Page 219 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Reciprocating Gas Engines and Compressors 201

the piston sounds should be checked daily for any unusual sounds or ex-
cessive piston slap. Excessive vapors from the crankcase breathers will
undoubtedly be the first sign of ring “blow-by.” When “blow-by” in-
creases to a dangerous level, the vapor will escape past the crankshaft
seals at the drive end of the frame. Other indicators of ring or liner trou-
ble are increased lube oil consumption, breakdown of lube oil, decreased
life- of the lube oil filter elements, increased crankcase pressure, high ex-
haust temperatures and the inability of the engine to carry load without
detonation.
Inlet, Exbust, and Gas Valves-The tappet clearance of any valve
should remain constant (after temperatures have leveled out) once it is
set. If it becomes necessary to readjust clearances after short periods of
operation, this is an indication of dangerous wear. The wear could be
anywhere in the valve operating gear, such as the cam, roller, roller bush-
ing, push rod ends, or tappet. When tappet clearance keeps changing,
these parts should be inspected immediately, because their failure can re-
sult in a complete engine wreck. If, on a four-cycle engine, inspection
shows these parts to be in good condition, the trouble will be found at the
valve seat or valve insert. Where hydraulic lifters are used, a noisy tap-
pet is a definite “red flag.” Burned or leaky valves can result in a rough-
running engine, increased exhaust temperatures, decrease in compres-
sion pressure and detonation. On four-cycle, turbocharged engines, the
first sign of valve trouble is unstable air manifold pressure accompanied
by intermittent misfiring and muffled detonation. This is caused by
burned combustion gases leaking into the air intake manifold, thereby
raising the pressure. The manifold regulator senses the increased pres-
sure and reduces the amount of air at a time when actually more air
would be needed to counteract the burned gases.
Nearly everyone is conscious of what valve trouble can do to a four-
cycle engine but some do not realize what faulty gas injection valves can
do to a two-cycle engine. One indication is a very hot pipe jumper be-
tween the gas header and the gas injection valve. This will be followed by
rough running, missing, and detonation. Some mechanics do not appreci-
ate the fact that a good seat is required. The safe way to ensure against
gas valve leakage is to pressure-test them by air on the bench. Soap and
water can be used to check the lapped seat and valve. If it is difficult to
get the seat to hold, the usual cause is excessive valve bushing wear. In-
sufficient valve tappet clearance will burn valve seats; too much clear-
ance is detrimental to cams, rollers, and tappets. There have been several
cases of operators experimenting with gas injection valve tappet clear-
ance which have led to cracked heads and liners and piston seizures. It
should be noted that gas injection valves are not exhaust valves; they are
designed to operate only in the cool part of the stroke. If allowed to be