Page 205 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Reciprocating Gas Engines and Compressors 187

Brightness of the liners is a healthy sign. Any distress to the rings and
wear bands will make dark streaks on the liner. There are two very im-
portant measurements that can be taken through the valve ports that will
give exact information for detecting wear of liners and wear bands. The
wear band is always larger in diameter than the piston, which means
there will be a space between the liner and the piston. When the piston is
installed, that space should be measured with feelers and then rechecked
after a specified number of hours of operation.
Following start-up or overhaul, the first inspection should take place
after no more than 48 hours of operation.
Subsequent periods of inspection will differ for each installation. Rota-
tion of the piston will be necessary if the feeler clearance decreases. The
liner diameter can be measured through the valve port. This measure-
ment will check not only for wear but for out-of-roundness from distor-
tion, which would indicate that the liner has been hot.

Valve Breakage

Valve failures have created much ill will toward reciprocating com-
pressors. Excessive breakage, especially at start-up, has left a bad im-
pression in the minds of many responsible people. Installation dirt, liq-
uid, and off-design temperatures and pressures necessary to get started
are largely responsible for failures that many times are mistakenly ana-
lyzed as incorrect materials, misapplication, corrosion, and incorrect de-
sign. On failures subsequent to start-up, the same reasons are given. This
thinking sometimes creates unnecessary change of design, materials and
sometimes a switch in suppliers. The point here is not to repudiate these
statements, because in some cases a change is required. Furthermore,
new ideas are healthy for the industry, provided they do not detour from
the very issues that are giving trouble. However, if proper practices and
basic principles are not recognized and observed, true progress on new
ideas will be blocked.
The proper practices and basic principles can be described by follow-
ing the sequence of events for start-up operation of a new installation.
An increase in the normal discharge temperature of the gas is a sure
sign of poor valve condition. Although there are others, e.g., noise and
reduction of capacity, discharge temperature is a very adequate indicator.
Generally, the start-up crews watch it very carefully and shut down for
repairs when it becomes too high. On the other hand, there are some who
put off replacing hot valves. They justify this by stating that the process
cannot be disturbed or they do not have time and it can wait. In addition
to direct damage to the valve assembly, a faulty valve can score compres-
sor rods and cause packing failure. This is especially true if the defective