Pumping Terms
Head
A measure of pressure, expressed in metres of head for centrifugal pumps. Water is used as the default where 10 meters (33.9 ft.) of water equals one atmosphere (14.7 psi. or 1 bar).
Flow
A measure of the liquid volume capacity of a pump. Given in litres per minute (l/min).
Pump performance curve
A diagram provided by a pump manufacturer to explain the relationship between the head and the flow rate of a pump.
Pipe friction loss
The positive head loss from the friction resistance between the pipe walls and the moving liquid. The faster a liquid is moving, or the thicker it is, the more it drags and creating losses.
Friction head/losses
The pressure expressed in metres of liquid needed to overcome the resistance to the flow in the pipe and fittings.
Total head
The amount of head including friction losses. This is calculated by adding together the total static head and friction head.
Total static head
The amount of head excluding friction losses. This is calculated by adding together discharge head and suction lift.
Pressure
The force exerted on the walls of a pipe by a liquid. Normally measured in imperial as pounds per square inch (psi) or metric as atmospheric air pressure (BAR).
Pressure drop
This refers to the loss of pressure between two points in a pipeline system. Generally occurs because of pipe friction loss.
Efficiency
A ratio of total power output to the total power input, expressed as a percent.
Best efficiency point (BEP)
This is the point where the power coming out of the pump is the closest to the power coming into the pump i.e. it is the BEP is the point at which the head (pressure) and flow converge to produce the greatest amount of output for the least amount of energy. At this point a pump is at its most mechanically reliable hence when selecting a pump for a duty the aim is to select one that works close to this point. Normally this would be within the middle third of the pump curve. Operating a pump outside this middle third can reduce the life span significantly and failure is not covered by any manufacturer’s warranty.
Net positive suction head available (NPSHa)
The point available to prevent cavitation of the pump. To calculate the NPSHa, you take the [Static Suction Head] plus [Suction Vessel Surface Pressure Head] minus [vapor pressure of your product] minus [friction losses in the suction piping, valves and fittings].
Net positive suction head required (NPSHr)
The point to stop a pump from cavitating. The NPSHr is generally supplied by the pump manufacturer.
Cavitation
The process in which cavities or bubbles form in the fluid low-pressure area and collapse in a higher pressure area of the pump - causing noise, damage to the pump, and loss of efficiency because it distorts the flow pattern. This occurs in centrifugal pumps when NPSHa is less than NPSHr.
Specific gravity
The ratio of the weight of a given volume of liquid to pure water. Pumping heavy liquids (specific gravity greater than 1.0) will require more horsepower or even be uneconomic to pump i.e. “if it does flow well it will not pump well”
Viscosity
A measure of a liquid's resistance to flow. Essentially it’s a how thick the liquid is. The viscosity determines the type of pump used and the speed it can run at.
Horsepower (HP)
The unit measurement on electric motor. 746 watts = 1HP.
Flooded Suction
In a flooded suction system, the liquid flows to the pump inlet from an elevated source by means of gravity. This is generally recommended for centrifugal pumps.
Discharge head
The vertical measurement from above the centre-line of the pump to the discharge point.
Suction lift
The vertical measurement from below the centre-line of the pump to the liquid level. Lift is achieved when atmospheric pressure acts on the liquid surface. This pushes liquid into the suction hose and the pump casing after it has created a vacuum.
Volt drop
The amount of voltage lost when using long power cables. This phenomenon can cause the cable to get hot (and be unsafe) and the motor to draw extra current. Overloads can trip and motors can be seriously damaged.
