Water Hammer

[Preparation Hell]

There appears to be a discrepancy regarding the formula for the speed of sound through a liquid ("a", which is used to calculated the critical periof for valve closure to prevent water hammer) between the "CERM 10th edition" and the "NCEES Civil Engineering Sample questions and solutions":

(1) Ex. 17.14, CERM P. 17.37:

a = sqrt(E/p), where a = Speed of Sound through a liquid, E= Bulk Modulus of Water, and p= density of water. The formula would include the gravitational convertion constant "gc" in the numerator in its U.S. units form.

(2) P. 502 (Water Resources), NCEES Sample Questions and Solutions:

a = 4720/sqrt(1 + C * K/E * d/e)

These two formulas yield two significantly different results. Curiously, when I work P. 502 using the CERM formula, I get 4,721 fps which is practically the numerator in the NCEES example's formula.

I would certainly appreciate it anyone can explain when one formula is to be used over the other, whether one of the formulas is out of date and no longer used, etc. Thanks for your time in advance and good luck in your continuing preparation.

 

[Guest]

^^^ The difference in equations is based on whether the pipe is considered to be rigid (first expression) or non-rigid (second expression).

JR

 

[Guest]

^^^ Good answer lusone:

JR

 

[WR/ENV_Instructor]

These two formulas yield two significantly different results. Curiously, when I work P. 502 using the CERM formula, I get 4,721 fps which is practically the numerator in the NCEES example's formula.

Check your units and numbers again.

The formulas should give similar results.