Calculate Total Head

Pump Head Calculation

There are TWO STEPS for the selection of submersible bore hole pumps:

Step 1 – Calculate the system head requirement

Franklin Electric Subdrive 75 Constant Pressure Pmp System

Use a Constant Pressure System to supply water to the house & garden directly from your bore. If you are worried about slight miscalculations in your total head, these systems can also run outside the recommended head for short periods.

Step 2 – Select a Pump

Step 1 – To ‘Calculate the System Head Requirement’, you will need to know 3 things:

  1. Static Head
  2. Pipe Friction Loss
  3. Discharge Pressure

The total of the above 3 things will give you your ‘Total System Head Requirement’ in meters.

You will use this figure along with the ‘Litres Per Minute’ (LPM) that you require for your system.

Take this information and look at the bore pump flow charts (also known as curves) to select a suitable bore pump.

Note* the LPM that you require must be less than your bore produces so that you don’t run out of water in the bore.  If you are not sure of the flow of the bore then it is safer to also purchase a bore pump controller with a ‘level control option’. The Level Controller will monitor the water level in the bore and shut it down before it runs dry.  This will also prolong the life of your pump.

How to Calculate the Total System Head

Bore Pump protection. Submersible motor protection devices

If you are not totally confident that you have calculated the ‘Total Head’ correctly, then it is advisable to purchase some type of motor protection device that will shut the pump down if it runs dry, cavitates, overheats or runs outside the recommended head. This will prolong the life of your pump.

 1 – First you need as much information on the bore and installation as possible. See ‘bore details’ at the bottom of the page.
2 – Basically the Total System Head is a combination of:

  1. the “Static Head” which is a measurement from the Static Water Level (or standing/draw down water level) to the highest point of the discharge outlet pipework.
  2. plus the Rising Main and Delivery Pipework “Frictional Loss”.
  3. plus the “Discharge Pressure Required” at the discharge outlet pipework.

Example:

Static Head = from standing/static/drawdown water level to the discharge pipework outlet at its highest point. Let’s say as an example it is 45 metres (450kpa)

Frictional Loss = losses in metres or kpa based on the ID Size and Type of Pipework by the required pumping flow rate in Litres/Second. Generally most pipe friction handbooks offer the friction loss per/100 metres. Let’s say the pipe is 32mm Polyethylene PN6.3 (PE63), PN8 (PE80) and P10 (PE100) has an internal pipe ID of 28.1mm, therefore at a Flow rate of 0.5 Litres/second (1.8m3/hr) the fictional loss will be 3.07 metres frictional loss per/100 metres length of pipework. If you have 1 km of discharge pipework the overall friction loss will be 3.07 x 10 = 30.7 metres loss.

internal pipe size diameter chart

Chart for measuring the internal diameter of poly pipe

Then you need to calculate the Bore Riser Pipe friction losses, from the discharge of the pump to the bore head works, which in this example we will say 5 metres.

Use a Pipe Friction Loss Calculator

Discharge Pressure Required = If you are operating reticulation sprinklers you generally require around 200 to 250kpa pressure or 20 to 25 metres. (You will need to confirm required system operating pressure).
PSI ÷ 1.42 = Meter Head eg: 35PSI ÷ 1.42 = 24.64m
Kpa ÷ 10 = Meter Head eg: 200kpa ÷ 10 = 20m
Therefore the Total Head will be:
  1. Static Head =  45 metres
  2. Friction Loss = 30.7 metres + 5 metres
  3. Discharge Pressure = 20.0 to 25.0 metres = 100.7 metres to 105.7 metres.

Total System Head Requirements = 100.7 to 106.4

calculate total head of bore pump

In this example the System Operating Pressure required is 200kpa.
Duty will therefore be 1.8m3/hr @ 100.7 metres. (example only… In reality you would not utilise pipework that introduces such high frictional losses.)

Note * There are other factors that need to be considered when calculating Total Head, such as Water Temperature, SG of Water, number of 45° & 90° Elbows installed, Isolation Valves, Check Valves, System Pipework such as long reticulation pipe runs. These losses also need to be factored into the calculation.
Remember, if you cannot answer the questions then it is impossible for you to calculate the system head requirement and therefore select a pump.

I trust the information provided meets with your immediate needs. Should you require further detail please do not hesitate to contact me.

 

IMPORTANT: If you are planning to use the pump to:
(1) Fill a tank (open discharge) AND
(2) Supply water for an irrigation system or taps, please let me know as there may be a huge difference between these 2 situations in Total head. It can be difficult choosing a pump that will run efficiently in both situations on the efficiency curve. I am not saying that it can’t be done… the pump selection just needs to be spot on!

calculate what size bore pump you need

BORE DETAILS
(a) Bore Depth (M)
(b) Pump Set Depth (M)
(c) Standing Water Level (M)
(d) Draw Down Level (M) (not many people have this info as it requires an expensive test, so we use Pumping Water Level)
(d) Pumping water level (M)
(e) Depth to Slots in bore casing (M)
(f) Bore casing size (diameter) (mm or inch)
(g) Elevation or height from the top of the bore to the highest point of discharge (tank, taps, house etc) (M)
(h) Flow of bore in LPM

PIPING
(i) Length of Pipe from bore pump to ground Level (M)
(j) Diameter of Pipe from bore pump to the top of the bore (mm)
(k) Distance from Bore to furthest tap (M)
Or…
(k) Distance from bore to tank (if only filling tank) (M)
(l) Diameter of pipe from bore to furthest tap or tank
How many elbows in pipework
How many check / non-return valves in pipework

DISCHARGE PRESSURE
(m) Max Service Pressure Required (Psi) (if you are running sprinklers, what psi do they require)?
(h) Flow of Water the bore produces (LPM)

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Need help to calculate the total head?
I can help!
I have been doing total dynamic head calculations and pump selections for over ten years.  You will need to fill in the quote form correctly. If there are any details that you are unsure of or have input incorrect details just to fill in the boxes please let me know in the message box at the bottom of the form.

Then submit the form.

Please make sure you have a current email address or contact phone number as I may have to contact you if I am unsure of any details submitted on the form.

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