Page 145 - Improving Machinery Reliability

Muchinery Reliability Audits arid Reviews 117

Pulsations

Pulsations can cause problems in rotating equipment as well as reciprocating
ma~hinery.~'.~* Pulsation resonances occur in piping systems and are a function of
the fluid properties and the piping, compressor, or pump geometry.
Pulsations can cause premature surge in centrifugal compressors and pumps if the
generated pulses, such as from stage stall,I6 match one of the pulsation resonances of
the system. The potential pulsating excitation mechanisms for piping systems are the
running speed component and its multiples, vane, and blade passing frequency and
those caused by flow excited (Strouhal frequency) phen~mena.'~
In the design stage, the acoustical natural frequencies of piping systems can be
calculated using either digital2s or analog modeling proced~res.~' A model of a pip-
ing system analyzed on a digital computer is given in Figure 3-30. The predicted

Test Points
A- Cylinder 41
B- PDF
c- PDL
D- ClO6ed End of Bypass
E- Hotrder Take-Off
F- HeLer Run Inlet Flng
C- 3' b4 Meter
K- PDM
1- Pump t2 Blocked

Figure 3-30. Digital computer model of pump system for pulsation analysis.