Evaluate Water Hammer on Existing System?

1/17/2025

I'm working on a project in an existing 7-story building with an existing 500 gpm 70 psi fire pump.

We are installing a new 4-story riser outside the pump room, and a question of water hammer/surge protection has come up. The existing equipment does not indicate any special considerations for this.

Most (not all) zone valve assemblies incorporate pressure relief valves, and all new zones will have them, as well. Hydrant test static was 65psi.

In general, what conditions would prompt you to consider the possibility of water hammer or pressure surge being an issue?

How do you go about evaluating this, if it is a potential issue?

Thanks in advance.

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4 Comments

Dan Wilder

1/17/2025 07:55:41 am

There should not be any issue....until there is an issue. You can show the current status by putting on a digital pressure recorder on the supply side of the incoming line for a couple days to see if there are any wild swings.

A majority of the system sits behind check valves (or should) so any start of a hammer will be dissipated from within individual zones and generally non-existent (unless you've got quarter turn valves that people are slamming shut, then those should be changed out).

I have seen hammer issues when jockey pumps are oversized...we found out the hard way when a project pump replacement started having issues and checking the data plate on the JP was towards the end of our diagnostic efforts (added PRV's to systems that did not have them, retested check valves that the existing ITM company said had passed that did not, added check valves on risers that only had butterfly valves as this was a multi-zone property).....changed the JP and everything came back to a normal condition...lesson learned!

Other times had issues when they were adjacent to car washes, typically 1-3 tap laterals away from the supply to the car wash were the problem buildings, and those were almost always smaller individual stand alone's like banks or medical offices.

Glenn Berger

1/17/2025 08:42:42 am

Water hammer should not be an issue except potentially at pressurization of the system. If it does occur you need to look for elevation changes in the piping network where air pockets may be formed. The other potential scenario is where a check valve is not seated correctly.

Since you have a pump on your system - if the pump is cycling then there is something that needs investigating.

Jack G

1/17/2025 09:21:47 am

Water hammer in a piping system is a large banging or knocking sound when the flow of water is stopped or change direction suddenly. It’s caused by a pressure surge, or shock wave that travels thru the pipe at the speed of sound ( as I learned in my college fluid mechanics course - Che eng) .
What causes it— a valve closing too quickly, a sudden stop by a jockey, Fire pumps , and a pressure wave resonates in the piping system.
The effects could be damage to the pipe and fittings, pumps and motors, can cause pressure spikes that are much higher than the pressure rating of the system, and could cause a pipe to burst.
How to correct for it ?
First I would install air relief valves at the top of each standpipe, along with a pressure relief valve set say at 175/psi— or psi rating of the system, , and at the highest point on each floor an air relief, ( are checked off from the standpipe systems ( required on wet systems - and pressure reliefs on system side of check) — the relief valve at the riser could release the spiked pressure but could leave trapped air in the system — causing a sponging and quick stop )
There are water hammer arrestors that are made for the fire pump discharge that could be added.
Make sure that the jockey pump settings are per NFPA 20. ( in a high rise especially the contractor sets the up on and off pressures lower due to elevation of the building) if the jockey pump comes on at a “ lower pressure than “ churn plus 5 psi” and off at say churn plus 10 or 15, it could cause a shock wave when it suddenly stops . ( longer run time.
In my opinion, the air relief valves along with pressure release valves will curtail water hammer.

Anthony

1/20/2025 07:48:22 am

Check out a wafer check specifically the Kennedy valve fig 806 is listed as a water hammer dampening device. Might be applicable here.

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Evaluate Water Hammer on Existing System?

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