Nailing water hammer

Reciprocating pumps are an essential component in many processing and production applications ranging from wastewater treatment to pharmaceutical manufacturing. However, regardless of the application, a problem that can have disastrous consequences both for pumps and processing plant is hydraulic surge, or “water hammer”.

Not to be confused with pulsation — the acceleration/deceleration of liquid — surge is the result of a rapid change in velocity. Surge occurs when the liquid flow suddenly stops, starts or encounters a rapid change in direction. The kinetic energy, released as pressure, can spike up to six times the pumping system’s operating pressure, destroying instrumentation, pumps, pipes, fittings and valves. The shock waves will travel the length of the pipe back to the pump and then reverse direction. Only when friction dissipates the pressure spike, or a component fails, will the oscillating motion cease.

The most commonly encountered causes are the action of quick-closing valves, back-surge, pump start-up and pump shutdown. Quick-closing valves and pump start-up have the potential to stop large volumes of energised fluid very suddenly and this must be avoided at all costs.

During pump start-up, the fluid in the pipe is static and must be accelerated. However, when the accelerated fluid comes into contact with static fluid already in the pipe it stops abruptly, causing a shock wave. Also, during pump start-up, the accelerated liquid causes any air trapped in the system to compress, creating a back pressure situation. The pipe vibrates and the collective fluid handling system is subjected to an immediate pressure boost, which can stress components beyond their limits.

When a pump is shut down, the liquid in the discharge line flowing at a constant velocity continues due to momentum and a low pressure area forms at the pump discharge. When the liquid’s momentum stops, its direction reverses and the liquid returns to the low pressure area creating a water hammer effect. Depending on the initial flow rate, the pipe gradient and the fluid mass, the pump casing may become stressed, with potential for failure. Also, pump seal integrity can be lost and the impeller can become warped.

Removing the threat of surge can be undertaken by: fitting pumps with motor controllers, installing rupture discs, introducing pressure relief valves or installing a surge suppresser. The first option, whereby the controller starts up and shuts down the pump slowly, has been shown to have a degree of success, but it does not come cheap.

Where a system includes quick-closing valves, fitting rupture discs and pressure relief valves are accepted solutions. Rupture discs will break at a lower pressure than any other components in the system and release the liquid into the immediate environment. Their suitability is limited when the liquid is potentially hazardous and cannot be released to the environment.

Pressure relief valves open when a predetermined pressure is reached. As the pressure spike increases, the valve opens and releases the liquid into a holding tank or return pipe. Valve sizing and relief pressure settings are critical to minimise the spike as rapidly as possible. It is an expensive approach and one that requires the valves to be tested on a regular basis.

A surge suppressor, however, addresses all these issues and counters the adverse effects of pump start- up/shutdown and quick closing valves. Similar in design and operation to a pulsation dampener — its difference lies primarily in sizing and pressurising — the latest surge suppressors contain an elastomer bladder inside a dome-shaped housing that separates the pumped liquid from a compressed gas charge. By capturing the gas charge, the proper pressure required for the specific application can be maintained.

During pump start up, the suppressor is pre-charged to 85% of the operating pressure. When the pump is started, liquid is initially pumped into the suppressor and then released into the pipeline to balance pressure as a steady flow is reached. When rapid pump shut- down occurs, a surge suppressor positioned close to the pump discharge, or check valve if one is installed in the system, will remove the possibility of a reverse flow spike or water hammer.

Properly sized and pre-charged to 50% of the operating pressure, the surge suppressor will accomplish two tasks. Firstly, being pre-charged, the vessel will accumulate fluid during pump operation and this accumulation will be released on shutdown to prevent column separation. Secondly, the suppressor will absorb the pressure spike generated as the water column reverses against the check valve or pump. In both cases, the surge suppressor provides protection and at a fraction of the cost of other solutions.

Where quick closing valves are involved, the surge suppressor is equally as effective because it absorbs the pressure spike by momentarily accumulating the flow of the liquid as the valve closes. Because of the speed of propagation of the transient wave created, the surge suppressor must be installed directly upstream of the quick closing valve and no further away than 10 pipe diameters.

Since the full capacity of the suppressor must be available to accept the accumulation of liquid when the valve closes, the suppressor must be pre-charged to 95-98% of the system pressure.