GE H301 Series, Operating Manual

Page: 101 / 137

H301/302 HSR Compressors | Rev. 2.2 | 91
Pressure Pulsations
Pressure pulsations are caused by piston motion, which generate the basic
fundamental pulsation frequency corresponding to crankshaft speed, gas
properties, and the flow pattern of the gas through the valves and piping.
The pulsation level is the product of piston speed and fluid density times the speed
of sound. Acoustic resistance determines the amplitude of the pressure pulsations.
Pressure pulsations can cause physical movement of compressor cylinders, piping,
and pulsation bottles or manifolds (across two cylinders). Pressure pulsation
can cause premature compressor valve failure. The presence of high frequency
vibration (usually 4 or more times operating speed) maybe the result of pressure
pulsations. The high frequency vibration maybe characterized by a physical
“buzzing” of the vibrating component. A tingling sensation may be felt when
touching the vibrating component. Removing the inlet and discharge piping from a
cylinder can help determine if the vibration is being caused by pressure pulsation.
Connect electronic analyzers with pressure transmitters to the process gas system
to measure pressure pulsations and frequencies.
Before and after removing the piping, operate the compressor over a speed range
and observe the cylinder vibration. A drop in the vibration level after the piping
is removed indicates pressure pulsations are present. Fix pressure pulsations by
installing:
• Orifice plates
•
Pulsation bottles or manifolds
• Piping supports
Torsional Vibration
Torsional vibrations are very difficult to detect without the use of special
instruments. If the compressor is in a torsionally critical operating speed, violent
V-belt movement at the free end of the driver (engine or electric motor) maybe
observed and gear train noise maybe present. At a critical operating speed,
torsional vibration of driver components maybe detected. Use the instrumentation
to confirm torsional vibration. During the design of the package, perform a
torsional vibration analysis (TVA) projection to determine optimum component
selection.
Move the driver speed through a wide range (900 to 1200 or 1800 RPM) to
determine if there is a possible torsional vibration present. While changing the
speed, observe the fan belts for “flapping”. If the fan belts flap at a specific speed
point, it indicates the presence of torsional vibration. Listen for a change of pitch
in the gear train. Torsional vibration may develop in compressor packages when
the engine mounted torsional vibration damper becomes ineffective due to age,
damage, loss of fluid, etc.
The measuring unit for amplitude is
pascal (Pa), millibar (mbar), pounds per
square inch (psi) or meter water column
(mWc).