Pumping Head Explained

pumpHead1Most people have an idea of what “Head” means when it comes to pumping water but often they get the detail wrong. So here is a simple explanation of pumping head:
Pumping head (also called Dynamic Head) is the pressure a pump must generate to cause water to flow from the outlet at the desired rate of flow. Although this is a pressure it is commonly expressed as a distance e.g. 100m head. In a pipe filled with water each 10m in height is equivalent to 14.72 psi pressure at the bottom

pumpHead2Simple! But how do we calculate it?
There are two parts to this.
Static head. This is the pressure at the pump if the pipe was full of water but not flowing. It is the vertical distance from the surface of the water being pumped to the highest point of the pipeline (usually the outlet). See diagrams.
pumpHead3

Friction Head. This is the resistance to flow caused by the pipe itself. The smaller the pipe, the longer the pipe or the faster the flow through the pipe more friction occurs. The pump needs to pump harder to overcome this friction and so the pressure goes up.
The Friction head can be determined using tables. The steps here are:

  1. Find the correct table for the type of pipe you are using.
  2. Calculate the speed of the flow needed at the outlet, in units shown in the table, usually litres/min or in litres per second.
  3. Find the row corresponding to the flow and go across till you reach the size of pipe you are using.
  4. Read the friction loss figure. Note that this is normally given as a loss per 1000 metres or feet of pipe.
  5. As you will not be pumping through exactly 1000 units of pipe you must adjust the figure.

Pipeline friction loss = loss from table x length of pipe /1000
The final result is reached by adding the two figures:

Pumping Head = Static Head + Pipeline Friction Loss

Friction Loss for Class PN6.3 Metric Polyethylene pipe

Litres Friction
Per 1000m pipe
loss number
Day Hour Second 25mm 32mm 40mm 50mm 63mm
1800 360 0.1 5.7 1.6 0.5
3600 720 0.2 20.4 5.7 1.9 0.6
5400 1080 0.3 43.3 12.2 4.1 1.4
7200 1440 0.4 73.8 20.7 6.9 2.3 0.7
9000 1800 0.5 111.6 31.3 10.5 3.5 1
10800 2160 0.6 156.4 43.9 14.7 4.9 1.4
12600 2520 0.7 208.1 58.4 19.5 6.6 1.8
14400 2880 0.8 266.4 74.8 25 8.4 2.2
16200 3240 0.9 331.4 93 31.1 10.5 2.7
18000 3600 1 402.8 113.1 37.7 12.7 3.3
21600 4320 1.2 158.5 52.9 17.8 4.5
25200 5040 1.4 210.9 70.4 23.7 5.9

Example: We need 9000 litres per day to be delivered to a tank 500m away and is 22 m higher than the water in the bore. We will be using a 40mm metric polypipe to carry the water.

Static head = difference in height between water in source and the outlet = 22m

Friction Head
Go down the table till you get to the flow rate needed.
9000 l / day = 0.5 l/sec (based on 5 hours pumping)

Now go across that row until you find the pipe size.
From table Friction loss number for 40mm pipe at 0.5 l/sec
= 10.5 m per 1000m pipe.
But we are only pumping through 500m pipe so the loss for our pipe is
Pipeline friction loss = loss from table x length of pipe /1000
= 10.5 x 500 / 1000
= 5.25m
Now calculating our pumping head:
Pumping Head = Static Head + Pipeline Friction Loss
= 22 + 5.25
= 27.25 m

So we will need a pump which can pump 9000 litres per day at a head of 27.25m