Setting the pressure tank PSI

I have a situation dealing with an RV pump providing water pressure in an older Cabin, which was retrofitted with all 1/2" PVC. The pump has an internal non-adjustable pressure switch. Cut in is at 30 PSI, cut out at 45 PSI, and the flow tops out at 5 GPM.

The pressure tank they want to add is small, I would say no more than 2 to 3 gallons. It's painted red with no spec label so I would say the tank is from China or Taiwan. It has an air fitting on top for adjustment or filling.

What should the charge on the tank be at for best performance at these pressures and flows? Can you tell me the math to use in figuring it out if I ever have to do this again?

Seems like a lot of the cabins in the area have these pumps and someone on the net sold a bunch of those tanks to folks around there with the pitch that they would keep the pumps from cycling so much, and being so small, were better matched to those RV pumps.

Thanks
Tenagles

Ten Eagles it almost sounds like the tank you are talking about is an thermal expansion tank.

Does it look like this...



In a slightly redder color of course.

Yes and no. That looks almost upside down, and doesnt have the flange and bolts on the main line that I assume are for allowing the bladder to be removed. The little nob on the opposite end you have is too short. This has a threaded valve like a bicycle tire on it, which I assume is for letting air in and out of the charged area outside of the bladder. I was also told that the seller had them for attaching to a household jet pump from China, you could buy the pump, pressure switch and tank as a unit, or the tank alone. Seems over there they dont use big pressure tanks for several reasons...

TE

Okay, I found the specs and photo of it on a China site @

http://www.made-in-china.com/showroom/tzyd0576/product-detailUobxeVRjamIp/China-Pressure-Tank-YD-24L-WS-.html

It says "Into pressure is 2.5bar, and highest pressure of 5bar." At elevation of 5600 feet that means "Into" is 33.907 PSI and "highest" is 70.167 PSI.

The only thing is I am not sure what they mean by "Into" pressure. I assume "Highest" is the most it can be pressurized safely before bursting. So I would also assume "Into" is where the tank starts taking in water once the pump has the lines pressurized to that level. Since the pump is set to start at 30 PSI it would then certainly run up the pressure to fill it from 33 to 45 PSI before the pressure switch shut it off, and then the pressure would stop discharging water from the tank at 33 on the way down, and then the pump would start running again as the lines got down to 30 PSI.

If this is the case, wouldnt I want to lower the precharge so that the "into" was lower than where the pump turned on at, so that there would be more drawdown capacity?

Calm down now Ten Eagles! LOL
I got to see it! I'm not all that well versed on wells and pumps... But I'm sure one of the guys who is will be along shortly. I've never seen a system that small, It must not be too fun with the limited flow and all. Let them advise you on the tank sizing.

The tank precharge should be 2 PSI. less than the pump cut in pressure.

I'm just concerned as to whether or, not these are rated for potable water. I'm a real sceptic on buying E*B*A*Y plumbing parts. Add in the chinese factor they're probably got lead paint.

1 Bar = 14.5 psi.

2.5 Bar = 36.25 psi is the factory precharge pressure.

5 Bar = 72.5 psi is the maximum safe operating pressure.

The precharge pressure should be reduced to 2 psi less than the START pressure of the pressure switch, or 28 psi if the pressure switch turn on at 30 psi.

The tank at the link is identified as 24 liters, which is a bit more than 6 gallons and will have a drawdown of about 1.5 gallons for your pressure range.

There is no good reason to use such a small tank with a 5 GPM pump. I would not buy one. There are lots of other tanks out there that are at least 20 gallons, and I would pick one of those. There is a substantial risk that the red tank will fail in one season, if it works at all.

tank

Unless the tank has a galvanized or glass lining it is going to rust in a potable water situation. A black steel one, which this could be, is only for closed system heating use.