Centrifugal Pump Basics
Selecting pumps: Using manufacturer’s data to choose the best pumps for your applications
Pump manufacturers provide a lot of technical data on their products, much of it in the form of graphs and tables. This article is intended to help buyers of centrifugal pumps interpret this data and to choose pumps that will meet the requirements of their applications in terms of performance, efficiency and reliability.
First, some useful definitions
Total developed head is a measure of the total pressure increase that a pump imparts to the fluid flowing through it. Head is measured in metres or feet. Head can be related to pressure by the formula:
|Where:||Po is the output pressure,|
|Pi is the intake pressure,|
|ρ is the density of the fluid, and|
|g is the acceleration due to gravity|
For centrifugal pumps, total developed head is largely determined by impeller design, impeller diameter, impeller speed and the number of impeller stages.
Flow Rate (Q)
Flow rate is the volume of fluid passing through a pump in an interval of time. Flow rate is measured in a variety of units, including cubic metres per hour, litres per second and US gallons per minute. Flow rate is largely determined by the rotational speed of the impeller and the size of the pump.
A pump’s characteristic curve shows the relationship between the head (H) and the flow rate (Q) when the pump is operating at a specific speed (RPM). Total developed head is highest at low flow rates and gradually decreases as flow rates become larger, as shown in the red line below.
The efficiency of a pump – usually represented by the Greek symbol “η” – is the ratio between the amount of energy imparted to the pumped fluid (usually a combination of potential and kinetic energy) and the mechanical energy provided by the pump’s motor. The level of efficiency that a pump can achieve depends on the flow rate, as shown in the following graph.
The flow rate Qopt is the point where the maximum pumping efficiency is achieved. This is also known as the “Best Efficiency Point” or BEP.
It is generally a good idea to operate pumps as close to their BEP or “sweet spot” as possible. Not only does this give the owner the best return on his energy dollar, but most pumps run most smoothly and have the longest service lives when operated near their BEP.
Optimal operating range
While characteristic curves may provide data for Q down to zero, it is not a good idea to operate centrifugal pumps a very low flow rates. There is always some heating of the fluid passing through a pump due to friction and if there isn’t enough flow to carry this heat away, the pump can overheat. In addition, running the pumps at very low flow rates can cause high bearing loads. For these reasons, manufacturers generally specify a minimum operating flow rate of 10% to 30% of the Qopt (Q at the BEP).
At the top end, manufacturers also recommend maximum flow rates, typically at around 110% Qopt.
When a centrifugal pump is running, there will be a local drop in fluid pressure in the immediate neighbourhood of the impeller intake. If the pressure in the fluid falls too low, the fluid may pass into a vapour phase (i.e. boil). When this happens, the vapour bubbles may obstruct or even completely block the fluid flow. The subsequent collapse of the vapour bubbles can also cause significant mechanical damage to the impeller and pump casing. To avoid this dangerous situation, fluid at the pump intake must be under an adequate level of positive external pressure to offset the pressure drop that occurs inside the pump. This required level of pressure is referred to as required NPSH (Net Positive Suction Head)
Each pump design will have a “Required NPSH” which is normally determined by testing, according to ISO 9906. A typical NPSH curve, shown below, increases at higher flow rates.
Available NPSH – which is a measure of the available fluid pressure at the pump intake – is a characteristic of the piping layout. Piping designers must ensure that available NPSH exceeds the required NPSH for the pump and operating point.
A real-life example
The following diagram is taken from the spec sheets for a KSB KRT submersible pump, a general-purpose pump widely used in water and wastewater facilities. This diagram is for a mid-size model with a 100mm outlet diameter.
In order to increase its versatility, the diameter of the impeller can be changed. The diagram show six characteristic curves, one for each size of impeller. All curves are for a pump operating at 1750 RPM.
In addition to the characteristic curves, this diagram also shows the minimum flow rate and efficiency figures for various operating points.
Selecting the right pump
Once the full operating envelope (H, Q and required NPSH) for a pump has been determined, it’s time to select the best piece of equipment for the job. This isn’t always a simple task, given the enormous variety of pumps available on the market. And, once an appropriate type of pump has been selected, it’s equally important to specify the optimal size and configuration. This might involve the selection of impeller types, seals, special corrosion or wear-resistant materials and so on.
KSB’s EasySelect® selection and configuration software can be a valuable tool for narrowing the search. Naturally the help of a skilled application engineer can be useful for confirming and refining the choice.