Pump Impellers

In a single suction impeller, the liquid enters the suction eye on one side only. A double suction impeller is, in effect, two single suction impellers arranged back to back in a single casing. The liquid enters the impeller simultaneously from both sides, while the two casing suction passageways are connected to a common suction passage and a single suction nozzle.

For the general service single stage, axially split casing design, a double suction impeller is favored because it is theoretically in an axial hydraulic balance and because the greater suction head. For small units, the single suction impeller is more practical for manufacturing reasons, as the waterways are not divided into two very narrow passages. It is also sometimes preferred for structural reasons. End suction pumps therefore use single suction impellers. Because an overhung impeller does not require the extension of a shaft into the impeller suction eye, single suction impellers are preferred for pumps handling suspended matter, such as sewage. In multistage pumps, single suction impellers are almost universally used because of the design and first cost complexity that double suction staging introduces.

Impeller are called radial vane or radial flow when the liquid pumped is made to discharge radially to the periphery. Impellers of thus type usually have a specific speed N, below 4200 rpm if single suction and below 6000 rpm if double suction.

Impellers can also be classified by the shape end form of their vanes:

• The straight-vane impeller
• The Francis-vane or screw-vane impeller
• The Mixed flow impeller
• The Propeller or axial flow impeller
  

The description of each type those impeller will discussed more detail.

Variable Displacement Flow Control

Rather than by speed control (changes in the number of displacements in a given period of time), flow may be varied by changing the volume displaced per stroke. Variable stroke are generally limited to small metering or controlled volume pumps. Although large capacity variable stroke pumps have been built, the complexity of the mechanism involved and the development of other effective and economical means of flow control have precluded their use.

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.