Did you ever wonder why the vibration monitor on your centrifugal air
compressor used a "2X" multiplier during startup? A "2X"
multiplier doubles the vibration values of the alert and shutdown set points.
Without this multiplier, it would be difficult to impossible for the compressor
to reach running speed without tripping on high vibration. The reason is the
rotors, at some speed or speeds exhibit a peak in their vibration. The speeds at
which this vibration peaks are called the critical speeds. Rotors may go through
several critical speeds on start up before reaching their operating speeds. If
this is the case, the rotor is known as a flexible shaft rotor. If, however, the
first critical speed is higher than the operating speed, the rotor is known as a
stiff shaft rotor
When the compressor stage operates between critical speeds, the operating
speed should be at least 10% away from the nearest critical speed in order to
assure safe and stable operation. Passing through a critical speed is by no
means harmful, in fact it is normal, if it is done quickly. When setting up centrifugal compressors to
run in snowmaking operations of remote ski areas, we sometimes have to extend the
start time in order to minimize the inrush current. Special attention is paid to
the compressor stage vibration signatures during ramp up to running speed. If
the compressor lingers too long at or near the critical speed, serious damage
can occur. This can also be a problem when the compressor is turbine driven.
Slow rolling to warm the turbine should be done at a speed approved by the
original equipment manufacturer and controlled by the turbine governor.
The critical speeds can be moved or adjusted in several ways to obtain the
10% operation margin required. The rotor system can be thought of as a
spring-mass-damper system. By changing any of these parameters, the critical
speed can be changed. Physically, this is accomplished in several ways. The
pinion can be modified by changing its stiffness, or the bearings can be located
either closer to each other or further apart (hard to do on a standard
compressor). The impeller can be modified, changing its mass. The oil
temperature or viscosity can be changed, adjusting the spring rate of the
system. The bearings can be changed or rotated to allow different clearances,
thus changing the damping.
If you have centrifugal compressor stage that seems unusually sensitive to
fouling or load changes, resulting in high vibration. It may be worth asking
what the stage critical speeds are and then comparing them to the operating
speed. Is the operating speed safely away from the critical speed?