Page 159 - Reciprocating Compressors Operation Maintenance

Operation and Maintenance of Reciprocating Compressors 145

Flow efficiency is excellent because of the high lift and streamlined
poppet head. This makes an ideal valve for low-pressure ratio, high-gas
density applications where valve losses are very important and sufficient
pressure drop can be generated to drive the valve open.
The poppet valve can be applied in other than gas transmission ser-
vice, but care must be taken to limit both valve lift and operating speed,
Poppet valves at speeds above 600-700 rpm have proven somewhat
unreliable.
Poppet valves require relatively large sealing surfaces because the
poppets are nonmetallic and because the seat is angled. In addition, the
poppets have to be relatively large. Thus poppet valves also work best at
high lifts; values of .250" or higher are common.
With plastic poppets, poppet valves can run with corrosive gas and
with relatively high impact velocities, but with the very high lifts
required for optimum performance they can only be used under condi-
tions of service that create relatively low-impact velocities; typically
low-speed, high-valve area and low-to-moderate gas density.
Because of the high lift and streamlined poppet shape, poppet valves
are very prone to flutter. Therefore, to be efficient and reliable, they
require relatively dense, medium-to-high pressure gas. Poppet valves
also tend to have high-clearance volumes that preclude their use for high-
pressure ratio applications.
Thus poppet valves are ideally suited to the low-speed, medium-pres-
sure, low-ratio efficiency service typical of gas transmission applications.
Table 3-5 gives an overview of the suitability of valve types for vari-
ous gas conditions.

TABLE 3-5
SUITABILITY OF VALVE TYPES FOR VARIOUS
GAS CONDITIONS

Gag Conditions
Valve Type Clean Dry Wet Dirty Corrosives
Feather (Strip) *
Channel (Strip) •
Plate • • *
Ring • • *
Disc (Poppet) * *