pressure surge calculation 1

[NOTWAL]

I am designing a system that involves 17 miles of 24-inch ductile iron transmission main. The surge I am worried about is the pressure that will develope from the quick closure of a fire hydrant or 6-inch hydrant valve. the 24-inch mainline gate valves take over 200 turns to close so they do not concern me. The system is fed by gravity so pump surge is not a problem. Does anyone know how to calculate the surge pressure developed in such a situation. I would like to prevent the PRV stations along this main from any future damage.

 

[cvg]

closing the hydrant valve will cause a pressure wave to return to the reservoir, when it hits the free surface it will dissipate. I don't think it will return. The momentum in the 24 inch line caused by the 6 inch hydrant lateral will also be quite low. I am assuming somewhere around 500 - 1000 gpm maximum through the hydrant which produces a flow velocity of about 0.7 feet per second in the transmission line. I would assume this is a negligble velocity and wouldn't cause problems.

 

[PUMPDESIGNER]

With what little I know about fire hydrants, I think cvg probably has it correct.

Here is the simplest form of the formula, which does not take into account any complications such as turns, temperature, gases, etc. As noted below, the time of closure is very problematic and not solved for in this formula. Apart from the linearity problem, this formula assumes no effect of pipe expansion/contraction, which for cast iron is about correct, so the result may be surprisingly close to reality.

Pressure Rise = .070 * (Velocity * Distance) / Time of Closure.

The formula requires American units of feet for velocity and distance. Distance is of course the length of line from source to point of closure. Time of closure is in seconds and assumes complete linearity, which is the problem with the formula, most of the flow is stopped at the last seconds of closure.

PUMPDESIGNER

 

[NOTWAL]

CVG-

Does this mean that if I assume the only movement in the pipe to be a fire hydrant, and that hydrant or valve hydrant were closed quickly that a pressure surge of some PSI (calculated by PUMPDESIGNERS calculation) will travel back through the main at ~0.7 fps back my connection point? Therefore, any PRV connection that I have between the closure and the connection will be subject to this surge as well.

Does anyone know of any free surge programs (basic modeling) that can be found on the internet?

Thanks for the advise.

NOTWAL

 

[PUMPDESIGNER]

Surges move through the line at the speed of sound in water.
Depending on temperature, dissolved gases, wall type, etc., this speed will be around 4,000-4,500 feet per second.

That little formula I gave you is not to be used for accurate calculations, but will give you a quicky analyses, and in some cases it will be accurate. Accurate calculations are complex and difficult, more like modeling. Consider that formula I gave you to be like a pocket cheapo calculator. Results with that little guy by the way tend to be higher than reality, not lower.

I am not personally aware of any freeware for accurate stuff. I think some have posted stuff on this forum before perhaps they will provide that link again.

PUMPDESIGNER

 

[cvg]

I calculated the 0.7 fps velocity based on the following assumptions:

a) the only flow out of the system was from the open fire hydrant

b) the water flowing to the hydrant is only flowing from the reservoir side of the hydrant connection (no loops in the system)

c) straight run upgradient from the fire hydrant to the gravity reservoir

With a looped system, additional flow demands and prv's in the main, this becomes more complicated. As PUMPDESIGNER stated, you may want to construct an accurate numerical model to help predict the surge.

This has been discussed many times in the forum. Do a search on key words for water hammer or try the following threads

thread378-79101
thread378-42680
thread164-70341

 

[Hookem]

With water velocites so low, hammer should not be of concern. Plus, who can close any valve "instantenously"?