Many industry problems exist today that result from harmful vibration generated by detrimental pressure pulsation.
This complex problem most commonly occurs within piping systems. The acoustical natural frequencies couple with pulsating driving forces, which in turn couple with the mechanical natural frequency of the structure. This phenomenon can wreak havoc on piping systems.
One classic example is reciprocating compressors that cause piping to vibrate. The reciprocating pump generates pressure pulsation caused by the motion of the piston. This pulsation coincides with the acoustical natural frequency of the piping system. In the most unfortunate circumstances, this acoustical frequency couples with the mechanical natural frequency and the system vibrates. The pulsation can cause fatigue cracks in the piping system that lead to failure.
There are several ways to fix the problem:
- Detune the system mechanically by adding supports or changing the structural configuration of the piping.
- Detune the system automatically by changing the geometry of the systems, adding pulsation bottles or perforated plates.
- Change the process conditions. It is also important to note that sometimes there are process transients caused by a molecular weight change.
Notwithstanding the aforementioned issues, there is one area of possible concern that is typically not addressed: the acoustical response inside the machinery casing. The difficulty in capturing and proving the existence of an acoustical resonance often results in failing to consider such as a contributing factor to the problem. As a result, when a compressor experiences a failure, this cause is often overlooked and typically surge or some other phenomena is attributed to the failure. Just like a piping system, the acoustical natural frequency of the case couples to the mechanical natural frequency of the blades and the blades fail. However, the pulsation may not always be present, as the acoustical response is a function of molecular weight. Often the "exciter" is the blade pass frequency, pocket pass or vane pass. Through instrumentation and acoustical modeling of the case using the finite element tool, one can effectively diagnose and remedy the problem.
These problems commonly exist in liquid ring, screw, axial and centrifugal compressors.
A good methodology for troubleshooting this problem is as follows:
- Review process data.
- Capture data on any transients.
- Perform a field study with a high-speed data acquisition system.
- Evaluate the exciter.
- Develop a finite element model of the case.
- Diagnose the problem.
- Remedy the problem by reconfiguring the hardware or changing the process conditions, or both.
Acoustical problems are some of the toughest problems to solve, mainly because the driving forces are typically the third, fourth or fifth modes. Simply changing the parts won't solve the problem. You need an experienced engineer to employ the above methodology.
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