Home Improvement Asked on April 25, 2021
I have Watts LF150A water hammer arrestors attached to the hot and cold supply valves for my washing machine (my LG washer is crazy about the water hammer). The LF150A instructions say to add pressure in the valve that is 5 psi less than the house water pressure.
Can someone explain – preferably like I’m a 5 year old – why 5 psi is the preferred amount? What would happen if it were 10, 15, or even 20 psi less than the house pressure? How much less effective would the arrestor be? At what psi would the valve stop arresting?
What would happen if there was more pressure than the house pressure?
My house pressure is 65psi and I’m really curious about the physics of this. Thanks!
Today's arrestors separate water from the shock absorbing gas (air) by means of a piston or membrane.
Separation was not always the case. Arrestors were also once implemented by vertical capped copper stand pipes pre-filled with air. Unfortunately the air pocket's absorption capability would eventually fail as the pocket filled with water over time.
Let's compare the piston with the membrane.
The gas (air) chamber of a piston arrestors is set to about the water pressure during flow, and it is placed near the "culprit" valve (washing machine etc...). The hammer or shock absorption is accomplished by movement of the piston and back pressure from the trapped air that compresses behind the piston.
The required absorption depends on the kinetic energy of the flowing water, which is determined by the volume of flow. This in turn depends on the water pressure and the diameter of piping and valve.
Arrestors are therefore sized based on flow, and thus pressure and diameter. Buy the right sized arrestor for your case; pretty easy to pick for a home appliances, and requires a bit calculation to size for back to back washrooms, rows of urinals etc...
The gas (air) chamber of a membrane arrestor is set lower to account for the elasticity in the membrane and still achieve the same shock absorption.
And since the elasticity does not depend on the water pressure (but perhaps water temperature), a membrane arrestor is set with a constant offset.
Correct answer by P2000 on April 25, 2021
The airspace in the arrester is creating a shock absorber just like on your car but I will use a bicycle tire to bring it to the kid level.
With the pressure a few psi below the actual the piston can move and absorb the shock of the water hammer.
If the pressure is above the water pressure the piston will be locked in place and not absorb the water hammer it would be like riding a bike with 120 psi air in the tires - you feel every grain of sand or rock you ride over.
If the pressure is too low in the arrester it’s not going to respond on the opposite side of too full - kind of like riding a bike with a flat tire the tire rolling around on the rim and not absorbing the shock
With the air just right at 5 below the arrester absorbs the shock of the water hammer like a bike tire rolling over a stone - you barely notice it.
Answered by Ed Beal on April 25, 2021
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