Charging Pressure Tank For A DI Pig

woodshed

I'm trying to charge a pressure tank with water to use as a DI pig. I tried using a 1/2 hp submersible utility pump but the highest pressure I can get is 20 psi. What type of pump do you guys use to charge the pressure tank with water?
I want to get 60-80 psi of water in the tank (4 gallons); I have air in the tank set at 40 psi.
Thanx, Rick

Ironman

Use house pressure from the potable cold water line. Even if you have glycol in the system and tank, the amount of water that gets added will be minimal.

woodshed

Bob - I'll be using DI water fed from a utility pump. No house potable water and no glycol.

Should I lower the air pressure in the expansion tank to 0-5 psi and then add DI water using a utility pump then increase the pressure in the expansion tank to 60 psi?

Ironman

I'd be afraid that the water would occupy too much of the tank's volume to attempt that.
My pump cart can create 60 psi, so a larger pump?

woodshed

Bob - I would fill the expansion tank to the maximum accepted volume. The expansion tank volume is 4.5 gallons and the maximum accepted volume is 2.5 gallons; I'd fill the expansion tank to 2.5 gallons.

hot_rod

Look for the bag type expansion tank then you do not have to worry about over stressing the diaphragm

I fill the tank to max pressure, usually 100 psi on Thermtrols. Then an Autofill valve to regulate

woodshed

Hot Rod - Do I first add DI water without any air pressure in the tank and then add air pressure after the DI water is in the tank?

If it's a 4.4 gallon tank can I fill the tank with 4.4 gallon of DI water or am I limited to the maximum accepted volume, 2.5 gallons?

Thanx, Rick

hot_rod

You will need to pump the fluid into the tank somehow. The diaphragm will be up against the nipple even with low or no pressure in the tank.

The diaphragm is crimped in about mid range of the tank. If you over pressurize them often you can stretch and damage that diaphragm. Not sure how much abuse they can take, cool water is possibly less stress.

The bag type expansion tanks contain a butyl or EPDM bladder and can take more acceptance, almost to the tank capacity.

Depends on how often you over stress the diaphragm, as to how long the tank will last, I suppose.

Here is a #15 DHW expansion tank. 12 psi pre-charge on the air, 40 psi water from my hose bib. I get exactly 1 gallon out of it. I have used these with solar glycol many times, no failures yet.

A Webstone valve on the tank allows you to fill and check fill pressure. A Caleffi Autofill allows you to regulate output psi into system. Cheap and easy to build.

woodshed

Hot Rod -

I'll be filling the expansion tank with DI water. My submersible utility pump puts out 20 psi. Should I let the air out of the expansion tank, fill the tank with DI water and then add air?

hot_rod

drop it below 20psi or you will not push any water in.

Try 5 psi on the air and fill it up. See how much comes back out, try it with 0 psi air, pump up and add 20 psi air.

I think you will get about the same draw down either way, but give it a try.

woodshed

Hot Rod -

So when I'm filling/purging the tubing and system piping with DI water, not with the pig but with the 20 psi utility pump, should I close the shut-off valve for the system expansion tank? That way I don't have to overcome the expansion tank pressure for the initial fill and purge?

hot_rod

The expansion tank will not put pressure into the system. If it is a bladder or diaphragm type tank, the air pre-charge is below the bladder in the tank.

Set the tank pre-charge pressure to what your final fill pressure will be.

The final fill pressure is determined by how high up above the fill point you need to lift the water.

It takes .433 psi to lift water 1 foot. So to lift the water 10 feet above the fill point you need 4.33 psi.

Often that .433 is round to .5 psi so the math is easier. So to fill a building with a radiator 30 feet above the fill, would require 15 psi. We want a positive 5 psi at the highest point in the system. this assures any air vents up top have positive pressure to operate and seal tightly.

Generally a two story building would use a 12 psi fill. Most expansion tanks and fill valves come preset at 12 psi. Check to confirm that, however.

If the building is a single story most installers still use 12 psi fill, no harm in having additional pressure, squeezes the air bubbles smaller

Determine the fill pressure for the system, adjust the air pre-charge and pump in the fluid, no need to isolate the expansion tank, most systems do not have expansion tank isolation, and if you add one be sure to provide a means to lock it open, or remove the handle to prevent isolating the tank from the system as it operates.

Your small fill pump will only add that 20 psi when all valves and vents are turned off. Generally when you purge you have a purge cock or vent open to dispel air. Once all the loops or zones run clear water, turn off the fill pump and valve, allow the AutoFill valve to pressurize the system to operating pressure conditions.