setting PRV and Expansion tank

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SGkent

SGkent

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Joined
Jul 19, 2016
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Location
near Sacramento, CA
Silly question but hours of research failed to answer it. The PRV that is installed is working fine and matches the pressure flow rate charts that Zurn-Wilkins has. Meaning that the fall in pressure between locked and open is within the charts they provide online. With no faucets on the pressure is say 60 PSI. With a heavy flow it will fall to just under 50 PSI. With a light flow 55 PSI.

Question 1: when setting the PRV to a recommended pressure, say 50 PSI, is that the bottom, top or no flow? 50 PSI locked up with no flow would be around 40 PSI with a heavy flow.

Question 2: When setting the expansion tank PSI, is it done to the locked up pressure, light flow or heavy flow?

Question 3: Is the expansion tank air pressure set with zero pressure in the lines or is it set with the system pressurized.

These questions do not appear in any articles or literature I can find. The articles and literature always says set the PRV to the desired pressure but that is a range according to load.
 
I found one article from a manufacture of PRV that said set the pressure lower than desired, then raise it slowly until it reaches the desired pressure, then check it again after opening and closing a faucet. That implies the recommended pressures are set at the lockset value, not the load value. I reset the PRV to 50 PSI. With load it drops within the specified chart that Zurn-Wilkins has on their website. I then ran the sprinklers, noted the pressure and set the expansion valve accordingly so it doesn't add pressure to the lines when they are opened. E.G., if the expansion valve is set to say 50 PSI like the lockset then when the sprinklers are opened the expansion tank will push water out of itself until the PRV flow value is reached. If the PRV flow drops to say 45 PSI while the sprinklers are on, when they switch off and pressure builds again to 50 PSI, That 5 PSI goes back into the expansion tank instead of causing a thud. Additionally if the water heater needs space as the water expands the tank still has lots of room for that. The water hammer in the kitchen is now completely gone and washer-dryer hoses wiggle a lot less when the washer is turned on and off. I still don't know if all this is correct but I suspect it is close. I don't know if the 5 PSI reduction on the expansion tank will lengthen or shorten its life. Kinda like running a tire at the lower end of the manufacturer specified PSI range for a softer ride around town at lower speeds.
 
To set the pressure on your expansion tank.
1] check the pressure of your system with a gauge
at the point where the expansion tank connects to the pipe

2] check the pre charge pressure of your tank,
charge it to the same psi as your system pressure
do this before you connect the tank to the system

3'] install the tank
your tank is charged to the correct psi of your system.

couple of very important points
check the pressure of your system where the tank connects..NOT a hose bib out side or your sink
charge the tank BEFORE you connect it

how to test the tank for a blown bag
press the air valve, if water comes out, the bag is busted
to check the air charge, use a tire gauge, on the air valve
hopefully, when you installed the tank, you wrote the charge on the tank with a sharpie
if not. pull the tank. check psi at the connection

WHY??? charge the tank before connecting???
if the bladder is 10 psi, and your system is 60 psi
the tank will fill 2/3 or 3/4 up with water, and the size of your expansion is greatly reduced
your gauge will show the correct psi for the bag,

why check psi at the point of connection?
because if you check somewhere else the pressure at that spot is different than where you are connecting the tank
 
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SGkent said:
Silly question but hours of research failed to answer it. The PRV that is installed is working fine and matches the pressure flow rate charts that Zurn-Wilkins has. Meaning that the fall in pressure between locked and open is within the charts they provide online. With no faucets on the pressure is say 60 PSI. With a heavy flow it will fall to just under 50 PSI. With a light flow 55 PSI.

Question 1: when setting the PRV to a recommended pressure, say 50 PSI, is that the bottom, top or no flow? 50 PSI locked up with no flow would be around 40 PSI with a heavy flow. The typical static(no flow) pressure.

Question 2: When setting the expansion tank PSI, is it done to the locked up pressure, light flow or heavy flow? At the static (highest) pressure.

Question 3: Is the expansion tank air pressure set with zero pressure in the lines or is it set with the system pressurized. With zero pressure on the system side. Or as mentioned above before installing, if it happens to be not connected.

These questions do not appear in any articles or literature I can find. The articles and literature always says set the PRV to the desired pressure but that is a range according to load. True! The desired pressure setting would be(or should be) based on the demand as well as being within recommended maximums.
Click to expand...
 
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Thank you. I should be able to turn house water off, depressurize the house water system, then set the tank while the entire system is depressurized. That should have the same effect as removing the tank. In this system I installed the best water pressure gauge I could find about 8" from the expansion tank on the line to the tank. That way I know what the water pressure is near the water heater and at the expansion tank at a glance. I print and attach a Dymo type label with date and pressure, what the gauge should be showing when set / date. Easy to peel off and replace.
 
SGkent, nice methodology!
With expansion tanks I put at point of installation: a LHBV, plus a tee with a hose bibb. It allows easy checking, draining/depressurising and future replacement without having to shut the house water off at all.
Webstone make a nice expansion tank valve (41672) that isolates and drains- really made for hydronics with an outlet for air separator but only in 1/2"... but great value and time saver, and I just cap the spare opening.
 
breplum said:
LHBV is Lever Handle Ball Valve
Click to expand...


I would like your opinion breplum.

if you depressurize the system as WE ALL suggested.
look at the ET location
I am thinking the ET is not drained because it is trapped
so, the tank if full of water, then the bladder is under the head pressure of the trapped water
therefore, the tank is not empty and the recharge will not be accurate
your thoughts?
 
tank does not spray any water when air pressure is released. Schrader valve is at the bottom. If there was air leaking past bladder it would show up in lower half, and spray when the Schrader valve is pressed. Air in the bladder has uphill run to main cold water source for water heater. Spoke with a long time plumber at the supply house south of the city today. He said that I should try opening all the valves and faucets in the house at the same time and let it run for 3 - 5 minutes then turn off one by one. That should make sure any air is not simply moving around in the lines as I go between faucets, sprinklers, and appliances. Also he feels the banging when the pressure valve is released is the flue gas baffle that has probably come loose over time. That noise is not the foghorn we get. I also spoke with Rainbird. They said that the fact the foghorn noise does not occur when the controller is unplugged means that the controller is probably going bad and randomly opening valves for a split second. The guy at the plumbing house thinks that the water heater may still be involved in making the sounds worse. He said the placement of the expansion tank is correct and it should dampen any normal hammer BUT that hammer is elusive and try running all the faucets etc at the same time to see if that helps. Air may have found a spot somewhere and the flow with one faucet at a time may not be enough to force it out.
 
frodo said:
I would like your opinion breplum.

if you depressurize the system as WE ALL suggested.
look at the ET location
I am thinking the ET is not drained because it is trapped
so, the tank if full of water, then the bladder is under the head pressure of the trapped water
therefore, the tank is not empty and the recharge will not be accurate
your thoughts?
Click to expand...
That is a good point and should be taken into consideration.
This wouldn't meet the zero pressure requirement as it would likely get over looked.
 
Already considered that. My 1% WIKI gauge is more accurate than most 10% air gauges that folks buy. A precision air gauge is typically only a 5% gauge.

The pressure depends on the water column height over the expansion tank bladder. Fresh water is slightly less weight than salt water. Two atmospheres in sea water (sea level plus one atmosphere) is a one inch column 33 feet tall (which is 396"). It is more like 34.xx in fresh water. The pipes above the expansion tank are less than a 1" column of water but lets use 1 inch and 33 feet to make the math simple. The height of the pipes in the attic are 43 inches above the expansion tank bladder. This means that the column of water is 43/396 of one atmosphere (actually slightly less because the pipe is 3/4" instead of 1", and fresh water. 43/396 is .1085 atmosphere. An atmosphere is 14.7 psi. 14.7 x .1085 = 1.6 PSI. Your 10% gauge (or 5% if you paid for a precsion gauge) isn't that accurate. I use a Wiki 4" mirrored 1% national standards lab traceable gauge, so my 50 PSI is accurate within .5 PSI. The average 10% gauge can be off 5 PSI at 50 PSI. Even a 5% gauge can be off by 2.5 PSI. The 1.6 PSI difference with the water above it is less than the average precision gauge can measure. I don't have a water gauge that is 1% so without both gauges being 1% accurate, setting the bladder to 50 PSI with a column of water on it 43 inches tall is within the margin of error.
 
Wow that's confusing.
What happened to...
2.31 feet of water is equivalent to 1 PSI.
Or 1 foot of water = 0.433 PSI.
 
the tank is a 2.5 gallon tank
so the water left inside is half of that
1.3 gallons
the pipe approx 4'
is 3.5 oz per ft
14 oz.

2 pounds of water are pressing on the bladder
just a little under 2 gallons,
so...the instructions say to pre charge and re charge on a empty tank
this tank in the position it is full of water tank.
 
frodo said:
the tank is a 2.5 gallon tank
so the water left inside is half of that
1.3 gallons
the pipe approx 4'
is 3.5 oz per ft
14 oz.

2 pounds of water are pressing on the bladder
just a little under 2 gallons,
so...the instructions say to pre charge and re charge on a empty tank
this tank in the position it is full of water tank.
Click to expand...

The load is spread out over the bladder size. That is why a diver who is 6' tall and one 5' tall experience the same water pressure at any given depth. So does the diver's gauge which might be 4" round. The coefficients are designed per square inch of water and surface. The bladder may have more water in it but it also has more surface area. The pipe above it are 3/4 which actually reduces the numbers by 25% in this calculation but I did not want to go there. PSI is pounds per square inch - not pounds per 3/4 square inch. :)

If water was 1 psi per 2.31 foot then a diver at 33 feet would be experiencing 76.23 PSI. At 33 feet of water column a diver experiences 14.7 PSI for the water and 14.7 for the atmosphere, which is 2 atmospheres. Sea water weights more than fresh water. Using your .433 per foot, 43 inches is 3,58 feet X .433 = 1.55 PSI. I said 1.6 so I guess the margin of error is even .05 PSI better. :)
 
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SGkent said:
The load is spread out over the bladder size. That is why a diver who is 6' tall and one 5' tall experience the same water pressure at any given depth. So does the diver's gauge which might be 4" round. The coefficients are designed per square inch of water and surface. The bladder may have more water in it but it also has more surface area. The pipe above it are 3/4 which actually reduces the numbers by 25% in this calculation but I did not want to go there. PSI is pounds per square inch - not pounds per 3/4 square inch. :)

If water was 1 psi per 2.31 foot then a diver at 33 feet would be experiencing 76.23 PSI. At 33 feet of water column a diver experiences 14.7 PSI for the water and 14.7 for the atmosphere, which is 2 atmospheres. Sea water weights more than fresh water. Using your .433 per foot, 43 inches is 3,58 feet X .433 = 1.55 PSI. I said 1.6 so I guess the margin of error is even .05 PSI better. :)
Click to expand...
What you say looks good with the exception of the reference to the 3/4" pipe size.
Not sure why the mention of, "pipe above it are 3/4 which actually reduces the numbers by 25%". First off the area of a 3/4" pipe is closer to 1/2 sq in. Secondly it doesn't matter what diameter the pipes are. As you said it's, "pounds per square inch". A foot high column of water 1/2" in dia has the same PSI at the bottom as a foot high tank of water 10 feet in dia.
 
frodo said:
the tank is a 2.5 gallon tank
so the water left inside is half of that
1.3 gallons
the pipe approx 4'
is 3.5 oz per ft
14 oz.

2 pounds of water are pressing on the bladder
just a little under 2 gallons,
so...the instructions say to pre charge and re charge on a empty tank
this tank in the position it is full of water tank.
Click to expand...
WHAT???????????
 
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