Page 147 - Root Cause Failure Analysis

Compressors 135

ume. This feature permits the use of these compressors in load-following applications.
However, care must be taken to prevent sudden, radical changes in speed.
Internal slip is simply the amount of gas that can flow through internal clearances
from the discharge back to the inlet. Obviously, internal wear will increase internal
slip.
Discharge pressure is relatively constant regardless of operating speed. With the
exception of slight pressure variations caused by atmospheric changes and back pres-
sure, a rotary positive-displacement compressor will provide a fixed discharge pres-
sure. Back pressure, which is caused by restrictions in the discharge piping or demand
from users of the compressed air or gas, can have a serious impact on compressor per-
formance.

If back pressure is too low or demand too high, the compressor will be unable to pro-
vide sufficient volume or pressure to the downstream systems. In this instance, the
discharge pressure will be noticeably lower than designed.

If back pressure is too high or demand too low, the compressor will generate a dis-
charge pressure higher than designed. It will continue to compress the air or gas until
it reaches the unload setting on the system’s relief valve or until the brake horsepower
required exceeds the maximum horsepower rating of the driver.
Installation

Installation requirements for rotary positive-displacement compressors are similar to
any rotating machine. As with centrifugal compressors, rotary positive-displacement
compressors must be fitted with pressure-relief devices to limit the discharge or inter-
stage pressures to a safe maximum for the equipment served.

In applications where demand varies, rotary positive-displacement compressors
require a downstream receiver tank or reservoir that minimizes the load-unload
cycling frequency of the compressor. The receiver tank should have sufficient volume
to permit acceptable unload frequencies for the compressor. Refer to the vendor’s
O&M manual for specific receiver-tank recommendations.

Operating Methods

All compressor types have moving parts, high noise levels, high pressures, and high-
temperature cylinder and discharge-piping surfaces.

Rotary positive-displacement compressors should be operated as baseloaded units.
They are especially sensitive to the repeated start-stop operation required by load-fol-
lowing applications. Generally, rotary positive-displacement compressors are
designed to unload about every six to eight hours. This unload cycle is needed to dis-
sipate the heat generated by the compression process. If the unload frequency is too
great, these compressors have a high probability of failure.