Frictional Loss Calculation using Hazen-Williams formula
A Frictional Loss calculation is used to find out how much pressure is lost through a run of discharge pipework. In applications such as domestic water boosting and moving drainage water (Submersible application) it is essential for the pump to provide the correct pressure to ensure the water gets to where it needs to. Here is a brief run down of what is required and how to work out this calculation using the Hazen-Williams formula
The calculation takes into account pipework lengths and diameter to work out how much pressure is required to move the water from the pump to where the water is required. Therefore the following details are required
Pipework Diameter – The wider the pipework diameter the less losses occur in the pipework, the narrower the pipework the more losses. If this is a new application and you haven't got existing pipe work in place the rule of thumb is to match the pipework diameter to the discharge port size of the pump (E.g. 50mm discharge connection on pump = 50mm pipework diameter)
Flow rate - The flow (or quantity as it sometimes referred to as) is the amount of water the pump can push through. Flow is normally measured in Litres (per sec, min or hour) and Meters cubed an hour (m3h, per hour). If you are unsure about the flow rate, check the pump curve of legend plate on the pump
Length of Pipework – The total amount of pipe work from the pump to where the outlet / end of the run is. (Including both horizontal and vertical pipework)
Elevation – The elevation (or Lift) is the total hight of the pipework from pump to the out let. For example, in a submersible pump application the list would be the hight of the vertical pipework from the pump to the rising main. If the application on an incline, you must include the height from the pump to the outlet even of the pump pipe work is horizontal.
Bends or Elbows – The amount of bends and elbows that are in the pipework also have a impact on the frictional loss calculations, please see below for more details
Material of Pipework – The material of the pipe work also effects the frictional. New plastic PVC is more effect at reducing frictional losses rather than decayed metal pipework.
With these pieces of information a pump manufacture or distributor will be able to work out the correct amount of pressure lost in the pipework. If you are feeling capable of doing the calculation your self, see below for the formula. Please note: use the below calculation as a rough guide only.
First you will need to find the correct factor to use in the calculation, this will be represented as Hr. The below table shows the Hr value for the chosen pipework diameter and the application flow rate. (For example if you were to use 25mm pipework and a application flow rate of 20 litres per min, the Hr value would be 4.79)
Total Length of pipework (In metres) ÷ 100 (to make the amount per 1m instead of 100m) x Hr Factor x Material multiplier (See below chart) =Pipework Run Losses (Total amount of losses due to pipework length, diameter and material)
Pipework Run Losses + Vertical lift (in metres) + Bends losses (In metres) = Total Friction Losses (Pressure loss in pipework)
The figure you are left with (Total Frictional Losses) is the total amount of pressure required to push the water from the discharge of the pump to the outlet