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Tuesday, December 1, 2009

Flow Control in Individual Pump

See Other Many Kinds of Pumps:
Centrifugal Pump
Vertical Pump
Sump Pump and Multistage Pump

Propeller Pump
Positive Displacement Pump
Reciprocating Pump
Diaphragm Pump
Piston Pump
Rotary Screw and Gear Pump
Jet Pump and Electromagnetic Pump

An inherent characteristic of positive displacement pumping of relatively incompressible liquids is that flow rate is proportional to displacement rate and independent of pressure levels. The capacity of a centrifugal pump operating at constant speed varies from a maximum flow at no developed pressure to zero flow at a definite limiting pressure known as shutoff head. The average capacity of positive displacement pumps at constant speed is within design limits of pressure, practically constant, even though flow rate pulsations do occur as individual displacement are forced into the discharge pipe.

Flow control of positive displacement pumps is accomplished by:
  • Changing the displacement rate
  • Changing the displacement volume
  • Changing the proportion of the displacement delivered into the piping system

Throttle Control in Direct-Acting Steam Pumps
Direct acting pumps are controlled by speed change, which is effected by throttling the flow of steam (or other motive gas) to the drive cylinder. The magnitude of excess force on the drive piston over that required for the driven, or pumping, piston to force pumpage into the piping system dictates the stroke rate and therefore the capacity of the pump. A sensor that detects the desired result of pumping (pressure, level, flow rate, and so on) may be used to modulate the steam throttle valve.

Speed Control in Power Driven Pumps
Speed modulation is the most common means of flow control for power driven pumps. The most rudimentary speed control is intermittent (start stop) operation. The average capacity over relatively long time periods depends upon the percentage of time the pump operates at 100% versus the percentage of time it operate at zero flow. Of course, consideration must be given to the frequency of starts because electric motors may overheat if there is insufficient time for cooling after the inrush of starting current.