What is water slam?

Water slam is a common problem associated with check valves.  Water slam occurs after pump stoppage, when flow reverses back towards the pump before the check valve is fully closed.  Once the check valve is closed there is an immediate stoppage of the reverse flow and a resultant water hammer potentially causing serious damage to the pumping system in the application. This action often causes a water hammer in the pipe.  The noise is not due to the impact of the disc hitting the seat, but it is the noise of the pipe stretching under the force of a water hammer.



The Problem with predicting water slam…


In severe pumping applications nearly all basic check valves will slam, in very mild applications hardly any check valves will slam. Between the two extremes is a vast and uncertain middle ground, this makes predicting check valve slam very difficult.


How to prevent check valve slam:


In order to prevent check valve slam the check valve must close either;

  • Extremely quickly and before appreciable reverse flow occurs
  • Extremely slowly once reverse flow has started


For the check valve to close quickly;

  • The disc should have low inertia and friction
  • Travel of the disc should be short
  • Motion of the disc should be assisted with springs


For the check valve to close slowly;

  • Check valve must have external devices like oil dashpots
  • Oil dashpots have been deemed effective at giving slow closure
  • However they significantly add to valve cost and may cause valve clogging in waste-water applications



The Solution to using check valves:


The main solution is to match the non-slam characteristics of the check valve to the pumping system

So before selecting a non-slam check valve the pumping system must be analysed and the deceleration of the liquid column after stoppage must be calculated.

It is the responsibility of the valve manufactures to give the closing characteristics of their valves so the engineer can estimate the maximum reverse speed that may occur.


Designers can make informed check valve decisions using characteristics such as:

  • Head loss
  • Laying length
  • Waterway design for fluids containing solids
  • Cost

The advantages of using Check Valves in your application:

  • Reduced cost
  • Low head loss
  • Special flow characteristics


The best check valve is not the one least likely to slam but the one that meets all the relevant criteria.



MGA Controls Ltd have an exclusive partnership with Valmatic Co. Valmatic are known for their valuable experience and success in manufacturing and supplying a wide range of Check Valves to many applications within the industry. MGA Controls Ltd are fully trained on the Valmatic product range and we believe their range of Check Valves to be the superior Check Valves in the market today.

For enquiries or more information, please contact our technical team on +44 (0) 8444 501123, or visit our Valmatic website pages.