Adjusting AVC in pneumatic tank

[tkeoki]

1) I understand the general principle of how the AVC works on a tank in combination with the check valve, snifter valve and bleeder valve; but how exactly does the AVC work when you vary the cut-in, cut-out pressure?

I've seen some AVC's online that are not adjustable, and some that say they are adjustable from 0 - 40 psi? How would each type affect the drawdown available in the tank?

2) Also, if the pneumatic tank is piped liked a bladder tank with the same opening being used for inlet and outlet, would the tank get waterlogged even if the check valve, snifter, and bleeder valve were all working? Wouldn't the air just bypass the tank if there was water being used, like a hose running, etc?

 

[LLigetfa]

In theory, a HP tank with working AVC will never get close to empty over the normal cycle like a captive air tank will so there is no "precharge" setting. Some HP tanks AVCs do have a setting that can vary the amount of air and more air means more drawdown. It is possible to have too much air that causes the air to burp forward into the house plumbing which can be a bad thing. The air/water blend can stir up sediment, knock a glass out of your hand, boil media out of a backwashing filter etc.

A HP tank should never be piped with a tank Tee. It needs to have separate inlet and outlet.

 

[Sponsor]

 

[tkeoki]

My current HP tank (Old and rusty--I'm in the process of replacing it) is piped with a tank tee, there's no AVC on the tank, but at the well head I have a check valve, snifter valve, and a bleeder orifice down the well (I'm not sure how far down). I don't know if all three of those components are working. I do need to manually add air from time to time.

I'd like to change to a non-HP tank, but the cheapest quote I've gotten for pulling the pump pipe the few feet needed to get rid of the bleeder orifice is about $500 (pump is on 350' 1-1/4' galvanized). About a year ago, I purchased the replacement tank (120 gallon--same size as old one) when it was on sale for $250. And I'm now getting the last few pieces of information I need before replacing it.

Can I change to a bladder tank without removing the bleeder orifice? Just remove the check valve and snifter? What's a typical bleeder orifice pressure rated to?

 

[Valveman]

If you remove the check valve the bleeder should not open. If it does it is just worn out and then you will have to pull the pump up and put a plug in place of the bleeder. It works most of the time without having to plug the bleeder, but you won't know until you try it.

 

[tkeoki]

OK, thanks,

If I'm thinking it through right, the bleeder shouldn't see any higher pressure without the check valve, it just sees that pressure for much longer periods of time. Is that right?

Hmm, so how can I utilize a spare unused 120 gallon HP tank?

 

[Reach4]

If I'm thinking it through right, the bleeder shouldn't see any higher pressure without the check valve, it just sees that pressure for much longer periods of time. Is that right?

Here is my understanding.

Because water is nearly incompressible, it does not take much of a leak to make the water pressure drop behind the check valve. That leak would have to be just a little more than what the check valve leaks. I expect that the bleeder orfices are set to leak a tiny amount when the water pressure is more than maybe 5 or 10 PSI, but to open more when the pressure drops below the threshold. With the checkvalve+snifter removed, the pressure tank would have to supply that just that tiny amount of water continually to keep the higher pressure present at the bleeder orifice. I don't know what the rate of that tiny leak per day would be. Could it be a gallon per day maybe? Just a wild guess.

Here is an example orifice in brass: http://www.aquascience.net/fittings/index.cfm?id=1311 The description says "As a pump turns on and builds water pressure, the orifice is forced closed with a ball. When the pump shuts off, pressure drops and the orifice is opened, allowing water above the orifice to drain out." They also come in rubber, but the description on the brass one was better in explaining.

 

[Valveman]

Bleeders are not suppose to leak any until the pressure is lowered and lets the brass ball drop away from the little hole. The weight of the brass ball helps if fall before the pressure gets to zero. It takes a few pounds to hold the weight of that brass ball against the hole. If the bleeder leaks enough to make the pump come back on to often, the bleeder is just worn out and needs to be removed.

When I was a kid my father and grandfather always had a pile of old pressure tanks. My grandfather would take a torch and split a tank vertically. Then he would lay each half on its side and attach four short legs to hold it a couple of feet off the ground. I spent many a cold morning pushing cows away and pouring cow feed in those troughs with a bucket.