How the shape of the pump impeller affects the pump curve 12-7
Pump people use the term specific speed to describe the shape of the pump’s impeller. In paper Volume 7 Number 3, I have shown you the specific speed formula and how the specific speed number relates to :
- The pump’s efficiency
- The NPSHR (net positive suction head required) to prevent cavitation.
- Suction specific speed.
- Possible motor overloading
- The pump cost
Please take a look at the following diagram. It describes how the specific speed number relates to the shape of the impeller :
In this paper we are going to see how the shape of the pump impeller affects both the slope of the pump curve and the amount of horsepower that will be consumed by the pump at various capacities.
Most of the pumps used in the process industry are of the Francis vane type with specific speed numbers between 1500 and 4000. That would be curves number two (#2) and three (#3) in the following diagram. These are the familiar curves you see on most of your pump prints.
The trick is to select the correct specific speed number so that the pump has a reasonable chance of accomplishing exactly what you want to do. In other words, the pump curve matches the system curve.
As an example :
- Some process systems require a high head, low capacity pump. A rotary positive displacement pump would be a natural for the application but often these pumps do not have enough capacity for the application. One look at the diagram above would show you that a lower specific speed impeller on a centrifugal pump might make sense in that application.
- Many boiler feed pumps need a curve with a constant head, but a varying capacity. In other words, a flat curve is necessary if the boiler pressure is going to remain a constant while the capacity or steam demand changes. A specific speed number of between 900 and 2200 looks like it would be a logical choice.
From the above diagram you can draw several conclusions :
- Low specific speed pumps are started with the discharge valve throttled to save power.
- High suction specific speed pumps are started with the discharge valve open to save power.
- Please keep in mind that any time you throttle a pump discharge, the pump is operarting off its best efficiency point and is subject to excessive radial forces that could deflect the shaft and cause a premature mechanical seal failure.
If you will refer to other papers I have written about this subject you will learn how the specific speed number of the impeller relates to pump efficiency and need for NPSHR (net positive suction head required)