Pumps are used to transfer fluid from one area to another. These are used all over the world for different services. Your home’s electric pump and hand pump are the most common examples of the pump. These pumps transfer water from wells into our building, circulate water in a central heating system, and execute other tasks to complete the pumping process. The pump head is a most famous physical feature of the pump. This concept is very easy to define, but it may be confusing when converted to an example that includes a real pump. This article explains a deep study of the pump head.
What is a Pump Head?
A pump head is a height up to which a pump can lift the water directly over it. In simple words, the head is the maximum height at which a pump can lift fluid. As the pump starts to pump water, the water produces resistance or pressure at a specific rate. So, you can measure the head as the differential pressure that a pump overcomes to lift the water.
Pounds per square inch and feet are the most commonly used units to represent a head (Pump curve calculators may suggest various units like a meter of head or bars).
In simple words, a pump head is an extreme height that a pump can gain by pumping fluid opposite to the gravity. The head depends on the capacity of the pump. If the pump has the capability to generate more pressure, it can transfer fluid up to a high height and create a higher head.
A simple example is when you have a vertical pipe extending up from the drain. A pump with a head of 4m will suck liquid from the outlet up to 4m of the pipe.
If the tank has a high amount of fluid, the greater the pump’s ability to pump water down the vertical drain pipe due to the pressure applied by the fluid in the suction tank.
Types of Pump Heads
The pump head has the following major types:
1) Discharge head of pump
The discharge head is a perpendicular distance that a liquid can be pumped. Let me clarify by an example, if your pump has a highest head of 18 ft, that doesn’t mean that you can only use 18ft of pipe.
2) Suction Head
Net Positive Suction Head (NPSH) is the amount of pressure that the liquid absorbs on the suction end of a pump. NPSH is calculated as the total liquid head at the impeller centerline minus the liquid vapor pressure. The main objective of the suction head is to classify and stop the working conditions that cause liquid to evaporate as it comes into the pump.
3) Static Head
A static head is also known as a vertical lift. It is a height that water travels as it flows by the drain pipe. To find this head, you need to find the vertical distance from where the water comes into the drain pump until the drainpipe changes from vertical to flat.
4) Total Head
The difference between the total suction head and total discharge head is known as the total head. The word “total” is intended to include vertical head (Z), velocity head (HV), and hydrostatic head (h).
The following formula is used to calculate the total head:
Total Head = Total discharge head – Total suction head
As you increase the amount of fluid in the suction tank, the discharge head also increases. A reduction in the suction tank fluid level lowers the discharge head. Therefore, the pumps suppliers and constructors also don’t let you know how much head a pump can generate because they don’t know how much fluid will be drawn into your suction tank. So, the total head doesn’t depend on the suction tank fluid level.
Pump head vs. Flow rate
The pump head and water flow are inversely proportional to each other. As the pump head increases, the water flow rate reduces.
The water pump system has zero flow at the maximum discharge head. This is because the pump uses all the force to rise the water already available in the system, and the pump can’t produce force to flow the new water.
As the pump head becomes equal to 0, the water moves at maximum speed. This is because the pump uses all the power to move the water flow instead of lifting it. Therefore, the pump head is zero, and water flows at a maximum rate.
When the flow rate increases, the pump head reduces and vice versa. This ratio produces a special graph for the working range of each pump. You can use this graph to select the right pump according to your application.
When the fluid enters the pumping system, friction also acts on it. The friction force between the sides of the flow pipes and the water further decreases the speed of the flow.
In the case of a flowing water pump system, the total head is as below:
Total head = Friction + (discharge head – suction head)
We won’t go into detail about pipe friction, but it’s vital to know that if you are pumping for a long distance, then the overall pump head will affect. Sharp bends in the pipe and roughness of the pipe surface also significantly affect the pump head.
Pressure vs. Head
Pressure is the force exerted on a unit area; for example, one pound-force applied to one square inch (psi). The pressure varies according to the density of the liquid.
While the head is a measure of energy. The head measures in terms of meter or feet.
The head is a height to which a pump delivers water. The pump head doesn’t depend on the liquid. This means various liquids with various densitiesrise to the same height.
In contrast, the pressure depends on the liquid—the liquid density affects it. The force per unit area of a liquid column of fixed height depends on the specific gravity. Therefore, in this case, the same head produces various pressures.
You can’t measure the head directly. The pressure is measured by using a manometer on the suction and pressure lines of the pump. Gauge readings represent the pressure difference exerted by the pump between the discharge and suction sides.