Hammering out problems

Wilo Pumps, water hammer
Pressure boosting is essential in tall buildings to boost the pressure of the mains water supply but can cause enormous problems if water flows get out of control.

The unmistakeable sound of water hammer is sure to make an experienced engineer shudder. Gary Wheatley discusses its causes and how to prevent it.

As water companies reduce the mains pressure of water to reduce leakage and more and more tall buildings are built, there is more and more reliance on pumps to deliver water at sufficient pressure to enable normal daily living and business operations to continue. With water provided under pressure in this way, a new potential threat has become apparent, hydraulic shock, which in a worst-case scenario can lead to serious failure of fittings within a building.

Some accusations have been made that the suppliers of booster pumps are at fault when such problems occur. Wilo UK has successfully worked with a wide range of contractors, local authorities and housing associations across the UK — installing its highly reliable, energy efficient, water booster sets to ensure a quality supply of potable water to higher levels — and has experience of how such problems can be avoided and prevented.

Noise in piping systems is never good! It is normally a symptom of a developing system failure and needs urgent attention. A hammering sound — known as water hammer — is the result of damaging vibrations caused by hydraulic shock. Whilst water hammer is annoying, hydraulic shock is a damaging force that needs to be addressed and eliminated. Hydraulic shock is the sudden elevation of inline pressure caused by a shock wave created by the sudden change in velocity of non-compressible liquid such as water. Most commonly, hydraulic shock occurs when a column of moving water is suddenly stopped, and the kinetic energy of the motion is changed to pressure energy that needs to be absorbed.

Take for example a building water supply that comes via a pump located at ground level, which for one reason or another is interrupted — perhaps because of a loss of power or perhaps the pump has ‘tripped’. The building’s rising main no longer has a constant supply and in most applications water will start to ‘drain down’ as water is drawn off. As the system drains a partial vacuum is created in the pipework. When the pump is restarted, it sees no back pressure as it forces the water to rise through the pipework at an uncontrollable rate, increasing the velocity rate of the moving column of water.

It is possible that the shock pressure equates to a value estimated to equal a figure taking velocity (in feet per second) and multiplying it by 60 to give the pressure in psi. Good engineering practice recommends that system velocities should not exceed between 5 to10 ft/s. Based on this recommendation, the hydraulic shock on a system could be as much as 600 psi (40 bar). If the system has not been designed to allow for this all-too-common occurrence, extensive damage can be caused to pipework, valves and other associated equipment.

There is a common misconception that the use of pressure-regulating valves or controls will solve the problem; however, they will still release water into systems at an uncontrollable rate. In short, the only true solution is to design the system to control the rate at which the water enters an empty or part-empty system, prevent the vacuum from forming in the system and control the rate at which the air leaves the system.

Controlling the flow of water into the system can prove difficult, as you do not want to affect the flow rate under normal working conditions. One approach is Wilo multi-pump sets fitted with a controlled fill system to limit the starting sequence after a forced shut down —reducing the rate the system is refilled.

Preventing a vacuum can be achieved by fitting CAVSA valves (combined air vent and surge arrestor) at the top of each riser to allow air to enter the system. These valves will also control the release of the air from the system when it starts to refill, therefore slowing down the rate the water enters the system.

There can be no doubt that addressing the potential problems associated with hydraulic shock is key to installing pressure-boosting sets in residential and commercial premises. Hydraulic shock is a potentially very expensive problem to address, so addressing it at the outset to ensure the problem does not arise makes huge sense.

Gary Wheatley is technical manager with Wilo UK.

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