General Considerations

Excessively high surge pressures can burst pipes and damage equipment. Low pressure surge can result in pipe collapse if the difference between internal and atmospheric pressure is high, cavitation and liquid column separation, and sub-atmospheric pressures unacceptable for drinking water pipelines.

If we consider the simplified case of the instantaneous waterhammer equation, pressure change is directly proportional to the fluid's density, wavespeed, and change in velocity. Therefore, a reduction in pressure response requires a reduction in density, wavespeed or velocity change.

While lowering the fluid density does reduce the pressure response, it is not usually a practical solution.

Note: The below suggestions are some common ways to mitigate pressure surge. This page is not intended to cover all possible means of reducing pressure change.

Lowering the Wavespeed

If the wavespeed of the fluid can be lowered, the pressure response will also be lowered. There are a couple of ways that the effective wavespeed can be lowered:

• Change the construction of the pipe to deform a greater amount under stress.

• The simplest way to accomplish this is to use a more flexible pipe material.

• It is also possible to do this by changing the pipe geometry. Under internal pressure, any pipe will want to become circular. An already circular pipe cannot deform to become "more" circular, however a non-circular pipe (such as a rectangular duct) can deform more easily. This deformation action increases the effective elasticity (decreases E), decreasing the wavespeed.

• Change how much the liquid deforms under stress.

• Entrain gas in the liquid. In effect, this makes the bulk modulus of the liquid lower.

Lowering the Rate of Velocity Change

The other practical way to reduce transient pressures is to reduce the rate of velocity changes in the system. There are a few ways that this can be accomplished:

• Slow down controlled equipment as much as possible.

  • Slow down the closure of valves. Making a valve closure as slow as possible will reduce the transient pressure response as much as possible.

    • It is important to note here that the closure time of a valve does not proportionally relate to the change in velocity at the valve except in special cases. An oversized valve may close almost all the way before any significant pressure rise is seen. This is because there may be very little pressure loss across the valve initially, leading to very small velocity change. The effective closure time may be significantly less than the entire actuation period. In cases like this the pressure response can often be dramatically improved by modifying the pressure loss vs. time characteristics of the closure, rather than the total closure time.

  • A speed controlled pump will cause fluid transients on speed change - slowing this speed change down reduces surge.

  • Any other similar change - variable flows, pressures, control setpoints, etc have a similar effect.

• Reduce the speed at which components adjust to the system.

  • Control Valves that quickly change position in an attempt to maintain control can worsen a transient event. Limiting their opening and closing rates can lessen this.

  • Relief Valves that are poorly designed can open and close very quickly, again propagating pressure waves they should mitigate.

  • Check Valve slam can be problematic - and preventing it may require changing the system or other equipment.

  • Tripping or starting pumps causes significant pressure response. This rate can be reduced by increasing the inertia of the pump and driver assembly - making the pump take longer to stop or start.

• Add equipment to mitigate high or low pressures.

  • If using this type of device, it is generally desirable to locate the device as close as possible to the cause of the transient. The practicality of the different options depend highly on the particular case of interest.

  • Relief Valves - Avoid excessive pressures by providing a means for fluid release. Inward relief valves can be used for low pressure situations.

  • Air Valves - Allowing ambient pressure gas into the pipe during low pressure surge raises the system pressure. Usually located at high elevation points along a pipeline.

  • Surge Tanks - Surge tanks can be an effective way to reduce surge pressures if the steady-state system pressures are relatively low. High steady-state pressures result in high hydraulic gradelines. Since the liquid level in a surge tank rises to the local hydraulic gradeline, they would have to be very tall if located in a high pressure region.

  • Gas Accumulators - Gas accumulators can be effective in spreading out the wave front of a transient and thus reducing peak pressures. Gas accumulators change the frequency response of the system and can in fact amplify pressure surge if not sized and located properly.

  • Check Valves - Appropriately located check valves can reduce transient pressures in the system by preventing reverse flow.

  • Parallel Valves - Rather than use one large valve to stop the flow, perhaps two or more valves can be used in parallel. The timing of the valve closures is staggered, thus allowing a portion of the flow to be stopped with each valve.

• Modify the piping

  • Increasing the diameter of a pipe decreases velocity for the same flow. With a lower initial velocity, the maximum potential velocity change is reduced.

  • Appropriately placed bypass piping can reduce pressure responses.

  • As with any wave phenomenon, the system may have resonant effects at certain frequencies. Changing the length or configuration of certain pipes can reduce (or increase) the pressure response. Adding dead end pipes tuned to a certain frequency is one method of accomplishing this.

The above and additional methods for surge reduction or discussed in detail in Swaffield et al.Swaffield, J. A., and Boldy, A. P., Pressure Surge in Pipe and Duct Systems, Avebury Technical, Hampshire, England, 1993. (1993, Chapter 6), Wylie et al.Wylie, E.B., V.L. Streeter & L. Suo, Fluid Transients in Systems, Prentice Hall, Englewood Hills, New Jersey, 1993. (1993, Chapter 10), and ChaudhryChaudhry, M. H., Applied Hydraulic Transients, 2nd edition, Van Nostrand Reinhold Company, New York, NY, 1987. (1987, Chapter 10).

Related Blogs

Raise the bar using Vacuum Breaker Valves

A Clever Way to Mitigate Waterhammer During a Valve Closure