Note: Descriptions are shown in the official language in which they were submitted.
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"STUFFING BOX FOR PROGRESSING CAVITY PUMP DRIVE"
FIELD OF THE INVENTION
The present invention relates generally to improvements in stuffing
box configurations for progressing cavity (PC) pump drive head installations.
BACKGROUND OF THE INVENTION
Surface drive heads for progressing cavity pumps require a stuffing
box to seal crude oil from leaking onto the ground where the polished rod
passes
from the crude oil passage in the wellhead to the drive head.
Due the abrasive sand particles present in crude oil and poor
alignment between the wellhead and stuffing box, leakage of crude oil from the
stuffing box is common in some applications. This costs oil companies money in
service time, down time and environmental clean up. It is especially a problem
with heavy crude oil wells in which the oil is often produced from semi-
consolidated sand formations since loose sand is readily transported to the
stuffing box by the viscosity of the crude oil. It is very difficult to make
stuffing
boxes that last as long as desirable by oil production companies. Costs
associated with stuffing box failures are one of the highest maintenance costs
on
many wells.
Conventional stuffing boxes are mounted below the drive head.
Conventional stuffing boxes are typically separate from the drive head and are
mounted in a wellhead frame such that they can be serviced from below the
drive head without removing it. A conventional stuffing box uses braided
packing
that is split so it can be replaced while the polished rod stays inside the
stuffing
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box. Since conventional stuffing boxes seal against the polished rod, which is
subject to wear, and due to poor alignment of the polished rod to the stuffing
box, leakage becomes somewhat inevitable. Due to this experience, users tend
to expect stuffing box leakage if the stuffing box uses braided packings.
In order to reduce or eliminate the leakage, high-pressure lip seals
have been used running against a hardened sleeve rather than against the
polished rod. Grenke in Canadian Patent No. 2,095,937 issued December 22,
1998 shows a typical stuffing box employing lip seals. These stuffing boxes
are
known in the industry as environmental stuffing boxes because they do not leak
at all until the lip seals fail. Since these high-pressure lip seals are not
split and
are mounted below the drive head, they cannot be replaced with the polished
rod
in place so the drive head must be removed to service the stuffing box. Since
the
drive head must be removed to service the lip seals, the wellhead frame has
been eliminated and the stuffing box is bolted directly to the bottom of the
drive
head on many drive heads now being produced. This type of stuffing box
directly
mounted to the drive head is shown in the above referenced Grenke patent. This
product is made by Grenco Industries. These types of stuffing boxes are
referred
to as integral.
There are many types of rotary lip seals that might be applied to
stuffing boxes for progressing cavity pumped wells. Grenco and other
competitors have had some field success with the type described as flanged
VARISEAL in the American Variseal catalog. American Variseal is a member of
Busak and Shamban Inc. This type of seal is made by a number of competitors.
Generally these seals are machined from reinforced TEFLON and they have a
preload spring between two lips. The flange is convenient for mounting the
seal
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and stabilizing it. Since the seals are TEFLON based, they can operate
without
lubrication.
Servicing of stuffing boxes is time consuming and difficult. In order
to service the environmental or integral stuffing boxes, the drive head must
be
removed which necessitates using a rig with two winch lines, one to support
the
drive head and the other to hold the polished rod. To save on rig time, the
stuffing box is typically replaced and the original stuffing box is sent back
to a
service shop for repair.
Recently, Oil Lift Technology Inc. has introduced top mounted
stuffing boxes to the industry, which allow the stuffing box to be serviced
from on
top of the drive head without removing the drive head from the well. These
types
of stuffing box are shown in Hult Canadian patent application 2,350,047 (the
"Oil
Lift Stuffing Box"). These top mounted stuffing boxes use a flexibly mounted
"floating" standpipe around which is a bearing supported shaft carrying the
rotary
stuffing box seals. Typically the primary rotary stuffing box seal is braided
packing since it has proven to last for a long time when running against the
hardened, flexibly mounted standpipe. Braided packings made from TEFLON
and graphite fibres and been used most frequently. KEVLAR cornered
packings are often used for the first and last packing rings to prevent
extrusion.
Packings of this type are generally self lubricating which can also be an
advantage in the present invention. Because the standpipe floats, it self
aligns to
the packing, reducing or eliminating run out and leakage compared to
conventional stuffing boxes. Packings have very low resilience so reduction of
run out is very important in prevention of leakage. In some cases the stuffing
box
is counter-pressurized, preferably by lubricating oil at a higher pressure
than the
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wellhead pressure so if there is any leakage through the primary rotary
stuffing
box seal, lubricating oil goes down the well rather than allowing well fluids
to leak
into the drive head. In the most difficult applications, the use of
pressurized
lubricating oil has proven very beneficial in extending stuffing box seal
life,
demonstrating many times the stuffing box seal life compared to non-
pressurized
stuffing boxes.
SUMMARY OF THE INVENTION
Applicant refers the reader to the Applicant's co-pending
applications: Canadian patent application 2,350,047 (Hult) filed on June 11,
2001
and laid open on December 9, 2001 and U.S. Patent Application Publication No.
US 2001/0050168 filed on June 11, 2001 and published on December 13, 2001.
The present invention relates to improving the performance and
serviceability of the Oil Lift Stuffing Box and to providing a series of
stuffing
boxes to retrofit to other wellhead drives either above or below the drive
head.
The present invention relates generally to improvements in stuffing
box configurations. The present invention also relates generally to
improvements
in seal configurations for stuffing boxes.
The present invention is applicable to top mounted stuffing boxes,
bottom mounted stuffing boxes, integral stuffing boxes and stand-alone
stuffing
boxes.
Stuffing boxes according to the present invention may either be
pressurized or non-pressurized.
Where the stuffing box is pressurized, the pressure may be applied
through a fluid medium. The fluid medium may be any suitable liquid or gas. In
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some applications, the fluid medium is preferably a lubricating fluid such as
lubricating oil so that the fluid medium is available to lubricate stuffing
box or
drive head components such as seals and bearings.
Where the stuffing box is pressurized, the pressure source may be
comprised of any suitable pressure source, including a hydraulic drive system
for
the well, a separate pump, a pressurized chamber such as a chargeable
pressure chamber, a pressure-intensifying cylinder, or combinations thereof.
The
pressure source may also consist of or be comprised of a hydraulic accumulator
for maintaining or stabilizing the pressurization of the stuffing box. It is
desirable
that the pressurization fluid be 50 to 500 psi above the wellhead pressure so
if
the primary seal leaks, pressurization fluid leaks toward the wellhead rather
than
allowing well fluid to enter the stuffing box or drive head housing.
Where the stuffing box is pressurized, two rotary seals may be
used with pressurization between the two seals. The first seal is a primary
seal
and has well fluid pressure on one side and pressurization fluid, preferably
at
higher pressure than the well fluid, on the opposite side. The second seal is
a
pressurization seal for containing or inhibiting the leakage of pressurization
fluid
within or from the stuffing box. The pressurization seal is subjected to
pressurization fluid on one side and little or no pressure on the opposite
side.
Both the primary seal and pressurization seal may be comprised of any type of
suitable rotary seal, including labyrinth seals, chevron packings, braided
packings, foil packings, O-rings, lip seals, rotary oil seals or combinations
thereof. Preferably the primary and pressurization seals are comprised of
braided packings because of the ease of service. In some cases, such as using
a pressurization fluid that is different than the lubricating fluid in the
stuffing box
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or drive head, even small leakage past the pressurization seal is
objectionable.
In these cases, the pressurization seal is preferably a high pressure lip seal
because these seals have lower leakage rates than braided packings. Where the
stuffing box is pressurized, a circulation path is preferably provided for
circulating
pressure fluid which does leak within or from the stuffing box. This
circulation
path may in some applications facilitate lubrication by the pressure fluid of
stuffing box or drive head components such as bearings or seals.
Where the stuffing box is non-pressurized, a controlled leakage
path is preferably provided for well fluids to prevent or inhibit such fluids
from
entering the stuffing box bearings or the drive head. Two rotary seals are
required with a leakage path for the escape of well fluids between these
seals.
The primary seal has well pressure on one side and is in communication with
the
leakage path on the opposite side so any well fluid that passes the primary
seal
escapes to the leakage path. The secondary seal is to prevent or inhibit well
fluids that escape past the primary seal from flowing into the drive head or
stuffing box housing, forcing said well fluids to drain out through the
leakage
path. The leakage path may comprise one or more passages and one or more
holes in components of the stuffing box or the drive head. Preferably the
leakage
path includes a lantern ring disposed adjacent to holes through the main shaft
thus permitting leakage to exit the drive head or stuffing box.
Stuffing boxes according to the present invention include rotary
seals. The rotary seals may be comprised of any suitable rotary seal,
including
labyrinth seals, chevron packings, braided packings, foil packings, O-rings,
lip
seals, chevron seals, rotary oil seals or any combination thereof. Preferably
the
rotary stuffing box seal is comprised of braided packings or lip seals or a
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combination of braided packings and lip seals.
Stuffing boxes according to the present invention may utilize a
rigidly mounted standpipe or a flexibly mounted "floating" standpipe for
improving
the performance of the stuffing box seal. Where a standpipe is utilized, the
standpipe may be either a single wall standpipe or a double wall standpipe. A
double wall standpipe is useful for facilitating a pressurized stuffing box in
which
the pressurization seal is serviceable from on top of the stuffing box or
drive
head. Preferably, the pressurization seal is comprised of braided packing or a
lip
seal or a combination thereof.
In order to pressurize the Oil Lift integral Stuffing Box illustrated by
prior art Figure 1, a labyrinth seal acting as the pressurization seal has
been
used between the drive gear (Figure 1 illustrates a labyrinth created by a
labyrinth ring sealing against the drive gear but the inner bearing race, the
shaft
itself, a bearing spacer or any concentric surface that rotates with the shaft
can
be used) and a labyrinth ring sealed to the drive head housing. A labyrinth
seal
has been used because it is non-wearing, but due to its location in the drive
head it is impossible to service without disassembling the drive head. It has
also
been found that good labyrinth sealing in that location is difficult to
achieve due
to run out between mating parts and the need for tight tolerances.
In one aspect of the present invention, the need for a non-
serviceable labyrinth seal located between the housing and main shaft (or an
equivalent) in pressurized stuffing boxes according to preferred embodiments
of
the invention has been eliminated by use of a double wall standpipe and a
rotary
seal instead of a labyrinth acting as the pressurization seal. The principle
is an
upper primary rotary seal and a lower rotary pressurization seal located in
the
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annulus between the standpipe and the shaft, with pressurization means
connected through passages in the standpipe communicating with the annular
area between the upper and lower seals, said seals being field serviceable by
removal and replacement through the top of the stuffing box or drive head. In
the
preferred embodiment, the upper and lower rotary seals are braided packings
separated by a preload spring or a lantern ring because of the ease of service
and durability of this type of seal. In some cases, such as using a
pressurization
fluid that is different than the lubricating fluid in the stuffing box or
drive head,
even small leakage past the pressurization seal is objectionable. In these
cases,
the pressurization seal is preferably a high pressure lip seal because these
seals
have lower leakage rates than braided packings.
Abrasive particles in the well fluid cause wear of the standpipe and
it must be periodically replaced. Another aspect of the present invention is
that
the standpipe can be inspected and replaced without removing the stuffing box
or drive head from the well.
Another aspect of the present invention is that in some preferred
embodiments, two different fluids can preferably be used inside the drive
head.
Hydraulic pressure, from the hydraulic system driving the drive head, can
preferably be used to pressurize the stuffing box. The lower bearings and
gears
can preferably be lubricated with gear oil. Unlike using a labyrinth seal as
the
pressurization seal, a pressurization seal such as braided packings or lip
seals
can be used in conjunction with a double walled standpipe so there is
negligible
flow of pressurization fluid into the lower bearings and gears of the stuffing
box
or drive head, thus keeping the hydraulic oil out of the gear oil in this
example.
In another aspect of the present invention, a non-pressurized
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stuffing box can be achieved using a flexibly mounted standpipe around which
is
a rotating shaft mounted on bearings in a housing. The primary rotary seal is
located in the annulus between the standpipe and the shaft. This configuration
can be used for a top mounted stuffing box as part of a drive head or as a
stand-
alone stuffing box that can be retrofitted below existing drive heads,
preferably in
a wellhead frame which supports a drive head above the stuffing box of the
present invention. Since there is no pressurization system, leakage of well
fluids
past the primary seal toward the stuffing box or drive head will occur. A
leakage
path is provided to allow escape of well fluids. A secondary seal is provided
to
prevent well fluids from entering the drive head or stuffing box housing.
Improvements in this system over Hult Canadian patent application 2,350,047
are shown in greater detail with reference to the drawings.
In some cases, it is not economic or practical to provide a pump to
pressurize the stuffing box. In these cases, a pressure intensification
cylinder
assembly can be added in conjunction with the stuffing box so that a pressure
fluid is made available at a pressure above the wellhead pressure.
In some cases, hydraulic pressure is readily available to provide for
stuffing box pressurization. However, a standpipe system requires a large main
shaft and large bearings, which may be too expensive for some applications. In
these cases, a bottom-mounted stuffing box with a pressurization system may be
an economic solution. The stuffing box may be integral with the drive head and
mounted on the bottom of the drive head by flanges, for example. The stuffing
box may also be a stand-alone stuffing box mounted in a wellhead frame with
the drive head mounted above the stuffing box on a wellhead frame.
In another aspect of the present invention, a stuffing box can be
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constructed with a non-rotating tubular shaft bearingly supporting a rotating
housing. The bearings may be lubricated with the pressurization fluid as it
travels
into the lower side of the primary rotary seal. This configuration is simpler
to
construct than a double wall standpipe but it uses more length and does not
align the standpipe and the housing as well as the double wall standpipe
configuration. This is because the housing is cantilevered from the bearings.
In another aspect of the present invention, a stuffing box comprises
a tubular housing with an internal bore to concentrically receive the polished
rod
therethrough. A tubular shaft is rotatably mounted in the housing by shaft
support bearings, and concentrically receives the polished rod for co-rotation
therewith. An annulus is created between the tubular shaft and the housing,
the
tubular shaft having a shaft extension projecting downward beyond the shaft
support bearings. A lower primary rotary seal is disposed in the annulus and
in
contact with well fluid on one side and pressurization fluid on its other
side. An
upper pressurization rotary seal is disposed in the annulus and acts to hold
pressurization fluid against the primary rotary seal. A leakage passage is
disposed in the annulus and acting to allow pressurization fluid that has
leaked
past the upper pressurization rotary seal to escape through a leakage path in
the
housing.
In another aspect of the present invention, a stuffing box comprises
a tubular housing with an internal bore to concentrically receive the polished
rod
therethrough and a tubular shaft, mounted in the housing by shaft support
bearings, and concentrically receiving the polished rod for co-rotation
therewith.
The tubular shaft creates an annulus between the tubular shaft and the
housing.
A static seal is provided between the polished rod and the tubular shaft for
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sealing well fluids. A primary rotary seal is disposed in the annulus and in
contact with well fluid on one side and a source of the pressurization fluid
on its
other side. A pressurization rotary seal disposed in the annulus and acts to
hold
pressurization fluid against the primary rotary seal; and a passage is
disposed in
the annulus and acts to collect pressurization fluid that has escaped past the
pressurization rotary seal.
BRIEF DESCRIPTION OF THE DRAWINGS
Aspects of the present invention demonstrating the concepts of the
present invention are illustrated, by way of example in the enclosed Figures,
in
which:
Figure 1 is a cross sectional view of the prior art stuffing box with
floating standpipe and labyrinth seal shown as Figure 6 in Hult Canadian
patent
application 2,350,047.
Figure 2 is a cross sectional view of the prior art stuffing box with
floating standpipe but no pressurization system, shown as Figure 8 in Hutt
Canadian patent application 2,350,047.
Figure 3 is a cross sectional view of the prior art stuffing box
pressurized from the hydraulic system, shown as Figure 9 in Hult Canadian
patent application 2,350,047.
Figure 4 is a cross sectional view of the preferred embodiment of a
stuffing box including a floating single wall standpipe but without a
pressurization
system.
Figure 5 is a cross sectional view of a preferred embodiment of a
stuffing box including a floating double wall standpipe and a pressurization
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system.
Figure 6 is a preferred embodiment of a stand-alone stuffing box
mounted in a wellhead frame, said stuffing box including a floating double
wall
standpipe and a pressurization system.
Figure 7 is a preferred embodiment of a stand-alone stuffing box
including a floating double wall standpipe and pressurization, said stuffing
box
mounted in a wellhead frame. Said pressurization source is a pressure-
intensifying cylinder built below the stuffing box, surrounding the polished
rod.
Figure 8 is a preferred embodiment of a stand-alone stuffing box
mounted in a wellhead frame using a floating single wall standpipe without a
pressurization system.
Figure 9 is a preferred embodiment of a stand alone stuffing box
constructed with a non-rotating tubular shaft bearingly supporting a rotating
housing.
Figure 10 is a preferred embodiment of a drive head with an
integral stuffing box mounted on the bottom of the drive head with a
pressurization system.
Figure 11 is a stand-alone stuffing box similar to and using the
same principles as the integral stuffing box shown in Figure 10.
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DESCRIPTION OF THE DRAWINGS AND OF PREFERRED EMBODIMENTS
Throughout the descriptions, components that have the same
function have the same number. For example, the function of static seals 126
are described in the description of Figure 4 so they are not described again
in
subsequent Figures, such as Figure 8. Since the number 126 is the same in both
Figures, the reader may assume that the function is the same in this and all
other Figures where the same number appears.
Figure 1 is a cross sectional view of the prior art stuffing box with
floating standpipe and labyrinth seal shown as Figure 6 in Hult Canadian
patent
application 2,350,047. Identification numbers in Figure 1 correspond to Figure
6
of the patent application.
Figure 2 is a cross sectional view of the prior art stuffing box with
floating standpipe but no pressurization system, shown as Figure 8 in Hult
Canadian patent application 2,350,047. Identification numbers in Figure 2
correspond to Figure 8 of the patent application.
Figure 3 is a cross sectional view of the prior art stuffing box
pressurized from the hydraulic system, shown as Figure 9 in Hult Canadian
patent application 2,350,047. Identification numbers in Figure 3 correspond to
Figure 9 of the patent application.
Figure 4 is a cross sectional view of the preferred embodiment of a
stuffing box with a floating single wall standpipe but without a
pressurization
system. It is an improvement compared to Figure 2 since braided packings or
high pressure lip seals can be used instead of the low pressure elastomeric
lip
seals shown in Figure 2. Braided packing materials and high pressure lip seals
made from reinforced TEFLON are self-lubricating whereas elastomeric lip
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seals are not and as a result they would wear out. Additionally, a high
pressure
lip seal can be fitted above the packings with benefits described below.
The preferred embodiment shown in Figure 4 will be used as a
reference to describe in detail the essential elements of a non-pressurized
stuffing box using a standpipe. Whether the stuffing box is separate from
(stand-
alone like Figure 6 and Figure 7) or is integrated into the drive head, the
essential elements are related. Although Figure 4 illustrates an integral
stuffing
box, a stand alone stuffing box can be constructed with the same elements. A
housing 52, often preferred (because of machining and assembly
considerations) with separable upper bearing cap 84, and separable lower
bearing cap 86, supports a rotating shaft 80. Separable bearing caps, if any,
are
considered part of the housing. A non-rotatable standpipe 92 is mounted
concentrically within the shaft and is detachably secured to the housing. The
polished rod 26 is received concentrically through the standpipe. Annular
passage 114 between the polished rod and the standpipe contains wellhead
pressure.
Annular passage 94 between the standpipe and the shaft can be
fitted with rotary seals. The top of the shaft has a removable drive cap 122
that is
drivingly connected to the polished rod 26 by a drive clamp 124. Below the
drive
cap are static seals 126 to prevent the escape of well fluids around the
polished
rod. Preferably the static seals are supported in a static seal carrier 110
which is
sealed to the shaft by seals 236. Seals 236 are preferably O-rings or similar
common seals. The static seal assembly is hereby defined as the static seals,
the static seal carrier and the seals 236. The drive cap, drive clamp,
polished
rod, shaft and static seal assembly, rotate together around the stationary
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standpipe. The static seals are referred to as 'static' because there is no
relative
rotary motion between the static seals and the polished rod and the static
seal
carrier. The only relative motion in the stuffing box is the rotary seals
rotating
against the standpipe. The standpipe preferably has a hardened surface to
reduce wear of the standpipe and the rotary seals.
By removing the drive clamp, drive cap and static seal assembly,
the rotary seals can be serviced from the top of the drive head or from the
top of
the stuffing box. Spring 118 serves to preload the primary seals 116 which are
preferably braided packings against the lantern ring 239. Once the spring is
removed, the lip seal assembly comprised of lip seal 305, lip seal carrier
302, lip
seal retainer 303 and O-ring seals 304 sealing the lip seal carrier to the
shaft can
be removed. Preferably the lip seal carrier has one or more tapped holes to
facilitate removal.
The primary rotary seal in the present embodiment is comprised of
a lip seal assembly acting first against well fluids and a set of packings
acting
once the lip seal has failed. The use of a lip seal in conjunction with
packings
provides substantial improvements in stuffing box life. Since lip seals have
very
little leakage and do a good job of excluding contaminants in the well fluid,
the lip
seal protects the packing from any wear until the lip seal fails. The packing
stays
like new. Once the lip seal fails, the packings take over the sealing role.
Essentially the stuffing box has two seals in series so the stuffing box life
is
equal to the lip seal life plus the packing life.
Two lip seals have been used in series in Grenke Canadian patent
2,095,937 but the use of packings provides a substantial advantage. When a lip
seal fails, leakage rates are very high and environmental damage can be
severe.
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A packing starts to leak slowly and operators have a chance to repair the
stuffing
box before substantial leakage can occur. Use of two lip seals per Grenke
provides longer stuffing box life and a resealable inspection port between the
two
lip seals can indicate when the first lip seal has failed. However, if
maintenance
checks are not done, both lip seals can fail, resulting in high leakage rates
of well
fluids and potential environmental damage. Use of packings prevents this.
Lip seals require accurate alignment between the rotating
components. Since the standpipe self aligns to the rotary seals, the lip seal
configuration in the present invention has substantial life advantages over
the
configuration used in Grenke Canadian patent 2,095,937. The Grenke
configuration has a shaft extension that is cantilevered from the bearings
supporting the shaft. Any misalignment at the bearings is multiplied at the
rotary
seals, unlike the present invention wherein the shaft is supported in bearings
spanning the stuffing box.
Below the packings 116 is an escape passage for well fluids
preferably comprised of a lantern ring 239 communicating with holes 238 though
the shaft. The lantern ring preferably has an upper and lower inner diameter
to
provide a running clearance to the standpipe. The lantern ring preferably has
an
upper and lower outer diameter to allow a sliding fit to the inside diameter
of the
shaft. The inner diameter and the outer diameter has a radially relieved
section
adjacent to radial holes 242 to allow well fluid that has leaked past the
packings
to escape more readily through holes 242 and then into holes 238 through the
shaft.
Below the lantern ring is the secondary rotary seal 300 which is
preferably a set of packings or another lip seal assembly as described above
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and shown in Figure 4 in the primary stuffing box seal location. Spacer ring
301
has a running clearance against the standpipe and serves to prevent the
packing
from extrusion into annular area 94. When a lip seal assembly is used as the
secondary rotary seal, the lip seal carrier can be integrated with the lantern
ring
to reduce the number of parts and the spacer ring is not required.
Figure 5 is a cross sectional view of a preferred embodiment of a
stuffing box using a floating double wall standpipe pressurization system. The
need for a labyrinth seal acting as the pressurization seal as shown in
Figures 1
and 3 has been eliminated by use of a double wall standpipe 306 to convey
pressurization fluid above a rotary seal, preferably a set of braided packings
or a
lip seal or combinations thereof, said rotary seal acting as the
pressurization
seal. Unlike the previous labyrinth seal shown in Figure 1, the pressurization
seal
in this embodiment can be serviced in the field without removing the drive
head
from the well. Also in this embodiment, the standpipe can be removed for
inspection and replacement without removing the drive head from the well.
In the Figure 5 embodiment, the pressurization fluid is conveyed by
a pressurization means such as a pump 72.
The preferred embodiment shown in Figure 5 will be used as a
reference to describe in detail the essential elements of a pressurized
stuffing
box using a double wall standpipe. Whether the stuffing box is separate from
(stand-alone like Figure 6 and Figure 7) or is integrated into the drive head
as
shown in this embodiment, the essential elements are related. Although Figure
5
illustrates an integral stuffing box, a stand-alone stuffing box such as
Figure 6
can be constructed with the same elements. A housing 52, often preferred
(because of machining and assembly considerations) with separable upper
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bearing cap 84, and separable lower bearing cap 86, supports a rotating shaft
80. Separable bearing caps, if any, are considered part of the housing and
will
be henceforth referred to as such. A non-rotatable standpipe 306 is mounted
concentrically within the shaft and is detachably secured to the housing. The
polished rod 26 is received concentrically through the standpipe. Annular
passage 114 between the polished rod and the standpipe contains wellhead
pressure.
Annular passage 94 between the standpipe and the shaft can be
fitted with rotary seals. The top of the shaft has a removable drive cap 122
that is
drivingly connected to the polished rod 26 by a drive clamp 124. The
connection
between the drive cap and the shaft can transmit torque and support axial
loads.
Below the drive cap are static seals 126 to prevent the escape of well fluids
around the polished rod. Preferably the static seals are supported in a static
seal
carrier 110 which is sealed to the shaft by seals 236. Seals 236 are
preferably 0-
rings or similar common seals. The static seal assembly is hereby defined as
the
static seals, the static seal carrier and the seals 236. The drive cap, drive
clamp,
polished rod, shaft and static seal assembly, rotate together around the
stationary standpipe. The static seals are referred to as 'static' because
there is
no relative rotary motion between the static seals and the polished rod and
the
static seal carrier. The only relative motion in the stuffing box is the
rotary seals
rotating against the standpipe. The standpipe preferably has a hardened
surface
to reduce wear of the standpipe and the rotary seals.
By removing the drive clamp, drive cap and static seal assembly,
the rotary seals can be serviced from the top of the drive head or from the
top of
the stuffing box in the case of a stand-alone stuffing box, without removal
from
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the well.
The primary rotary seals are preferably packings 116 or a
combination of packings and lip seals as shown in Figure 4. Below the packing
116 is a packing pusher ring 308 which has a running clearance against the
standpipe and serves to prevent the packing from extrusion into annular area
94.
Preload spring 118 acts with the pressurization fluid to push the packing
toward
the static seal carrier 110.
Below the spring is the pressurization rotary seal 307 which is
preferably a set of packings or a lip seal assembly as described above and
shown in Figure 4 in the primary seal location. Spacer ring 308 above the
packing 307 and spacer ring 301 below packing 307 have a running clearance
against the standpipe and serve to prevent the packing from extrusion into
annular area 94. The spacer rings are not required when a lip seal assembly
serves as the pressurization seal.
The standpipe in this embodiment is called double walled because
that is the preferred method of its construction. Other methods of
construction
would be possible as long as the standpipe functions to communicate pressure
from a pressure supply to the stuffing box between the pressurization rotary
seal
and the primary rotary seal as described herein. Functionally, the double
walled
standpipe has internal passages to communicate pressure from the
pressurization system to the annular area 94 between the primary rotary seal
and the pressurization seal. A pressure connection to a passage in the housing
is made where the standpipe is secured to the housing. Generally the inner
wall
is sealed to the housing and the outer wall is sealed to the housing and fluid
is
conveyed from the housing between these two seals, shown as items 354 and
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355. Fluid is then conveyed in the annulus 321 between the outer and inner
wall
of the standpipe and then is conveyed radially through holes or passages 322
through the outer wall into annular passage 94 between the primary seal and
pressurization seal.
By use of a double walled standpipe, both the pressurization seal
and the primary seal can be replaced in the field without removing the drive
head
or stuffing box from the well. This is not possible with the labyrinth located
in the
position of Figure 1.
Abrasive particles in the well fluid cause wear of the standpipe and
it must be periodically replaced. Another aspect of the present embodiment of
the invention is that the standpipe can be inspected and replaced without
removing the stuffing box or drive head from the well by releasing retaining
fastener 309 which is preferably a special bolt that fits radially into a
retention
hole or other suitable shape 310 in the standpipe. When the retaining fastener
is
in place the standpipe is prevented from rotation or axial movement. The
retaining fastener is fitted with clearance into the retention hole to permit
the
standpipe to tilt to better align the standpipe to the rotary seals carried by
the
shaft.
The principle of configuring the standpipe securing means so the
standpipe can be inspected or replaced can also be applied to the single wall
standpipe shown in Figure 4. In this case the standpipe requires only a single
seal and a retention hole so it can be radially secured as described herein.
Figure 8 illustrates the principle.
Figure 6 is a preferred embodiment of a stand-alone stuffing box
mounted in a wellhead frame using a floating double wall standpipe and
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pressurization system. The drive head in this and all stand alone stuffing
boxes
is mounted on the top of the wellhead frame.
The essential elements of this stand-alone stuffing box are the
same as a stuffing box integrated into the drive head in Figure 5. The
description
of Figure 5 applies to this stuffing box as well.
The principle whether integrated into a drive head or in a stand-
alone stuffing box is an upper primary rotary seal and a lower rotary
pressurization seal located in the annulus between the standpipe and the
shaft,
with pressurization means connected via inlet passage 316 through passages in
the standpipe communicating with the annular area between the upper and lower
seals, said seals being field serviceable by removal and replacement from the
top of the stuffing box or drive head. In the preferred embodiment, the upper
and
lower rotary seals are preferably braided packings separated by a preload
spring
or a lantern ring because of the ease of service and durability of this type
of seal.
In some cases, the pressurization seal is preferably a high pressure lip seal
because these seals have lower leakage rates than braided packings and they
take less axial length. In the preferred embodiment, the stuffing box would be
pressurized off the hydraulic system that is powering the drive head. The
pressure from the hydraulic system is preferably reduced down to 50 to 500 psi
above the wellhead pressure by the built in pressure-reducing valve 315. A
check valve 393 is preferably used with pressurized stuffing boxes since it
locks
fluid into the annular area between the primary and pressurization seals and
prevents shifting of these seals when well servicing may cause high wellhead
pressure.
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Pressurization fluid that escapes past the pressurization seal is
preferably returned to the pressurization source though fluid passage 317.
Housing 52, non-rotatable standpipe 306, polished rod 26, annular
passage 114, annular passage 94, static seals 126, static seal carrier 110,
seals
236, static seal assembly, primary rotary stuffing box seals 116, packing
pusher
ring 308, preload spring 118, pressurization rotary stuffing box seal 307 and
spacer ring 308 function as described in the description of Figure 5.
When the stuffing box is integrated into the drive head, the
polished rod clamp supports the polished rod load and transmits torque from
the
drive head to the polished rod. When the stuffing box is a stand-alone
version,
the polished rod is still supported and driven by the drive head. However, for
the
stand-alone version, the stuffing box is driven by the polished rod. Very
little
torque is required to drive the stuffing box so the drive clamp and its
connection
to the drive cap do not need to be as robust. The bearings 312 and 313 are not
large enough to support the axial load of the polished rod so it is important
that
the rod clamp 124 does not rest against the drive cap 122 and apply axial
load.
Axial clearance space 323 should be visually apparent so an operator can be
sure axial load is not being applied to the stuffing box bearings. The
stuffing box
functions the same in both cases.
Removable drive cap 122 is preferably secured to shaft 80 by
fasteners 318. Preferably the fastener is an Allen head bolt that can protrude
above the drive cap and be driven by corresponding recesses in drive clamp
124. Alternately, the drive cap and static seal carrier might be combined and
the
main shaft could be internally threaded to connect the combined static seal
carrier/drive cap to the shaft. Other methods of connecting the drive cap to
the
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shaft and transmitting torque from the drive clamp to the drive cap can be
used.
Determination of which connection is preferable depends on cost and space
considerations.
In the preferred embodiment, spacer ring 301 has been eliminated
but rather the shaft is made with a close running fit at location 320.
The passage 321 between the inner and outer walls of the
standpipe and the passage 322 through the outer wall leading to the area
between the seals are more readily apparent in Figure 6 than in Figure 5 but
the
passages are present in both embodiments and function the same in both.
Figure 7 is a preferred embodiment of a stand-alone stuffing box
mounted in a wellhead frame using a floating double wall standpipe similar to
Figure 6. The stuffing box functions identically to Figure 6, only the source
of
pressurization is different. In this embodiment, the pressurization source is
a
pressure-intensifying cylinder assembly located below the stuffing box,
surrounding the polished rod. Grease or oil under pressure is pumped through
valve 338 into the upper chamber 336 to push the piston 325 down. Wellhead
pressure in annular passage 114 pushes on the bottom of the piston, urging the
piston upward. Since the piston area on the wellhead side is larger than on
the
stuffing box side, oil or grease feeds into the stuffing box through passage
341 at
higher pressure than the wellhead pressure. By mounting the cylinder assembly
between the stuffing box and the wellhead, heat is conducted into the cylinder
to
prevent the cylinder from freezing. There are no separate fluid lines to
freeze off
in cold weather or be damaged during well servicing. It will be appreciated
that
this pressurization system can be used whether the stuffing box is a stand-
alone
version or is built into the drive head. This pressurization system could be
used
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with any stuffing box that can employ a pressurization system.
Pressurization fluid that escapes past the pressurization seal is
preferably returned to the pressurization source though fluid passage 395.
Components of the pressure intensification cylinder are a piston
325 fitting into cylindrical bore 328 of intensifier housing 326. The
intensifier
housing has a smaller diameter at bore 327 than at 328. The piston is shown at
the bottom of its stroke. Seal 331 located between the inside of the piston
and
extension tube 324 acts against well pressure. Well pressure also acts against
seals 330 between the piston and bore 328 of the intensifier housing. Fluid
contained in cavity 336 acts on the small side of the piston and is therefore
at a
higher pressure than the well fluid. Seal 329 between bore 327 and the piston
and seal 398 between the extension tube and the inner diameter of the piston
are acted on by the pressurization fluid.
Extension tube 324 may be part of housing 326, but for ease of
manufacturing it may be sealed to and secured to the housing. Figure 7
illustrates an O-ring seal 339 with bolts 340 securing the tube to the housing
but
many other methods are possible. Passage 337 is a breather hole to allow air
to
escape or flow into the area between the external seals on the piston. O-ring
seals 354 and 355 have the same function as with all the double wall standpipe
embodiments. They act to seal the standpipe to the housing in two places with
pressurization fluid flowing into the passage 321 between the two seals.
For ease of manufacturing, Figure 7 illustrates a step in the
cylinder housing bore but a piston having a larger area on the bottom side
than
the top side can also be achieved by a stepped extension tube and a cylinder
housing with a straight bore.
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Figure 8 is a preferred embodiment of a stand-alone stuffing box
mounted in a wellhead frame using a floating single wall standpipe with a
pressurization system. Space is often a constraint when retrofitting stuffing
boxes to existing equipment. In general terms, the sealing system is
equivalent
to Figure 4, except the pressurization seal 347 has been removed from the
annulus between the shaft and the standpipe and is relocated to the annulus
between the shaft and the housing. The lantern ring has been eliminated since
the leakage path past the primary rotary seal is between the shaft and the
standpipe. Elimination of the lantern ring and relocating of the secondary
seal
saves axial length and this is an advantage where space is constrained.
However, the pressurization seal cannot be field serviced without removal and
disassembly of the stuffing box.
Pressurization fluid is introduced through fluid passage 399.
Pressurization fluid pressure may be indicated on pressure gauge 314.
Pressurization seal 347 is preferably a high pressure lip seal. It may be
fitted into
a groove or retained by, for example, a spacer ring 348 and a retaining ring
such
as a snap ring 349. A single wall standpipe 92 is secured to housing 52 by
special fastener 309 which prevents rotary and axial displacement. The special
fastener is sealed to housing 52 to prevent loss of well fluids. As with
embodiments shown in Figures 4, 5, 6, and 7, the standpipe can be fastened to
permit inspection and replacement through the top of the stuffing box
stuffing.
Figure 4 is not shown with the upwardly removable standpipe but it can be done
in the same manner illustrated by Figure 8.
Preferably, the primary seal is comprised of a high pressure lip 305
seal acting first against wellhead pressure in series with packings 116 acting
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once the lip seal has failed. The principles have already been described under
the description of Figure 4. Alternately, only the high pressure lip seal or
only
packings may be used. The advantage of packings is that they are split and can
thus be replaced without removing the drive head from the wellhead frame 311.
In this embodiment, bearings 312 and 313 are preferably greased.
Grease nipple 346 and grease relief 345 are for purposes of adding grease to
the housing. Alternately, the bearings may be in an oil bath. Housing cap 344
can be removed for repair of seals or bearings. Primary seals 305 and 116 can
be serviced from above the stuffing box as previously described.
Figure 9 is a preferred embodiment of a stand alone stuffing box
constructed with a non-rotating tubular shaft 357 bearingly supporting a
rotating
housing 356. The bearings 358 and 359 can be lubricated with the
pressurization
fluid as it travels toward the lower side of the primary seal 116 along fluid
passages 368 and 369. This configuration is simpler to construct than a double
wall standpipe but it uses more length and does not align the standpipe and
the
body as well as the double wall standpipe configuration because the primary
seal and pressurization seal are outside the bearing supports and self
alignment
is not possible. The primary rotary seal 116 is field serviceable without
removing
the stuffing box from the well but the pressurization seal 360 is not. It may
be
preferable to use a high pressure lip seal as the pressurization seal to save
axial
space. Pressurization fluid that escapes past the pressurization seal is
preferably
returned to the pressurization source though fluid passage 367. Collection of
leaked pressurization fluid is provided for by oil seal 361 which is
preferably
protected by flinger seal 362.
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Figure 10 is a preferred embodiment of a drive head with an
integral stuffing box mounted on the bottom of the drive head with a
pressurization system. In some cases, hydraulic pressure is readily available
to
provide for stuffing box pressurization. However, the standpipe system
requires
a large shaft and large bearings, which may be too expensive for some
applications. In these cases, a bottom-mounted stuffing box with a
pressurization
system may be an economic solution. This can be done with the stuffing box
integral with the drive head or as a stand-alone stuffing box mounted in a
wellhead frame as shown in Figure 11. In this preferred embodiment shown in
Figures 10 and 11, there are a pressurization seal and a primary seal
preferably
comprising two sets of packings separated by a packing preload spring that
acts
as a lantern ring. The packings run on a hard sleeve that is supported on an
extension 383 of the main shaft 80 of the drive head. The main shaft is
supported by bearings 379, 380 and the stuffing box is fitted to the drive
head
with a pilot diameter 400 to align the rotating shaft with the rotary stuffing
box
seals. Although alignment is not as good as with a floating standpipe, this is
a
cost effective solution, suitable in conditions where stuffing box wear is not
severe. In this embodiment the primary rotary seal 384 is located at the
bottom
of the stuffing box. The upper seal 385 is the pressurization seal. Since the
pressurization seal is sealing against lubricant, wear of the pressurization
seal
and shaft extension 383 is generally not severe. It may be preferable to use
one
or more lip seals as the pressurization seal rather than packings because they
need less space and have no leakage.
Lubricant leakage passing through the pressurization seal should
not be allowed to enter the housing 52 through the lower shaft seal 387. For
this
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CA 02711206 2010-07-28
reason a spacer ring 386 is placed above the pressurization seal 385 to allow
pressurization fluid to escape through passage 382. Pressurization fluid
enters
the stuffing box through passage 381 and pushes against both sets of packings
together with preload spring 118. Packing pusher 372 loads the pressurization
packing 385 while spacer ring 389 pushes against primary packing 384. Spacer
ring 388 or an equivalent shape in stuffing box housing 401 prevents packing
extrusion.
Figure 11 is a stand-alone stuffing box similar to and using the
same principles as the integral stuffing box shown in Figure 10 except in this
case the stuffing box is driven by the polished rod.
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