Stress Engineering Services

  PROCESS TECHNOLOGY GROUP    

WATERHAMMER ANALYSIS

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Waterhammer problems are among the most prevalent and most serious flow problems seen in the process industries.  The large pressures generated during a waterhammer event can lead to catastrophic failure of high capital expense components such as pumps, turbines, and large-diameter valves.  The cost of the downtime required to repair a waterhammer failure can easily exceed the capital cost of replacing equipment.  Furthermore, because of the transient nature of waterhammer events it is often difficult to localize the cause of the event.


Stress Engineering maintains special-purpose analysis programs to assist our clients in diagnosing the cause of waterhammer events and in designing systems to avoid such problems.  Our engineers are skilled in transient flow design and we maintain an active practice in the field of forensic engineering and accident investigation.



Waterhammer Example

 

An example of the combination of computational fluid dynamics (CFD) with traditional waterhammer calculations is presented below.  In this example a butterfly valve on the discharge from a storage tank is suddenly closed, generating a waterhammer event. 

 

 

In order to compute a realistic value for the maximum pressure generated by the valve closing, an accurate assessment of the pressure - flow - position behavior of the valve is required.  While this can be obtained through testing, equally accurate results can be obtained from 3D computer models.  The figures below show such a model of the butterfly valve.  The plot on the left shows velocity vectors when the valve is in the 45 degree position. The right-hand plot shows pathlines of flow through the valve.  These are the paths that particles in the flow take.  In addition to the waterhammer application, the results in these plots can be used to improve the design to reduce all of the following:


   cavitation
   erosion
   sediment deposits

 

 

The critical result for the waterhammer analysis is the pressure drop versus valve position.  The figures below show the computed pressure distribution on the mid-plane through the valve for a constant mass flow rate.  In these plots red represents high pressure and blue low pressure.  Using this data a discharge coefficient can be computed for each valve position.  This discharge coefficient provides the critical piece of missing information for the waterhammer analysis.

 

 

The waterhammer analysis is conducted using a special-purpose computer program that can be modified to simulate piping systems of any degree of complexity.  Figures (a), (b), and (c) below show the computed pressure on the upstream valve face as a result of three different valve closure rates. In each figure the red line shows the valve discharge coefficient, ranging from 1.0 when the valve is fully open to 0.0 when the valve is fully closed.  The blue line shows the pressure on the valve as it is closed and held shut; the pressures are presented as multiples of the supply head H0.


Figure (a) shows the pressures pulses that are generated when the liquid flow is stopped by an instantaneous valve closure. The pressure and rarefaction waves alternate as the flow energy is gradually dissipated. The piping system must be capable of sustaining both the over-pressure and the suction conditions that continue for many seconds following valve closure.


Figure (b) is an example of a linear valve closing. This simplification is sometimes made when specific details of the valve flow loss characteristics are not known precisely. The linear reduction in flow rate provides a reduction in both peak over-pressure and peak suction as compared to the instantaneous valve closure case.


Figure (c) shows the pressure and suction waves that occur at the valve face as a result of the valve flow loss characteristics predicted by the CFD model. Among the three cases analyzed the calculation based on the CFD-predicted flow condition yields the most realistic and smallest pressure and suction peaks.  The additional design margin provided by the CFD analysis can provide significant cost savings without reducing the factor of safety used in the design.

 


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