The thermal expansion is caused by water expanding as it is being heated.

Water does not compress

The expansion tank has an aircharge which will compress and it allows the expanding water to enter the tank with only a slight compression of the air charge. If the air charge wasn’t there the system pressure could go as high as 150 psi causing the T&P to discharge,

The compression of the air when the water enters may only raise the pressure a pound or, 2 since air can compress.

Water hammer is where the water is flowing at a velocity then a valve is suddenly closed bringing the flow to a sudden stop.

If you imagine the flowing water as a hammer swinging then the closing of the valve as the moment the hammer impacts, that is water hammer.

The water hammer may hit several times each one with less force similar to a hammer bouncing.

A thermal expansion tank may absorb water hammer with the air charge cushioning the effect but usually it is not in a location to do so.

Hammer arrestors should be placed just before the fast closing valve for maximum effectiveness.

Water hammer has a tremendous amount of force behind it and can be quite damaging. When I was playing fireman years ago we had a new deputy chief in our fire department and early on he was running a department drill. He had all the trucks pumping away like mad flowing huge volumes of water. Our truck alone was flowing about 2,000 GPM off a hydrant and there were at least 4 others there doing a similar amount for a total of around 8,000 GPM.

At some point the new wet behind the ears deputy chief decided to end the drill and rather than call each pumper first asking them to reduce their pressure then shut down in increments he calls over the radio, “All units shut down.” which of course each pump operator immediately did.

It was surreal as I the Captain was running towards our truck yelling, “No!”

But it was too late the every truck operator chopped the throttle and as soon as the rpm’s died down to idle closed the valves…

I arrived to see the gauge on the suction side of the pump bouncing up in the 450 psi range.

This would not have been a true indication either because the gauges are dampened by a needle valve to keep the needle from fluttering. Who knows what the actual spike reached?

Shortly after that the water companies were on the scene replacing a main that was broken in several places.

That is water hammer on a grand scale!

I find the mistaken sentence :

“The tank is designed to take in the excess water pressure that may be cause by the thermal expansion or more commonly called water hammer.”

This is incorrect. Water hammer is the sudden pressure spike caused by quick closing of a valve in a pipe with flowing water and the pressure spike is caused by the momentum of water flow being suddenly stopped. This often makes an audible slamming sound throughout the piping system and is potentially damaging. It is true that thermal expansion can also cause excess pressure and is potentially very dangerous, but that is not what is meant by “water hammer” – except insofar as the term is mistakenly used and repeated through misunderstanding which is unfortunately common.

Regards,
Bill Dempster