A worked calculation of how much water a length of pipe holds, useful for filling, flushing and dosing.
Knowing how much water a pipe run holds is handy for filling, flushing and working out a disinfection dose. The method is just area times length, with care over units.
Step 1 - the formula. Volume = cross-section area x length, and for a round pipe the bore area = 0.7854 x diameter x diameter, where 0.7854 is pi divided by 4.
Step 2 - inputs. Take an internal bore of 20 millimetres and a run of 25 metres for this example. Work in metres, so the bore is 0.020 m.
Step 3 - area. Area = 0.7854 x 0.020 x 0.020 = 0.7854 x 0.0004 = 0.00031416 square metres.
Step 4 - volume. Volume = area x length = 0.00031416 x 25 = 0.0078540 cubic metres.
Step 5 - convert to litres. There are 1000 litres in a cubic metre, so 0.0078540 x 1000 = about 7.85 litres in the run.
Step 6 - use it. The result is the water inventory of this pipe run. It is not, by itself, a complete flushing volume or a disinfectant dose: follow the specified flow, duration, concentration and contact time. Use the true internal bore, and include fittings and any cylinder or cistern separately when calculating the volume to be treated.