GB2062185A

GB2062185A - Valve stem assembly

Info

Publication number: GB2062185A
Application number: GB8033824A
Authority: GB
Original assignee: Rockwell International Corp
Current assignee: Boeing North American Inc
Priority date: 1979-10-31
Filing date: 1980-10-20
Publication date: 1981-05-20
Also published as: NL8005911A; BE885959A; BR8006938A; AR222913A1; MX152586A; FR2468818B1; FR2468818A1; DE3039331A1; CA1144136A; GB2062185B

Abstract

A valve stem assembly for a valve which utilizes stem packing material which is capable of creep throughout its effective life when subject to a predetermined axial load. The stem packing material 18 is retained within the stem opening 44 against an internal shoulder 46 upon initial installation to produce the predetermined axial load thereon and the stem is capable of axial movement under normal valve operating pressure to cause a radially extending shoulder 38 thereof to apply the predetermined axial load when the stem packing material becomes subject to creep at some time after initial installation. Surfaces 42, 48 allow for back seating of the stem. <IMAGE>

Description

SPECIFICATION Valve stem assembly Background of the Invention Field of the Invention This invention relates to a valve stem assembly and more specifically to such an assembly which is capable of applying a predetermined axial load to the stem packing material needed for effective sealing even if the stem packing material is capable of creep throughout its effective life.

Description of the PriorArt In the valve art, there exists a number of means intended to insure adequate sealing around the valve stem during continued valve life. However, for a number of valves the stem packing material utilized is properly resilient for sealing but includes the undesired feature of being capable of creep during its effective life when subjected to an axial load needed for effective sealing. To compensate for the creep of the packing material, some valves utilize springs to apply the axial load as disclosed in United States Patents 3,717,323 and 3,916,943. However, the spring loading of the packing material appears to apply a force on the stem or valve isolation member which might interfere with smooth valve operation.Other valve stem assemblies include adjusting means on the stem or on the packing retention device which might be tighter during the life of the packing material to periodically increase the load thereon to compensate for creeping. United States Patents 3,030,067; 3,030,068; 3,384,341; 3,409,268 and 3,920,026 disclose valves employing some of these features. Although a number of valves includes valve stem assemblies which utilized system pressure within the valve to produce the desired axial load, these systems are often subject to stem leakage if full system pressure is not employed. Additionally, any number of these valve configurations discussed hereinabove are so arranged to make a stem backseating feature impractical when such a feature would be desirable for stem packing protection at pressures above normal operating pressure.Therefore, there continues to be a need for a simple valve stem configuration allowing effective use of stem packing material that is subject to creep which will not interfere with normal valve use and incudes backseating protection.

Summary of the Invention It is, therefore, an object of the invention to provide a valve stem assembly which allows effective use of a stem packing material capable of creep without requiring spring means or periodic adjustment to insure an adequate axial load is applied to the packing material.

It is a further object to provide a valve stem assembly of the type described which includes a backseating feature for protection of the stem packing material.

These and other objects are provided in a preferred embodiment of the invention in the form of a valve stem assembly for a valve of the type which includes a body having an inlet and an outlet, a valve isolation member mounted within the body for rotation about an axis of the valve between open and closed positions and stem packing material which is capable of creep throughout its effective life when subjected to a predetermined axial load needed for effective sealing. The valve stem assembly includes a rotatable stem extending along the axis through an opening in the body which is coupled to the valve isolation member to produce the rotation thereof. The stem is capable of limited axial movement independent of the axial position of the isolation member relative to the body.The stem has an intermediate section with a first diameter to be closely received within an intermediate region of the opening. The outer section of the stem is reduced to have a second diameter less than the first diameter so that a radially extending shoulder is located between the outer section and the intermediate section. An elongated inner end of the stem includes a radially extending backseating surface adjacent the intermediate section. The opening is enlarged to include an outer region thereof outwardly of the intermediate region to have a third diameter greater than the first diameter and to provide an internal shoulder of the body which is generally aligned with the radially extending shoulder. The opening is enlarged inwardly of the intermediate region to provide a seating surface within the body capable of mating with the backseating surface of the inner end of the stem.A packing retention device is secured around the outer section of the stem to retain the stem packing material within the outer region of the opening. It is a fixed axial distance from the interior shoulder to apply a predetermined axial load to the system packing material therebetween upon initial installation within the valve. The stem is capable of the axial movement toward the packing retention device under normal valve operating pressure without the backseating surface being forced into contact with the seating surface to cause the radially extending shoulder and the packing retention device to apply the predetermined axial load to the stem packing material therebetween when the stem packing material becomes subject to creep at some time after initial installation.

Brief Description of the Drawing Figure 1 is a fragmentary, side view partially in section of a valve including a preferred embodiment of the valve stem assembly of the present invention.

Detailed Description of the Preferred Embodiment As seen in Figure 1, the preferred valve stem assembly 10 is shown to be utilized in a valve 12 having a body 14 with an inlet and outlet (not shown). A valve isolation member 16, which is a plug in the preferred embodiment, is mounted within the body 14 for rotation about the axis of the valve between open and closed positions. The stem packing 18 is of a material such as an expanded graphite based packing material. This material as well as other material often used in valves of this type is generally resilient and capable of radial expansion to make sealing contact with a stem and body cavity when subjected to a predetermined axial load. However, the packing material can disadvantageously experience creep during its effective life under such a load which could interfere with seal in some installations.The present invention is intended to insure that effective sealing will be maintained even if significant creeping of the stem packing material 18 after initial installation is experienced.

The valve stem assembly 10 of the present invention includes a rotatable stem 20 extending along the valve axis through an opening 22 in the body 14. The stem 20 is coupled at 24 to the valve isolation member 16 in a manner which allows rotation of the stem 20 to rotate the valve isolation member 16 without any direct contact therebetween in the axial direction. The stem 20 and valve isolation member 1 6 have flat, axially extending working surfaces at their adjacent ends with a collar 26 therebetween which acts on these surfaces to transmit only the rotational movement of the stem to the isolation member. Therefore, the stem 20 is capable of limited axial movement within the opening 20 independent of the axial position of the isolation member 1 6 relative the body 14.The spring 28 within the collar 26 does bias the stem 20 upwardly and the isolation member 10 downwardly but represents only one of the forces acting on each during valve operation and does not prevent their independent axial positioning. The method of coupling insures that the stem and isolation member are both capable of independent axial positioning which is desirable for effective valve operation.

The stem 20 has an intermediate section 30 which is closely received within an intermediate region 32 of the opening 22. The stem is reduced at its outer section 34 to have a diameter which is less than the diameter of the intermediate section 30. A radially extending shoulder 36 is formed at the transition between the intermediate section 30 and the outer section 34. The shoulder 36 includes a thrust bearing 38, the purpose of which will become obvious from the explanation of the invention hereinbelow.

An enlarged inner end 40 of the stem 20 includes a radially extending backseating surface 42 adjacent the intermediate section 30. The enlarged inner end 40 effectively limits the axial movement of the stem 20 and insures its retention within the opening 22.

The opening 22 is enlarged to include an outer region 44 outwardly of the intermediate region 32. The outer region 44 has a diameter larger than the diameter of the intermediate section 30 and thus larger than the diameter of the intermediate region 32 which closely receives it. An internal shoulder 46 formed in the body 14 produces the transition between regions of the opening 22. The internal shoulder 46 is generally aligned with the radially extending shoulder 36 during initial installation. The significance of this alignment will be discussed hereinbelow.

The opening 22 is also enlarged inwardly of the intermediate region 30 to provide a seating surface 48 within the body 14. The seating surface 48 is capable of mating with the backseating surface 42 of the inner end 40 of the stem 20. However, when the shoulders 36 and 46 are generally aligned during normal valve operation no contact between the backseating surface 42 and the seating surface 48 exists. It can be seen that a significant amount of axial movement is required before backseating of the stem 20 would actually occur.

With the stem 20 properly installed within the opening 20 and coupled to the isolation member 16, stem packing material 1 8 must be installed around the outer section 34 of the stem 20 within the outer region 44 of the opening 22. A ring 50 is first positioned against the shoulders 36 and 46 in the preferred installation although some packing material 1 8 might have a sufficient integrity so that the ring 50 would not be necessary. In either case, the stem packing 1 8 is initially sized to insure that it will be preloaded for sealing when a packing retention means 52 is secured around the outer section 34 of the stem 20. The packing retention means 52 of the preferred embodiment of Figure 1 includes a gland 54 which makes contact with the upper portion of the stem packing 18 and a gland retainer 56.The gland retainer 56 is rigidly bolted to the body 14 such that a fixed axial distance exists between the packing retention means 52 and the shoulder 46 at initial installation and throughout valve operation. This fixed axial distance when coupled with the initial sizing of stem packing 1 8 insures that the stem packing 18 is subjected to the predetermined axial load upon initial installation for effective sealing. It should be noted that the sealing will be effected independent of the actual pressure within the valve at all levels up to normal operating pressure.

As thus explained, the valve stem assembly 10 will result in effective stem sealing as long as the shoulders 36 and 46 remain aligned. The shoulder 36 will move axially above the shoulder 46 only if the pressure within the valve exceeds normal operating pressure. In this case, higher pressure on the stem 20 will cause it to move axially, further compressing the stem packing 1 8 until backseating occurs. The backseating of the stem 20 should protect the stem packing and prevent its failure.

However, as discussed hereinabove, at some time after initial installation, the stem packing 18 may become subject to creep. Since the axial distance between the shoulder 46 and the packing retention means 52 is fixed, this creep would effectively reduce the axial load on the seal packing 1 8 as its axial dimensions tend to decrease. Since the stem 20 is capable of axial movement toward the packing retention means 52, normal valve operating pressure would act on the stem 20 to raise the radially extending shoulder 36 above the shoulder 46. As a result, the shoulder 36 would force the ring 50 upwardly to compensate for the decreased dimensions of the stem packing 1 8 after creep occurs. It should be noted that a significant amount of this axial movement which will maintain the stem packing 1 8 at the desired predetermined axial load can exist prior to backseating of the stem 20.

As thus explained, the present invention includes a valve stem assembly which allows stem packing material to be preloaded for effective sealing independent of valve system pressure, allows continued sealing at normal operating pressure even after the packing material begins to creep with age, and maintains a stem backseating capability throughout valve operation.

Claims

CLAIMS 1. A valve stem assembly for a valve of the type which includes a body having an inlet and an outlet, a valve isolation member mounted within said body for rotation about an axis of said valve between open and closed positions and stem packing material which is capable of creep throughout its effective life when subjected to a predetermined axial load needed for effective sealing; said valve stem assembly comprising: a rotatable stem extending along said axis through an opening in said body and coupled to said valve isolation member to produce said rotation thereof, said stem being capable of limited axial movement independent of the axial position of said isolation member relative to said body; said stem having an intermediate section with a first diameter to be closely received within an intermediate region of said opening; said stem being reduced at its outer section to have a second diameter less than said first diameter and a radially extending shoulder between said outer section and said intermediate section; said stem having an enlarged inner end to include a radially extending backseating surface adjacent said intermediate section;; said opening being enlarged to include an outer region thereof outwardly of said intermediate region to have a third diameter greater than said first diameter and to provide an internal shoulder of said body which is generally aligned with said radially extending shoulder; said opening being enlarged inwardly of said intermediate region to provide a seating surface within said body capable of mating with said backseating surface of said inner end of said stem; packing retention means being secured around said outer section of said stem to retain said stem packing material within said outer region of said opening, said packing retention means being a fixed axial distance from said interior shoulder to apply said predetermined axial load to said stem packing material therebetween upon initial installation within said valve; and said stem being capable of said axial movement toward said packing retention means under normal valve operating pressure without said backseating surface being forced into contact with said seating surface to cause said radially extending shoulder and said packing retention means to apply said predetermined axial load to said stem packing material therebetween when said stem packing material becomes subject to said creep at some time after initial installation. 2. The valve stem assembly as set forth in claim 1, further including a ring around said outer section of said stem to overlie said radially extending shoulder and said internal shoulder so that said ring directly transmits said predetermined axial load to said stem packing material between said ring and said packing retention means. 3. A valve stem assembly substantially as herein before described with reference to and as shown in the accompanying drawing. New claims or amendments to claims filed on 27th February 1981 Superseded claims 1 to 3 New or amended claims:-

1. A valve stem assembly for a valve which includes a body having an inlet and an outlet, a valve isolation member mounted within said body for rotation about an axis of said valve between open and closed positions and stem packing material which is subject to creep when subjected to an axial load needed for effective sealing for extended periods of time; said valve stem assembly comprising: a rotatable stem extending along said axis through an opening in said body and coupled to said valve isolation member to produce said rotation thereof, said stem being capable of limited axial movement independent of the axial position of said isolation member relative to said body: said stem having an intermediate section with a first diameter closely received within an intermediate region of said opening; said stem being reduced at its outer section to have a second diameter less than said first diameter and a radially extending shoulder between said outer section and said intermediate section; said stem having an enlarged inner end to include a radially extending backseating surface adjacent said intermediate section; said opening being enlarged to include an outer region thereof outwardly of said intermediate region to have a third diameter greater than said first diameter and to provide an internal shoulder on said body which is generally aligned with said radially extending shoulder; said opening being enlarged inwardly of said intermediate region to provide a seating surface within said body capable of mating with said backseating surface of said inner end of said stem; stem packing material around said stem within said outer region of said opening; packing retention means secured around said outer section of said stem to retain said stem packing material within said outer region of said opening, said packing retention means being a fixed axial distance from said interior shoulder to apply an initial axial load to said stem packing material therebetween upon initial installation within said valve; and said stem being capable of said axial movement toward said packing retention means under normal valve operating pressure without said backseating surface being forced into contact with said seating surface to cause said radially extending shoulder and said packing retention means to apply an axial load to said stem packing material therebetween under the influence of fluid pressure within said body which remains effective when said stem packing material becomes subject to said creep at some time after initial installation.

2. An assembly as in Claim 1, further including a ring around said outer section of said stem to overlie said radially extending shoulder and said internal shoulder so that said ring directly transmits the axial load to said stem packing material between said ring and said packing retention means.

3. A valve comprised of a body, a valve plug within said body movable between valve open and closed positions, an opening in said body, a rotatable stem extending through said opening and mounted therein for limited axial movement with respect to said body, the inner end of said stem being drive connected to said plug for rotating said plug while permitting relative axial movement between said stem and said plug, a body backseating surface on said body, a stem backseating surface on said stem adapted to contact said body backseating surface upon limited axial movement of said stem, a shoulder on said body surrounding said stem, stem packing in said opening surrounding said stem, packing retention means secured to said body, said packing being axially compressed a predetermined initial amount between said shoulder and said retention means, and abutment means on said stem for applying an axial thrust on said packing under the influence of fluid pressure within said body acting on said stem.

4. A valve as in Claim 3 in which said backseating surfaces are normally axially displaced from each other.

5. A valve as in Claim 3 or 5 in which a metallic ring is interposed between said packing and said shoulder and said abutment means contacts said metallic ring.

6. A valve defined in Claim 3, 4 or 5 together with means to limit the amount of initial compression placed on said packing by said shoulder and said retention means.

7. A valve stem assembly substantially as hereinbefore described with reference to and as shown in the accompanying drawing.

8. A valve substantially as hereinbefore described with reference to and as shown in the accompanying drawing.