Page 155 - Practical Machinery Management for Process Plants Major Process Equipment Maintenance and Repair

Reciprocating and Liquid Ring kcuum Pumps 137

will affect the volume of gas drawn into the machine and the compression
ratio, Service liquid absorbs the heat of compression and must be cooled.
This is accomplished by running the liquid through external coolers, or
by make-up liquid, or by using a once-through system. Usually the ser-
vice liquid circulating system employs a discharge vessel in which gas
and liquid separation occurs. The level of liquid in this vessel is main-
tained at the level of the shaft centerline to ensure the correct amount of
liquid is in the machine.
As mentioned, service liquid temperature also has a profound effect on
the efficiency and capacity of liquid ring machines. As the temperature of
the service liquid rises, so does its vapor pressure. This increases the par-
tial pressure of the service liquid vapor in the machine and reduces the
volume available for the process gas.
Final discharge pressure, where it can vary, can also affect overall per-
formance. If the process gas contains a condensible vapor and the dis-
charge pressure is high enough at compression temperatures to allow
condensation, some liquid will condense. When this liquid leaks through
running clearances back to suction, it can flash off and reduce inlet or
suction capacity.
Starting of liquid ring machines must be done with the machine only
half full of liquid. Failure to maintain the correct level for starting can
result in either reduced capacity (level too low) or overloading (level too
high). The latter is more serious as it can result in driver overload, belt
wear, or coupling failure. These machines have only a limited capability
to handle liquids in the process stream.
Large volumes of liquid in the process gas or vapor stream can over-
load the machine. The reader will appreciate that a volume of liquid
greater than the volume between vanes at the discharge openings cannot
be compressed. High vibration, overload, and machine failure can result.
Particulate matter or solids in the process stream can be handled in small
quantities. Solids can lodge between running faces and cause wear, and
eventually open up clearances to reduce capacity. Large quantities of sol-
ids can plug up internal clearances and passages, reducing the capacity of
the machine and possibly seizing it. Excess heat can be generated by the
closing up of running clearances. This can cause excess thermal growth
of the rotor and further wear.
Cavitation damage can often be found in the suction porting and on
vacuum pump rotors. During operation this can be detected by the char-
acteristic sound of “gravel on steel.” Some vacuum pump systems em-
ploy an ejector in their suction lines to boost the suction pressure that the
machine sees. Motive fluid for the ejector can be taken from the pump
discharge. Where cavitation is found it may be worthwhile to consider
the use of a suction line ejector, raising the suction pressure (where pro-