Water hammer

What is water hammer?

Expansion joints damaged by water hammer

Water hammer - Blown expansion joints

Water hammer is caused by a sudden increase in pressure that generates a shock wave in pipelines or equipments. In many cases, this high pressure exceeds normal operating conditions and can induce a mechanical deformation and even a rupture of materials. This blow spreads in the water at the speed of sound. The water hammer is recognized by the sudden banging that it provokes. There are two kinds of water hammer:

  • Hydraulic water hammers in installations carrying liquids

  • Thermal water hammers in steam and condensate installations as well as in superheated water tanks

In this last situation, the intensity of water hammers depend on the surface of contact between steam and water, on the relative speeds of water and steam, and on their temperature difference.

Hydraulic water hammers

When there is a a sudden stop of the steam system (as happens when the rapid shut-off of a valve suddenly blocks the movement), the force engendered by the mass and the speed (kinetic energy) of the column of liquid creates a pressure blow upstream of the valve. Since liquids are incompressible, they cannot absorb this blow of pressure, which spreads in all directions. Downstream from the valve, the flowing movement does not stop immediately and induces a partial vacuum next to the valve. This vacuum then sucks the liquid upstream. The liquid bounces against the valve, thus creating another shock  wave. This shock wave is replicated a number of times before fading away.

Thermal water hammers

Thermal water hammer is produced when a hot steam bubble meets cold condensate. The steam bubble condensates very rapidly, causing a vacuum, and the condensate brutally irrupts and clashes. We say that these steam bubbles implode. The steam bubbles can be dragged by the surface of hot condensate towards colder condensate. The steam bubbles can result from a decrease in the pressure of the condensate, be dragged towards colder condensate and then implode.

Consequences of water hammer

It is important not to underestimate the effects of water hammer. Their impact can have the following consequences:

  • Collapsed elements in all designs of steam traps
  • Overloaded steam gauges
  • Distorted internal mechanisms
  • Split steam traps
  • Broken pipe joints
  • Broken valves
  • Damage in heat exchanger internal tubes
  • Broken pipe welding or pipes
  • Ripped pipe supports/fasteners

In extreme cases, water hammer not only damages equipment, but it can also be dangerous for plant personnel.

Water hammer noises are alarm signals indicating that something is wrong in the steam system and that it should be remedied. By eliminating water hammer, you benefit from a safer work environment and you will reduce maintenance costs and production off-time.

Causes of water hammer

Water hammer forming in a pipe

Water hammer forming in a pipe

  • Poor planning of an installation (banking-up of unwanted condensate at low points, negative slopes, etc.)
  • Sudden closing or opening of valves
  • Use of non-standard equipments
  • Faulty operation of regulating equipment or drain valve (insufficient draining)
  • Speed of the steam flow higher than speed of the condensate flow : this causes a dragging of water drops that later meets obstacles and cause hydraulic water hammers. Even very small quantities of water, when they are thrown at high velocity, can give rise to considerable forces. For example, this is what can happen on the blades of a turbine.