Note: Descriptions are shown in the official language in which they were submitted.
~104467
Backqround of the Invention
Field of the Invention. This invention relates to a
force limiting device for a valve actuator rod and, more
specifically, such a device which produces a maximum tensile
5 force thereon less than would normally be produced on a piston
of a valve actuator associated with the actuator rod when the
actuator has located the valve in a backseated position.
Description of the Prior Art. There have heretofore been
utilized a number of means for closing large valves in the
; 10 feedwater and steam systems of power plants. It is presently
felt that a valve actuator incorporating a hydraulic cylinder
and a gas accumulator affords an excellent means for rapid
closure of these valves. Hydraulic oil is directed to a
piston within the hydraulic cylinder to open the valve in
15 opposition to a precharged source of high pressure gas which
acts on the other side of the piston. A charge of high
pressure gas is maintained in an accumulator to insure a
sufficient quantity is available to act on the piston to
rapidly close the valve when the high pressure is relieved.
While this system produces adequate force for opening and
closing the valves, it has been found that the dimensions of
such actuators and the pressures required are capable of
producing a large tensile force on the actuator rod when the
valve is in an open, backseated position. Accordingly, there
; 25 have heretofore been utilized various force limiting devices
in the form of couplings between the actuator rod and the
valve stem to protect the valve during backseating. While
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these devices have reliably limited the tensile force, they
have required an additional element and thus an added expense
to the valve-actuator configuration. There remains a continu-
ing need for a simple, reliable, inexpensive means for utiliz-
ing such a valve actuator while insuring the valve will not bedamaged during backseating.
Summary of the Invention
It is, therefore, an object of the invention to provide a
force limiting device for an actuator rod which does not re-
quire a force limiting coupling between the actuator rod andthe valve stem.
It is another object of the invention to provide a force
limiting device of the type described which is safe and reli-
able and relatively inexpensive to manufacture.
These and other objects of the invention are provided by
the preferred embodiment thereof in the form of a force limit-
ing device for an actuator rod of a valve actuator of the type
which includes hydraulic fluid from a first source being se-
lectively applied to and discharged from a cylinder at a first
side of a piston means mounted therein and connected to the
actuator rod as fluid from a second source acts in opposition
thereto on a second side of the piston means. The piston means
has a central cavity therein which includes a cylindrical bore
having a diameter which narrows into an opening at the first
side of the piston means having a lesser cross sectional area
than the bore. The actuator rod is slidably received within
the opening to extend from the first side of the piston means
through a hole in a first end of the cylinder and terminates at
an extended end thereof which is outwardly of the cylinder and
capable of being joined to a valve stem. An enlarged piston
head is secured at a first side thereof to the other end of the
rod and is disposed within the cylindrical bore for retention
therein by a portion of the piston means around the opening.
The piston head makes sliding, sealed contact with an interior
of the bore for limited axial movement therein. There is
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included means for communicating with the bore to cause a pre-
determined fluid pressure from a third source different from
the first and second sources to be applied to a second side of
the piston head. The hydraulic fluid producing a pressure on
the piston head is opposed by the predetermined fluid pressure
to produce a maximum tensile force on the actuator rod when
the piston means is being held against a piston stop at a
second end of the cylinder by the hydraulic fluid to produce a
force on the piston means greater than the maximum tensile
force.
Brief Description of the Drawings
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Figure 1 is a sectional side view of a prior art valve
actuator including a coupling between the actuator rod and the
valve stem with the valve shown in a backseated position.
Figure 2 is a fragmentary sectional side view of the pre-
ferred force limiting device including various features of the
invention and demonstrates the position of the elements at the
time the valve is being opened.
Figure 3 is a fragmentary sectional side view of the em-
- 20 bodiment of Figure 2 shown in a position just prior to back-
seating with little or no flow pressure differential across
the valve.
Figure 4 is a fragmentary sectional side view of the em-
bodiment of Figure 1 shown in a position for backseating.
Figure 5 is a sectional side view of an alternative em-
bodiment of the invention with the elements thereof shown in a
position for backseating.
Detailed Description of the Preferred Embodiment
As can be seen in Figure 1, a prior art valve actuator 10
without any form of lost motion device includes a hydraulic
cylinder 12 which is mounted on a yoke 14 of a valve 16. A
piston 18 is slidably mounted within the cylinder 12 and
includes an actuation rod 20 which extends from a first side
22 of the piston 18 through an opening 24 in a first end 26 of
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the cylinder 12. The rod 20 has an extended end 28 which is
joined by a coupling device 30 to a valve stem 32.
The valve stem 32 extends through packing 33 of a bonnet
34 of the valve 16 to operate a closure member 36, in this case
a gate device, of a valve 16. As shown, the piston 18 and thus
the closure device 36 are in the open position.
The prior art valve actuator 10 further includes an ac-
cumulator 38 which, preferably, is spherical in shape and in-
cludes a pressure wall 40 which is intersected at an opening
42 therein by the cylinder 12. The cylinder 12 is fixedly,
sealably joined to the pressure wall 40 and is positioned rel-
ative thereto to include a second end 44 which is disposed
within the interior 46 of the accumulator 38. The second end
44 of the cylinder 12 is in this prior art embodiment opened to
allow high pressure gas within the interior 46 of the accumu-
lator 38 to act on a second side 48 of the piston 18.
During normal valve operation, hydraulic fluid is sup-
plied to the interior of the cylinder 12 through an access
hole 50 to act on the first side 22 of the piston 18 to cause
it to move axially in opposition to high pressure gas of the
accumulator 38 until the valve 16 is in the open position as
shown in Figure 1. When closure of the valve 16 is desired,
the hydraulic fluid of the cylinder 12 will be allowed to
rapidly discharge through the access hole 50, allowing the
high pressure gas to act on the second side 48 of the piston 18
to cause it to move axially in the closed direction.
As thus disclosed, the prior art valve actuator 10
normally provides a reliable means for opening and closing a
valve but the relatively rigid coupling 30 will cause all of
the force on the piston 18 to be applied to the closure device
36 which could damage the valve during backseating if the
pressure of the hydraulic fluid and the high pressure gas are
not accurately maintained. Additionally, it can be seen that
any effort to limit the force applied to the backseat would be
greatly complicated if the high pressure gas of the accumula-
tor were allowed to escape through leakage or rupture of the
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accumulator. Accordingly, some valve actuators of this type
have been improved to include a partial closure over the
second end 44 of the cylinder 12 to limit the upward movement
of the piston 18 when the valve is in the opened position.
S However, the distance between the closure at the second end 44
and the backseat of the valve 16 could not be reasonably manu-
factured with acceptable tolerances to insure that excessive
force would not be applied to the back seat. Obviously, there
would be expansion and contraction of the actuator rod 20 and
the valve stem 32 so that either full force by the hydraulic
fluid would be applied to the backseat or potentially no force
would be applied to the backseat if the piston 18 were against
the closure and the rod and stem too long to bring the closure
element 36 to the backseated position.
Therefore, there have been heretofore utilized different
coupling devices from the coupling device 30 shown in Figure
1. These devices (not shown) utilize some form of spring or
biasing device in a lost motion fashion to allow the piston 18
to fully seat against the closure at the second end 44 of the
cylinder 12 while transmitting a predetermined force to the
valve stem 32 to provide proper spacing and pressure for
backseating. The present invention is intended to replace
sùch a coupling device so that a simpler, more direct coupling
device, such as coupling device 30, might be utilized while
still providing for proper backseating.
Also, as seen in Figure 1 of the prior art valve actuator
10, there is provided a feature which is intended to insure
that hydraulic oil will not be allowed to leak into the inte-
rior 46 of the accumulator 38. The piston 18 is provided a
pair of sealing rings 52 generally for this purpose but should
the sealing ring 52 adjacent the first side 22 allow hydraulic
fluid to leak thereby,a means is provided for preventing the
hydraulic fluid from leaking by the other sealing ring 52.
Accordingly, radially extending passages 54 communicate the
space between the sealing rings 52 with the axial passage 56
of the actuator rod 20. The passage 56 extends to the extended
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end 28 of the actuator rod 20 to terminate at a fitting 58
which can selectively be opened to the atmosphere. Therefore,
should leakage occur as described above, the hydraulic fluid
would pass through the passages 54 and the axial passage 56 to
be discharged to atmosphere through the fitting 58. Since the
interior 46 of the accumulator 38 is at a significantly higher
pressure than these passages, the hydraulic fluid would not be
able to leak by the sealing ring 52 adjacent the second side 48
of the piston 18.
As seen in Figure 2, the preferred force limiting device
60 can be utilized in a valve actuator 62 which again has a
cylinder 64 similar to cylinder 12 mentioned above. Unless
otherwise indicated~ the various elements of the actuator 62
will be identical to the elements as described hereinabove for
the prior art actuator 10.
The force limiting device 60 includes a piston means 66
mounted within the cylinder 64 for sliding axial movement
therein for generally positioning the valve closure element.
However, the preferred piston means 66 has a central cavity 68
which includes a cylindrical bore 70 having a diameter D and
narrows into an opening 72 at a first side 74 of the piston
means 66 and can be seen to have a lesser cross sectional area
than does the bore 70. An actuator rod 76 extends through the
opening 72 to extend from the first side 74 through a hole 78
in a first end 80 of the cylinder 64 and terminates at an
extended end thereof which is outwardly of the cylinder 64 and
is capable of being joined to a valve stem as was the case in
the prior art valve actuator 10. An enlarged piston head 82 is
secured at a first side 84 thereof to the other end 86 of the
rod 76 and is disposed within the cylindrical bore 70 for
retention therein by a portion of the piston means 66 around
the opening 72. The piston head 82 includes a sealing ring 88
and makes sliding, sealed contact with an interior of the bore
70 for limiting axial movement therein.
While it can be seen that in the preferred force limiting
device 60 the rod 76 and the opening 72 have a circular cross
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section, it will be seen that other cross sectional shapes
might be reasonably employed without altering the effective-
ness of the invention.
The piston means 66 is generally formed of two elements,
a first piston element 90 which includes the central cavity 68
and a second piston element 92 which is bolted thereto with a
plurality of bolts 94 to generally enclose the cylindrical
bore 70. Each element 90, 92 has a sealing ring 96 to provide
the piston means 66 with a general sealing configuration as
shown in the prior art valve actuator 10. Again, the actuator
rod 76 includes an axial passage 98 to provide a leakage path
from a space between the sealing rings 96 through radially
extending passages 100 (perhaps better seen in Figure 3) and
the cylindrical bore 70 for communication with the atmosphere.
The elements as thus described are shown from a different
angular position in Figures 3 and 4. However, the piston
means 66 is shown in a different location in each of Figures 2,
3 and 4 for a better explanation and understanding of the
force limiting device 60 of the preferred embodiment. Accord-
ingly, each of these figures will be separately explained for
a better understanding of the forces which are acting on the
piston means 66 and the piston head 82 to enable the preferred
embodiment to satisfy the objectives of the invention. As
seen in Figure 2, the valve would still be in a closed position
and hydraulic fluid has been supplied through an access hole
102 to the interior of the cylinder 64 to apply an upward force
to the piston means 66. While designed lea~age through
opening 72 by actuator rod 76 will cause the hydraulic fluid
to act on the first side 84 of the piston head 82, the amount
of force required for lifting the valve closure element from
its seat is sufficiently large so that the force being re-
quired is actually generated by the hydraulic fluid acting on
; the piston means 66 rather than on the piston head 82. As a
result, the piston head 82 is positioned axially within the
bore 70 for direct contact between the head 82 and the piston
element 90. The large force created is thereby transmitted
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through the piston head 82 and the actuator rod 76 to cause the
valve to open. It should be understood that the force being
applied to the piston means 66 is generally equal to the
pressure of the hydraulic fluid times the effective area of
the first side 74 of the piston 66 less the pressure of the
high pressure gas from the accumulator times the effective
area of a second side 104 of the piston means 66. Similarly,
the force being applied to the piston head 82 by the hydraulic
fluid will be the pressure of the hydraulic fluid times the
effective area of the first side 84 of the piston head 82 less
the pressure (in this preferred embodiment, the atmospheric
pressure) in the bore 70 times the area of a second side 106 of
the piston head 82. While the resulting force in each case
will tend to change depending on the amount of force being
applied by the hydraulic fluid, during the supply of hydraulic
fluid required to initially open the valve, the resulting
force on the piston means will be greater. Therefore, direct
contact between the piston means 66 and the piston head 82
will transfer the force needed to the valve stem for initially
opening the valve. However, once the initial opening force
required is reduced, the pressure of the hydraulic fluid will
similarly reduce as the piston means 66 is allowed to move
axially up the cylinder 64. At this lower force condltion for
the hydraulic fluid the resulting areas and pressures are such
that the primary force moving the valve stem and the closure
element is created at the piston head 82. Therefore, the
piston head 82 will move axially within the bore 70 to a posi-
tion as generally shown in Figure 3. The amount of force
during this travel of the valve stem and closure element will
generally be the force which is required to overcome friction
in the packing which seals the extension of the valve stem
through the body assuming that the flow press differentials
are relatively low.
As shown in Figure 3, the hydraulic fluid has caused the
piston means 66 to travel axially in the cylinder 64 toward
the second end 108 thereof to position the valve at backseat-
ing. In this position, just prior to backseating, the
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hydraulic fluld is still producing a resulting force on the
piston head 82 and actuator rod 76 which is greater than the
resulting force acting on the piston means 66. As a result,
the piston head 82 is still disposed upwardly within the
cylindrical bore 70.
With the upward movement of the actuator rod 76 now re-
stricted by the closure element being held against the back-
seat, as seen in Figure 4, the pressure of the hydraulic fluid
being supplied to the cylinder 64 continues to increase to
insure that there is adequate force applied to the valve dur-
ing backseating. This force on the piston head 82 and rod 76
will again be determined by the area of the first side 84 of
the piston head 82, the second side 106 of the piston head 82
and the respective pressures acting thereon. The increase in
lS hydraulic fluid pressure will also act on the effective area
of the first side 74 of the piston means 66. This force from
beneath the piston head 82 will be opposed by the pressure of
the high pressure gas acting on the second side 104 of the
piston means 66 but will result in a significant positive up-
ward force on the piston head 82 to cause it to continue up-
wardly within the cylinder 64 until it makes contact with a
piston stop 110 at the second end 108 of the cylinder 64.
Although the preferred force limiting device 60 might be
utilized for any type of valve independent of size, it is
presently being considered for globe valves and gate valves
ranging from 6 inches (15.24 cm~ to 32 inches (81.28 cm).
~lowever, it would be helpful to present some parameters ex-
pected for a typical valve configuration. It has, for exam-
ple, been determined that for a 16 inch (40.64 cm~ gate valve
the desired backseating force could range between 11,000
(4988.7 kg) and 12,000 (544.2 kg) pounds. A typical hydraulic
system being used to operate a 16 inch (40.64 cm) gate valve
would be expected to operate at as high a pressure as 4,500
p.s.i. (316 kg/sq. cm). This level of pressure is sufficient
to create an opening force (that needed to lift the gate ini-
tially off the seat) in the range of 130,000 ~58957 kg) to
140,000 pounds (63492 kg) of force. Obviously, it can be seen
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that having a hydraulic system capable of applying a tensile
force of this magnitude to the actuator rod would be unaccept-
able for backseating. Therefore, such a force is utilized
when the elements of the valve actuator are in the position as
shown in Figure 2. However, when the valve is backseated as
shown ln Figure 4, the piston means 66 is being held against
the piston stop 110 by the hydraulic fluid producing a signif-
icantly higher upward force on the piston means 66 than can
the hydraulic oil as it acts independently on the piston head
82. Hydraulic fluid in this condition produces a pressure on
the piston head 82 which is opposed by a predetermined fluid
pressure, atmosphere pressure in the preferred embodiment, to
produce a maximum tensile force on the actuator rod 76 which
is significantly less than the hydraulic fluid system is cap-
able of providing and actually does provide during initialopening of the valve.
Specific dimensions and areas for the various surfaces
mentioned above would have to be determined according to the
specific requirements for the valve with which the valve actu-
ator is employed. Therefore, each force limiting device would
have to be specifically designed for its valve depending on
the expected forces required to unseat the valve, forces re-
quired for backseating the valve and forces required for
rapidly closing the valve.
Therefore, it can be seen that the preferred force limit-
ing device 60 can be employed to limit the tensile force being
applied to an actuator rod during a backseating condition and
that the system can also provide for the leakoff feature de-
scribed above which would prevent hydraulic fluid from leaking
by both sealing rings 96 into the interior 46 of the accumula-
tor 38. To further demonstrate the applicability of the in-
vention, it should be shown that other means can be provided
for communicating fluid pressure from an external source for
the proper application of force to the piston head 82~
As seen in Figure 5, there also exists a different valve
actuator 120 which utilizes hydraulic fluid and high pressure
gas in an accumulator 122 in the manner generally described
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above. However, the valve actuator 120 utilizes a pair of
pistons 124, 126 which are independently, slidably mounted
within a cylinder 128. During normal operation, both pistons
move axially within the cylinder 128 with the hydraulic fluid
being applied to a first side 130 of the piston 124 and high
pressure gas being applied to a first side 132 of the piston
126 through access holes 134 which extend through an end
closure device 136 rigidly fastened to the end of the cylinder
128.
However, this valve actuator 120 includes structure for
allowing hydraulic fluid to be utilized to close a valve
should the actuator 122 develop a leak or be ruptured to
significantly reduce the pressure of the high pressure gas
therein to a level which would adversely affect closure of the
valve.
The valve actuator 120 includes a tubular housing 138
which is secured to the end closure 136 and extends therefrom
to the upper portion of the accumulator 122. The tubular
housing 138 has an extended end fitting 140 external of the
accumulator 122 which is joined to a piping system which can
be vented to atmosphere. The piston 126 includes a hollow
tube 142 extending from its first side 132 to be received
within the interior of the tubular housing 138. Sealing means
144 is provided between the tubular housing 138 and the hollow
tube 142 at the end closure 136 to allow sealed, sliding
contact between the hollow tube 142 and the tubular housing
138. The hollow tube 142 is sufficiently long to maintain
this contact throughout axial travel of the piston 126.
A radially extending groove 146 is provided in at least
one of the abutting surfaces of the pistons 124, 126.
Therefore, if high pressure gas is unavailable to close the
valve, hydraulic fluid can be supplied to the tubular housing
138 through the extended end 140. The hydraulic fluid will
continue to pass through the hollow interior of the tube 142
to act on the upper surface of the piston 124. When hydraulic
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fluid is supplied between the pistons 124, 126, the piston 126
will be held against the end closure 136 and the piston 124
will be caused to move downward for closure of the valve.
As explained, it can be seen how the actuator 120 can be
caused to close a valve if high pressure gas is lost and if an
actuator rod were rigidly secured to the piston 124 in a
configuration in which the present invention was not being
utilized.
However, as seen in Figure 5, a pressure limiting device
150 is installed in piston 124 and again includes an actuator
rod 152 with an enlarged piston head 154 thereon. The piston
154 is sealably and slidably disposed with a cylindrical bore
156 in piston 124 which narrows to an opening 158 at the first
side 130 of the piston 124. During normal operation, the
piping associated with fitting 140 would be opened to atmos-
phere so that the space between the sealing rings associated
with pistons 124, 126 would be vented for accommodating
hydraulic leakage and atmospheric pressure would be acting on
the upper side 160 of the piston head 154. The relative posi-
tion of the piston head 154 within the cylindrical bore 156would be same throughout movement of the pistons 124, 126 as
was the case in the embodiment shown in Figures 2-4.
If the high pressure gas of the actuator 120 were un-
available, the valve could be closed by closing the vent
associated with the fitting 140 and introducing hydraulic
fluid to the tubular housing 138. Supplying hydraulic fluid
between pistons 124 and 126 would allow the piston 124 to be
moved axially for closing the valve. The hydraulic fluid
would cause the piston head 124 to be positioned downwardly
; 30 with respect to piston head 154 to cause a stop collar 170
threadedly mounted within the bore 156 to make contact with
and apply a force to piston head 154 and the actuator rod 152
sufficient to close the valve.
Having thus shown two embodiments of the invention, which
can be incorporated in a valve actuator of the type which
utilizes a hydraulic-high pressure gas configuration, it
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should be pointed out that other types of actuators might also
be properly fitted with a force limiting device of the present
invention. For example, if a valve actuator were of the type
which simply used a hydraulic cylinder and selectively applied
hydraulic fluid at opposite sides of a piston member slidably
disposed thereint a force limiting device of the present in-
vention could similarly be utilized to limit the tensile force
being applied to the actuator rod during backseating. It
should be clear that even though hydraulic fluid might be
introduced to both sides of the piston, a central cavity could
be provided which could be vented to atmosphere to again
control the amount of tensile force on the actuator rod when
the piston is positively located against a cylinder and stop
in the direction for opening the valve.
Although in the preferred embodiment the source of fluid
pressure being applied to the piston head has been air at
atmospheric pressure, it should also be obvious that any other
system utilizing a gas or fluid at a predetermined pressure
could be employed with the same results. Clearly, alterations
could be made to the invention as explained in the preferred
embodiment while still being included within the scope of the
invention as claimed.
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