expansion tank logging

vinny50

I have a job where the dfacilities people are blaming logging of conventional expansion tanks are shutting off the boilers in the building.  We did a job where we tied into the hot water system which was existing and refilled the system. The tanks have been drained twice by facilities people, amd still we are logging water but not poppuing relief valves.   I think the filll is over feeding.  Keep in mind these are not bladder tanks but steel tanks.

Mark Eatherton

Automatic air vents anywhere?

If there are any automatic air vents in the system, they WILL allow the air out of the tank, and will result in a water logged compression tank.



If there are A.V.s within the system, best bet would be to replace the existing non bladder tanks with bladder tanks. That or eliminate ALL air vents.



A continually leaking pressure reducing valve will cause the relief valves to pop off, unless their setting is above the highest potable water pressure available.



The loss of the PONPC could lead to poor pump performance, but shouldn't bring the system to a screaching halt.



ME

icesailor

Waterlogged Expansion Tanks:

If it isn't blowing the relief valves on the boilers, it isn't overfilling by the PRVs'.

The expansion tanks, do they have air relief bleed plugs on the side so when you drain the water from the tank, you can break the vacuum in the tank? If you do, I find that I must put Teflon tape and paste on the plugs. If they are leaking, they leak the air out of the tank.

The other thing I would consider is tank is leaking air. Use an air compressor and do an air test on the tank. Drain the water and isolate it from the heat system. If the pressure starts to rise in the system, let some water out through the boiler bottom drain. Blow the water out of the tank and pump it with air to 50#+. See if it looses air in an hour. If it is tight, it is leaking through the expansion tank fill/drain valve. Sometimes, I have had to just replace that valve. Some have ground joint plug screws and you need to use some kind of lubricant on the ground joint. If someone in the past used a compound that gets hard, you may not be able to get it air tight and the air will leak out through the tube that goes to the top of the tank. You may be able to use some gas leak detector fluid to see if it bubbles.

There's one style that looks like a 1/2" brass plug stuck in the bottom of the tank. Those are the worst offender.

The maintenance guys draining the tanks tell me that this is probably the problem. If they don't know how the tank drain works, they won't drain it right.

Mike Kusiak_2

Are they really draining the tanks?

Many maintenance people don't realize that they must not only drain the water from the tank, but also let air in to break the vacuum. If they only connect a hose and drain water until it stops flowing, the tank may still still contain a lot of water and no additional air.



So in the end they have really accomplished nothing, and the tank is still waterlogged.

icesailor

Expansion Tank Waterlogging:

ME,

I don't understand how auto vents somewhere in a heating system can remove the air from a hydro-pneumatic tank. I understand about air absorption over time but I can not understand how the air will be removed by auto vents. I can see and do see auto vents adding air to systems but not to the tanks. I have seen far more in older tanks where auto drain fittings were used and the tanks were draining air. The culprit was either the tank was starting to leak air at the top or the drain plug for the vacuum breaking, air entrance tube.

I always blow out the water with my portable air compressor and pre-charge the tank. If I just drain it and fill it, I don't get a lot of air in the tank when it is compressed. Then, check it with gas leak detector fluid.

The plugs that are tapered like a needle jet on a carburetor are usually wrecked from over tightening. They then leak.

Just My Experience:

Mark Eatherton

AV's and non captive expansion tanks

Chris, Where ever air has a tendency to accumulate (high parts of the system or poor piping practices) an unknowing technician will employ a device to get rid of air binding, that being an Auto Vent. You understand air going back into suspension. If it is coming from the expansion tank, and is being expelled by the A.V.'s, then the cushion goes away, pressure increases, and relief valves start popping.



In a well designed system, any air that gets out of the tank is recaptured and redirected back to the tank to maintain the compressible air. In a poorly designed system, the air gets out, causes air binding and trapping, and is problematic.



The basic rule is, if you have a non captive expansion tank, you can;t have any automatic air elimination devices anywhere in teh system.



If you have a diaphragmatic or bladder type of expansion tank, you MUST have a means of eliminating the air out of the system, Plan B is preferable in my professional opinion. The problem comes from unqualified technicians throwing solutions at the "problem" out of their van's stock.



If the cushion tank had leaks at any of the tanks tappings, once the air has been expelled, then a water leak should follow... Unless it is a hygroscopic air vent.



As for precharging the non captive tank with compressed air, I would think this may have a tendency to introduce air into the system when system pressure drops. The normal method is to COMPLETELY drain the tank and allow atmospheric pressure equalization. This way, when the system cools down, there is a good reserve of water between the contained air, and the fluid tapping connection on the bottom of the tank.



But if that works for you, then so be it.



ME

icesailor

Logging:

I understand that, But this system as far as I can tell, has been around a long time. This may or may not be a recent problem. I find what I describe on old tanks. I have a very slow leak in a tire valve on my John Deer tractor. I pump it up to 32# and in two months, it will be at 24#. It is a very slow leak.

And the original poster and others have mentioned that the maintenance personnel may not be draining the tank properly and letting air in. I've seen that before. 

Mark Eatherton

Your tractor tires...

and even the bladder on a diaphragm tank, WILL lose air pressure right through the walls of the tire. It is the propensity of pressure to seek balance, and Mother Natures despise of differentials (air pressure, heat, humidity, oxygen content etc).



The strange thing on OUR molecular thought levels is, that if air can make it in and out of a given material (rubber plastic etc) why can't water do the same thing? It has to do with the size of the molecule of water versus the molecules of O2.



One can make it through the sieve, in either direction (air), while the other gets stuck (water).



If the gap is large enough (needle and seat affair that you reference) if it lets air out, it could be large enough to allow water out too. Or not...



As for the maintenance guys NOT doing a proper job of servicing the expansion tank, that is an ABSOLUTE real possibility, but I can't understand why it would cause the heating system to cease working as was proffered by the original poster...



Maybe I misunderstood :-)



ME

Gordy

B&G atf airtrol

Tank fitting is a must on the old style tanks. They insure proper water level in the tank, and prevent gravity migration of the cold air saturated water in the tank going out in the system.  With out it hot water from the system migrates up into the tank soaking up the air in the tank as it cools. While the cooler oxygen saturated water in the tank migrates down into the system with in the same pipe.



Gordy

icesailor

Expansion Tanks:

Those are the ones I'm talking about. See that little brass plug in the bottom of the valve? Unless you get it really sealed, it will leak the air out of the tank. I wrap the plug with a few wraps of Teflon tape and Rectorseal #5 and screw it in. Not excessively tight. I promise you that the maintenance personnel do not do this. It leaks. You are being blamed for something that isn't your fault.

If you want to prove it, if you have a portable air compressor, make up a rig like I have so you can connect a "rig" that you can connect to your air compressor and connect to the tank with a double hose connection/washing machine hose. That tank must have a boiler drain on the other end of the tank to put a hose on. Unless you remove the plug, you will not drain the tank. But, if you put a hose on the tank and let whatever water comes out, then hook the compressor to it and blow some air into the tank. Then hook up the hose and drain. Do this for a couple of cycles and then leave the tank precharged with 30# or more of pressure. Check that plug with some gas leak detector and see if it makes bubbles.

I have a gauge on my rig. I leave the empty tank with a 10# precharge.

It works for me.

Ron Jr._3

Not sure of the physics

but here's the instructions for B+G's Airtrol System ...........



http://www.bellgossett.com/literature/files/599.pdf



 It doesn't specifically state to remove air eliminating devices on an Airtrol system  ...........



But there is not one air can in any diagram in that .pdf file . Plenty of airscoops with the venting port going directly to the tank though . 



And quoting page 2 , part 8 ......



" However , if there are leaks in the system through which air may be lost , the compression tank can lose it's air and become water logged " .



I'd assume any air elimination device can be considered a leak in this scenerio . I'd also assume this statement hold true for any compression tank system ( even ones without Airtrol fittings ) .