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Pump terminology

Centrifugal pumps

What is a centrifugal pump?
The key element of a centrifugal pump is the impeller which rotates in a pump casing. The pump impeller is equipped with blades and is mounted on the pump shaft. The pump casing of the centrifugal pump is fitted with a suction and discharge connection.

How do centrifugal pumps work?
When the impeller in the centrifugal pump rotates quickly centrifugal forces are created. The fluid is forced to the outer contour of the impeller, causing a difference in pressure. The fluid then starts moving and a flow is created through the centrifugal pump from the suction to the discharge connection.
When to use a centrifugal pump?  
A centrifugal pump is used for pumping fluids with a low viscosity (< 500 cSt.*). You can easily test whether a certain fluid can be pumped with a centrifugal pump. Just fill a measuring cup with the fluid to be pumped and place a wooden pencil upright in the middle, if it falls to one side, you can use a centrifugal pump. If the pencil remains upright in the center, then choose a positive displacement pump.

*For reference, viscosity of water at 20 °C is about 1 cSt.

Positive displacement pumps

Positive displacement pumps are normally used for pumping highly viscous fluids. An exception to this is the piston pump that we use for wellpoint dewatering. A piston pump is a positive displacement pump with a number of unique features, including good suction capabilities, making piston pumps the best choice for wellpoint dewatering.

How do piston pumps work?
With piston pumps, a piston moves back and forth inside a cylinder. As a result, the space in front of and behind the piston alternates, becoming larger and then smaller, a valve system then takes care of the rest. During the suction of the fluid, the discharge valves close and the suction valves automatically open. A vacuum is created on the suction side, and the fluid flows into the pump casing. When the piston moves in the other direction, the suction valves close, the discharge valves open and the fluid is pumped out.

Pump curve

Each centrifugal pump model has its own pump curve. The performance curve of a pump is a curved line on a graph that shows the pump performance at a specific speed. The vertical line indicates the maximum head (pressure) that the pump delivers in relation to the pump capacity which is shown on the horizontal line. Most centrifugal pumps are tested by their manufacturers with water, as the specific gravity of the fluid also determines the performance curve of a pump. Additional information such as the pump power, pump efficiency and NPSH can be obtained from the pump curve.

Would you like to learn more about how to read pump curves? Then contact us.

System curve

A system curve of a pump is a graphical representation of the total dynamic head required at various flow rates to move the fluid from A to B. In the system curve of the pump we also show the performance curve of the pump to check whether the selected centrifugal pump is suitable for the application.

Pump flow

Pump flow (Q) concerns the amount of fluid that is pumped per unit of time. Pump flow is also often referred to as flow rate or capacity and indicated in m³/hour or l/min. The pump flow can be measured with a water meter or using our calculator.

What is pump head?

Pump head (H) is the maximum height that a pump can move a fluid against gravity. Head is the pressure that the pump must deliver to reach a certain pump flow. In the pump curve, the head is usually expressed in mwc (meter of water column). Other measuring units such as bar and Pascal are also used.

A commonly used abbreviation for the pump head is TDH, which stands for Total Dynamic Head. This is the total head of the pipe system consisting of the vertical height difference (static head) + the pipe resistance (dynamic head).

Pump head is measured with a vacuum gauge and a pressure gauge, both values are then added together.

Pump professionals still use the term geodetic head, this is the vertical distance between both liquid levels on the suction and discharge side. Manometric head is also a term still in use. This concerns the sum of the manometric suction pressure plus the manometric discharge pressure.

Pump power

Pump power (P) is shown in the pump curve or specifications of the pump. The transferred pump power, also called the shaft power, is indicated in kW.

Pump efficiency (ηis indicated in percentages and determines the variable costs of your pump. The higher the pump efficiency, the lower the transferred pump power and the lower the energy consumption of the pump!

Each pump model has its own pump efficiency curve. Always select the duty point as close to the highest efficiency point as possible. In pump terms we are talking about the BEP (Best Efficiency Point).

Best Efficiency Point Pump

BEP stands for the Best Efficiency Point of the pump. This is a point or work area in the pump curve with the highest pump efficiency. On the BEP, the pumpset works optimally with the minimum of internal turbulence and/or flow losses. Select the duty point of the pump as close to the BEP as possible to minimize energy consumption and maintenance costs.

What is NPSH in pumps?

This stands for Net Positive Suction Head. This is a concept related to the inflow conditions of a pump, and is expressed in m (meters) or mwc (meters of water column).

We can distinguish between:
  • Net Positive Suction Head Available (NPSHa) - this is the pressure available to the suction flange of the pump and is determined by the system in which the pump is installed.    
  • Net Positive Suction Head Required (NPSHr) - this is the pressure the pump needs on the suction flange to operate without cavitation, we can view this on the pump performance curves.
NPSHa should always be higher than NPSHr in order to prevent cavitation damage to your pump.

Friction loss in pipe

As soon as a fluid flows through a pipe or hose, pressure losses occur. This is known as friction loss which is caused by friction along the tube wall. There are a number of factors that influence friction loss; the pump flow, length and diameter of the pipe and the pipe material. It's easy to calculate friction loss using our pump head calculator.

Pumps in parallel

Pumps in parallel is when several pumps are connected next to each other on the same pipeline to increase the pump flow. For example, if you install two of the same centrifugal pumps in parallel, you double the pump flow but the pump head will remain the same. In such cases it’s important to be aware of the friction loss in the pipe, and to always install a non-return valve in the discharge pipe after each pump.

Pumps in series

With pumps in series, several pumps are connected one after the other to the same pipeline in order to increase the pump head. For example, if you install two of the same centrifugal pumps in series, you will double the pump head with the pump flow remaining the same. The discharge pipe of the first centrifugal pump is installed on the suction connection of the second centrifugal pump. Always check that the material is suitable for a pumps in series set up in terms of the maximum pressures and in order to prevent water hammer.

Most common measurement units used in pumping

m³/hour           pump flow in cubic meters per hour
l/min                pump flow in liters per minute
mwc                pump head in meters water column
rpm                 pump speed in revolutions per minute
η                     pump efficiency in percentage
kW                  engine or pump power in kilowatts (1 kW = 1.36 pk = 1000 Watt)
g/kWh             fuel consumption diesel driven pumps in grams per kilowatt hour
dB(A)              sound level pump in decibels (A) stands for the A scale