The vibration of a system involves the transfer of its potential energy to kinetic energy and of kinetic energy to potential energy, alternately. If the system is damped, some energy is dissipated in each cycle of vibration and must be replaced by an external source if a state of steady vibration is to be maintained.
Every piece of equipment, regardless of size or configuration, has a natural or resonant frequency. Compressors are no exception and, in fact, numerous vibration analysis techniques are employed to predict compressor vibration during the design phase and to identify the source of high vibration in the operation phase. If the resonant frequency (critical speed) is below the operating frequency or speed, the unit is considered to have a flexible shaft. If the resonant frequency is above the operating speed, the unit is said to have a stiff shaft. Almost all axial compressors are considered to have a flexible shaft; that is, the normal operating speed is above the resonant (or undamped critical speed) frequency. The resonant frequency of centrifugal compressors falls on either side of the undamped critical speed line depending on the number of impellers involved and the shaft size. Blowers, integralengine-reciprocating compressors and screw compressors generally fall into the stiff shaft category. Separable reciprocating compressors trains (driver & compressor) may fall into either flexible or stiff shaft category depending on the number of cylinders and the type of driver selected. Vibration is a major consideration in the design of compressor rotor assemblies.
Rotor designs vary widely among compressor types and, if fact, they vary even between compressors of similar frame size. Physical factors such as number, size, arrangement of impellers or pistons, and bearing spans, bearing housing design, torque requirements, coupling selection and system parameters such as volume flow, casing pressure rating, operating speed ranges all affect the natural frequency and therefore rotor design.
The vibration to be eliminated or reduced can be in the form of one or more forms of disturbance displacement, velocity, acceleration, and transmitted force. The following methods are discussed to eliminate/reduce vibration at the source:
- Balancing of rotating machines single- and two-plane balancing.
- Controlling the response and stability of rotating shafts.
- Balancing of reciprocating engines.
- Reducing vibration caused by impacts due to clearances in the joints of machines and mechanisms.
The following methods are discussed to reduce transmission of vibration from the source:
- Changing the natural frequency of the system when the forcing frequency cannot be altered.
- Introducing a power-dissipation mechanism by adding dashpots or viscoelastic materials.
- Designing an isolator which changes the stiffness/damping of the system.
- Using an active control technique.
- Designing a vibration absorber by adding an auxiliary mass to absorb the vibration energy of the original mass.