Page 183 - Fluid mechanics, heat transfer, and mass transfer

FANS, BLOWERS, AND COMPRESSORS 161

– Mechanical damage can include the following: . What is the effect of wetness of inlet gas on compressor
1. Radial bearing load during the initial phase of performance? How is the wetness minimized/
surging. A side load is placed on the rotor that acts eliminated?
perpendicular to the axis. & Results in damage to the compressor in the form of
2. Thrust bearing load due to loading and unloading. pitting corrosion, salt deposits and diluted lubricants.
3. Seal rubbing. & Use of liquid knockout drums on suction side fitted
4. Stationary and rotating part contact if thrust bear- with mist eliminators, which are installed in the
ing is overloaded. knockout drum.
. How is surge controlled? & The mist eliminators should have appropriate speci-
fications free from incorrect installation, overload-
& The only way to prevent surging is to recycle or
ing, uneven velocity profiles, high liquid viscosity,
blowdown a portion of the flow to keep the compres- waxy deposits, liquid slugs, foaming, and several
sor away from its surge limit. other possibilities.
& Unfortunately, compressing extra flow results in a . “Efficiencies for compressors are generally more than
severe economic penalty. for pumps.” True/False?
& The control system must be able to accurately de- & True. Compressor efficiencies are in the range of
termine the operating point of the compressor as to 60–80%, whereas for pumps, efficiencies are in the
provide adequate, but not excessive, recycle flow. range of 40–60%.
. What types of compressors experience surge problems? . What are the possible causes of centrifugal compressors
& The surge problem is inherent in dynamic compres- problems?
sors—centrifugal and axial, as distinguished from & Table 5.12 presents a summary of possible centrif-
positive displacement types. ugal compressor problems.

TABLE 5.13 Summary of Reciprocating Compressor Problems
Trouble Probable Cause(s)
Compressor will not start Power supply failure; switchgear or starting panel; low oil pressure shut down switch; control
panel.
Motor will not synchronize Low voltage; excessive starting torque; incorrect power factor; excitation voltage failure.
Low oil pressure Oil pump failure; oil foaming from counterweights striking oil surface; cold oil; dirty oil filter;
interior frame oil leaks; excessiveleakage at bearing shim tabs and/or bearings; improper low
oil pressure switch setting; low gear oil pump bypass/relief valve setting; defective pressure
gauge; plugged oil sump strainer; defective oil relief valve.
Noise in cylinder Loose piston; piston striking outer head or frame end of cylinder; loose crosshead lock nut;
broken or leaking valve(s); worn or broken piston rings or expanders; improper valve
position.
Excessive packing leakage (blue rings). Worn packing rings; improper lube oil and/or insufficient lube rate; dirt in packing; excessive
rate of pressure increase; packing rings assembled incorrectly; improper ring side or end gap
clearance; plugged packing vent system, scored piston rod; excessive piston rod runout.
Packing overheating Lubrication failure; improper lube oil and/or insufficient lube rate; insufficient cooling.
Excessive carbon on valves Excessive lube oil; improper lube oil (too light, high carbon residue); oil carryover from inlet
system or previous stage; broken or leaking valves causing high temperature; excessive
temperature due to high-pressure ratio across cylinders.
Relief valve popping Faulty relief valve, Leaking suction valves or rings on next higher stage, Obstruction (foreign
material, rags), blind or valve closed in discharge line.
High discharge temperature Excessive ratio on cylinder due to leaking inlet valves or rings on next higher stage; fouled
intercooler/piping; leaking discharge valves or piston rings; high inlet temperature; fouled
water jackets on cylinder improper lube oil and/or lube rate.
Frame knocks Loose crosshead pin; pin caps or crosshead shoes; loose/worn main; crankpin or crosshead
bearings; low oil pressure; cold oil; incorrect oil; knock is actually from cylinder end.
Crankshaft oil seal leaks Faulty seal installation; clogged drain hole.
Piston rod oil scraper leaks Worn scraper rings; scrapers incorrectly assembled. Worn/scored rod; improper fit of rings to
rod/side clearance.
Courtesy: GPSA Engineering Data Book, 12th ed.