Page 111 - Valve Selection Handbook

98 Valve Selection Handbook

plug out of the body seat by means of a hinged lever arrangement prior to
it being rotated and reseating it after it is rotated to the desired position.
The plug may be rubber faced, and is, by its taper, normally self-locking.
This particular valve is a multiport companion valve to the eccentric plug
valve shown in Figure 3-53.

Antistatic Device

In plug valves, seats and packings made of a polymeric material such
as PTFE can electrically insulate the plug and the valve stem from the
valve body. Under these conditions, friction from the flowing fluid may
generate an electrostatic charge in the plug and the stem that is high
enough to produce an incendiary spark. This possibility is more pro-
nounced with two-phase flow. If the fluid handled by the valve is flam-
mable, the valve must be provided with an antistatic device, which
achieves electrical continuity between the plug, stem, and the valve body.
Plug Valves for Fire Exposure

Plug valves, which may be exposed to a plant fire when handling a
flammable fluid, must remain essentially fluid-tight internally and exter-
nally and be operable during and after a fire. This degree of resistance to
fire damage is particularly difficult to achieve when the plug valve nor-
mally relies on polymers for seat and stem seal. Common practice in this
case is to provide the valve with an auxiliary metal seat in close proximi-
ty to the plug, against which the plug can float after the soft seat has dis-
integrated. The soft stem packing can readily be replaced with a fire-
resistant packing.
The requirements for testing and evaluating the performance of valves
exposed to fire are similar to those for ball valves, described on pages 108
and 109.
Multiport Configuration

Plug valves adapt readily to multiport configurations such as those
shown in Figure 3-59. The valves may be designed for transflow, in
which case the second flow passage opens before the first closes; or for
non-transflow, in which case the first flow passage closes before the sec-
ond opens. The transflow sequence is intended for duties in which the
flow cannot be momentarily interrupted; for example, on the outlet of a
positive displacement pump that is not protected by a relief valve. The