Compression Tank Questions - What do I have?

Al Roethlisberger

First, let me say that I finally received Dan's excellent books today, "How Come?" and "Pumping Away"(along with a few others) and I read them cover to cover this afternoon. This answered several questions, but also raised a few new ones as I plan for my tune-up of my old overhead gravity system following its conversion to a sealed circulator system around 1991.


So, my first question is: What type of compression tank do I have, a diaphram or plain steel tank?

I believe my old tank is a plain steel tank, but am not 100% certain.

I have attached two photos that show the three bungs on the tank for fitting connections, and although I believe this is a simple steel(non-diaphram) tank, I wanted to confirm.

The tank is cylindrical, and has a convex cap on one end as seen in the attached photos, and a concave end on the other end near the chimney. You can see the fittings as connected.

Can you help me confirm which type of tank this is?

Also, based on my calculations, this tank is approximately 22.72 gallons. According to Dan's book "How Come?", and my calculations for EDR for the system, I need about 32 gallons for a steel tank or 17.6 gallons for a diaphram tank.

If this is just a plain steel tank, it is obviously a bit undersized by almost 10 gallons, but is the difference significant enough to warrant replacing it?

If a diaphram tank, it is obviously oversized(is that bad?), and also warrants a host of other questions such as how to fill this tank to balance the diaphram as currently there is no air valve anywhere(maybe goes where that blocked off bung is?).

Finally, a two part question: Being an overhead gravity system, I have an automatic air vent at the very top of the system, and no "air separator" of any kind anywhere else. So, I cannot use something like an Airtrol tank fitting to automatically keep the tank from becoming water logged.

So:

1) After draining the system, how do I fill this tank correctly if a plain steel tank, and how do I check if it is filled appropriately and not water logged?

2) Do I need an actual "air seperator" if I already have an automatic air-vent at the top of the system? I would think not, but want to check. And by the way, my boiler does not have an outlet from a built in air seperator or baffle.

Thanks!
Al

Mike T., Swampeast MO

Certainly a "plain" expansion tank, but it's connected improperly!

The system connection should come in at the bottom with the drain connection at the end (where the system connection is now).

If my personal experience is anything, the galvanized piping should be removed and replaced with black steel/iron or copper.

Al Roethlisberger

Thanks, a few related questions


Hi Mike,

Thanks so much for the clarification for the type of tank.

So, still, three questions:

1) How do I properly drain and refill this tank to ensure and verify the proper 2/3-1/3 water/air ratio?

2) I assume I do not need an air-seperator installed before the compression tank since I have an automatic air-vent at the top of the system in the attic?

3) Is this mis-sized enough by approx 10 gallons to require a new tank?

Thanks again!
Al

brucewo1b

Al

Usually just draining the tank and refilling with the drain valve closed will get the proper ratio, and as long as the pressure in the system is staying relitively stable it will be ok but if the pressure increases too much when runnng in the dead of winter then you will need a bigger volume tank.

Mike T., Swampeast MO

What's the length and diameter of that tank?

Al Roethlisberger

Length and diameter is 47.5\" Length x 12\" Diameter


Mike,

The length and diameter is 47.5" Length x 12" Diameter.

The 47.5" is approximate due to welds, and the concave/convex ends. But measuring from edge to edge lengthwise, ignoring the concave/convex ends, it comes to approximately 47.5"

Thanks
Al

Mike T., Swampeast MO

[Probably] a 24-gallon tank.

If undersized you'll notice a considerable rise in pressure when the system is quite hot. 4# or 5# rise in pressure really isn't a problem.

I've never seen (let alone worked on) an overhead system in my life. I can only speculate that air will naturally rise to the top--thus the need for an automatic air bleeder at the top once the open tank in the attic was removed. Likely no real need for an air separator.

An Air-Trol fitting is another matter entirely. Its purpose is to prevent gravity circulation from the system into the tank thus preventing air-laden water from the tank making its way into the system where the air can come out of solution. The circulator can't move water in and out of the tank, but gravity can...

An Air-Trol fitting MUST however be installed at the bottom connection! If installed at the end (current system connection) it would do nothing!

Al Roethlisberger

OK to slightly oversize a tank?


Thanks Mike.

So here's a question: Since one rarely can buy the exact size tank that the calculations generate for a given system, is it better to oversize or slightly oversize the tank(s)? And by what % under/over is usually not considered a big deal?

Thanks
Al

Uni R_2

The only thing...

The only thing an over-sized expansion tank should hurt is your wallet.