting means for moving said
lifting means axially through said housing means;
e. valving means having first and second axially spaced ends
disposed within said housing means and a moveable axially within a
valve bore in said housing means to form a sliding seal with a
portion of said valve bore to open and close said outlet means;
f. first circulating means adjacent said first housing means end
for moving fluid into and out of said housing means as said valving
means is moved; and
g. lower openings included in said first circulating means
extending through said housing means into said valve bore adjacent
and above the uppermost sealing point of said valve bore and
valving means when said valving means is at its lowermost point in
said valve bore whereby fluid and solid particles may flow into and
out of said valve bore at axially spaced locations for preventing
particle buildup in said valve bore.
7. A fluid pump as defined in claim 6 further including second
circulating means adjacent said second housing means end for moving
fluid into and out of said housing means as said lifting means is
moved with said inlet means being adjacent and below the lowermost
sealing point between said piston means and said piston bore when
said piston means is at its uppermost point in said piston bore
whereby fluid and solid particles may flow into and out of said
piston bore at axially spaced housing locations to prevent particle
buildup in said piston bore.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to means for moving fluids
from one location to another. More specifically, the present
invention relates to a pump for elevating fluids and, in
particular, to a pump for raising subterranean petroleum fluids
through production tubing in completed oil wells.
2. Description of the Prior Art
A conventional oil well includes a cased well bore with one or more
strings of tubing extending downwardly through the casing into the
oil or other petroleum fluid contained in the subsurface mineral
formation to be produced. The casing is perforated at the level of
the production zone to permit fluid flow from the formation into
the casing, and the lower end of the tubing string is generally
open to provide entry for the fluid in the casing.
One type of pump conventionally employed in structures of the type
described is wedged into an internal constriction or seating nipple
formed internally of the tubing below the fluid level. A metallic
enlargement on the external body of the pump prevents it form
travelling below the seating nipple and resilient seal rings on the
body of the pump housing act to form a leakproof seal between the
seating nipple and pump. The pump is generally driven by a
mechanical linkage of metal rods, referred to in the trade as
sucker rods which extend from the pump to the well surface. The
sucker rod linkage is powered in a reciprocating motion by a
conventional mechanical apparatus usually called a pumping unit
located at the well surface.
The conventional pump itself generally includes a housing through
which a piston is reciprocated by the sucker rod linkage. In its
simplest form, the conventional pump of the type described often
includes a number of ball and seat valves with one such valve in
the piston and another at the inlet port of the housing. On the
upstroke of the plunger, the ball in the inlet port valve is drawn
away from its seat and the ball of the outlet port valve is forced
over its seat to draw fluid from below the sealing nipple and into
the housing. On the piston's downstroke, the ball in the inlet
valve is forced onto its seat and the ball in the piston valve
moves away from its seat to allow the piston to move downwardly
through the fluid contained in the housing. On the subsequent
upstroke, the closing of the piston valve forces the fluid above
the piston out of the housing through the outlet ports and into the
tubing above the sealing nipple and simultaneously fills the
housing below the piston with fluid. Repetition of this cycle
eventually fills the tubing string and causes the fluid to flow to
the surface.
The previously described pump or some variation thereof is probably
the most widely employed in applications where it is desired to
drive a subsurface pump by a surface powered, mechanical linkage.
One of the most significant problems in pumps of this type is
caused by wear of the ball and seat valves. The fluid produced from
many formations contains minute, abrasive particles such as sand
which lodge between the ball and the seat and wear away the valve
components. Over a period of time, the sealing efficiency of the
valves is reduced to such an extent that the pump must be removed
and repaired or replaced. In some wells, where the production fluid
is particularly sandy or corrosive, pumps of the type described
must be replaced at frequent intervals. It is, of course, evident
that removing and repairing or replacing a pump, and the associated
losses caused by reduced production time can be significant expense
factors.
Yet another loss in conventional pumps is associated with the
reduction in fluid output caused by the faulty seating of the
valves during their periods of production. Thus, in some cases, it
may be more economical to continue to operate the pump at a reduced
efficiency caused by valve wear rather than to have the pump
removed and repaired.
Ball valve pumps are also relatively limited in theoretical
efficiency and cycling rate due to their inherent principle of
operation. Any increase in the amount of fluid which can be
produced by such a pump usually involves an increase in the driving
power and pump dimensions and includes a corresponding decrease in
efficiency. Moreover, the valve closure time required for the ball
and seat type valves restricts the speed of the pumping cycle and
thereby further limits the maximum production rate of pumps
employing these valves.
SUMMARY OF THE INVENTION
The pump of the present invention employs sliding pistons as a
valving mechanism rather than the conventional ball and seat
arrangement. In the pump of the present invention, a small
clearance is provided between pistons and bores and is filled with
the fluid being pumped to produce an effective seal between the two
components. The clearance between the piston and the pump bore
eliminates metal-to-metal contact and thereby reduces valve wear
and resultant leakage.
In its broadest aspect, the pump of the present invention employs a
first piston which is directly moved by an external power source
and a slave piston which is moved by the resultant pressure
differentials created by the movement of the first piston. In one
embodiment, where the pump is to be employed in a conventional oil
well structure, the first or lower piston is fixed to the lower end
of a cylindrical rod and the upper piston, which has a tubular
form, is adapted to slide over the rod. O-rings are carried
internally of the upper piston to form a sliding seal with the rod.
The two pistons and a portion of the rod are carried in the bore of
a tubular housing where the reciprocating axial movement of the rod
produces a similar movement in the lower piston. The upper piston
is, however, free to slide over the rod and its axial movement is
controlled by the resulting pressure differentials created across
its length. These pressure differentials act to move the upper
piston to appropriately open or close the outlet ports of the pump
which in turn permits fluid to be expelled through the outlet ports
and prevents return flow into the pump.
The bore of the pump housing is provided with internal restrictions
which limit the axial movement of the upper piston whereby a low
pressure area is created between the two pistons as the axial
distance between them increases during the downstroke of the lower
piston. When the lower piston has moved below the inlet ports,
fluid is forced through the inlet ports to fill the low pressure
area. The operation of the pump thereby effects a positive flow
into the pump bore which in turn permits an increase in the pumping
rate.
The design of the pump also produces a continuous fluid flow over
the operating surfaces of the pump to lubricate and prevent any
particle buildup. As a result, the pump operates more smoothly and
wear is reduced.
The lower end of the pump is equipped with a pump guide having a
pointed, external contour to prevent premature lodging of the pump
as it is lowered through the tubing and into the seating nipple.
The pump guide and the upper portion of the pump assembly are also
provided with ports which communicate with the housing bore to
prevent a fluid lock and to form a pathway for the lubricating and
washing action of the fluid.
The pump of the present invention is durable and capable of
efficiently producing more volume than is presently possible with
conventional ball and seat type pumps with the same power input.
The design of the pump produces a smoothness of operation which
reduces sucker-rod breakage often associated with conventional
pumps, increases efficiency and reduces the cost of manufacturing
the pump.
BRIEF DESCRIPTION OF THE DRAWings
FIG. 1 is a partial elevation, partially in section illustrating a
standard oil well structure equipped with the preferred form of the
pump of the present invention;
FIG. 2 is an elevation, partially in section illustrating the pump
of FIG. 1 at the bottom of its stroke;
FIG. 3 is an elevation, partially in section illustrating the pump
of FIG. 1 at the mid-point of its upstroke;
FIG. 4 is an elevation, partially in section, illustrating the pump
of FIG. 1 at the top of its stroke;
FIG. 5 is an elevation, in section, illustrating a second form of
the pump of the present invention.
DESCRIPTION OF THE PREFERRED EMBodiments
With reference to FIG. 1 of the drawings, the pump of the present
invention, designated generally at 10, is disposed in a
conventional well structure which includes a string of casing C.
The casing C is perforated to permit petroleum fluid P to flow into
the casing from a subsurface formation F.
A second conduit string T, generally referred to as tubing, extends
through the casing C and into the petroleum fluid P in the casing.
The tubing T is provided with an internal restriction or seating
nipple, indicated generally at N, which forms a seal with and
supports the pump 10.
As with conventional pumps, the pump 10 is driven by a
reciprocating linkage S of sucker rods to raise fluid from below
the seating nipple N and expel it into the tubing T above the
nipple. This pumping action eventually fills the tubing T and
forces fluid through a production line L at the well surface.
Details in the construction of the pump 10 of the present invention
may best be described by reference to FIG. 2 of the drawings. The
pump 10 includes a first lower, cylindrical piston 11 threadedly
secured to a cylindrical rod 12. The upper end of the rod 12 is
threadedly secured to the lower end of the sucker-rod linkage S. A
second tubular piston 13 is carried about the rod 12 between the
piston 11 and sucker-rod linkage S and is free to move axially with
respect to the rod. An O-ring bushing 13a is threadedly secured
within the upper end of the piston 13 and carries two resilient
O-rings 13b which form a sliding seal between the piston 13 and the
rod 12. A plural part housing indicated generally at 14 surrounds
the pistons 11 and 13 and a portion of the rod 12. The housing 14
includes an upper housing section 14a, a hold-down body 14b, and a
lower housing section 14c. Threads formed on the upper and lower
housing sections 14a and 14c respectively are firmly engaged with
cooperating threads formed at either end of the hold-down body 14b
as illustrated in the drawings.
The hold-down body 14b includes a radially enlarged box section
14b' extending outwardly from a smaller shank portion 14b". The
external dimensions of the section 14b' are greater than the
internal dimensions of the seating nipple N which prevents the pump
10 from moving below the seating nipple in the tubing string T. The
external dimensions of the pump 10 below the section 14b' are less
than the internal dimensions of the seating nipple N which thereby
permits the lower portions of the pump 10 to extend below the
seating nipple as illustrated in FIG. 1 of the drawings.
The shank 14b" of the hold-down body 14b supports a conventional
sealing assembly which includes a plurality of resilient seating
cups 15 axially spaced from each other by spacers 16. The seating
cups 15 and spacers 16 are fixed against axial movement with
respect to the hold-down body 14b by means of a jam nut 17 which
engages threads on the shank 14b" to hold the sealing assembly
securely against the enlarged section 14b'. As best illustrated in
FIG. 1, the seating cups form a leak-proof seal between the pump 10
and the seating nipple N which isolates the internal tubing area
above the nipple N from the area below the nipple.
A composite central bore extends through the housing 14 and is
formed by the internal cylindrical surfaces 14d, 14e, and 14f of
the housing sections 14a, 14b, and 14c respectively. The axial
length of the bore surface 14f is substantially greater than that
of the bore surface 14d since the axial distance travelled by the
piston 11 is greater than that travelled by the piston 13. A rod
guide 18 is threadedly engaged with internal threads formed in the
upper end of the housing section 14a for restricting lateral
movement of the rod 12. The lower axial end of the rod guide 18
also acts to limit the upward axial movement of the piston 13. At
the lower end of the housing 14, a pump guide 19 is threadedly
engaged with internal threads formed in the lower end of the
housing section 14c and is employed primarily for preventing the
pump from prematurely lodging as it is being lowered through the
tubing T and into the seating nipple N.
A small clearance 20 is provided between the outer surface of the
piston 11 and the bore surface 14f with a similar clearance 21
provided between the piston 13 and and the bore surface 14d. When
the pump 10 is submerged, clearances 20 and 21 are filled with the
fluid being pumped which produces an effective seal between the
pistons and the bore surfaces. The two clearances also provide
lubrication of the housing bore surfaces and pistons to reduce wear
and add to the smoothness of operation of the pump.
A series of inlet ports 14g are provided about the circumferences
of the lower housing section 14c for admitting fluid below the
seating nipple N into the housing bore. The pump guide 19 is
provided with an axial bore 19a which intersects four radial
circulating ports 19b. When the lower end of the piston 11 is below
the bottom of the inlet ports 14g during any portion of the pumping
cycle, the bore 19a and ports 19b provide a flow path to prevent
fluid lock of the piston 11. The resultant fluid movement through
the bore 19a and ports 19b also creates a washing action which
prevents any accumulation of particles in the pump and lubricates
the lower portion of the bore surface 14f.
A series of outlet ports 14h are provided about the circumference
of the upper housing section 14a for exhausting fluid from the
housing bore and into the tubing string T above the seating nipple
N. Immediately above the outlet ports 14h, a series of circulation
ports 14i is also provided about the circumference of the housing
section 14a. The ports 14i cooperate with four circulating ports
18a which extend through the rod guide 18 to provide a pathway for
fluid contained within the bore housing above the piston 13. The
resultant fluid flow through the ports 14i and 18a acts to prevent
the piston 13 from being fluid locked and to lubricate the upper
portion of the bore surface 14d as well as to prevent any particle
build-up in and about the pump 10.
In the initial installation of the pump 10 of the present invention
in a conventional oil well structure, the pump is lowered through
the tubing string T until the enlarged section 14b' of the
hold-down body 14b engages the restriction formed by the seating
nipple N. During the descent through the tubing string T, the
external contour of the pump guide 19 assists in directing the pump
past tubing deviations and obstacles formed internally of the
tubing and thereby prevents the pump 10 from prematurely lodging
before it is properly positioned in the seating nipple. With the
pump 10 positioned in the seating nipple N as illustrated in FIG.
1, the resilient seating cups 15 form a leakproof seal which
isolates the area below the seating nipple from the internal area
of the tubing T above the nipple.
With the initial positioning of the pump being thus described, the
operation of the pump 10 may best be understood by reference to
FIGS. 2-4 which illustrate the pump structure at various phases of
the stroke. With initial reference to FIG. 2, the pistons 11 and 13
are illustrated at the bottom of the pumping stroke. In this
position, the lower piston 11 is below the inlet ports 14g which
permits fluid in the tubing T below the nipple N to flow into and
fill the housing bore in the area between the two pistons 11 and
13. The upper piston 13 simultaneously covers the outlet ports 14h
to prevent the fluid in the tubing T above the seating nipple N
from flowing into the housing bore. It will be understood that the
sliding seal formed by the O-rings 13b prevents any fluid flow
between the rod 12 and the piston 13 whereby the fluid in the
tubing T above the nipple N is completely sealed away from the
internal housing area between the two pistons 11 and 13.
As best illustrated with reference to FIG. 2, when the sucker-rod
linkage S and attached rod 12 begin the upstroke of the pumping
cycle, the piston 11 is pulled upwardly, and after a short length
of travel, closes over and seals the inlet ports 14g to trap the
fluid contained within the housing bore between the two pistons.
Continued upward movement of the piston 11 lifts the fluid trapped
in the housing bore and the fluid in turn transmits this lifting
force to the upper piston causing it to move upwardly through the
housing bore until it engages the lower end of the rod guide 18.
The sliding seal formed by the O-rings 13b permits the rod 12 to
continue to move upwardly through the stationary piston 13. When
the bottom of the upper piston 13 has thus been raised above the
outlet ports 14h, and is in the position illustrated in FIG. 3, the
outlet ports are thereby opened and the fluid in the housing bore
is forced out through the outlet ports by continued upward movement
of the piston 11 as it completes its upstroke.
After the piston 11 reaches the top of its upstroke, as illustrated
in FIG. 4, the movement of the sucker-rod linkage S cycles and
begins to drive the piston 11 downwardly through the downstroke.
The downward movement of the piston 11 produces an initial lowering
of pressure in the housing bore between the two pistons and thereby
creates a resultant force which immediately moves the piston 13
downwardly to close the outlet ports 14h. An internal restriction
formed by the upper axial end of the hold-down body 14b prevents
the piston 13 from moving below the outlet ports 14h and holds the
piston 13 in position for the next upstroke.
As the rod 12 continues to move downwardly through the sliding seal
in the stationary piston 13, an increasing low pressure area is
created in the housing bore between the two pistons as they move
further apart. When the top of the lower piston 11 has moved below
the top of the inlet ports 14g, fluid rushes into the inlet ports
to fill the housing bore in the low pressure area between the two
pistons. The driving force transmitted through the sucker-rod
linkage S then reverses and the pumping cycle is repeated.
FIG. 5 of the drawings illustrates a second form of the pump of the
present invention indicated generally at 100. The reference
characters employed in FIG. 5 are 10 times the value of the
characters employed in identifying corresponding features of the
form of the pump illustrated in FIGS. 1-4. The embodiment of FIG. 5
is adapted to be driven by a rod 120 which extends into the bottom
of the pump housing 140. Circulating ports 180a extend through the
rod guide 180 to act as relief ports for fluid trapped below the
lower piston 110. The upper piston 130 has a solid cross sectional
area, and its axial motion is directly governed by the pressure
differentials existing within the bore of the housing 140. The top
of the housing 140 is capped by a fitting 210 which is provided
with circulating ports 210a. The ports 210a serve essentially the
same purpose as the ports 18a in the embodiment of FIGS. 1-4.
The operation of the embodiment of the present invention
illustrated in FIG. 5 corresponds to that previously described with
reference to the form illustrated in FIGS. 1-4. It will, of course,
be understood that any suitable means (not shown) may be provided
for driving the rod 120 through the required reciprocating axial
movement.
While the pump of the present invention has been described as being
particularly suited for use in oil well structures, it will be
understood that the pump is of general utility wherever it is
required that fluids of any type be transported from one place to
another. Thus, by way of example rather than limitation, the pump
may be employed in a water well or in driving fluids through a
horizontal pipe line. In any of its applications, it will be
understood that the power source may be a conventional
reciprocating sucker-rod linkage where suitable, or it may be some
other device or mechanism for imparting reciprocating motion to the
rod 12 or 120. It will also be appreciated that the structure of
the pump may be modified without departing from the present
invention. As an example, the sealing assembly described for
positioning and sealing the pump in a seating nipple may take on
various conventional forms, and in some applications may be
completely unnecessary. In the way of further example, the pistons
11 and 13, which has been illustrated and described as being
tubular or cylindrical in form, may in fact assume various other
forms. It will also be understood that even though the two pistons
have been illustrated as having approximately the same lateral and
axial dimensions, they may in fact have differing dimensions
provided they appropriately conform to the internal dimensions of
the respective portions of the housing bore. In suitable
applications, the pistons 11 and/or 13 may also employ annular
gaskets or O-rings or other suitable means for forming a friction
seal with the internal surfaces of the housing bore.
The foregoing disclosure and description of the invention is
illustrative and explanatory thereof, and various changes in the
size, shape and materials as well as in the details of the
illustrated construction may be made within the scope of the
appended claims without departing from the spirit of the
invention.
* * * * *