Page 26 - Valve Selection Handbook

Fundamentals 13

API Std 600 lists seating materials and their combinations frequently
used in steel valves.

Sealing with Sealants

The leakage passages between metal seatings can be closed by sealants
injected into the space between the seatings after the valve has been
closed. One metal-seated valve that relies completely on this sealing
method is the lubricated plug valve. The injection of a sealant to the seat-
ings is used also in some other types of valves to provide an emergency
seat seal after the original seat seal has failed.

Soft Seatings

In the case of soft seatings, one or both seating faces may consist of a soft
material such as plastic or rubber. Because these materials conform readily
to the mating face, soft seated valves can achieve an extremely high degree
of fluid tightness. Also, the high degree of fluid tightness can be achieved
repeatedly. On the debit side, the application of these materials is limited by
their degree of compatibility with the fluid and by temperature.
A sometimes unexpected limitation of soft seating materials exists in
situations in which the valve shuts off a system that is suddenly filled
with gas at high pressure. The high-pressure gas entering the closed sys-
tem acts like a piston on the gas that filled the system. The heat of com-
pression can be high enough to disintegrate the soft seating material.
Table 2-2 indicates the magnitude of the temperature rise that can
occur. This particular list gives the experimentally determined tempera-
ture rise of oxygen that has been suddenly pressurized from an initial
state of atmospheric pressure and 15°C. 4

Table 2-2
Experimentally Determined Temperature Rise of Oxygen Due to
Sudden Pressurizing from an Initial State of Atmospheric Pressure
and 15°C
Sudden Temperature
['ressure Rise Rlise
2
25 Bar(3601b/in ) 375°C (705°F)
2
50 Bar (725 lb/in ) 490°C (915°F)
2
100 Bar ( 1450 lb/in ) 630°C (1165°F)
2
150 Bar (2175 lb/in ) 730°C (1345°F)
2
200 Bar (2900 lb/in ) 790°C (1455°F)