# Pump head pressure vs flow relationship

### What is the difference between pressure and head?

On every single water pump, flow rate and pump head are the two most important specifications. Flow rate is how Flow rate will follow if you've got good head pressure. Going back to the pump flow curve Duckie posted. Pressure, in conjunction with flow rate and power, is used to describe pump The pump curve illustrates the available total head at a given flow rate of the pump. The relationship between head and pressure can be characterized by the we try to identify a pump that has a total discharge head at the flow.

To begin, what is head? Assume that you have a pump designed to move water clamped into a process line.

You have a suction line and a discharge line, both running horizontally. The pump is then turned on. Once the pump is running, it will move the fluid to some height measured in feet. That height to which the pump can raise the water to is its head.

### What is Pump Head? How is it Different than Pressure? | Holland Applied Technologies

Is head affected by the suction conditions? The answer to that question is yes. If you lower the suction level, the head measured will be less. The opposite is true if you raise it.

Well, a pump is not very smart. It has no electronics other than a motor coupled to it. It is mindlessly providing energy. The motor converts electric energy into mechanical energy and that mechanical energy is used by the pump to impart that energy pressure into the fluid.

If we raise or lower the suction level of the fluid, we are adjusting the potential energy of the fluid. The potential energy of ten foot column of water is greater than the potential energy of a five foot column of water.

## Info: Pump Head vs Flow Rate

Centrifugal pumps are like that, they can pump water up to a height and create pressure at the discharge without flow going through them; they are just sitting there churning up the same water. The total head at zero flow is the maximum head also called the shut-off head, the total head decreases as the flow increases.

The plot of total head vs. Figure 6 Typical curve of total head vs. To buy the correct pump for your application you first have to know what total head you need and at what flow rate. Follow these links to get a good idea on how to establish this, it's not difficult, and for home owners all you need is a couple of key pointers. The static head requirement is often the main component of total head, how high do you need to get the water based on the level of the suction tank?

The next important consideration is how much friction do you need to allow for, this depends on the length of pipes and their diameter. The sum of the static head and friction head will give you the total head.

## What is Pump Head? How is it Different than Pressure?

The total head and your flow requirement will allow you to buy the right pump. You are looking for a pump to operate in the area shown in Figure 6. When you look at the curve in Figure 6 it seems counter-intuitive that the maximum flow should occur at the minimum head. After all if you have a high head, which means a high pressure, shouldn't that push more water through the pipes.

Think of it this way, the pump is always turning at the same speed regardless of whether the pipe is fully open or whether the valve at the end of the pipe is closed. When you close the valve the energy that the pump imparts to the water now goes into increasing the pressure since there is no place for the water to go.

As pressure increases, total head increase and reaches a maximum at zero flow.

• Pressure vs head, what is the difference?

This is also why it is not a good idea to let a pump run with a discharge valve closed. The energy that goes into the water to produce pressure also produces heat and since there is no flow the heat cannot be dissipated, the end result can be a very hot pump indeed. All residential pump systems have a pressure switch, the switch cuts off the power to the pump when the pressure gets to a certain level.

We need to make a distinction between a system with no flow and a system with flow. The difference is flow produces friction. Figure 8 Total head with flow. 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 then 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? What are the 2 major components of total head?

If you increase the total static head what will happen to the flow? If you decrease the discharge static head, what will happen to the flow? Check for the continuation on November Download this file to see the answers. Why use the term head as opposed to pressure?

There are some pump manufacturer's that use pressure i. 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.