The invention relates to stem seals for movable stems, and more particularly to a stem cylinder seals and a stem shoulder seal for movable stems.
Many machines or devices include stems, variously referred as pins, shafts, spindles, etc., extending from a fluid-containing member to transmit power into or out of it. All these stems or spindles need sealing to prevent the fluid from leaking around the stem, but some stems dispose one seal either at the stem cylinder or at the stem shoulder, and some stems dispose both the two.
For example, valves, as the controlling unit for fluid conveying, have a valving member, such as the ball in ball valves, the gate in gate valves, etc. The valving member is interposed in a flow path, and has an open position, which allows medium to flow through the valve, and a closed position, which prevents medium from flowing through the valve. The shifting of the valving member between the two positions is realized by a stem extending out of the valve. The stem needs sealing to prevent medium from leaking around the stem out of the valve into atmosphere. However, some valves need only one seal either for stem cylinders or for stem shoulders, and other valves need both the two.
Stems, such as stems for ball valves or gate valves, shall not be ejected through the pressure boundary by internal pressure when the stem packing and/or retainer have been removed in accordance with the related standards, so that the stem shall have a stop shoulder to be pressed against the inside of stem exits to prevent the stem from being removed through the stem exit opening at the bottom of the stuffing box of valves. That is to say, the stem, particularly the stem of ball valves, has two seals, one stem shoulder seal located at the inside of the stem exit and one stem cylinder seal starting at the outside of the stem exit and extending through the stuffing box. When assembled, the stuffing around the stem is compressed against the bottom of the stuffing box starting at the outside of the stem exit by glands, Belleville washers and the nut engaged by thread with the stem to provide a cylinder seal for the stem, and at the same time the stop shoulder of the stem is pulled tight against the inside of the stem exit opposite the bottom of the stuffing box to provide a shoulder seal for the stem.
The conventional metal stem stop shoulder is a plain (see
The conventional packing as stem cylinder seals is a group of either plain rings (see
It is well known that any packing material in stuffing boxes, if its supporting and its compressing planes both are square to stem axis, can have a radial movement only when it is axially compressed to yield and deform, whereas, if both are a cone not friction-locked for the radial movement, only the local material in contact with the conical surfaces may radially be compressed to move because forces are axially decreasedly transmitted in packing material with frictional resistance. That is to say, there are altogether two ways to enable the packing to yield an efficient radial sealing movement; one is to enable the whole packing to fully yield and deform to get some packing material a bi-radial movement, and the other is to enable the whole packing to suitably radially shrink to get the packing material a uni-radial movement. To accomplish a stem cylinder seal, the packing in bi-radial movements needs a full radial restriction from boxes, and the packing in uni-radial movements needs full support and compression from cones. The bi-radial movement subject to the yield and deformation of material needs the packing to be compressed to a fully yielded and deformed state, and so it is possible to exhaust the allowable strength of packing material regardless of whether the working pressure is low or high. The uni-radial movement subject to a shrinking deformation may only need the packing material to deliver a suitable strength in accordance with the sealing requirement or may not need to exhaust the allowable strength for no reason. The grouped square-sectional rings shown in
The first, it is well known that any sealing packing shall be easily deformed under compression; otherwise it can not be used as the sealing stuff filling any unevenness in the jointing surface. So it is beyond any doubt that any V-packing rings and the other shaped packing rings in sets are no rigid body, and will deform to become a packing with an integral-sectional function or to change into a non-wedged body from a wedged body and lose the mechanical characteristic as their original sectional shapes when compressed to some extent, one embodiment of which is that these grouped rings, after compressed to some extent, may not be separated without an externally stripping force, and another embodiment of which is not that the more they are compressed, the higher medium pressure they withstand after compressed to some extent and far before compressed to a broken state.
The second, it is imaginable that any shaped-packing rings in sets will have two opposite deformations at the same time from different partial sections when compressed to deform; one may be a radial increase of partial sections doing sealing work, and the other may be a radial decrease of partial sections doing unsealing work. For example, female Vees always expand to do sealing work, and male Vees always contract to do unsealing work. So any set of packing rings with shaped-sections will always work in such a way that the more they are compressed, the fewer the sealing surface becomes, and the more concentrated the sealing stress becomes to fast exceed the material strength limit and result in a sealing failure, even if they do not yet lose their mechanical characteristics when compressed to some extent. For example, the intermediate V-ring always has a female Vee at its one side, and a male Vee at its other side, with its female Vee expanded to do sealing work and with its male contracted to do unsealing work during being compressed. If Vees do not yet lose their wedging function when compressed to some extent, they may finally have only one external edge circle of female Vees doing sealing work, which both wastes sealing material and has no working reliability. Therefore, any set of packing rings with a shaped-section, particularly for high pressure medium service, should be designed or considered according to some non-wedged bodies or finally regarded as some stuffing material without any wedging function; if not so designed, they will have a worst material-utilizing ratio and a worst working reliability.
The last, it can be seen from the above-analyzed that all the boxed seals are to have their packing fully stuffed into and conformed to the space between the box wall and the stem to finally become one packing with an integral-sectional function and without any interference with its both stuffed box and sealed stem regardless of whether it is in sets or in groups or not, i.e. any packing of all the boxed seals finally relies on transverse strains given by Poisson's ratio to provides radial sealing stresses. The Poisson's ratio of usual sealing material is less than 0.5, such as PTFE with a Poisson's ratio of 0.46, and so the axial strain and stress in the boxed sealing packing are respectively at least 2 times its radial strain and stress; in other words, the maximum load stress in the boxed sealing packing will be at least 2 times its stem-sealing stress after compressed to some extent if the resistance to packing deforming motion is neglected; i.e. the boxed seal may have only a half strength (allowable stress) of sealing material used for stem cylinder seals at most. Thus, to make use of a limited sealing stress or capacity of the boxed sealing packing for a higher medium pressure, the boxed seal has to have a medium-leaking path extended by increasing the axial height of packing, whereas increasing the height is equivalent to decreasing the sealing stress. To keep the sealing stress not changed, it has to have an axial load increased again. To keep the axial stress within the material's allowable stress after increasing the axial load, it has to have an axially force-receiving area increased by increasing the radial dimension of packing again. However, the larger dimension or the more material increased, the more maldistribution of strains and stresses of packing, and the more sealing material wasted. Therefore, the boxed seal is only of one inefficient sealing construction.
Besides, the boxed seal has a stem-embracing force both axially maldistributed because the compressing force is axially progressively decreased, and radially maldistributed because the stem, the packing, the box and the gland can not be in a coaxial or symmetrical assembly, and so when compressed to obtain an integral seal, will have a packing over-compressed to be easy to be worn at some points. That is to say, the boxed seal design has a lower material availability and a lower material wearing resistance. The sealing power is axial to make at first directly the stem shoulder seal operative and then indirectly the stem cylinder seal operative, particularly for the plain packing ring design (see
Clause 7.1.1 of ASME B16.34-2004 specifies that valve shells shall withstand a minimum of 1.5 times pressure rating with the valve in the partially open position or including the stem packing, but clause 7.1.3 additionally specifies that leakage through the stem packing shall not be the cause for rejection, and that stem seals shall be capable of retaining pressure at least equal to the 38° C. rating without visible leakage when incapable of withstanding 1.5 times rating. So specify API 6D/ISO 14313 and the other valve standards. That is to say, the prior stem sealing art can not meet the actual requirement so that the valve standards have to make a concession to stem seals by lowering the valve reliability.
In Europe, valve manufacturers have to add one or two O-sealing rings on the stem with the prior packing seal in order to meet the requirement from German TA Luft (Technical Instructions on Air Quality Control).
If the stem-embracing component for stem cylinder seals could be provided by an integral bushing or ring fully supported and compressed between two cones but not by a set of packing rings fully compressed to yield and deform in stuffing boxes, a selection of the two conical angles could adjust both the magnitude of the resultant force radially compressing on the sealing bushing or ring and the matching characteristic of stem cylinder seals and stem shoulder seals. If the radial resultant stress on the sealing bushing or ring could be adjusted to one not less than any other directional stresses, mightn't the stem cylinder seal have all the strength (allowable stress) of sealing material used for the sealing of the stem cylinder and double meet the requirements from ASME standards and TA Luft instructions? If there was a stem shoulder seal matching with such a stem cylinder seal, mightn't the two combined stem seals dually double meet the requirements from ASME standards and TA Luft instructions?
The first object of the invention is to provide a cylinder seal with radially stem-embracing components for movable spindles, shafts or stems. The second object of the invention is to provide a shoulder seal for movable spindles, shafts or stems. A third object of the invention is to provide for movable spindles, shafts or stems a combined stem seal including a cylinder seal and a shoulder seal, which can individually withstand a maximum of 1.5 times valve pressure rating and can double meet the requirements from American ASME standards and German TA Luft instructions.
The stem cylinder seal of the invention is a triangle-sectional bushing or ring of stem cylinder seals, compressed fully within two opposing conical sockets around a movable stem therethrough and not needing any restriction from stuffing box, one of the said conical sockets or the upper socket being integrated in a gland, and the other or the lower socket being integrated at the outside of the thrust step of an exit opening of the said movable stem extending out of a cavity; wherein the section of the said bushing is a truncated triangle, the untruncated side of the said truncated triangle being the generatrix of the cylindrical inner surface of the said bushing used for sealing the said stem cylinder, a truncated side of the said truncated triangle being the generatrix of a taper outer surface used for sealing the said lower conical socket, the other truncated side of the said truncated triangle being the generatrix of the other taper outer surface of the said bushing used for receiving the compression from the said gland, and the truncating side of the said truncated triangle being the generatrix of the short cylindrical outer surface of the said bushing used for providing a wearing and compressing allowance for the said bushing. A radial force compounded by the compressing force from the gland and the reacting force from the lower conical socket is applied to the cylindrical outer surface formed by the truncating side, and so the stem cylinder seal assembly of the invention does not need any stuffing box again, or the sealing material of the invention does not again need any restriction from stuffing boxes and, being able to swing free with stems, has a radially stem-embracing component so evenly and so symmetrically as to be able to provide a seal high efficient, sensitively compensated and wear-resistant for movable stem cylinders. Besides, the triangle-sectional bushing or ring of stem cylinder seals has no air bubble, and so has a high reliability when temperature changes.
It has been proved by tests that the triangle-sectional bushing or ring used as the seal of valve stems can withstand the same pressure as the burst pressure of valve bodies without failure. In general, the burst pressure of valve bodies may be equal to 4 times pressure rating of the valve. That is to say, the triangle-sectional bushing stem seal have the same reliability as the valve body, and can double meet the requirements from German TA Luft instructions.
The stem shoulder seal of the invention is a ball wedge/spherical socket mating arrangement, comprising a ball wedge or spherical shoulder integrated at a movable stem end and a spherical socket integrated at the inside of the thrust step of an exit opening of the said movable stem extending out of a cavity, wherein the said spherical shoulder is so diametrically either equal to or economically slightly bigger than the said spherical socket and the sphere's center of the said spherical socket is so economically slightly off the edge circle plane of the said spherical socket for a distance δ as to ensure the first mating contact or the initial mating contact is only at the said edge circle and close to the sphere's great circle (whose center is the same as the sphere's center) of the said spherical shoulder or as to make the said spherical shoulder be a small angle of wedges relative to the said spherical socket or as to make the said stem shoulder seal be a positive ball wedge/spherical socket mating arrangement with a small wedging angle tending to zero degree, which will tend to result in an infinite force for the said ball wedge or spherical shoulder to wedge the said spherical socket under a small operating force passing the center of the said spherical shoulder; whereby the mating of a hardened metal ball wedge and a soft metal spherical socket will, like a seal of metal to non-metal, accomplish sealing as soon as the said ball wedge touches the said spherical socket by a small operating force, and further more the said mating contact, provided the ball wedge is still adequately round, will be always on the spherical surface of the said hardened ball wedge without any diametrical change and will be getting more and more as operated, no matter how the said ball wedge rotates and deflects and how the said ball wedge squeezes and wears its mating socket during each operations, i.e. the rotating and the deflecting of the said ball wedge do not affect the integrity of seals, and the more wear, the tighter the closure of the mating and the more resistant to the wear.
It has been proved that the mating of ball wedges and spherical sockets used as the valve stem stop shoulder seal can withstand the same pressure as the burst pressure of valve bodies without failure. In general, the burst pressure of valve bodies may be equal to 4 times pressure rating of the valve. That is to say, the mating of ball wedges and spherical sockets, when used as the stem stop shoulder seal, have the same reliability as the valve body, and can double meet the requirements from German TA Luft instructions.
It is imaginable that a movable stem with a triangle-sectional bushing or ring of stem cylinder seals and a ball wedge/spherical socket mating arrangement of stem shoulder seals can dually double meet the requirements from American German TA Luft instructions.
a is a conventional stem assembly including a group of plain packing rings for stem cylinder seals and a mating of plain shoulders and gaskets for stem shoulder seals, shown in cross-sectional elevation.
b is a conventional stem assembly including a set of V-packing rings for stem cylinder seals and a mating of taper or ball shoulders and conical or spherical gaskets for stem shoulder seals, shown in cross-sectional elevation.
What is shown in
What
The triangle-sectional bushing or ring of stem cylinder seals of the invention, as mentioned above, can not need any intermediate cylindrical periphery used for receiving a radial restriction from stuffing box, but has to still need a short length of intermediate cylindrical periphery used for providing a wearing and compressing allowance for itself, and therefore its section, as shown in
As shown in
In the traditional stem assembly of
The ball valve shown in
What is shown in
The above-mentioned ball valve and needle valve are only two valve examples used to describe the designs in accordance with the invention. Anybody skilled in the art can follow the above-mentioned descriptions to use the triangle-sectional bushing or ring as the stem cylinder seal in other valves and machines, and use the ball wedge/spherical socket mating arrangement as the stem shoulder seal and the closure pair in other valves and as the spindle thrust bearing and seals in other machines.
Number | Date | Country | Kind |
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2005 1 0097905 | Aug 2005 | CN | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/CN2006/002156 | 8/23/2006 | WO | 00 | 4/8/2008 |
Publishing Document | Publishing Date | Country | Kind |
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WO2007/022721 | 3/1/2007 | WO | A |
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Entry |
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Technical Committee ISO/TC 10, Technical drawings—Tolerancing of linear and angular dimensions, ISO 406 Second Edition, Oct. 1, 1987, International Organization for Standardization, Switzerland. |
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Number | Date | Country | |
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20080217568 A1 | Sep 2008 | US |