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!