Page 112 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Pump Best Practices Best Practice 2 .21

Clearly stating impact of problem on plant pro fit.
Prepare a brief statement of:
The problem
Action plan and confirmation of success (past experience)
Cost of failure to date
Cost of solution
The impact on plant profit (loss)
Be confident!
Be professional!
Provide timely updates and final report on completion.

Fig 2.20.24 Obtain and maintain management support by .

Implementation of the action plan
Fig 2.20.23 Piping considerations
Correction of pipe stress problems is usually the most difficult
problem to obtain action plan implementation for. Why? It is
time. This usually requires a complete new foundation be costly, exposes the plant to possible safety problems and can
designed and installed at the next T&I. A short-term fixcan result in a process unit shutdown. Usually, it should be planned
be to install stainless steel shims to temporarily correct the for a T&I. I have found that the guidelines in Figure 2.20.23
problem. provide the best probability of implementation.

Best
Best
Best Practice 2.21Practice 2.21Practice 2.21
Pre-commissioning e ensure that every centrifugal pump Lessons Learned
operates in the equipment reliability operating envelope Many new pumps are selected with impeller diameters that
(EROE) and change impeller diameter if required. are too large for field operation parameters. This can result
The hydraulic calculations used to determine the pump head re- in driver overload and possible cavitation.
quired for centrifugal pumps will only approximate the field conditions, Many new plants commissioned do not confirm that centrifugal
and can be conservative, which will result in lower field head required pumps are operating in the EROE, which results in pump overload and
than noted on the pump data sheet. cavitation. This lack of action has resulted in low MTBFs and frequent
Lower pump head required can force centrifugal pumps to operate tripping of motor driven pumps.
at greater flow than the design point.
Always confirm that new pumps are operating within the EROE, as Benchmarks
previously noted in this chapter, and take corrective action as noted This best practice has been followed during pre-commissioning of all
below if required. new centrifugal pump installations since the mid-1980s to optimize
Most centrifugal pumps drivers are sized for þ10% power and can centrifugal pump unit safety and reliability. This best practice has en-
be overloaded if the pump flow is greater than the design flow. sured pump unit MTBFs in excess of 80 months.
The most cost effective solution to prevent driver overload is to
reduce (cut or trim) the pump impeller diameter, to arrive at the desired
pump flow at the actual field process head required conditions.

B.P. 2.21. Supporting Material
3
Where: Q = Flow, in m /hr or GPM
Q 1 N 1 D 1
= =
The affinity laws Q 2 N 2 D 2 N = Speed, RPM
2 2 H = Head in m-kg force/kg mass
H 1 N 1 D 1
= = or ft-lb force/lb mass
Let us examine the effect of speed and/or impeller diameter H 2 N 2 D 2 Hp = Power in kW or BHP
change on the performance of centrifugal pumps. 3 3 D = Impeller diameter in mm
HP 1 N 1 D 1
The affinity laws e or fan laws, as they are sometimes re- = = or inches
HP 1 N 2 D 2
ferred to e play an important role in determining centrifugal
pump performance when operating conditions change. They are
also used for scale-up purposes when performance parameters Fig 2.21.1 Affinity law relationships
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