Analysis of alternative machines

Reciprocating engines and compressors can be diagnosed with high accuracy from the dynamic signal of pressure, ultrasound and vibration.

In piston compressors and combustion engines, vibration analysis by FFT is not an effective technique, since the vibration spectrum is very rich in peaks and it is impossible to discern the different mechanical problems present there.

The analysis of cylinder pressure traces is the fundamental technique in the predictive diagnosis of this type of machine. Cylinder pressure can be represented as a closed curve with respect to the volume swept by the piston (P-V) or as an open curve with respect to the crankshaft angle (P-a).

The P-V curve is used to calculate power (IHP) and to perform a machine performance analysis, as a technique to evaluate the operating condition by comparison with nominal or design parameters.

The open P-a curve is used for multichannel waveform analysis by superimposing typical mechanical condition variables (vibration and ultrasound) on the cylinder pressure trace.

Typical diagnostic charts for reciprocating machines are:

• Pressure versus cylinder volume (P-V) graph.
• Pressure versus crankshaft angle (P-a) graph.
• Superimposed graph of BF vibrations (0-200 Hz) on the trace (P-a).
• Superimposed graph of AF vibrations (1-20 kHz) on the trace (P-a).
• Superimposed ultrasound plot (40-60 kHz) on the trace (P-a).
• RPM angular kinematics monitor (torsional vibration analysis).
• Pressure FFT plot (pulse analysis).
• Secondary and Primary Ignition Graph (in M.E.P.).

Related products

Advanced monitoring system for reciprocating compressors Prognost

Contact one of our specialist predictive maintenance engineers for advice on how to apply predictive maintenance by alternative machine analysis: info@preditec.com