Page 93 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Pump Best Practices Best Practice 2 .12
attain the predicted value. If this is not possible, the following
alternatives exist: Piping runs directly vertically or horizontally into pump without
high pockets that can cause vapor formation
Operate pump at lower flow rate to reduce NPSH R Minimum straight suction pipe runs of:
Modify pump to reduce NPSH R at operating flow rate
Three (3) pipe diameters – single suction
A summary of this discussion is presented in Figure Five (5) pipe diameters – double suction
2.12.27. Double suction pumps should have pipe elbows perpendicular
to the pump shaft
The ‘belly’ of an eccentric reducer should be in the bottom
location

Obtain predicted value of NPSH A
Calculate NPSH A using actual conditions Fig 2.12.30 Inlet piping arrangements to avoid hydraulic
If NPSH A actual < NPSH A predicted: disturbances caused by process piping
Increase NPSH A if possible
Operate pump at lower flow rate (if cost effective)
Modify pump to reduce NPSH R
Low flow hydraulic disturbance test
Fig 2.12.27 Confirmation of NPSH A and recommended action
The test outlined in Figure 2.12.31 will determine if the liquid
disturbance is caused by low flow circulation or temperature
rise.
Internal pressure loss test

The test outlined in Figure 2.12.28 will confirm if the liquid
disturbance is caused by pump inlet pressure losses resulting Open pump discharge control valve or bypass to increase flow
from high liquid velocity. If pump noise significantly reduces and conditions noted in
Figure 2.12.26 become stable, cause of either low flow
recirculation or temperature rise cavitation is confirmed
Calculate liquid flow temperature rise to confirm if recirculation is
Close pump discharge control valve to reduce flow the root cause
If pump noise significantly reduces and conditions noted in Figure
2.12.26 become stable, cause is confirmed
Fig 2.12.31 Test to confirm low flow hydraulic disturbances

Fig 2.12.28 Test to confirm high velocity cavitation
If high velocity cavitation is confirmed and the stated value of
NPSH A is confirmed, possible solutions are presented in Increase pump flow rate, if possible, until quiet operation is
achieved
Figure 2.12.29. Modify inlet volute to increase impeller inlet velocity (if sufficient
NPSH A exists)
Install impeller with reduced eye area (assuming sufficient
Increase NPSH A until quiet operation is achieved NPSH A exists)*
Reduce pump throughout if cost effective Install minimum flow bypass to increase flow rate (to eliminate
Operate two pumps in parallel low flow temperature rise)
Increase impeller eye area *Note: wear ring modifications are required
Impeller material change
Purchase new pump with acceptable NPSH R
Fig 2.12.32 Solutions e low flow hydraulic disturbances

Fig 2.12.29 High velocity cavitation solutions
Possible solutions are presented in Figure 2.12.32.
Modification of impeller eye area is not always possible and Internal volute and/or impeller eye modifications are not
the pump vendor must be consulted to confirm it is possible and always possible. The pump vendor must be consulted to confirm
that satisfactory results have been achieved. these modifications are acceptable and satisfactory results have
Before leaving this subject, mention of the inlet piping ar- been achieved.
rangement is required. Suggested inlet piping arrangements are
presented in Figure 2.12.30. Justification of proposed action plan
Failure to conform to the guidelines presented here can lead
to hydraulic disturbances caused by:
Regardless of the cause of hydraulic disturbances, the
Entrained vapor problem cannot be resolved without management endorse-
Additional internal DP caused by turbulence ment of a cost effective action plan. As previously men-
Liquid separation from impeller vanes. tioned, all action plans must be justified by cost savings. In

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