Page 90 - Valve Selection Handbook

Manual Valves 77

Valve Bypass

The seating load of the larger parallel gate valves (except those with
floating seats) can become so high at high fluid pressures that friction
between the seatings can make it difficult to raise the disc from the
closed position. Such valves are therefore frequently provided with a
valved bypass line, which is used to relieve the seating load prior to
opening the valve. There are no fast rules about when to employ a
bypass, and the manufacturer's recommendation may be sought. Some
standards of gate valves contain recommendations on the minimum size
of the bypass.
In the case of gases and vapors, such as steam, that condense in the
cold downstream system, the pressurization of the downstream system
can be considerably retarded. In this instance, the size of the bypass line
should be larger than the minimum recommended size.

Pressure-Equalizing Connection

In the case of the conventional double-seated parallel gate valves
shown in Figure 3-25 and Figure 3-26, thermal expansion of a liquid
trapped in the closed valve chamber will force the upstream and down-
stream discs into more intimate contact with their seats, and cause the
pressure in the valve chamber to rise. The higher seating stress makes it
in turn more difficult to raise the discs, and the pressure in the valve
chamber may quickly become high enough to cause a bonnet flange joint
to leak or the valve body to deform. Thus, if such valves are used to han-
dle a liquid with high thermal expansion, they must have a pressure-
equalizing connection that connects the valve chamber with the upstream
piping.
The pressure rise in the valve chamber may also be caused by the
revaporation of trapped condensate, as in the case in which these valves
are closed against steam. Both the valve chamber and the upstream pip-
ing are initially under pressure and filled with steam. Eventually, the
steam will cool, condense, and be replaced to some extent with air.
Upon restart, the steam will enter the upstream piping and, since the
upstream seat is not normally fluid-tight against the upstream pressure,
will enter the valve chamber. Some of the new steam will also condense
initially until the valve body and the upstream piping have reached the
saturation temperature of the steam.