Designing for Water Hammer

Previously, we have discussed the basics of water hammer, so today I would like to look at how professionals quantify it and take steps to design around it.

To accurately calculate the pressure increase due to water hammer by abruptly closing a valve, you must know: water velocity, pipe length, pipe diameter, pipe temperature, material of construction of the pipe, wall thickness and time taken to close the valve.

The many factors make for some complicated formulas but do not despair, there are multiple  web sites online that can assist you in calculating the effect of water hammer. C However , caution is required as some of these do not take into account all of the factors. Especially important is the pipe information. This info, when included, factors in the flexibility of the pipe walls and how the pipe can absorb water hammer.

The many factors make for some complicated formulas but do not despair, there are multiple  web sites online that can assist you in calculating the effect of water hammer. C However , caution is required as some of these do not take into account all of the factors. Especially important is the pipe information. This info, when included, factors in the flexibility of the pipe walls and how the pipe can absorb water hammer.

Pipe manufacturers often provide excellent programs.  Performance Pipe’s “Plexi Calc” program seems to be one of the most commonly used although no one to date has been able to explain why pipe length or valve closing rate is not included as a variable.

Assuming there is no access to online programs, the following is an example of how to calculate a pressure spike. (Example utilizes the same base Formula used in some online programs.)

Caution

It should be noted that:

1. It is limited to use on clear water as it has no input for liquid specific gravity

2. It has no input for the modulus of elasticity of the pipe material

3. It incorporates no safety factor

If we are flowing 1200 usgpm at 300 psi through 7000 ft of 6 inch ID steel pipe and we close a valve in a time span of one second what is the pressure surge?

First, calculate the flow velocity in the pipe:

The formula for calculating  the pressure spike in a pipe is:

Using Barlow’s formula to estimate the burst pressure of the pipe.

P = 2 s t / (do SF)

Where:

P = max. working pressure (psig)

s = material strength (psi)…………………………… For 1018 mild steel, it is approximately 63,800

t = wall thickness (in)…………………………………. For 6 inch schedule 40  – 0.280

do = outside diameter (in)……………………………. For 6 inch schedule 40  –  6.63

SF = safety factor (in general 1.5 to 10)………….. Assume no safety factor to get a burst

In this example, our water hammer was calculated at 922 psi. The burst pressure calculates out at 5388 psi. Dividing burst pressure of 5388 by water hammer pressure of 922 psi results in a safety factor of 5.8.

A safety factor of 5 is normally deemed acceptable, so in this example, the pipe itself would be able to handle any water hammer generated by a one-second closing rate of the valve.

In addition to using components designed to handle the calculated water hammer, designers can also incorporate components that will absorb or cushion the effects of water hammer.

The most common device is an accumulator. This device has a volume of gas (usually air)  trapped within a chamber. The air, being compressible, cushions the water hammer and acts to absorb the shock wave.

The two copper pipes on the wall behind your clothes washing machine are an example of simple accumulators. The capped pipes, which seem to go nowhere, contain air that cushions the water hammer formed when the solenoid valves within the washing machine close.

Industrial accumulators often have pistons to prevent the loss of entrapped air back into the process fluid.

In slurry systems, the solids would quickly destroy the moving piston, so it is replaced by a bladder charged with air or nitrogen.

Another ingenious way to avoid water hammer is to install a length of flexible dredge sleeve in locations where hammering is likely to occur. The dredge sleeve will expand on impact, absorbing the hammer. (Caution: too much pressure may rupture the hose.)

I hope today’s discussion gives you a better understanding of the factors engineers use when designing around water hammer issues. If you are concerned about the potential for water hammer, I highly recommend seeking help from a qualified engineer.

Cheers

RJ