Page 99 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Pump Best Practices Best Practice 2 .16

Best Practice 2.16

Use automatic minimum flow bypass valves, for centrifugal Lessons Learned
pumps, whenever the pumped liquid specific gravity is less The majority of EP&Cs and end users do not have specific
than 0.7 and/or for hot fluids. requirements for the use of automatic minimum flow
Liquids having a specific gravity below 0.7 are known as high vapor bypass valves. This fact leads to low MTBFs for pumps that
pressure liquids, and they will become a vapor under atmospheric can handle high vapor pressure liquids.
conditions.
Many pumps which handle fluids with high vapor pressures (spe-
These liquids also are usually close to their boiling point at field
cific gravity less than 0.7) do not have automatic minimum flow bypass
conditions, and therefore have a low net positive suction head re-
valves or manual bypass valves without any indication to operators in
quirement and are susceptible to cavitation and recirculation.
the control room of low flow. Pumps operating with high vapor pres-
Automatic minimum flow bypass valves should be used in all ap-
sure liquids at low flow conditions are prone to impeller and wear ring
plications involving such fluids, for each pump.
failure and seizure.
The minimum flow set point should be based on suction-specific
speed calculations, and the pump vendor’s recommendation for the Benchmarks
exact fluid being pumped. This best practice has been used in pump applications since the
As a general rule, the minimum flow point should not be lower than
the point at which the NPSH required curve ends at low flow on the 1990s, which has resulted in pumps of the highest safety and reliability
and MTBFs exceeding 80 months.
pump curve.

B.P. 2.16. Supporting Material
Centrifugal pump:
Protecting the pump Minimum flow bypass:
Manual
Automatic by external electronic or pneumatic signal
Control and protection are interrelated. Since the objective of Motor overload protection
the control function is to maximize production throughput Size driver for end of curve power
at minimum cost, it can sometimes cause the pump to operation Install orifice in discharge to increase system resistance and
in a region which can cause harmful effects (refer to limit pump operation
Figure 2.16.1). Flow or pressure limiting device
Overspeed (variable speed driver)
Governor
Positive displacement pump
Centrifugal pump: Overpressure/overload
At or below minimum flow operation: Relief valve sized for fullflow
Overheating Overspeed protection (direct acting pump)
High radial loads
Internal recirculation damage
Fig 2.16.2 Pump protection methods
End of curve flow
Electric motor overload
Turbine or engine overspeed
Damage to pump/driver
Positive displacement pump
Closed discharge valve Centrifugal pump protection systems
Damage caused by overpressure Since centrifugal pump flow varies inversely with pump head
Electric motor overload damage required, the pumps must be protected against low flow oper-
Overspeed of direct acting pump ation and high horsepower requirements (high pump flow).
Damage to pump
Minimum flow bypass systems and proper driver sizing and/or
protection systems are therefore required.
Fig 2.16.1 Consider these factors for pump protection
Minimum flow bypass protection
Depending on the characteristics of the pumped fluid, contin-
Pump protection systems uous operation at low flow conditions can damage a centrifugal
pump. Figure 2.16.3 shows the three (3) types of minimum flow
Now that the potential effects of operating a pump outside bypass systems e manual, automatic external and automatic
a ‘safe region’ have been identified, an overview to illustrate the internal. As a rule of thumb, minimum flow protection should
various methods available to protect each type of pump will be be provided for any pump application that can continuously
provided (refer to Figure 2.16.2). operate below the pump vendor’s minimum specified flow rate.

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