What Is Water Hammer? Causes and Prevention on Ships

Water hammer is the violent pressure surge in a pipe when moving water is suddenly stopped or a slug of water is driven along the pipe at high speed. On ships it is heard as a heavy bang or knocking in a steam line, and at its worst it splits valve bodies, blows gaskets and tears pipes from their supports.

In a steam system the result is a release of steam at high temperature in an occupied engine room, which is why every engineer learns to warm and drain steam lines before opening them up.

Where it happens on a ship

• Steam lines: the boiler's main and auxiliary steam lines, heating lines for fuel oil tanks and pipes, steam tracing, and the cargo heating coils on tankers.
• Condensate and feed lines, where steam meets cold water.
• Liquid systems such as the sea water, fresh water and fire main lines, when a valve is closed quickly or a pump is started or stopped suddenly.

How it happens in a steam line

When a steam line is shut down, the steam inside condenses as the pipe cools, and the condensate collects at the low points. If the steam valve is then opened quickly, steam rushes into a pipe partly full of water. It picks up slugs of condensate and drives them along at steam velocity, which can be tens of metres a second, until they hit a bend, a valve or a closed end.

Water is almost incompressible, so the slug stops almost instantly and its momentum turns into a pressure spike against whatever it hits.

A second form is condensation-induced water hammer. A pocket of steam trapped over cold condensate collapses as it condenses, the water on either side rushes in to fill the space, and the two columns meet with a bang. This can happen even with valves open, wherever steam and cold water come together in a poorly drained line.

How it happens in a water line

In a pipe full of flowing water, closing a valve quickly stops the water in front of it and sends a pressure wave back along the pipe at roughly 1,000 to 1,400 metres a second in steel pipe, a little below the speed of sound in open water. The rise in pressure is roughly the water's density × the wave speed × the change in flow velocity.

Stopping water flowing at 2 metres a second can therefore produce a surge of roughly 20 to 30 bar on top of the line's normal pressure, if the valve shuts faster than a pressure wave can travel to the end of the pipe and back. That is more than many low-pressure systems are built to take.

Signs of water hammer

• Banging or knocking in the pipe, especially when a line is first opened.
• Pipes jumping or vibrating, and loose or bent pipe supports.
• Weeping flanges and blown gaskets after start-up.
• Damaged steam traps, gauges and valve internals.

Preventing it in steam lines

The main defence is to get the water out before the steam goes in, and to let the steam in slowly.

• Check that the isolating valve upstream is not leaking steam or condensate into the dead line, then open the drains at the low points of the line before admitting steam.
• Crack the steam valve open slightly, or open the small bypass valve around the main stop valve where one is fitted, so a small flow of steam warms the pipe gradually. Warming the line also produces condensate, which the open drains remove.
• Keep the drains open until the sound at the drain changes from water to dry steam.
• Open the steam valve further in stages, and close the drains once the line is hot and dry.

System design and maintenance also matter:

• Steam traps at the low points and ends of lines remove condensate continuously, so traps need regular checking; a failed trap is a common cause of water collecting in a line.
• Steam lines should slope down in the direction of flow towards the drain points, with no sagging sections where water can lie.
• Insulation reduces condensation in long lines.
• Heating coils and tracing lines are drained and warmed slowly when they are brought back into use, for example before heating cargo or a fuel tank.

Preventing it in water lines

• Close and open valves slowly, particularly on long pipelines.
• Start centrifugal pumps against a closed or nearly closed discharge valve and open it gradually, unless the system is designed otherwise.
• Vent air from lines before putting them into service; trapped air pockets make surges worse.
• Where quick-closing valves or sudden pump stops cannot be avoided, the system may need surge arrestors or air vessels to absorb the pressure wave.

Consequences

Valves with cast iron bodies are particularly vulnerable, because cast iron is brittle under shock loads. A cracked valve body or blown joint on a steam line can cause serious scalding to anyone nearby, and water hammer and trapped condensate have killed seafarers. In 2002 one person died and another was injured on the Queen Elizabeth 2 when steam and hot water escaped from a pipeline in which condensate could not be drained (MAIB report 17/2003). Warming a long steam line properly takes time and should not be rushed. For the boiler end of the steam system, see boiler mountings and how marine boilers work.