Page 222 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

204 Major Process Equipment Maintenance and Repair

very conscious of its importance, but there seems to be confusion among
operators and mechanics as to how to accomplish it.
Some people periodically check the exhaust temperatures and peak fir-
ing pressures of each cylinder and, if they are not normal, adjust the air-
to-fuel ratio controls or change the amount of gas supplied to certain cyl-
inders. This practice of making adjustments without first determining the
reason for unbalance is not sound. There are others who will change the
same adjustments to relieve a detonating engine with the assertion that
the cause will be investigated later. This approach can be dangerous and
in the meantime causes unnecessary work. An engine installation should
be viewed as an engine with a certain number of adjusting knobs on it.
The simple installations may have one or two adjustments. In order to get
the “last squeal out of the pig,” modern engines have several adjusting
knobs. The point is that there is only one position for each knob and,
once they are set in that position, they should never need changing be-
cause adjusting screws of gas valves or air-to-fuel ratio controls do not
wear. A change in air-to-fuel ratio or balance is not caused by adjust-
ments changing but is due to some malfunction of the engine.
For example, in the case of a two-cycle engine, an increase in air mani-
fold pressure indicates carbon in either the intake and/or the exhaust
ports. Carbon does not form in equal amounts in all cylinders, which up-
sets engine balance. The balance may be restored by a change in adjust-
ments but it will last for only a day or two. In other words, it is impossi-
ble to keep an engine in balance with carbon in the ports. The proper
procedure is to watch for signs that carbon is forming so that port clean-
ing can be scheduled. In addition to carbon in the ports, other malfunc-
tions such as faulty ignition, valves, turbochargers, blowers, and piston
rings will upset engine balance. Consequently, the correct approach to
engine balance is to repair the assembly that is causing the unbalance.
Engine Safety Devices-The trend in new installations is toward com-
plete or partial automation-in these cases, plant designers include a
safety shutdown device for every engine function. The instruments are
called on to do a very difficult job because they do not operate for long
periods. In the meantime they collect rust, dirt, moisture, and in some
instances oxidized oil-and when called on to protect the engine they
may not function. Also, the problem of false shutdowns is sometimes
temporarily put off by blocking out the instrument. This has proved
costly and embarrassing in many instances. It should be a hard and fast
rule in all installations that the engines be shut down at least every six
months by actual operation of every shutdown device.
This section may impart the feeling that if engines are to be maintained
as suggested, there would not be any time left for running. However, re-
member that throughout, the theme is to watch for signs and make in-