Simple questions on tanks`

[Eric Johnson]

I'm planning a couple of years ahead when I relocate into a place where I'm going to put in hot water storage. A guy near there sells all kinds of used propane tanks. The ones I'm interested are 330 gallon tanks. They're 30 inches in diameter, which unlike a 500 gallon tank, will fit through the cellar door. So, I'm thinking 3 330s ought to do the trick. If I have 10 feet of headroom (might), I'll stand them up.

In any event, will I need to have plumbing fittings welded onto the tank or is the existing tapping sufficient? I'm guessing you'd want outlets near the top and near the bottom. These tanks are supposedly "aired out." I assume that means they don't have any gas residue. In that case, would it be OK to weld them? Also, are there locations on a tank where you wouldn't want to weld in a fitting because it would weaken the tank?

That's three questions. The final one is: I know there are different ways to connect the tanks, but I'm guessing it would be: boiler supply to the bottom of the first tank, with the draw from the top of the first tank going into the bottom of the second tank, etc. Does that make sense? How big should the connecting piping be?

 

[jebatty]

will I need to have plumbing fittings welded onto the tank or is the existing tapping sufficient? I'm guessing you'd want outlets near the top and near the bottom

Think hydronics. What volume of water do you need to move, and what is the pipe/fittings size that is required to do that? Next, what will happen to the hot supply water and where will it go in the tank, and where will the cool return water come from? Ideally your fittings should at least be the same size as the required piping, and supply normally is at the top and return from the bottom, as this aids stratification. Depending on your system design, you may have more than one supply point and more than one return point. At least one bottom fitting should be as close to the bottom as possible, as it will be very difficult to heat the water below that point due to stratification. If the tanks are installed horizontal, it makes a lot of sense to have hot supply on one end and cool return on the opposite end.

I greatly prefer the boiler/tank circuit to be independent of the tank/system circuit. Let the boiler's only job be to charge the tank. Then the tank supplies the system. I think that eliminates possible problems in system performance and increases system design flexibility.

These tanks are supposedly "aired out." I assume that means they don't have any gas residue. In that case, would it be OK to weld them?

My tank which I flushed with hot water before using it in my system had lots of crud that came out in the water. So I then also installed a hot water filter to catch remaining crud when the tank was place in operation. And lots more crud was caught. After about a year of use the tank stopped shedding crud. Even if aired out, the tank likely is dirty and good cleaning is in order before using. The cleaning should take care of any flamability issues. I doubt a good weld will weaken the tank.

know there are different ways to connect the tanks, but I'm guessing it would be: boiler supply to the bottom of the first tank, with the draw from the top of the first tank going into the bottom of the second tank, etc. Does that make sense? How big should the connecting piping be?

Your "series" design can work depending on your system needs, also a "parallel" design can work. Design your system, determine its needs, and then decide which is best. Size of piping? What volume of water do you need to move, and what is the pipe/fittings size that is required to do that?

 

[mr.fixit]

If there is any possibility of any other heat sources added at a later date, be sure to put in additional fittings to begin with. Wouldn't add alot of cost and alot easier now than later.
I added a pellet boiler later which meant accessing the tank, draining it,drilling and welding ports in it, not to fun.
I put the pellet boilers supply fitting right on top of the tank and the return not quite midway down so it only has a couple hundred gallons to work with.

 

[Eric Johnson]

Tanks, guys. I like the water filter idea.

 

[maple1]

My tanks were open when I got them - basically means, the ports weren't plugged. It still smelled, but the smell wasn't flammable. I even tried to see if I could get some flame action from them as soon as I got them home. Nothing but disappointment there - lol. I hosed mine out with a pressure washer with a gutter wand on it - but I don't think I had to, nothing much came out that I could see. It might depend on how long the tanks have been out of service & sitting open. I installed a simple Wye strainer between the bottom port, and the loading unit cold in port. My tanks (stacked horizontal) are also not quite level - they have a slight slope away from the bottom port. So dirt should settle in the opposite end of the bottom tank and stay away from most of the water flow.

Your boiler supply should come from boiler to top of tanks. Your system supply should also come from top of the same tank that the boiler supply sees first. I used one tapping with a T on it for both, others use separate tappings for each. The separate fitting approach might be safer, or I should maybe say more tolerant of system design boo-boos or shortcomings. But I would also try to not do any more cutting & welding than is absolutely necessary - existing ports might suffice depending on the particular tank. I think my two 330's each had different existing port layouts. How you would plumb them depends on horizontal vs. vertical. I think anything but stacked horizontal I would plumb parrallel, reverse return.

Two more hints - use a pro welder, and make sure you don't end up with cast fittings to weld on by mistake.

 

[woodsmaster]

Tanks I got had been sitting open for a while. I got a mouse nest out of one of them, and one still had some mersapen in it. I filled them completely with water and a few gallons of bleach and got lots of crud and stuff out of them. I you fill them completely then there is no chance of anything flammable left in them.

 

[ewdudley]

In any event you need to plumb all three tanks in parallel, with one hot water manifold servicing the tops of the tanks, and one manifold for the bottoms.

I did three vertical tanks for my install in 2009. In the years since I think I understand the requirements better and can perhaps offer some insights.

In my current setup there is a manifold across the top and across the bottom of all three tanks, with tees connecting to ports welded and the top/bottom of each tank. Each manifold has two tees; one between the first and second tank and one between the second and third. So in reverse-return fashion the supply from the boiler enters the closest tee on the top manifold and returns from the furthest tee on the bottom manifold, and vice versa for the system supply and return lines.

This was crazy. It added six inches or so of plumbing to the top and bottom of the tanks, plus a shload of expensive unions and other fittings. And devising pedestals for the tanks that would allow access to the manifold and piping on the bottom was a full tilt pain in the butt.

Since then I've come across the idea of placing the ports on the sides of the tanks and then using internal riser and dip tubes that go off at an angle to points close to the tops and bottoms of the tanks, with diffusers if you're convinced that jetting is a major concern, which it is not. It should be straightforward to connect a bent pipe or pipe assembly with a 45 degree elbow that connects to a double tapped reducing bushing screwed into a forged steel half coupling welded to the side of the tank.

I there are not suitable ports on the sides of the tanks then it is a simple matter to attach forged steel half couplings. Shape the couplings to the curve of the tank with a grinder, then construct the manifolds. Lay the tanks down side-by-side and tack weld the couplings in place first without cutting the holes. Disassemble the manifolds, go to town on welding the couplings in place, pressure test each one with air and soap bubbles if needed, then cut through into the tanks with a Lennox or comparable quality (not Kobalt) hole saw.

The boiler supply should connect to one end of the top manifold and the boiler return should connect to the opposite end of the bottom manifold. The system supply and return can tee in close to the manifolds on the same ends of the manifolds as the boiler, or they can connect to the opposite ends.

If you're planning on using gravity flow for power-fail heat-dump purposes then 1.5 inch would normally be big enough, otherwise 1.25 inch, assuming 45kW or so. Reducing down to one inch for the internal riser/dip tubes would not be a problem since the flow is being split three ways.

As for safety, if you weld the couplings before cutting the holes that should help. And propane is not like oil. A tank that has been used to store oil can blow up after twenty years of venting. Nevertheless there have been a few people killed by 'safe' propane tanks. I filled my from the top with car exhaust and gave it a whirl. YOLO.

You would also need small ports and PEX piping for venting air, and maybe temperature probe ports, but external sensors taped to the tanks under the insulation will do the job quite well and a lot easier.

If you want to introduce solar heated water then maybe a manifold across the middle of the tanks, but fossil fuel boilers can connect in parallel with the wood boiler using the same manifold at the top. But for a fossil fuel boiler return you might want separate return manifold with risers that go up to a point a foot or more below the supply risers so the fossil fuel boiler only has to heat a limited portion of the storage tank as a short-cycle mitigation buffer.

 

[Tennman]

All manifolds and black iron piping on our storage system is 1 1/2" which keeps pump horsepower to a minimum. The only thing I'd do different would be to slightly raise and lower the demand supply/return ports. Below are the features of our tanks that I'd definitely do again.

- We welded long nipples, at least 6" long to the tank instead of threaded fittings. We do not have any threaded joints under our 3.5-4" of foam. All welds were tested at ~15-20 psi with soapy spray to catch any pin holes. Our company's best welder did my tanks and there were several small pin holes that needed to be repaired. Make sure the welder does leak checks.

- For our bottom port, a long nipple is welded tangent to the bottom of the dome. In other words, the bottom nipple is level with the floor so the bottom dome can be about 2-3" off the floor for the foam coating. So our bottom manifold (the boiler supply manifold) has tees on these two nipples for drain/fill hose bibs as well as the tee in the middle where the piping returns to the boiler.

- The boiler supply nipples come straight out the top of our tanks for the boiler supply manifold that's on the very top. I've read about people doing "dip" tubes" here so the hottest water coming from the boiler is injected deeper into the tank. I wish I had done more research on this because it makes sense to inject the hottest water not just into the top of the dome. BUT, I didn't and ours works great as is.... But maybe it could have been better. I'll never know now, but I have no complains. For my education opinions here are welcome.

- We welded (per my direction) the demand nipples slightly below the dome-to-barrel weld of the tank. I hindsight, I wish I had welded them slightly above that weld to get higher into the hotter stratified water. But, here again, I may be leaving some btus in the very top of the tank that don't make it to the house, but that very hottest water at the top definitely warms the water below close to where we pull it from to go to the house. Again, here this decision might not be the "optimal" but it works just fine!

- I would definitely use four angle legs welded to the sides again for easy access when spraying with foam and to see if any leaks develop for repair or access. I've seen rolled rings for bases, but I really like being able see/inspect and know if ever a leak developed we could do an in place weld repair.

When we added the BioWIN we added check valves to the top manifolds so both boilers can be run... not simultaneously, but one at a time. It was trivial to add the second boiler the way our tanks were assembled. Bottom line, it seems the difference in performance between a "perfect" arrangement and what we did must be quite small. Because ours is working great. Here's a pic. Hope it helps. I welcome any comments on how others would do differently. But I'm extremely pleased how this is working. I've read over the years how many storage systems are plumbed and it seems how this thermal "flywheel" is done is very forgiving. Best wishes.

 

[Bob Rohr]

I'm working on a two pipe tank conversion to a 500 gallon LP. Drawing not to scale.

Tank is pressurized, approximately 70 gallons of expansion space up top.
Solar drainback pumps from bottom, returns into air space to break siphon.
Wood boiler pumps from bottom into top, 3 way thermostatic valve at boiler for protection.
DHW via plate HX, accessing approx 25 gallons at very top (hot point) of tank.
Mod con boiler only heats the very upper portion, again about 25 gallons is heated by backup backup mod con.
Both radiant loads and radiators are taken off via ODR pump stations, maybe the Viega Mix Blocks or Taco 3 way motorized valve.

With a tall shop I can stand it up in a tire rim, maybe a corrugated culvert ring for base.

Here is the key to two pipe. Large header size! This section becomes your low loss header. Keep connections close to tank, close together. I capped the ends of the 3" tube and cut slots to evenly disperse the flow. The tube design and vertical tank should provide best stratification.

For dual tanks, same concept could be applied.