What is Head
WHAT IS HEAD?
Then it doesn’t matter what the suction tank level is, they are telling you only what the pump can do regardless of the water supply pressure at the suction.
The next important consideration is how much friction do you need to allow for, this depends on the length of pipes and their diameter.
Assume we have a system such as in Figure 7 where the discharge pipe is high enough that no flow can come out. In other words the pump cannot develop enough pressure to push water out of the pipe.
Now suppose you cut a piece off the pipe end, this will lower the discharge head or the height at which the liquid is pumped as in Figure 8.
This lowers the overall total head and flow starts to come out of the pipe.
Since we have flow we now have friction and the influence of friction is known as friction head.
This is exactly as predicted by the curve in Figure 6.
In a system with flow, the total head is the difference between the discharge and the suction head plus the friction head and the sum is less than the shut-off head.
Suction and discharge static head are often combined. The difference between discharge and suction static head is the total static head (see Figure 8).
What happens to the pressure at the discharge of the pump when the flow increases, or when the discharge valve goes from a fully closed position to fully open?
ANSWER: IT DECREASES
What are the 2 major components of total head?
ANSWER: A STATIC HEAD & FRICTION HEAD
If you increase the total static head what will happen to the flow?
ANSWER: THE FLOW WILL DECREASE
If you decrease the discharge static head, what will happen to the flow?
ANSWER: IT WILL INCREASE
Why use the term head as opposed to pressure?
There are some pump manufacturer’s that use pressure (i.e. differential pressure or difference of pressure at the discharge vs. the suction) to characterise their pumps but not many.
Head is a very useful and practical term to use when evaluating a pump’s capacity to do a job.
Many pump applications involve pumping to a higher level. If you have to pump liquid up 30 ft. and your pump doesn’t have at least 30 feet of head then there is no chance that your system will work.
Your pump will have to have at least 30 ft. of total head plus the friction loss for you to get the require flow at the discharge point.
Also, head is independent of the type of liquid pumped as long as the viscosity is low and similar to water. If you are pumping sea water or some heavy acid, the head achieved by the pump in Figure 2 will be the same as that for water.
The pressure at the discharge of the pump however will be higher. The explanation for this involves a little bit of math, nothing heavy, and the realisation that head is a form of energy (potential energy) just like a cyclist at the top of a hill has potential energy. Also pressure is another form of energy like soda pop under pressure in a can. The explanation can be found here.