Understanding The Basics of Water Hammers

Wikipedia defines a water hammer (or, more generally, a fluid hammer) as a pressure surge or wave caused when a fluid in motion is forced to stop or change direction suddenly (momentum change).

The pressure surge or shock wave generated can be very large and therefore destructive. It is not uncommon for water hammers to split pipes or tear apart Victaulic fittings.

For example, a 6-inch pipe 2000 ft long contains 392 cubic feet (2930 USGPM) which weighs approx 25000 lbs. If the flow rate is 880 USGPM the velocity is 10 ft /sec. (7 mph). Suddenly closing a valve in that line creates a force similar to that of a semi-trailer truck hitting a brick wall at 7 mph.

The most common source of water hammer is the abrupt closing of a valve.  Probably second to this would be the hammer associated with the rapid start or stoppage of a pump.

However, these are not the only sources –

an abrupt change in pipe direction may cause issues

when steam and cold water are mixed the results can be devastating and deadly steam can also accelerate slugs of water to up to 100 times the rate of straight water

In addition to the destructive nature of the initial impact, the potential shock wave can wreak havoc with a multitude of components upstream of the impact zone. This is based on the compressibility of the liquid. I know most people think of water as incompressible but this is not quite true, it is very slightly compressible!!!  Therefore the example that likens a column of water hitting a closed valve to that of a semi-truck hitting a brick wall is not quite accurate.

A unit of water at ambient temperature will decrease in volume by about 0.0000034 percent when pressure is increased by 1 psi.  This equates to approx 10% at 40,000 psi.

In addition, water normally contains at least 2% air (that’s how fish breathe). This significantly adds to water’s compressibility.

The good news is the compressibility very slightly cushions the initial impact of a water hammer. The bad news it creates a kind of bounce-back force as the compressed liquid expands back to normal volume.  This creates a shock wave.

A shock wave created by abruptly closing a valve downstream of a pump will travel in the path of least resistance. The valve basically blocks the shock wave from traveling downstream so the reflected wave travels back up the liquid toward the pump.

This shock wave travels at the speed of sound in water, approximately 4,500 ft/sec ( 1.5 km per second )

Damage caused by water hammer is not limited to just one item. It can damage any part of a system that acts to contain the liquid. This can include; pipes, casings, seals, Victaulic fittings, gaskets etc.

The magnitude of the wave is greatest at the point of inception (the cause of the water hammer), with friction causing the wave to diminish over the distance travelled.

The best way to avoid a water hammer is to operate it in such a fashion that you don’t create a hammer.  This usually entails slow closures of valves and controlled start-ups to slowly fill empty lines.  However if you can’t avoid it,  and you are worried about it, look for an upcoming post where we will discuss designing for avoidance of water hammer.

Cheers

RJ