Hydraulic Gradient and Water Velocity


Hydraulic Gradient and Water Velocity determine pressure loss in a water supply system.

Hydraulic Gradient and Water Velocity.

The loss of pressure in a water system can be calculated. Both the Hydraulic Gradient and Water velocity have an effect on the pressure in a water system.

Hydraulic Gradient (or pressure gradient)

Plumbers must consider the flow of water from the open end of a pipe and this is dependent upon:-

  • The head of water acting on the outlet,
  • the amount of head or pressure absorbed by the friction created when the water passes through the pipes & fittings. This reduction is known as the hydraulic gradient.

When a plumber designs a water system all draw off points must be below the hydraulic gradient or pressure gradient.

Hydraulic gradient measures the available pressure drop in K pa per metre of pipe run. An allowance of between 10-20{b8e2f19214e0244469773d78fe272f89c5960306b526116d03458cf23172a218}  for fittings. Hydraulic Gradient is important when determining pipe sizes for water main reticulation in a plumbing system.

Hydraulic Gradient diagram showing how hydraulic gradient is determined.

Water Velocity

Noisy Pipes

Noise created by water flowing through small bore pipes can be a major problem to plumbers and home owners.

Velocity is the speed at which water moves through a pipe or system and this affects the noise generated in the pipes. So plumbers consider water velocity  when installing or modifying a plumbing installation.

A flow rate of 1.8 litres per second through a 20 mm pipe is acceptable. A flow rate of 0.6 litres per second through a 15 mm pipe, though  is unacceptable. This is because of the increase in in noise.

Considerations by plumbers.

The recommended maximum velocities for water services is between 1.0 and 2.2 metres per second

Australian Plumbing Standard AS 3500 refers to maximum velocities in certain situations and plumbers adhere to this standard.

Plumbers select pipe sizes to achieve a  velocity of the flow of water of 2.4 metres per second externally and 1.6 metres per second internally.

Plumbers check that all the pipes are large enough in diameter because they need to ensure the rate of flow is within the capacity of the pipe.

Velocity = √2 x 9.8 x head

 Location of water mains

Plumbers must locate the mains because they to make a connection.

Often conduits are laid across the road facilitate installation of property services by plumbers.

Most Local Councils do not permit excavation of the road surface unless all else fails because of the damage and cost of recitifcations.
In older areas, the water main is quite likely to be located within the roadway, and this does require excavation to permit connection.

Water main depth varies between 450 – 600 mm but has been found up to 3 metres.

Plumbers locate water mains by looking  for markers attached to power poles and fences because they indicate the location of the mains.

For more information on flow velocities..click here…


This article is intended as general information and not as plumbing advice or diagnosis. Contact a licensed plumber or drainer for specific advice for your particular circumstances because this article is only general information.

or call Wilmot Plumbers Newcastle

0432 428 532

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