Fixed speed motor centrifugal pumps

For the ordinary fixed-speed motor centrifugal pump, I suggest the following rules when selecting a control valve ... 

Most induction ac motors are fixed-speed. However, a large number of motor applications would benefit if the motor speed could be adjusted to match process requirements. Motor speed controls are the devices which, when properly applied, can tap most of the potential energy savings in motor systems. Motor speed controls are particularly attractive in applications where there is variable fluid flow. In many centrifugal pump, fan, and compressor applications mechanical power grows roughly with the cube of the fluid flow. To move 80 percent of the nominal flow only half of the power is required. Centrifugal loads are therefore excellent candidates for motor speed control. Other loads that may benefit from the use of motor speed controls include conveyers, traction drives, winders, machine tools and robotics. 

A steam turbine is a machine with an ancient genealogy. It is a direct descendant of the overshot water wheel, used to kick off the industrial revolution in England, and the windmill still used in Portugal. Turbines are widely used in process plants to drive everything from 2-hp pumps to 20,000-hp centrifugal compressors. They are versatile machines, in that they are intrinsically variable-speed devices. Electric motors are intrinsically fixed-speed machines. It is true that there are a variety of ways to convert AC (alternating-current) motors to variable speed, but they are all expensive and complex. 

Whereas energy can be saved by running steam pumps at a reduced speed, conventional motor-driven centrifugal pumps run at a constant speed. The electricity used by a motor does not depend on the motor horsepower rating but is a function of the head and flow developed by the pump. The pump curve supplied by the vendor (see Fig. 9-1) fixes this relationship for a particular-sized impeller. 

Installation only of a larger control valve, or larger valve trim, will save zero energy regardless of the result of the above calculation. In addition to modification of the control valve, the upstream centrifugal pump must also be modified. Assuming that the pump is driven by a fixed-speed AC motor, the size of the pump s impeller has to be reduced. The smaller impeller plus the larger control valve will result in ... 

---