Valve sealing

[novice123]

Kindly explain in detail about the valve sealing between disc and body , especially

1)type of materials mostly used in sealing .
2) effect of fluid temp. on sealing materials.
3) metal to metal sealing.
4) soft sealing material vs hard sealing material .

of valves especially gate and plug valve. More beneficial if pictures were included . All are welcome to include any thing they know about valves sealing , so that every one can be benefited.

[mechcolor]

For gate valves all the seating surfaces through which sealing is ensured is called trim. All the trim materials for gate valve as per API 600 standard should conform to the requirements mentioned in para 6.2.1. For plug valves, API 599 covers all the requirements for materials and refer ASME B16.34 or ASME B16.42.

You point wise reply is as follows:

1) Type of materials which are normally used for trim generally include 13Cr, 18Cr-8Ni (SS 304 type), 25Cr-20Ni, 19Cr-29Ni etc. Valve trim material is selected based upon the applicable owner's specifications. Hardness of the two sealing surfaces is extremely important and the acceptable values of hardness are given in applicable valve standard. Section 3 of API 599 specifies materials requirements for plug valves.

2) Temperature conditions do require certain specific trim materials due to effect of thermal expansion. Thermal co-efficient of the two sealing materials needs to be considered for higher temperature services. Many of the valves only leak on high temperature and provide excellent sealing at normal temperatures. You may also be interested in looking in ASME B16.34 for pressure temperature ratings of valves.

3) Valves provide closure / sealing thru metal to metal contact. The sealing is always tested thru closure tests as specified in different standards. You may like to see this thread regarding sealing integrity / closure testing of valve http://www.thepetrostreet.com/forums/vi ... osure+test. API 598 covers plug valves' closure test also as API 599 refers that.

4) Normally different combinations are used in selecting trim material, it depends upon the manufacturer and the owner's specifications that how they want valve sealing to be. Selection of combination is to be made in order to achieve required sealing through utilizing soft sealing and hard sealing material.

[arcpro]

The choice of material to be used for valve sealing disc, gate, seat largely depends upon the pressure differential, service conditions, corrosive / non-corrosive type of fluid, temperature. Valve manufacturers offer different set of valve internals (trim) material against different service conditions.

Service temperature is also a factor needs to be considered while choosing a valve internal material as mentioned in point 1. Material charts selection guides are available by different suppliers according to their experience which suggest you the appropriate material for valve trim against service type and conditions. Stellited valve internals are also used just to maintain hardness at higher temperature conditions.

[ashfaqanwer]

Attached find an excellent reference document on valve materials selection.