We have a consultant's report on an ESD type valve in a 10" condensate export pipe which closes in the period of 10 seconds. They are predicting column separation, a pressure rise of around 2000kPa and the attendant huge water hammer forces.

I have read the caesar II manual section on water hammer. I am prepared to believe the rise in pressure, but have trouble with the hammer force prediction. The speed of sound in the fluid is, lets say, 1000m/s, the max. length between adjacent elbows is about 20m, thus the pressure wave passes between elbows in about 20ms. Because the valve closes so slowly, the pressure wave on the upstream side has such a long ramp-up in comparison to the 20ms that the net pressure difference between adjacent elbow pairs is always going to be very small, and so too the net force. As pointed out in the manual the closing time should be smaller than 2L/c for significant hammer. The only exception to this I can see is if the pressure wave in fact carries a large negative peak behind it which could cause vapourisation /column separation. Does anyone have comments?

On the downstream side of the valve the pressure variation is immediately negative as described in the manual so I would expect that vapourisation/column separation could occur and large forces could result. Are these forces more difficult to predict than those due to pressure rise ?

regards
Martin Blackman
Pipe stress engineer