Page 273 - Root Cause Failure Analysis
P. 273

Compressors      261

                    to the impact as each piston reaches the top and bottom dead-center of its stroke. The
                    energy levels also are influenced by the unbalanced forces generated by nonopposed pis-
                    tons and looseness in the piston rads, wrist pins, and journals of the compressor. In most
                    cases, the dominant vibration frequency is the second harmonic (2X) of the main crank-
                    shaft's rotating speed. Again, this results from the impact that occurs when each piston
                    changes direction (i.e., two impacts occur during one complete crankshaft rotation).

                    Valves

                    Valve failure is the dominant failure mode for reciprocating compressors. Because of
                    their high cyclic rate, which exceeds 80 million cycles per year, inlet and discharge
                    valves tend to work harden and crack.

                    Lubrication System

                    Poor maintenance of lubrication-system components, such as filters and strainers, typ-
                    ically causes premature failure. Such maintenance is crucial to reciprocating compres-
                    sors because they rely on the lubrication system to provide a uniform oil film between
                    closely fitting parts (e.g., piston rings and the cylinder wall). Partial or complete fail-
                    ure of the lube system results in catastrophic failure of the compressor.

                    Pulsation

                    Reciprocating compressors generate pulses of  compressed air or gas that  are  dis-
                    charged into the piping that transports the air or gas to its point(s) of use. This pulsa-
                    tion often generates resonance in the piping system, and pulse impact (i.e., standing
                    waves) can severely damage other machinery connected to the compressed-air sys-
                    tem. While this behavior does not cause the compressor to fail, it must be prevented to
                    protect other plant equipment. Note, however, that most compressed-air systems do
                    not use pulsation dampers.
                    Each time the compressor discharges compressed  air, the air tends to act like a compres-
                    sion spring. Because it rapidly expands to fill the discharge piping's available volume,
                    the pulse of high-pressure air can cause serious damage. The pulsation wavelength, h,
                    from a compressor having a double-acting piston design can be determined by

                                                  60a
                                             A=-=-      34,050
                                                  2n       n
                    where
                          h =  wavelength, ft;
                          a =  speed of sound = 1,135 Wsec;
                          n =  compressor speed, revolutions/min.

                    For a double-acting piston design, a compressor running at 1,200 rpm will generate a
                    standing wave of  28.4 ft. In  other words, a shock load equivalent to the discharge
   268   269   270   271   272   273   274   275   276   277   278