Page 121 - Valve Selection Handbook

108 Valve Selection Handbook

tics at low temperatures, which tends to resist the opening of the
upstream seat seal.

Antistatic Device

The polymeric seats and packings used in ball valves can electrically
insulate the ball and the stem from the valve body. Ball valves may there-
fore have to be provided with an antistatic device for the same reason as
described on page 98 for plug valves. Figure 3-67 shows a typical anti-
static device consisting of spring-loaded plungers—one fitted between
the tongue of the stem and the ball, and a second between the stem and
the body.

Figure 3-67. Antistatic Device for Grounding Stem to Ball and Stem to Body.
(Courtesy of Worcester Valve Co., Ltd.)

Ball Valves for Fire Exposure

The soft seals for seat and stem commonly used in ball valves will dis-
integrate if the valve is exposed to fire for a long enough period. If such
valves are used for flammable fluids, they must be designed so that loss of
the soft seals due to an external fire does not result in gross internal and
external valve leakage. Such designs provide emergency seals for seat and
stem that come into operation after the primary seals have failed.
The emergency seat seal may be provided by a sharp-edged or cham-
fered secondary metal seat in close proximity to the ball, so that the ball