Page 105 - Valve Selection Handbook

92 Valve Selection Handbook

Cylindrical Plug Valves

The use to which plug valves can be put depends to some extent on the
way the seal between the plug and the valve body is produced. In the
case of cylindrical plug valves, four sealing methods are frequently
employed: by a sealing compound, by expanding the plug, by O-rings,
and by wedging an eccentrically shaped plug into the seat.
The cylindrical plug valve shown in Figure 3-49 is a lubricated plug
valve in which the seat seal depends on the presence of a sealing compound
between the plug and the valve body. The sealing compound is introduced
to the seatings through the shank of the plug by a screw or an injection gun.
Thus, it is possible to restore a defective seat seal by injecting an additional
amount of sealing compound while the valve is in service.
Because the seating surfaces are protected in the fully open position
from contact with the flowing fluid, and a damaged seat seal can easily
be restored, lubricated plug valves have been found to be particularly
suitable for abrasive fluids. However, lubricated plug valves are not
intended for throttling, although they are sometimes used for this pur-
pose. Because throttling removes the sealing compound from the
exposed seating surfaces, the seat seal must be restored, in this case, each
time the valve is closed.
Unfortunately, the manual maintenance of the sealing compound is
often a human problem. Automatic injection can overcome this problem,
but it adds to the cost of installation. When the plug has become immov-
able in the valve body due to lack of maintenance or improper selection
of the sealing compound, or because crystallization has occurred between
the seatings, the valve must be cleaned or repaired.
The seat seal of the cylindrical plug valves shown in Figure 3-50 and
Figure 3-51 depends on the ability of the plug to expand against the seat.
The plug of the valve shown in Figure 3-50 is fitted for this purpose
with a packing sleeve, which is tightened against the seat by a follower
nut. The packing commonly consists of compressed asbestos or solid
PTFE. If the packing needs retightening to restore the seat tightness, this
must be carried out while the valve is in the closed position to prevent
the packing from expanding into the flow passage. The valve is made
only in small sizes, but may be used for fairly high pressures and temper-
atures. Typical applications for the valve are the isolation of pressure
gauges and level gauges.
The plug of the valve shown in Figure 3-51 is split into two halves and
spread apart by a wedge, which may be adjusted from the outside. The