Note: Descriptions are shown in the official language in which they were submitted.
CA 02286872 1999-10-18
HYDRAULICALLY OPERATED PRESSURE RELIEF VALVE
BACKGROUND OF THE DISCLOSURE
The present disclosure is directed to a pressure relief valve installed
with a mud pump. The function and operation of this valve will become
more apparent on a review of the context in which the device is used. To
set the stage, consider the operation of a typical triplex mud pump which
is delivering a large volume of mud flow for a drilling rig. The mud is
delivered to the drill stem to flow down the string of drill pipe and out
through the drill bit appended to the lower end of the drill stem. It flows
through the drill bit. The flow of mud cools the drill bit and reduces the
temperature so that it lasts longer. Moreover, the mud flow is jetted out
through a set of openings in the drill bit so that the mud hydraulically
washes away the face of the well borehole if it is formed of soft materials.
In addition, it washes away rock chips and cuttings which are generated as
the drill bit advances. Then, the mud flow must return to the surface in
the annular space on the outside of the drill stem and on the interior of
the open hole formed by the drilling process. While portions of the
borehole may be cased from the surface, the mud flow must be of
sufficient velocity that the mud is returned to the surface so that chips and
cuttings which are inherently heavier than the mud are flushed to the
surface and delivered. This requires a substantial flow velocity. The
cooling necessary also requires a substantial velocity. The mud flow
velocities required mandate a high volume of mud. It is not uncommon
that a triplex mud pump will deliver 500 or even 1,000 gallons per minute
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through the drill stem. This substantial flow must be delivered under
control. As will be understood, the flow path down through the drill stem
and back through the annular space describes a U-tube. The U-tube will
therefore prompt a return at the surface but not with adequate pressure.
Pressure levels at the pump side have to be raised to get the velocity and
volumetric throughput desired. It is not uncommon for the triplex pump
to be required to deliver mud flow at 1,000 psi and even higher. The
wellhead pressures at the pump must be much higher if there is
substantial flow pressure resistance along the flow path. The pump
therefore is often operated at a very high pressure.
The drill stem in a deep well is an impediment to flow, thereby
resulting in higher back pressures. The impediment to flow is overcome
by applying greater pressures at the surface. In this regard, the mud
pump typically could be operated at pressures as high as 5,000 psi output.
Because of the great variety of circumstances in which the drilling rig may
be used, the output pressure of the mud pump may vary widely. In one
regard, the mud pump output pressure will vary with the change in pump
speed. Sometimes, the prime mover for the mud pump will not run
smoothly. The mud pump itself has a characteristic pressure peak
signature. Normally, mud pumps are constructed with three large
cylinders which provide three pressure peaks during each cycle of
operation. These pressure peaks can be excursions as great as 200 or 300
psi on top of the prevailing baseline pressure.
Because of variations in motor speed, because of the three cylinder
construction involved, the mud pump output pressure will vary
significantly. The present disclosure is directed to a regulator which is a
pressure relief valve able to control and stabilize the pressure downstream
CA 02286872 1999-10-18
of the triplex mud pump. The mud pump output manifold is input to the
pressure relief valve of the present disclosure. That valve is operated so
that the output pressure is controlled to a desired level. The output
pressure accomplished by the present regulator enables the system to
operate with a controlled pressure level. This is important to insure that
the pressure experienced down hole at the drill bit and in the formations
penetrated by the drill bit is regulated. Mud pressure at the bottom of the
drill stem is an important factor. The pressure has to be maintained at a
certain range to prevent formation damage. As the well is drilled, there is
a tendency for the liquids in the mud to migrate into the formation and to
form a residue known as a mud cake in the well borehole. The mud cake
is generally desirable. The depth of penetration into the formation by the
liquid solvent is not as desirable in that it may force the hydrocarbons in
the formation back away from the well borehole and make it more difficult
to start production flowing. That is one problem with excessive bottom
hole pressure. Where the bottom hole pressure is inadequate, there is the
risk that gas flow will start, and the gas will then cut or thin the drilling
mud. Typically this happens with gas bubbles entrained in the mud which
are small in size at the bottom of the borehole but which become larger at
the surface where the hydrostatic head on the mud is less. This thinning
of the drilling fluid describes a process known as gas cutting. When the
mud is gas cut, it is not as reliable in operation because the standing
column of mud in the well borehole does not have the necessary weight.
In other words, it tends to froth and reduces the bottom hole pressure,
creating a dangerous condition. The bottom hole pressure is in part
controlled by the mud pump pressure. It is therefore rather important to
regulate and control the mud pump pressure.
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The apparatus of this disclosure is briefly summarized as a pressure
regulator having a movable element which is dynamically responsive to
pump output pressure. The movable element is located in the mud flow
path and is installed so that it modulates the mud flow. Dynamically,
movement is occasioned by the movable element which is in a slight
pressure balanced condition. On one side, pump pressure is applied. On
the other side, the same pump pressure is applied but the second side is
less in diameter and has a reduced cross-sectional area. There is a
compensation applied to this side for balance purposes. That compensation
is provided by a piston rod extending to a remote chamber, and the
chamber itself is provided with a hydraulic fluid pressure balance across
the chamber. Through the use of a restricted flow path, movement of the
rod is controlled so that it modulates the forces applied to the valve
element. This accomplishes a desired setting under the control of
hydraulic fluid. That is provided with a reduced hydraulic pressure.
There is a multiplier which reduces the hydraulic pressure required for an
operating range.
BRIEF DESCRIPTION OF THE DRAWINGS '
So that the manner in which the above recited features, advantages
and objects of the present invention are attained and can be understood in
detail, a more particular description of the invention, briefly summarized
above, may be had by reference to the embodiments thereof which are
illustrated in the appended drawings.
It is to be noted, however, that the appended drawings illustrate
only typical embodiments of this invention and are therefore not to be
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considered limiting of its scope, for the invention may admit to other
equally effective embodiments.
The single drawing in this disclosure is a pressure relief valve shown
in sectional view where the section cut line is along a diameter of the
cylindrical housing thereof showing internal details of construction, and
further including a schematic connecting the valve of this disclosure to a
pump and in a mud flow system.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Attention is directed to the only drawing where the numeral 10
identifies the pressure relief valve of the present disclosure which is
connected in a mud flow system. The mud flow system will be described
to provide the context. The mud flow system is shown in schematic form
for simplification. The system includes a triplex mud pump 12 which
delivers a flow of mud through an output line, typically around three
inches in diameter. It is delivered into the valve 10 of this disclosure. The
regulated output is delivered to the rig mud line 14. The mud line 14
extends upwardly in the derrick and connects to the top end of the drill
stem 16. The drill stem is supported in the derrick on a cable extending
through the crown block and down to the traveling block which supports
the swivel and mud line connected to the swivel. The drill stem at the top
end incorporates a Kelly which is a pipe having a square profile. The
square shaped Kelly extends through the rotary table which imparts
rotation to the drill stem. The drill stem includes one or more joints of
drill pipe. At the bottom, it includes one or more drill collars which are
drill pipe which an extra thick wall to enhance weight and stiffness. The
bottom-most component in the drill stem is the drill bit.
CA 02286872 1999-10-18
The mud flow is returned to the surface in the annular space on the
outside of the drill stem. The mud is delivered up through this annular
space and flows through the blowout preventor stack under the drilling
rig. It is collected in a mud recovery line and delivered away from the
drilling rig to a degasser 18. Sometimes, a degasser is replaced by a shale
shaker or other screening device. After degassing, it is then delivered into
a set of mud pits 20. The heavier cuttings are permitted to fall to the
bottom of the mud pits and the lighter drilling mud is recovered from the
top of the mud pits. This helps remove some, perhaps most of the cuttings.
The mud is then recycled by delivery from the mud pits 20 back through
the pump 12 and flows through the same route again. At various locations,
mud line pressure measuring instruments 22 form an indication of mud
pressure.
The triplex pump 12 delivers mud with peaks resultant from the
three pistons sequentially operating in the pump. The three pistons
provide pressure peaks creating a ripple. If the pump is operated at 100
rpm, there will be three peaks per cycle or 300 peaks per minute.
Variations in pump pressure occur for these and many other reasons. It is
therefore desirable that they be smooth. The valve 10 is able to
accomplish this.
Going now to the valve in particular, and tracing through its
construction form bottom to top, the pump 12 is connected through a
suitable flow line connected to valve inlet opening assembly 24. That has
the form of a hollow sub which is provided with an internal passage 26. It
is constructed with an external flange 28 to enable attachment by a set of
bolts 30 located on a flange circle. The number of bolts, thickness of the
flange and other details of this sort are controlled so that the sub 24 is
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constructed in accordance with industry standards. The sub 24 is bolted at
the flange to the valve body 32. The body 32 is provided with an enlarged
internal passage at 34. This enables a sleeve 36 to be inserted for locking
purposes. The sleeve locks in position a lower seal ring 38. The seal ring
38 is similar to the upper seal ring 40 except that they are at opposite
ends of a movable valve element 42. The upper seal ring 40 is locked in
place with a similar sleeve 44. The sleeves 36 and 44 serve the same
function and are relatively similar in construction.
The valve body 32 is an elongate cylindrical construction having an
appended and integrally constructed outlet housing 46. The housing 46
connects serially to an output sub 48. Together, they have a common
passage indicated at 50. They are also joined with appropriate bolts on a
flange bolt circle, the bolts 52 being incorporated in appropriate number
and rating. This locates the valve outlet 54 for connection with the rig
mud line 14 as previously mentioned. The upper end of the valve body is
closed over by a hydraulic chamber in a cylinder 58. The internal
chambers will be detailed later. At the lower end, it incorporates a flange
60. The flange 60 aligns against the top face 62 and is joined to it again
with bolts connecting to the flange. The bolts 64 are similar to the bolts 30
previously mentioned. Coming back now to the movable valve element 42,
it will be observed that the passage 26 is extended inside the sleeve 44.
This defines the passage 66 above the movable element. The element 42
can move upwardly and downwardly in the aligned passages 26 and 66
because they have the same diameter. The element 42 serves as the
modulating valve element in the system. It moves in the passages 26 and
66 which have equal cross-sectional areas. The passage 66 enables mud
under pressure to act on the top side of the valve element. The valve
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element 42 is provided with the internal passages 68 which communicate
to the top end of the valve element. This enables it to operate in a
pressure balanced mode except for the loss of area in a piston rod 70. That
loss of area will be detailed below.
The pistol rod 70 extends up through a captured cylinder head 72.
That cylinder head is located at the bottom end of the chamber 58. The
piston rod 70 is joined to a guide bushing 74 in the chamber 76. The
bushing 74 is also a seal ring support structure so that the chamber 76 can
fill with hydraulic oil which does not escape at the bottom end of that
chamber. Moreover, the bushing 74 serves as a compression piston for
hydraulic oil in the chamber 76 as will be described. The piston rod
connects with an upper rod 80. The rod 80 has an outlet passage 82 which
defines the outlet end of a flow path through a check valve 84. The check
valve 84 is a one way valve which permits flow from the top of the
drawings towards the bottom, i.e., out through the passage 82. This flow
path will come into significance momentarily. The system includes a pair
of hydraulic lines. The hydraulic line 86 is an input to fill the chamber 76.
The piston rod 80 is enlarged at the upper end slightly, this being
identified as the piston rod extension 90 which has slightly larger
diameter. It is however received in an upper head 92 which is drilled to a
diameter sealing against the larger rod diameter 90. With a suitable seal
ring around the rod 90, flow along the piston rod 90 is prevented so long
as the seal ring is in the upper head 92. The seal ring however will break
clear of that and permit filling of the upper chamber 94. The chamber 94
is under a floating piston 96. The piston 96 is below an air cushion in the
chamber 98. The chamber 98 con~~eniently is provided with air at low
pressure which pressure can be regulated to some low value such as
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CA 02286872 1999-10-18
atmospheric. The inlet to the chamber 98 enables a reset pulse to be
applied for moving the valve element to the fully closed position.
Hydraulic oil in the upper chamber 94 is permitted to escape through an
outlet control valve 78 which is adjustable to control the hydraulic oil
dump from the system.
Consider operation of the equipment as described. The pump
increases pressure until the movable valve element is forced upwardly.
Equal pressures are applied to the top and bottom of the element 42.
However, the cross-sectional -area of the top side of the element 42 is less
than the cross-sectional area of the bottom side. While the passages 26
and 66 are equal in diameter, the top side area is reduced by the amount
of cross-sectional area involved in the piston rod 70. That creates a force
differential forcing the piston rod 70 upwardly. The controlling force of
the system is applied through the rod 70. As just stated, the valve
element 42 will open because there is a greater force acting to open it
rather than to close it. It starts opening in response to this increased mud
pressure. As it moves, there will be a change in the force at the upper end
of the piston rod 70. This involves the application of forces serially
applied to the rod 70 from 'the rod 80 and that from the enlarged rod 90.
To consider how this occurs, assume for the moment that the rod 70 moves
upwardly. Hydraulic oil first fills the chamber 76. As the rod 80 moves
upwardly, the chamber 76 pressure is held steady until the seal ring on
the rod enlargement 90 pops through and disengages at the upper end.
The seal ring on the enlarged rod 90 breaks free of the surrounding
divider wall 92. This then permits the chamber 94 to be filled by flow
along the rod 90. Oil under pressure will move the free floating piston 96
upwardly in the chamber 98. The piston 96 separates the oil from the air
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in the chamber 98. One force acting on the piston rod is derived primarily
from the hydraulic oil pressure on the piston 74. That force reduces to
zero on the loss of hydraulic fluid from the chamber 76 into the upper
chamber 94. While that movement may occur, the force on the rod 70 acts
against the movable valve element 42 to control upward movement. This
modulates the opening of the valve element 42.
The piston rod 70 will close the valve element 42 at least partially
because the hydraulic fluid in the upper chamber 94 is vented through the
check valve 84 and out. through the port 82 to refill the chamber 76. That
flow path enables relatively quick response (downward movement). For
movement in the opposite direction, there is a buildup of force in the
piston rod 70 until the time that the piston rod 90 clears and the seal
breaks free. In other words, a small upward movement of the rods 70, 80
and 90 prior to the seal disengagement is then accompanied by the quick
flow of hydraulic fluid from the chamber 76 up into the chamber 94.
There is a restrictive aspect to this upward flow of hydraulic fluid around
the rod 90 because the rod 90 blocks most of the opening This restricted
flow path results in controlled filling of the upper chamber 94.
The valve 78 is used to control the hydraulic pressure in the system.
This is an adjustable mechanism which permits the valve element to be
raised or lowered. The valve element 78 is therefore controllably set so
that the pressure in the quiescent state in the chamber 76 is regulated.
In normal operation assume that the valve element 42 is in the down
position. Pressure at the inlet side builds up to force the valve element 42
upwardly. This force is transmitted to the piston rod 70, the piston rod 80
and the piston rod 90. They move together as a unit. This raises the
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pressure in the chamber 76. Assume for the moment that a force balance
is achieved. That will then stop movement of the valve element 42.
If the pressure below the valve element 42 continues to increase, the
pressure in the chamber 76 is raised. Eventually, that pressure will exceed
the setting of the adjustable valve element 78 which is then forced open.
This delivers hydraulic oil through the flow line 86 out of the chamber 76.
The oil flows through the valve 78 and into the chamber 94. As oil flows
out of the chamber 76, the piston 74 moves upwardly along with the
piston rods 70, 80 and 90. When the seal around the rod 90 breaks free,
there is a large flow of oil so that the piston rod 70 and connected
equipment will rise, ultimately moving to the point that the chamber 76 is
substantially empty of hydraulic oil. The oil is pumped out. This occurs in
conjunction with opening the seal 38 around the valve element 42. This
opens the controllable flow path. As previously mentioned, resetting this
control under an external source by delivery of an air pressure pulse into
the top end of the chamber 98. Ordinarily, that chamber is permitted to
float at atmospheric pressure. It can be used for a control line which
delivers a closing pulse.
Wear parts are readily replaced. The seal rings 38 and 40 probably
wear most of all. They can be easily replaced by simple disassembly to
retrieve the sleeves 36 and 44 which lock them in position. Moreover, the
system is constructed so that drilling fluid is kept below the relatively
thick head 72. In fact, loss of drilling fluid along the piston rod 70 into
the
chambers above is prevented. Hydraulic fluid in the chamber 76 is not
permitted to get into the head 72 and commingle with the drilling fluid. In
summary, service is easily accomplished. These wear parts are easily
removed and placed. Periodic maintenance of the system typically
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involves replacement of selected parts. As desired, other components can
be periodically replaced but they generally have a fairly durable life.
One context for the present apparatus is control of mud flow. In that
instance, it is connected downstream from a typical triplex mud pump at a
drilling rig. Normally, it is used to regulate the pressure of the mud which
is applied to the downhole drill stem extending into the well borehole for
lubrication of the drill bit and removal of the cuttings. For any number of
reasons, the dynamic pressure load on the mud system may fluctuate. For
instance, the mud pressure at the surface can drop because the well
borehole penetrates accidentally into a void or other low pressure
formation which will drop the mud pressure momentarily. On the opposite
hand, the drill bit may penetrate into a high pressure zone so that gas is
produced at very high pressure, thereby raising the pressures encountered
by the mud flow. Any number of reasons can be suggested in addition to
these whereby the pressure will fluctuate.
Pressure fluctuations are commonplace in operation. Such pressure
variations are apt to cause all types of problems. The apparatus 10 of this
disclosure helps overcome these problems. Drilling mud can be very
abrasive to the pump and any metering device controlling the pump. The
construction of the present device uses replaceable seal members 38 and
40 so that the valve element can move readily in them to mud flow
isolation at full closure. The valve element is relatively dynamic. The
response time for movement is relatively quick. In general terms the
control mechanism illustrated in the drawings finds great use in control of
the other incompressible adhesive fluid by the modulation of the valve
element 42.
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While the foregoing is directed to the preferred embodiment of the
present invention, other and further embodiments of the invention may be
devised without departing from the basic scope thereof, and the scope
thereof is determined by the claims which follow.
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