Page 401 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Be st Practice 7 .7 Lube, Seal and Control Oil System Best Practices

Fig 7.7.11 Screw pump performance (Courtesy of IMO Industries)

- Suction Conditions e The pump must be assured of liquid flow The same damage can occur from entrained gas (process
at minimum acceptable conditions during operation. If gas in seal oil systems or air) in the pumped liquid. Sources
strainers are installed in the pump suction a low suction are:
pressure alarm switch is recommended downstream of the
strainer. - Insufficient degassing in the reservoir
- Air carried into the system by pump suction system leaks and
The pump must always move the fluid in its liquid state. pump seal leaks, since most auxiliary system pumps operate
To ensure this, the net positive suction system head under a slight vacuum on the suction side.
available (NPSH A ) must exceed the net positive suction
head required (NPSH R ) by the pump. The NPSH R of any The effect will be the same as cavitation, since the suction
pump is simply the internal pump pressure drop from the end of the pump will be operating on a compressible fluid before
pump suction flange to the pumping element (screw, gear it is compressed by the energy available in the pump. In both
impeller). cases, potential damage can best be detected by the high-
The net positive suction head available (NPSH A ) is the pitched sound or whine emitted. Recommended action
absolute pressure at the specified suction fluid conditions above concerning pump cavitation and aeration is noted in
the liquid vapor pressure. Any liquid will change to a vapor when Figure 7.7.12.
its surrounding pressure is less than its vapor pressure. There-
fore, if the pressure drop internal to a pump (NPSH R ) equals or Fluid cleanliness
exceeds the available pressure surrounding the liquid, the liquid Pump life is definitely related to pumped liquid contamination
will become a vapor, so causing pump cavitation. Continued level. Simply put, the cleaner the system, the longer the pumps
operation under these conditions will cause loss of pump will last. Attention is drawn to the practice of using pump
lubrication, pump damage and possible failure, since the original strainers. It is our opinion that an effective auxiliary system and
incompressible fluid (liquid) supporting the rotors is now component ensurance program as detailed in this section proves
compressible, and cannot support the load required to separate to be a more reliable alternative to ensure pump reliability than
the rotors. the installation of pump suction strainers or filters.

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