Page 149 - Root Cause Failure Analysis

Compressors 137

12,000 hp. Pressure capabilities range from low vacuums at intake to special com-
pressors capable of 60,000 psig or higher.

Reciprocating compressors are classified as constant-volume, variable-pressure
machines. They are the most efficient type of compressor and can be used for partial-
load, or reduced-capacity, applications.

Because of the reciprocating pistons and unbalanced rotating parts, the unit tends to
shake. Therefore, it is necessary to provide a mounting that stabilizes the installation.
The extent of this requirement depends on the type and size of the compressor.

Reciprocating compressors should be supplied with clean gas, so inlet filters are rec-
ommended in all applications. The filters cannot satisfactorily handle liquids entrained
in the gas, although vapors are no problem if condensation within the cylinders does
not take place. Liquids will destroy the lubrication and cause excessive wear.

Reciprocating compressors deliver a pulsating flow of gas that can damage down-
stream equipment or machinery. This sometimes is a disadvantage, but pulsation
dampers can be used to alleviate the problem.

Conjiguration
Certain design fundamentals should be clearly understood before analyzing the oper-
ating condition of reciprocating compressors. These fundamentals include frame and
running gear, inlet and discharge valves, cylinder cooling, and cylinder orientation.

Frame and Running Gear Two basic factors guide frame and running gear design.
The first factor is the maximum horsepower to be transmitted through the shaft and
running gear to the cylinder pistons. The second factor is the load imposed on the
frame parts by the pressure differential between the two sides of each piston. This
often is called pin loud because the full force is exerted directly on the crosshead and
crankpin. These two factors determine the size of bearings, connecting rods, frame,
and bolts that must be used throughout the compressor and its support structure.

Cylinder Design Compression efficiency depends entirely on the design of the cyl-
inder and its valves. Unless the valve area is sufficient to allow gas to enter and leave
the cylinder without undue restriction, efficiency cannot be high. Valve placement for
free flow of the gas in and out of the cylinder also is important.

Both efficiency and maintenance are influenced by the degree of cooling during com-
pression. The method of cylinder cooling must be consistent with the service
intended.

The cylinders and all the parts must be designed to withstand the maximum applica-
tion pressure. The most economical materials that will give the proper strength and
the longest service under the design conditions generally are used.