Page 145 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Compressor Best Practices Be st Practice 3.4
Refer to Figure 3.3.10 and remember: probes. Note that a response curve must be plotted for each set
of unbalance locations and unbalance amount.
- The compressor must go through NC 1
Figure 3.3.11 shows the rotor response for the non-drive end
- The shaft vibration increases at NC 1 (usually 2 ,3 or more)
- The vibration at center span is approximately 10 the probe (N.D.E.) set of probes with the first set of unbalance.
vibration Figure 3.3.12 shows the rotor response for the drive end set of
probes (D.E.). The operating speed range of this example is
Therefore: 6,000 e 8,000 rpm.
Vibration at the mid span during the first critical speed
will be:
Measured rotor response
¼ð7:0 milsÞ ð2:0Þ ð10Þ
Probe value NC 1 amplification Mode shape difference During a shop test, the rotor response of every turbo-com-
¼ 140 mils!! pressor rotor is measured during acceleration to maximum
Normal clearance between the rotor and interstage labyrinths speed and deceleration to minimum speed. Values are plotted
is typically 40 mils! This vibration exposes the diaphragms, on the same coordinates as for the rotor response analysis. The
which are usually cast iron, to breakage. One final comment; plot of shaft vibration and phase angle of unbalance vs. shaft
during shutdown, the rate of rotor speed decrease CANNOT be speed is known as a bode plot.
controlled as in the case of start-up. It depends on rotor inertia, Bode plots represent the actual signature (rotor response) of
load in the compressor, the process system characteristics and a rotor for a given condition of unbalance and support stiffness.
the control and protection system. If the vibration at the probe They indicate the location of critical speeds, the change of shaft
locations is high, the best advice is to stop the compressor whilst vibration with speed and the phase angle of unbalance at any
fully loaded, which will reduce the time in the critical speed speed. A bode plot is a dynamic or transient signature of vi-
range as much as possible. Yes, the compressor will surge, but bration for a rotor system, and is unique to that system for the
the short duration will not normally damage the compressor. recorded time frame. Bode plots should be recorded during
Figures 3.3.11 and 3.3.12 present the primary output of every planned start-up and shutdown of every turbo-compres-
a rotor response study. sor. As discussed in this section, the bode plot will provide
Rotor response plots display vibration amplitude, measured valuable information concerning shaft vibration and phase angle
at the probes, vs. shaft speed for the horizontal and vertical at any shaft speed.

Best Practice 3.4Practice 3.4Practice 3.4
Best
Best
Limit non-lubricated reciprocating compressor piston Lessons Learned
speed to below 600 ft/minute for optimum compressor Failure to limit piston speed in non-lubricated re-
non-lubricated compressor reliability (greater than 94%). ciprocating compressor applications has led to valve,
Non-lubricated reciprocating compressors have the lowest re- packing and ring MTBFs of less than 6 months.
liabilities; only around 92%.
Invest in an extra cylinder if necessary to keep piston speeds below Benchmarks
600 ft/minute for maximum packing, valve and piston ring/rider band
life. I have used this best practice since the mid-1970s to result in non-
lubricated reciprocating compressor reliabilities greater than 94%.
Packing, valve, piston ring and rider band MTBFs have exceeded 24
months.

B.P. 3.4. Supporting Material In this section the functions of each major component of
a reciprocating compressor are defined. That is, what the pur-
Non-lubricated reciprocating compressors have the lowest re- pose of each component is or “What It Does”. With this un-
liability (92% or lower) amongst compressor types because they, derstanding, you will be in a better position to know if the
like all reciprocating compressors, have a number of wear parts compressor is performing its duty correctly. We will present
but also do not have any directed packing and cylinder lubrica- each major component, state its function, operating limits and
tion. Packing MTBFs can be typically less than 6 months. Re- what to look for. After presenting general information for each
ducing the piston rod speeds below 600 ft/minute has proven to component, we will present specific information concerning site
optimize packing, valve, piston ring and rider band MTBFs. compressors. We start with the crank shaft.

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