Page 206 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

188 Major Process Equipment Maintenance and Repair

valve is in the crank or frame end of the cylinder. The leaky valve seems
to have a torch effect on the rod and packing. A faulty valve in either end
of the cylinder can scuff the piston and liner and actually cause a serious
piston seizure and possible subsequent crankcase explosion. The length
of time a hot valve can be operated without damage cannot be established
for all installations. Furthermore, the period of time cannot be set by as-
signing a limit to the discharge temperature. This all depends on the size,
class, and design of the cylinder, the gas being pumped, and the pressure
ratios involved. In addition to installation dirt, another common occur-
rence is trouble with the cap gasket (Item 11, Figure 5-7). Where corrosive
gas is involved, a poor choice of gasket material has caused trouble. Also,
regardless of material, this gasket is sometimes incorrectly installed. It is
also common for some people to use the old gasket after overhaul. Regard-
less of what made the gasket defective, one of the very serious consequences
of leakage at that point is that the operator often further tightens the cap nuts
(Item 10, Figure 5-7). When the leak continues, the tendency is to put an
extension on the wrench and further tighten the nuts. The result is a distorted
valve seat (illustrated in the exploded view to the left in Figure 5-7). This
distortion is also characteristic of a valve installed in a cocked position. A
valve plate cannot conform to a distorted seat without fatigue and breakage.
These examples are a few known reasons for valve failures but, con-
tinuing the description of proper practices, we can now address the topic
of removing valves for repair. Most plates are steel, and the seats and
guards (Items 2 and 4, Figure 5-7) are either steel or cast iron and, in
almost every case, the seat or the guard or both are marked with a notice-
able dent caused by breakage of the metal plate. At this point, there are
three different approaches to restoration of the valve assembly. One is to
put in a new plate (Item 3, Figure 5-7) and leave the guard and seat as is.
In another, when the operator understands that a plate cannot seat on a
damaged seat without fatigue and breakage, he machines or laps it. What
most people fail to realize is that the impact of the plate on the guard
upon opening is as severe as the action of the plate returning to the seat.

Figure 5-7. Reciprocating compressor valve.