When requesting a pump, the supplier will need basic information to provide the correct pump selection.
This is crucial in terms of selecting the correct pump for HEAD (The maximum height in static lift) and FLOW (Normally in liters per minute). A centrifugal pump performance curve is a useful tool which enables anyone to understand how a pump will perform. The supplier needs to be able to peg your pump duty to a specific pump curve. This will determine the most efficient pump selection for the duty.
A pump develops a specific PRESSURE (PSI or BAR measurement translated into feet or meters head)at a specific FLOW (normally represented in gallons per minute or liters per minute). This value is dependent on the design of the hydraulic components of the pump. Pump selection is based on a specific flow in liters per minute to head in meters.
A note on establishing flow.
If you do not know the flow you require it is relatively simple to determine.
Take it one step at a time and add all the outlets (Taps or sprinklers together). For industrial applications, for washing, heating or cooling, the equipment will have a flow and pressure data detailed on the design plate.
Flow in a system must be established by understanding the requirement of the entire system.
In our example we will use a house which has two bathrooms, a dishwasher, a washing machine, an electric geyser in the roof and a guest toilet with hand basin. Also a sink in the kitchen and an outside shower for the pool area.
So we have the following outlets :-
Bathroom hand basin taps 4.
Bath taps 4.
Shower taps 6.
Toilet cistern 3.
Guest basin tap 2.
Sink in kitchen 2.
Washing machine 1.
Dishwasher 1.
TOTAL 23.
If we assume each tap will be required to deliver 2 gallons per minute (GPM) at 44 PSI or 3 bar.
We simply need to determine how many outlets we would expect to operate at any one time.
It is unlikely that all toilets, all showers, all baths etc will be on at the same time.
A good method is to take a third of the total outlets as your flow required for any single moment in time.
In our example that represents 23 divided by 3 or 7.6 make it 8 outlets. This means a flow requirement of 8 X 2 = 16 GPM or 60 liters per minute.
A note on the concept of PUMP HEAD.
Think about the highest point of your body, the top of your HEAD. It is the same idea when speaking pump selection. Head refers to the measurement in feet or meters from the center line of the pump (the pump shaft center line), to the highest point to which the unit is expected to deliver fluid.
The above definition of head is limited to what is known as the Static head. In other words, this is the measurement of the vertical height which should never change, it is static. If the pump needs to deliver water from a tank at the bottom of the garden to the geyser in the roof. The garden has a steep bank from the house to the lower area where the tank is located. Measure the height from the base of the tank to the exact spot where the geyser rests in the roof, that is static head.
This measurement is static head, it does not change.
However when it comes to fluid, determining the total head the pump will need to develop in order to deliver the required flow to that vertical height, there are some extra variables which will effect the head.
These variables are where the calculation of total dynamic head becomes a little more involved. We are not going to get into a huge technical discussion with formula and major math here. All we need to recognize is that there are dynamic forces at work. These forces affect the performance of the pump which one needs to use when developing a system curve. Doing this right will enable you to determine the right pump for your needs
The total dynamic head is a combination of static head, friction losses in the pipe system, called friction head, and pressure losses caused by the equipment through which one is delivering the fluid. These losses change in measurement depending on the volume of fluid which is being pumped at any one time. As such, the losses are dynamic, they change in relation to flow rate.
In our example, the tank at the bottom of the yard and the geyser in the roof, this the static head.
Friction Head
Friction head results from the desired volume of water flowing through a specific pipe size over the total distance from the pump to the geyser. Added to that we will also need to add the required working pressure for the geyser.
So lets see how we can make the concept a lot more simple, lets look at a picture.
