Page 68 - Subyek Teknik Mesin - Forsthoffers Best Practice Handbook for Rotating Machinery by William E Forsthoffer

Be st Practice 2 .6 Pump Best Practices
B.P. 2.6. Supporting Material consisting of a vessel into which the produced product flows,
the objective then is to remove (pump) the throughput from
the vessel at the same rate of flow as it enters the vessel. If this
Objectives of the operating company is accomplished, the following parameters will remain constant
in the vessel:
During normal operation, the prime objective of the operating Flow in and out
company is to maintain maximum reliability and maximum Level
product throughput at minimum operating (maximum pump Pressure
efficiency) cost (refer to Figure 2.6.1).
Any point in any process can be thought of as an “equivalent
vessel”. By controlling either flow, level or pressure, the oper-
ating company’s objective will be realized (refer to Figure 2.6.3).
Therefore the objective of any pump control system will be to
maintain a constant, controlled variable set point (flow, level or
pressure). The output of the controller can then either vary:

Head required (the process)
Head produced (the pump characteristics)
These options are shown in Figure 2.6.4 for a centrifugal
pump and Figure 2.6.5 for a PD pump.

Adjusting head (energy) required

Head required in a pumping system can be changed by adjusting
the discharge system resistance using pressure control flow
control or level control (refer to Figures 2.6.4 and 2.6.5).
Each of these methods results in closing a throttle valve in
the discharge piping which increases the head (energy) re-
quired and reduces the flow rate. This action requires more
Fig 2.6.1 The end user’s objectives energy (head) to overcome the increased system resistance
(refer to Figure 2.6.6).
To meet the operating company objectives, a particular Throttling the discharge of a positive displacement pump
pump is selected, on the basis of the optimum process flow rate increases the discharge pressure, and consequently the power
and the required system head (energy). Since the pump char- required to overcome the increased head required while passing
acteristics will change with wear, erosion or fouling, and the the same flow rate through the pump. This scheme is not effi-
system characteristics will vary, a reliable control and protection cient and is usually replaced by a “bypass” throttling arrange-
system must be selected to continuously meet the objectives ment (refer to Figure 2.6.5).
noted in Figure 2.6.1 regardless of whether the selected pump is
dynamic or positive displacement. Regardless of type, the op-
erating company’s objectives are met by meeting process Effects of throttling pump suction
throughput requirements in the most efficient manner.
There are only two options available to vary the process Generally, throttling the pump suction will produce harmful
throughput requirements (refer to Figure 2.6.2). effects and should be avoided (absolute pressure is reduced at
Before proceeding, it is helpful to introduce the concept of the impeller inlet), except in certain cases involving series pump
an equivalent vessel. Another way to state the operating operation and ‘hot well’ condensate pumps specifically designed
company’s objectives is to state that they want to process all for such services (refer to Figure 2.6.7 for the effects of suction
the throughput produced. If we visualize any process system as throttling).

Adjust the head required
Adjust the head produced

Fig 2.6.2 Methods for varying pump throughput

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