Expansion Tanks

PN

I understand how expansion tanks work on small domestic boilers that have an air charge in them. Not sure about the ones you see in commercial systems that are just big tanks hanging off the ceiling. Don't see anyplace to check or pressurize with air. How do they work? Do they have to be drained at some point?

Empire_2

Tank

There are many articles to help you out, and it works on the same principal as smaller tanks, but water volume in a major consideration as to how large the tank should be.

Uni R

Canuck

Large tanks....

work on the same principle as small tanks - they allow a pneumatic cushion for expanding hydraulics caused by heat. Sometimes they are pre-charged (i.e.- someone goes to the bother of putting some air pressure in the tank before the system is flooded). Many times the tank is simply empty, without a pre-charge, when the system is filled. Due to the fact that the air inside the tank is trapped, it is pressurized by the air and/or water that enters into it, as the system fills. Although the following is not precise, it is a good, general, rule-of-thumb: For every 15 psi on the system, the tank will lose half of its volume to water; (i.e - at 15 psi, the tank will be half full of water, at 30 psi, the tank will be 3/4 full of water [the half that it was at - at 15 psi plus another half of the empty half at 30 psi]. It follows then, that at 45 psi, the tank will be 7/8 full. Certainly the benefit of pre-charge can be seen at systems with pressure higher than 30 psi. It also follows that if you have a system at 15 psi and the tank is 15/16 full - you're waterlogged and need to drain out the tank completely - and refill.

Mark Eatherton1

Articles...

from www.contractormag.com under my name.


EATHERTON ON HYDRONICS


BY MARK EATHERTON , Hydronic Heating Contractor

Why expansion tanks need to be understood

Perhaps one of the least understood functional areas of a closed-loop hydronic heating system is the expansion tank. This one item receives the least amount of attention of all the components in the heating system.
Much of this inattention is due to a general lack of knowledge. Here’s some information to help you to better understand the operation of expansion tanks.

Water, as we all know, expands when it is heated. Conversely, it contracts when cooled. If we don’t have some means to compensate for all this expansion and contraction, the fluid pressure in our systems will fluctuate wildly.

The first expansion tanks on the old gravity systems were nothing more than copper-lined, open-top tanks located above the highest part of the heating system. As the water expanded, it filled the tank in the attic with the expanding fluid. If the tank overflowed because of excessive fill, the excess water spilled into the gutter of the attic. No big deal.

When the water cooled down, the tank volume decreased but still maintained a full heating system. Filling of the system was done via manual fill, and a gauge in the mechanical room indicated how many “feet” of water were above the boiler. A mark on the gauge usually indicated cold fill level.

As time went on, and systems evolved, a need arose for higher operating temperatures. In order to achieve this, a “closed” pressurized system had to be created. Hence, the open expansion tank went the way of the dinosaurs.

People knew early on that they must have an air charge inside the tank because water is incompressible. The early tanks were basically a cylindrical steel tank, just like the vessel in a gas-fired water heater. They were connected to the boiler because this is where it was thought that the air that was in suspension in the water had the greatest potential of coming out of solution.

The air was recovered from the water and sent to the expansion tank to maintain the “cushion” that could be compressed when the fluid volume of the system expanded.

This type of system is called an “air in” system. Other than initial purging, all the air that was in the system will remain in the system and is supposed to be recovered at the boiler and sent back into the tank. Many fittings were developed to ensure that this was what happened, and that the air didn’t escape from the tank when the system cooled down.

This type of system should never have an auto vent installed anywhere on the system. If it does have one, remove it. It’s letting the air that’s supposed to be recovered and sent back to the cushion escape. Consequently, the expansion tank becomes water logged, and then the relief valve starts popping off.

Conversely, a system with a captive air bladder type of expansion tank is considered an “air out” system and must have an air elimination device installed. This device should be installed at a place where the velocity of the water is the lowest and the potential for air coming out of suspension is the greatest.

This “air out” method is the “new” type of system. Its operation is much quieter than the old “air in” systems and the potential for ongoing internal corrosion of ferrous components is held to a minimum. Remember, rust never sleeps.

See you next month for the second installment of Expansion Tanks 101. Until then, keep your caps dry and Happy Hydronicing!

Mark Eatherton is a Denver-based hydronics contractor. He can be reached via e-mail at guruofbtus@ mindspring.com or by phone at 303/778-7772.




Expansion Tanks 101: pressure and air vents

If you’re considering converting an old, non-captive-type expansion tank to a captive-air expansion tank, an easy rule of thumb is to replace the existing tank with the model that is equal to the existing tank’s volume. For example, if a 15-gal. tank is hanging from the ceiling, replace it with a No. 15 bladder-type tank.
At the time that you do this, you must install an auto vent to get rid of the system air. To recap, on old non-captive compression tanks, automatic air vents are not allowed. If it’s a captive-bladder expansion tank, auto vents are a must. The new tanks come with a factory pre-charge of 12 psi. Why, you ask? Because the manufacturer assumes that in a residential setting there will never be more than two floor levels above the boiler. In order to “lift” the water to a height high enough to completely fill the system, it requires 0.5 psi of pressure to raise water 1 ft. of vertical height. Hence, the water in a two-story house would have to be pressurized to 10 psi in order to fill the system.

You typically want some residual pressure in order to “burp” any bubbles out of the top of the system. Hence, 20 ft. times 0.5 psi plus 2 lb. residual pressure equals 12 psi. That’s also the reason the factory ships the pressure regulators preset to 12 psi.

What if the system height is say, 30 ft. high? With 0 psi on the water side of the tank, the tank’s air charge should be pressurized to the system’s anticipated fill pressure. In this case, 30 ft. times 0.5 = 15 psi, plus 2 lb. residual = 17 psi. Then, you must also adjust the pressure regulator to compensate for the additional pressure necessary to lift the column of water to the top of the system. What happens if you leave the pressure of the tank at 12 psi but raise the fill pressure to 17 psi? You begin using the acceptance capacity of the tank up front, which means you’ll have that much less on the other end when the water gets hot and expands. You will see an increase in pressure once the system gets hot. It may not be enough to blow the relief valve, but it will increase.

If you do it right, increases in system pressure are held to a minimum. In some cases, overfill can be used to your advantage, but you want to make sure that you don’t overfill or you’ll ruin the expansion tank and the relief valve.

If you are dealing with a hydronic heating system that has non-oxygen barrier tube, you must use an expansion tank designed for potable water. This tank will have a phenolic lining on all metal parts to keep the oxygen that’s in suspension from eating the tank’s shell. Also, don’t forget to adjust the tank’s air charge downward. Potable water tanks typically come from the factory with a 45-psi pre-charge.

Although the tank manufacturers have told some contractors that it doesn’t matter what direction you install the tank, common sense should tell you that it does matter. Every drawing I’ve ever seen has shown the tank nipple on the top of the expansion tank. This keeps air away from where the water is supposed to be.

In a retrofit, where the old ceiling-hung expansion tank’s pipe goes up, install a tee in a normal configuration, bush down the end run and install an auto vent. Then on the branch, install a nipple long enough to give you clearance for the diameter of the tank and install an elbow facing down. Hang the tank right there, and don’t forget to properly support it.

If you’re using a floor-mounted tank, the piping will be similar. Install an auto vent at the top of the down comer to ensure no air gets to where there is only supposed to be water.

Isolation valves are cheap when it comes to expansion tanks. A tank is one of those items that is guaranteed to fail at some time. Why not make it easy on the next person who has to change it out? Besides, you might just be that next person.

Technically speaking, with captive-air bladder-type expansion tanks, over sizing is not a major concern. The cost differential between a No. 15 and a No. 30 is so negligible it’s not worth considering. Go with the bigger tank. If the tank is oversized, it allows you to slightly over fill the system. I can’t remember when I last used a No. 15 tank. If memory serves me correctly, it was back in the heyday of solar.

If we follow the instructions that come with certain fill valves and we shut off the make-up water, the oxygen comes out of suspension and is expelled from the system. Then the system volume and pressure are going to decrease, and you’ll be back there re-pressurizing the system. If you’ve calculated a system fill pressure of 12 psi and you manually fill the system to 20 psi, as the oxygen comes out of suspension the diaphragm will send a bit of water back into the system to compensate.

I know for a fact that the statement about shutting off the water to the heating system comes from the old days of “air in” systems and no low-water fuel cutoff.

With today’s “air out” systems, shutting off the make-up valve is a guaranteed way to get a callback. I guess that if you build that into your installation price it’s ok. It gives you the opportunity to see the boiler room again to see how things are going. I personally don’t have any spare time, so I leave the make-up valve on. I also install a low-water fuel cutoff device so I can sleep at night.

EATHERTON ON HYDRONICS


BY MARK EATHERTON , Hydronic Heating Contractor

Expansion Tanks 101: the facts and myths

On some commercial jobs, and extremely large residential jobs, you may run into a computerized system called Expan-Flex. This is a microprocessor-controlled expansion tank that changes the system fill pressures and the diaphragm charge pressures to compensate for expansion and contraction. Although it works quite well, it has too many moving parts for my taste. Air compressor, solenoid valves, computer, etc.
I’ve also seen non-captive tanks that were charged with nitrogen. This particular arrangement was on a high-rise system that provided heating and cooling from the same loop. The expansion tank had to compensate not only for big expansion factors, but also big contraction factors. The tank of nitrogen has to be replaced on occasion.

Here are some other often-overlooked facts and myths about bladder-type expansion tanks. Contrary to popular belief, the bladders do get stuck and don’t allow for proper operation. Rare, but nonetheless it happens.

The factory always sets the air pressure to exactly 12 psi. Wrong. The factory sometimes over- or under-shoots its target. Always check the air pressure prior to installation with a tire gauge and adjust if necessary.

Do you know what A.S.M.E. stands for? A Substantial Monetary Exchange.

You can check the air pressure of the diaphragm with the tank on line and normal fill pressure. Wrong. The water-side pressure must be at 0 psi in order to get an accurate read of the air pressure on the diaphragm.

Tanks never lose their air charge. Wrong. Tanks do lose their charge, either through the Schrader valve, external leak in the tank or through the diaphragm.

Captive air tanks never get waterlogged. Wrong. If you’ve either lost the air charge or the diaphragm, the tank will be waterlogged. Be careful. Treat every expansion tank as if it were a loaded expansion tank. I’ve smashed more knuckles than I can tell you from unanticipated weight gains of supposedly empty expansion tanks. (Ouch!)

Expansion tanks can’t have their air charge raised above a certain point. True. If the tank is only rated for 30 psi, it shouldn’t be pressurized to more than 30 psi. If you need more than 30 psi, then you need an A.S.M.E.-rated tank. Do you know what A.S.M.E. stands for? A Substantial Monetary Exchange.

Expansion tanks are always located at the bottom of the system. Wrong. The tank connection should be located as close to the pumping station as possible. In the case of some commercial systems, however, the tank may be located on the top floor of the system to avoid static pressure.

What does this do to the head-generating capacity of the pump? If the expansion tank connection is half way between the suction and discharge ports of a pump, then half the pressure will be in the form of positive pressure at the discharge of the pump and half will be in the form of negative pressure at the suction port of the pump.

If the expansion tank system connection is located at the suction side of the pump, even though the tank is located 12 stories above, then 100% of the pump’s differential pressure will be in the form of positive pressure as sensed at the discharge of the pump.

There’s enough to talk about on this subject to do a whole other story. That will come in a future article.

Anyway, that’s what I know about expansion tanks.

Oh, yeah, one last detail. If you come across an old non-captive expansion tank that is waterlogged, and you decide to “service” the tank by draining it, drain it completely and allow the air pressure on the inside to equal the air pressure on the outside or you haven’t fully completed your job.

Also, carry some self-tapping saddle valves on your service truck. When you connect the drain hose to the expansion tank and drain the tank into the floor drain, eventually you’ll get to the “glug” stage where the tank is trying to suck air back up the same hose from which the water is draining. Simply connect the saddle valve to the tank riser between the tank isolation valve and the expansion tank and open it to allow air in to relieve the vacuum. The tank will be empty in no time flat and you won’t be standing there at $100+ dollars per hour listening to the glug.

Congratulations! You’ve completed Expansion Tanks 101. Now, take the knowledge you’ve learned and go out and use it.

See you next month. Until then, Happy Hydronicing!

Sweet_2

Dude

How much do I owe you?

tls_9

Mark...

Great Essay!!!
Lots of good stuff that too few people ever think of.

ever think of teaching?

tom

Mark Eatherton1

No Charge...

I've already been paid for the articles:-)

Hi Tommy, I don't have time to teach, too busy educating people here at The Wall:-)

Good to see you around again.

ME

dan_21

http://fhaspapp.ittind.com/homeowners/CompressionTank.htm


http://www.bellgossett.com/Press/compress.htm

dan_21

more info

the following sites contain additional info

http://fhaspapp.ittind.com/homeowners/CompressionTank.htm

http://www.bellgossett.com/Press/compress.htm