Page 84 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Be st Practice 2 .12 Pump Best Practices
Justification of required action will be easier if the operating operating companies have documented case histories of problem
company looks beyond the project costs, and examines what pumps that will provide facts relating to the ‘total cost’ of op-
the actual cost (that could occur through lost production) erating problem pumps for the life of a project (refer to
will be if a pump experiences hydraulic disturbances. Most Figure 2.11.6).

Determine the cost effectiveness of pump selection not
only on the project (capital investment) costs, but on the
cost to the operating company of unreliable pumps

Fig 2.11.6 Justifying the selection of trouble free pumps

Best
Best Practice 2.12Practice 2.12
Require that suction specific speed for low NPSH required units) will result in a pump of low reliability and high
pumps is less than 10,000 (US units) to minimize the pos- maintenance.
sibility of recirculation. The writer has personally experienced recirculation at the rated
Low NPSH required for centrifugal pumps is attained by having operating point with a double flow impeller that was not checked for
a large impeller suction area and/or by using a double suction impeller suction specific speed (the suction specific speed exceeded 17,000).
(which has twice the suction area of a conventional impeller). The recirculation forces resulted in the following damage to the pump:
Recirculation is a hydraulic disturbance, which is the result of low
Impeller damage
fluid velocity into the impeller.
Shaft breakage
The increase of impeller suction area to reduce cavitation can
Bearing failure
produce recirculation if the increase in impeller suction area is too
Seal failure
great.
Suction specific speed predicts the probability of recirculation.
Benchmarks
Lessons Learned I have used this best practice since the mid 1980s, to ensure that
Selecting a pump for low NPSH required values without pumps are not susceptible to recirculation at flows in the operating
assuring the suction specific speed is below 10,000 (US range of the pump ( 50% of the pump best efficiency point). Pump
MTBFs using this best practice have exceeded 80 months.

B.P. 2.12. Supporting Material (see Figure 2.12.2). If the centrifugal pump operates only in the
‘heart of the curve’, also known as the equipment reliability
Liquid disturbances in centrifugal pumps are the major cause of operating envelope (EROE), costly field problems caused by
low pump reliability. The pressures generated by cavitation can liquid disturbances will be eliminated.
exceed 689,500 kpa (100,000 psi). Cavitation caused by many
different factors is responsible for pump lost service time as Maintaining a liquid inside a pump
a result of various pump component failures shown in
Figure 2.12.1. Design objectives

All pumps are designed to increase the energy of a fluid while
maintaining the fluid in its liquid state. Each centrifugal pump is
Bearings designed to produce a specific amount of head at a specific flow
Seals rate based on a specified fluid density. Once the pump is
Impellers
Shaft designed, any reduction in the fluid density will result in a re-
Wear rings duction in flow rate since the pump now requires greater head
(energy) and can produce this value only at a reduced flow rate
(see Figure 2.12.3).
Fig 2.12.1 Cavitation caused pump component failures One way to rapidly reduce the density of a fluid is to change
its phase. If a liquid suddenly changes to a vapor, the impeller
Proper pump selection guidelines require a centrifugal pump head required to meet the same process differential pressure
to be selected in the ‘heart of the curve’, and that sufficient requirements increases by a factor of 300 or more (see
NPSH AVAILABLE be present to avoid mechanical damage Figure 2.12.4). As a result, the pump will not be able to move

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