Page 80 - Valve Selection Handbook

Manual Valves 67

Figure 3-22. Piston Valve,
Adapted for Draining Vessels,
Seat Packing Mounted in
Valve Body. (Courtesy of
Yarway Corporation.)

represents the seat packing, while the upper packing seals the piston to
the atmosphere. The bonnet serves thereby as the gland that permits both
packings to be tightened through tightening of the cover bolts. Disc
springs under the nuts of the cover bolts minimize variations in packing
stress due to thermal contraction and expansion of the valve parts. When
one of the packings leaks, the fluid seal can be restored by retightening
the bolts. Retightening must be carried out while the valve is closed to
prevent an unrestrained expansion of the seat packing into the valve bore.
The valve shown in Figure 3-21 differs from the one in Figure 3-20
only in that the piston is pressure balanced. The two packings around the
piston are both seat packings, and a separate packing is provided for the
stem. The purpose of balancing the piston is to minimize the operating
effort in large valves operating against high fluid pressures.
The packing train of the valve shown in Figure 3-22 is likewise
stressed through the bonnet in conjunction with springs under the nuts of
the cover bolts, or with a spring between the bonnet and the packing.
However, as the piston moves into the final closing position, a shoulder
on the piston contacts a compression ring on top of the packing so that
any further progression of the piston tightens the packing still further.