Page 160 - Reciprocating Compressors Operation Maintenance

1 46 Reciprocating Compressors: Operation and Maintenance

VALVE MATERIALS

Valve components are made of various materials depending on the
operating pressures and type of gas to be handled.
Materials used for valve seats and guards include:

«Cast iron—most common for low and medium pressures
« Ductile iron—for medium pressures
• Cast steel—for high pressures
• Steel (bar stock)—for high pressures

Depending on the pressures, combinations of the above are commonly
used, that is, cast iron guards with the seat made of ductile iron or steel.
For highly corrosive gases, various alloy materials are frequently used.
Among these are stainless steel (316, 410, 420), 17-4 PH stainless, high
nickel and cast iron.
Valve seats and guards machined from steel bar stock typically con-
form to AISI 4140 or 1141. Some valve suppliers use 410 stainless bar
stock for seats and guards.

Sealing Elements

Materials used for sealing elements fall into two classes: high strength
steels and plastic composites.
Feather valve strips, valve channels, and springs are made of steel,
usually 410 stainless that is stress relieved, hardened, and tempered for
maximum wear resistance. For more corrosive applications, channel may
be made from 316 stainless. Feather valve strips may be made of Inconel
17-4 PH materials for greater corrosion resistance.
As indicated in Table 3-6, valve discs or plates may be metallic or
plastic composite. The advantage of metal plates is their ability to with-
stand high temperatures and high differential pressures.
Their disadvantages are:

• Even stainless steels are subject to corrosion attack by substances
frequently found in the gas compressed (for example, sulfides and
chlorides).
• They fail by impact at relatively low-impact velocities (typically 20
to 25 ft/sec.).