Pressure, Head, and Friction Loss

Use Pressure to Measure Height

Pressure is a useful way to determine head - both Static Head (in a delivery pipeline) and Total Dynamic Head, in a pipeline being pumped. It is also a good diagnostic tool for checking the pumping system after it is installed.

There is a direct relationship between Head and Pressure*.

Pressure (kPa) = Height (Mts) x Gravity (m/s2)

The value for Gravitational Acceleration at sea level is 9.81 Metres per second per second. For convenience we use the number 10. So the formula to calculate pressure can be re-stated to calculate the height of a column of water (or the maximum height of a pipeline holding water). The formula is:-

If the pressure in a static pipeline (that is the pipe is full with water, but not flowing) is say 400 KiloPascals (kPa), the height of the water in the line is 40 Metres. If the pressure is read while the water is flowing in the pipeline then the pressure will indicate Total Dynamic Head. The difference between the flowing pressure and the static pressure, is the Friction Loss in the pipeline, at that flow rate. This number will increase if the flow rate increases.

Some useful conversions are :-

KiloPascals KPa	Pounds per Sq Inch (PSI)	Metres of Water	Feet of Water
1	0.145	0.102	0.335
6.895	1	0.703	2.31
9.810	1.420	1	3.28
2.984	0.433	0.305	1

* The real formula actually includes Density, but as we usually pump water and the density if water is 1, we ignore this part of the equation.

Other useful formula