# Explanation of relationship between pressure and flow rate

Discussion in 'Plumbing and Central Heating' started by Milleniumaire, 17 Jan 2010.

1. ### Milleniumaire

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Could someone please explain the relationship between water pressure and flow rate.
For example, assuming the cold water feed into a house is split and fed in parallel through a 22mm pipe and a 15mm pipe of the same length, what is the relationship between the pipe diameter, pressure and flow rate. Are the following statements correct:

The water pressure will be higher in the 15mm pipe as it is of a narrower diameter.
The flow rate will be the same in both pipes as that is dependant on the flow rate of the mains cold water supply and isn't affected by the pressure.

If a 10 litre bucket was placed under the open end of each pipe, then I assume they would both fill at the same rate, despite one pipe forcing the water out at a higher pressure than the other.

As the mains pressure is increased or decreased, the pressure in both pipes would increase or decrease. What would happen to the flow rate?

Similarly, if the mains flow rate is increased or decreased, the flow rate in both pipes would increase or decrease. What would happen to the pressure in the pipes?

2. ### baxpoti

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Pressure is fairly constant, as provided by the local water company.

Flow rate will be better with larger bore pipework (less restriction)

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4. ### David0123

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Get yourself a large diameter cylinder 10 metres long. Stand it on end and fill it with water. If you measure the pressure at the top just under the water level it will be nothing (nothing above atmospheric pressure) Go to the bottom and the pressure will be one bar above atmospheric.

Now stick 1 metre lengths of 15mm and 22mm dia pipe horizontally out the side of the cylinder at the bottom. If you put a bung in the end of each pipe and measure the pressure at the end of either pipe it will be the same, one bar. No water is flowing so we just measure the static pressure.

Now pull the bungs out. If you measure the water pressure at the bottom of cylinder it will be one bar still. The pressure at the open end of the pipe will be close to atmospheric. Water is being pushed down the pipes by that one bar difference between the pressure inside the cylinder and atmospheric pressure outside. The 22mm dia pipe is more than twice the cross section of the 15mm dia pipe so its bucket will fill more than twice as fast.

If you went and got a 20 metre long cylinder and did the same thing the pressure at the bottom would be two bar. The pressure difference pushing water along the pipes would be twice as much and roughly water will flow twice as fast. The bucket under the 22mm pipe will still fill twice as quickly as the bucket under the 15mm pipe

• Thanks x 1

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