Note: Descriptions are shown in the official language in which they were submitted.
CA 02264497 1999-03-02
1 "VALVE CAGE AND BALL FOR A RECIPROCATING PUMP"
2
3 FIELD OF THE INVENTION
4 This invention relates to improvements to a valve, ball seat and
ball used in a subterranean reciprocating pump.
6
7 BACKGROUND OF THE INVENTION
8 A reciprocating, positive displacement pump is typically used
9 for pumping subterranean fluids containing fine solids to surface. The pump
comprises a cylindrical pump barrel, piston and piston rod. The bore of the
11 pump barrel forms a compression chamber. The piston rod reciprocates up-
12 and-down in the compression chamber to complete one pumping cycle. The
13 piston rod is suspended from a rod string or reciprocating production
tubing.
14 The barrel is anchored in the casing of a well. A standing valve is located
at
the bottom of the barrel. A piston and travelling valve are located at the
16 bottom of the piston rod. To begin a pumping cycle, on the upstroke of the
17 piston rod, the standing valve opens in and permits fluid to fill the
18 compression chamber of the pump barrel. On the downstroke of the piston
19 rod, the standing valve closes and the travelling valve opens to permit
compressed fluid in the barrel to enter the piston rod. This completes the
21 pumping cycle. During the next up stroke of the piston rod, the fluid
within
22 the rod is incrementally advanced up the well, while new fluid is drawn
into
23 the compression chamber.
24 The standing and travelling valves are typically identical and
comprise a form of ball check valve. More particularly, having reference to
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1 prior art valve shown in Figs. 1, 1 a, a valve 30 comprises a guide race or
cage 32.
2 The cage is generally tubular in configuration and defines an axial bore
3 extending therethough. A ball 34 is fitted within the bore of the cage.
Adjacent
4 the cage's lower end is positioned a ball seat ring 36. The seat ring serves
two
functions: one, to retain the ball within the bore; and two, to form a seal
with
6 the ball. Adjacent the cage's upper end is a horizontal bar or stop
extending
7 across the bore for preventing exit of the ball.
8 With the advent of horizontal wells, the bore of more
9 contemporary valve cages include longitudinal guide ribs 38 which
form a race of substantially constant diameter for closely guiding the ball so
11 that only moves axially. Presumably this is to aid in guiding the ball
directly
12 and concentrically onto the race during the closing portion of the cycle.
Fluid
13 flows between the ribs and around the ball.
14 Typically, the ball seat is sandwiched between the cage and a
lock ring or seat retainer 40 screwed into the bottom of the cage. As shown in
16 Fig. 1, there is a sharp or abrupt transition between the bores of the lock
17 ring, the ball seat ring and the guide race of the cage.
18 In Alberta, Canada, valves must contend with difficult
19 environments. More particularly, pumping scenarios include:
= cold production of heavy oils containing large fractions of
21 sand and potentially sandstone solids;
22 = Steam injected and other thermal wells in which contained
23 water tends to flash as pressure is reduced; or
24 = slant or horizontal wells in which the valves are required to
perform on their sides.
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I Application of conventional valves in the above scenarios often
2 result in low efficiencies. Conventional wisdom in this art specifies that
the
3 diameter of the ball should be at least'/2 inch larger than the diameter of
the
4 ball seat ring so as to avoid sticking or jamming of the ball.
Unfortunately, the resulting abrupt transition for fluid entering
6 the small diameter of the ball seat, and leaving it, results in high
pressure
7 drop. In cold production of heavy oil, this severely limits the intake of
virgin
8 oil to the pump barrel. In thermal cases, the pressure drops usually causes
9 contained water to flash to steam, causing a 1000-fold increase in volume,
displacing oil and vapor-locking the pump or at least reducing pumping
11 efficiencies.
12 To combat the low efficiencies associated with thermal
13 applications, operators sometimes compensate by landing the pump deeper
14 in the well, often lower than the heel or deep in the horizontal portion of
horizontal well, thereby increasing the hydrostatic head or pressure. There
16 is significant cost associated with such a remedy. Landing a pump deeper in
17 a well requires more time, greater quantities of production tubing, and
18 requires actuation of the pump around the heel, resulting in high cyclical
19 stresses in the rod or production string. Wear associated with cyclical
movement through the heel has prompted implementation of expensive
21 tubing and rod rotation devices.
22 Again, in cold production scenarios, large particle debris such
23 as sandstone often become lodged between the ball, the cage's bore, and
24 the guide ribs.
3
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1 Ideally, pressure drop should be minimized, the flow passages
2 through the valve should be maximized, and should be achieved without
3 compromising valve integrity. It is known that ball-type check valves are
4 associated with a pressure drop equivalent to that experienced in about 150
pipe diameters.
6 In some prior art implementations, efforts have been made to
7 utilize the largest possible valve cage and ball combination. Unfortunately,
8 as the size of the valve increases, the ball becomes so heavy that excessive
9 pressure drops are incurred merely to lift the ball. A 3.75 inch diameter
ball
weighs almost 8 pounds (3.5 kg). Some prior art remedies include
11 manufacture of the ball of lightweight and very expensive titanium - being
12 only about 56% the weight of steel alloys.
13
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1 SUMMARY OF THE INVENTION
2 Generally, the objectives of minimizing pressure drop through
3 the valve while maximizing flow passage therethrough are met by improving
4 the conventional valve in the following manner:
= managing the profile of the bore through the valve from its
6 inlet through its exit for avoiding abrupt transitions and the
7 associated entrance and exit losses, more particularly by
8 forming a continuous and tangential inside wall throughout;
9 = maximizing the diameter of bore of the ball seat;
= minimizing turbulence through the fluid bore and around the
11 ball;
12 = eliminating guide ribs for enabling lateral movement of the
13 ball, thereby creating the largest possible particle passing
14 bore size; and
= minimizing the weight and inertia of ball.
16 By implementing the above factors, the improved valve
17 demonstrates increased pump efficiency and thereby has a reduced need for
18 the pump to be landed so deep in a well. In horizontal wells, this
translates
19 into the ability to land the pump higher in the well, more specifically in
the
vertical portion of the well above the heel, and thereby receive all of the
21 benefits associated thereof. Additionally, in vertical wells, the pump can
be
22 repositioned above the perforations and avoid sanding and debris issues.
23 In a broad aspect, an improved valve is provided for a
24 reciprocating pump. The valve comprises a cage containing a ball and ball
stop at a fluid inlet, a ball seat and a seat retainer. A contiguous bore is
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1 formed therethrough. The bore has an inside wall which is formed of three
2 continuous profiles. The diameter of the bore diminishes from the fluid
inlet
3 of the seat retainer to the ball seat, then increases in the cage to the
fluid
4 outlet. Bore transitions are managed so as to be substantially tangential
throughout.
6 Preferably, the cross-sectional area of the cage is about twice
7 that of the cross-section area of the ball. Further, it is preferable to
maximize
8 the bore of the ball seat to be within 1/8 inch of the diameter of the ball.
9 Even more preferably, the ball is manufactures so as to be hollow, thereby
reducing its inertia.
6
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1 A BRIEF DESCRIPTION OF THE DRAWINGS
2 Fig. 1 is a cross-sectional side view of a valve according to the
3 prior art;
4 Fig. 1 a is a cross-sectional view of the valves of Fig. 1 cut
through the valve along lines I-1;
6 Fig. 2 is an exploded, cross-sectional view of a valve according
7 to one embodiment of the invention.
8 Fig. 3 is an assembled, cross-sectional view of the valve of Fig.
9 2, shown in operation wherein fluid is passing past the ball. The ball is
shown to having one quadrant partially cut away to illustrate the optional
11 hollow ball embodiment; and
12 Fig. 4 is an assembled, cross-sectional view of the valve of Fig.
13 2, shown in operation wherein the ball is seated and flow is blocked. The
left
14 cross section shows a stepwise inside wall transition and the right cross-
section depicts a smoother continuous profile.
16
17 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
18 As shown in the exploded view of Fig. 2, one embodiment of
19 the valve of the present invention is a valve assembly 1 comprising a
cylindrical valve body 2, a valve guide insert or cage 3, a ball 4, a ball
seat 5,
21 and a seat retainer 6.
22 Throughout, despite the potential for orientations of the valve 1
23 in horizontal well applications, terms such as top, upward and their
24 counterparts relate to orientations of the valve as if it was installed in
a
vertical implementation.
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1 As shown in the assembled views of Figs. 3 and 4, a fluid bore
2 7 is formed through the valve assembly, and listed in order of the direction
of
3 fluid flow, is continuous through each of the seat retainer 6, the seat 5,
the
4 cage 3, and the body 2.
The valve body 2 forms an assembly bore 8 for accepting the
6 valve guide insert or cage 3. The assembly bore can accept a variety of
7 cages dependent upon the application.
8 The ball 4 is located within and is movable within the cage's
9 fluid bore 7. A stop pin 10 extends across the top of the cage 3 to prevent
upward escape of the ball 4. The ball seat 5 is fitted at the bottom of the
11 cage to prevent downward escape of the ball 4. The ball seat 5 and cage 3
12 are retained within the valve body 2 by the seat retainer 6. The outer
13 circumference of the seat retainer 6 is threaded 12a for complementary
14 engagement with an inner threaded circumference 12b of the lower end of
the valve body 2.
16 The exterior of the valve body shown in Figs. 2 - 4, and the
17 means for affixing the valve body to the pump, are shown as configured for
a
18 standing valve. The exterior of a valve body, and means for affixing the
19 valve as a traveling valve, is not shown however, the interior of the valve
1,
and more particularly the fluid bore 7, are the same whether implemented as
21 a standing, or a traveling valve.
22 The fluid bore 7 comprises an inside wall 13. The interfaces
23 14a-14c of the wall 13 between each component are continuous, namely:
24 the interface 14a between the seat retainer 6 and the ball seat 5; the
interface 14b between the ball seat 5 and the cage 3; and possibly an
8
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1 interface 14c between the cage 3 and the valve body 2. Very simply, this is
2 accomplished by removing any intermediate and abrupt transitions and
3 thereby minimizing associated loss of head (increased pressure drop). More
4 specifically, the diameter of the fluid bore 7 at the interfaces 14a, 14b,
14c are
matched and the transition is made as close to tangent as possible.
6 As shown in Figs. 3 & 4, machining techniques result in step
7 transitions S1,S2,S3 on the profile of the inside wall 13. Referring to Fig.
4,
8 the left cross-section shows the step transitions S1,S2,S3. The right cross-
9 section shows two continuous transitions S4,S5 which is accomplished using
numerical machining techniques.
11 Accordingly, there is minimal formation of low-pressure areas,
12 or more particularly, there is minimization of entrance and exit loss
pressure
13 drop which results from the formation of vena-contracta flow. The profiles
of
14 the inside wall between each of the seat retainer, the ball seat, the cage,
and
the valve body are continuous and substantially tangent.
16 More particularly, the fluid bore 7 in the seat retainer 6 forms a
17 first profile P1 or bell-like fluid intake which tapers inwardly from the
bottom
18 16 to the ball seat 5 (diminishing diameter). The diameter of the fluid
bore 7
19 of the seat retainer 6 at the interface 14a matches that of the ball seat 5
and
is substantially tangent thereto.
21 The ball seat 5 comprises a circumferential seal 17, typically
22 manufactured of a satellite alloy. The inside wall 13 of the ball seat 5
forms a
23 second profile P2 having an outwardly tapering discharge (increasing
24 diameter). The diameter of the seat's bore 7 and circumferential seal 17 is
maximized. In contradistinction to the practice of the prior art, the
difference
26
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I between the diameter of the ball 4 and the diameter of the bore 7 of the
ball
2 seat 5, is less than the prior art 1/2 inch, most preferably in the order of
1/8
3 inch on the diameter. For example, the bore 7 of the ball seat 5 for a 3.75
4 inch ball has a diameter of 3.625 inches. It is clear upon examination that
the energy loss or pressure drop though the ball seat 5 and past the ball 4 is
6 significantly reduced (compare Fig. 1 and Fig. 3) as the flow is no longer
7 subjected to such an abrupt transition at interface 14b, nor is it deflected
8 laterally as much as it is in prior art valves.
9 The circumferential seal 17 is beveled steeply (only 11 degrees
off the axis) to match the deep-seating of the ball 4 in the ball seat 5.
11 The diameter of the second profile P2 at the discharge of the
12 ball seat 5 matches the fluid bore 7 at the inlet of the cage 3 and is
13 substantially tangent thereto.
14 The inside wall 13 of the cage 3 forms a third profile P3 which
has a progressively diminishing side wall thickness between the ball seat 5
16 and stop pin 10 (increasing diameter). Accordingly the diameter of the
fluid
17 bore 7 increases upwardly towards the stop pin 10. This provides maximal
18 flow area and minimizes pressure drop.
19 Preferably, the bore 7 of the ball seat 5 is as large as possible,
but the corresponding ball 4 should not be made too large for the fluid bore 7
21 in the cage.
22 More specifically, the diameter of the ball 4 is restricted such
23 that the cross-sectional area of the fluid bore 7 around the ball during
fluid
24 flow is about equal to or greater than two times the ball's cross-sectional
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1 area. Note that the ball rises in the cage during fluid flow to rest against
the
2 stop pin 9.
3 First then, the fluid bore 7 is made as large as it can be. Then
4 the ball's size is chosen so that the projected area of the ball 4 is
about'/ of
the cross-sectional area of the bore 7 of the cage 3 when the ball 4 is
6 against the stop pin 9. The size of the ball seat 5 is then chosen so that
the
7 diameter of the bore 7 through the ball seat 5 is only about 1/8" less that
the
8 diameter of the ball 4. Preferably, the area of the bore through the ball
seat
9 is comparable to the free area in the cage's bore 7 around the ball 4.
The stop pin 9 itself comprises a slender member extending
11 across the center of the fluid bore, blocking only enough of the fluid bore
to
12 prevent egress of the ball. Accordingly, as shown in Fig. 3, the distance
13 between the inside wall and the stop pin is greater that the radius of the
ball
14 so that the unseated ball will rest either one side of the pin or the
other. As a
result, lateral ball movement is arrested and the ball is unable to spin or
16 flutter during high fluid flow, avoiding undesirable turbulence, pressure
drop
17 and associated gas breakout.
18 The relationship between the diameter of the ball 4, the
19 increasing diameter fluid bore 7 in the cage 3, and the distance between
the
ball seat 5 and stop pin 9 is designed to minimize the axial travel of the
ball 4
21 and thereby provide the fastest ball response time for engaging and
22 disengaging the ball seat (closing and opening respectively).
23 Specifically, no prior art guide ribs are provided, thereby
24 permitting the ball to move laterally within the cage and thereby expose or
form the largest possible flow passage through the cage 3, improving fluid
11
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1 flow with the lowest associated pressure drop, and for passing the largest
2 possible entrained particles. As a result, screening of the valve inlet and
3 accumulation of debris (particles) is avoided.
4 In a second embodiment, the normally solid ball 4 is
manufactured in a hollow form. By reducing the weight of the ball, less
6 pressure drop is incurred in lifting the ball from its seat and thereby
further
7 reduces the opportunity for gas breakout or flashing to occur. The reduced
8 inertia of the ball also improves response of the ball to opening and
closing.
9 While hollow balls are known in the prior art of steam traps, the
known balls have insufficient wall thickness to survive in the environment of
11 a subterranean reciprocating pump. Accordingly, novel manufacturing
12 techniques are employed comprising machining of concave hemispherical
13 hollows 20 in each of two matched metal blanks (not shown). Suitable
14 materials of construction are 52100 chromium alloy or 440C stainless steel.
The two blanks are welded together with the hollows 20 facing. The welded
16 blanks are machined to form a sphere. The sphere is treated to
17 homogenize, stress relieve and harden the sphere. Finally, the sphere is
18 ground and surface hardened in known ball-bearing forming processes to
19 form a perfectly sphere and the valve ball 4.
21 Example I
22 A test valve constructed according to the first embodiment was
23 placed in a producing well in Northern Alberta to replace a prior art
valve.
24 The well was subjected to steam injection being a candidate for low
efficiency pumping. Prior art valves had been used and were of the straight
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1 bore, guide-rib type having 3.75" balls and 3.25" ball seats. These prior
art
2 valves required regular replacement on about a one month cycle.
3 Specifically, the valve seats were "washing-out".
4 The test valve was formed with an inside wall profile as shown
in Fig. 3. The maximum bore between the ball seat and fluid outlet had a
6 diameter of about 5.25 inches for a cross sectional area of 21.64 square
7 inches. A 3.75 inch ball was provided having a cross-sectional projected
8 area of 11.04 inches. Accordingly the bore's cross-section was 1.96 times
9 the ball's projected area so that as much free cross-sectional area remained
through the bore as was blocked by the ball. For the 5.25 inch cage, use of
11 a 4 inch ball would have been inappropriate, resulting in a ratio of bore-
to-
12 ball of only 1.76. This would have meant a further 14% decrease in the
13 remaining free cross-sectional area in the bore due to the 4 inch ball.
14 A large diameter bore ball seat was used; being 3.625 inches
in diameter or only 1/8 inch smaller in diameter than the ball. This seat
16 provided a flow cross-sectional area of 10.32 square inches, or as much as
17 97% of the free cross-sectional around the ball.
18 As of the date of this application, the test valve has operated
19 for 4 months without sticking or service of any kind, and continues to
operate
continuously, 24 hours/day with comparable pump efficiencies.
13
