Bladder expansion tank press

triggerhappy24

 I keep getting different answers to this question from different people. 

On a heating system with a bladder expansion tank. How much air pressure do I put into the bladder? The system runs at 60lbs in the summer and 75lbs in the winter. 120 degrees in summer and 160 degrees in winter. 

pecmsg

triggerhappy24 said:

 I keep getting different answers to this question from different people. 

On a heating system with a bladder expansion tank. How much air pressure do I put into the bladder? The system runs at 60lbs in the summer and 75lbs in the winter. 120 degrees in summer and 160 degrees in winter. 

You have a system that runs 60 PSIG?

STEVEusaPA

Show me a picture of your gauge. I think you're reading the gauge wrong.
The correct pressure for space heating is the cold pressure your boiler needs to have, based on the height from your gauge to the highest point of space heating in your system, + about 5%.
For domestic water, the same pressure as your incoming domestic supply, after a pressure reducing valve, if any.
If you're running 60-70 psi on residential domestic heating, you have 2 major issues...1, your relief valve isn't working or it's the wrong one, and 2 double check your life insurance policy, make sure it's paid up.

JUGHNE

What is the MAWP of the boiler?
What is the pressure relief valve rated for?
This may not be the usual residential install.

triggerhappy24

Ok so maybe I should explain this better. This is a commercial heating system. We use high temperature hot water run threw a heat exchanger to heat the heating hot water. Yes, the system runs at 60 psig in the summer at 120 degrees and them 75psig in the winter at 180 degrees. One pump in the summer 2 in the winter. 

EdTheHeaterMan

You may be reading the pressure gauge incorrectly. If the 60 is actually "Feet" or "Foot H2O" or "Feet Head" the actual pressure is about 25 PSI. Still a little high but that may be a result of an overloaded expansion tank or an over pressure of the expansion tank bladder. The proper air pressure should be measured with the tank disconnected from the system. or at least the system pressure lowered (during the test) below the tank air pressure.

For example: remove water from the closed system until the pressure is around 5 PSI, then use a tire gauge to measure the air pressure in the tank. It should read say between 10 and 15 PSI (industry standard is 12 PSI) If you measure the tank air pressure with 20 or 25 PSI of water pressure connected to the tank. The air in the tank will be compressed by the water pressure and the air pressure will be the same as the water pressure.

(I'm trying to find an illustration I used in my hydronics class. Will add this when I find it)

The boiler cold pressure should be high enough to provide some pressure at the highest point of the system. Since the weight of 28 inches water will exert one pound of pressure on the bottom of that column of water, it is safe to say that 28 feet of water will exert 12 pounds of pressure on the bottom of that water column. Since most heating systems are in the basements of two story homes the industry has adopted this 12 psi standard. But you don't need to keep that in your home, if you have a 3 story home and the highest radiator is 30 feet above the boiler gauge in the basement, you will need to set the Cold Boiler Static Pressure at 16 or 18 PSI. in order to get the water to the top floor radiators.

Now if the boiler cold static pressure must be 18 PSI, you will need to adjust the air pressure charge to 18 PSI to get the full acceptance capacity of the diaphragm type tank. If you also have a pressure reducing valve (automatic) feeder that pressure should also be adjusted to 18 PSI.

I hope this helps

Yours truly,
Mr. Ed

triggerhappy24

If you are asking for the static pressure then yes. My static pressure is 20psig. The 60 psig I run is with the pump running. This is not on a boiler it is on a high temp hot water heat exchanger. We are running 375 degrees at 250lbs at our boiler in our plant.

This is also NOT a home. It is a commercial building heating system! 

Please stop telling me I'm not reading the Guage properly. If that was the case I would find another career. I may not be a genius, but it doesn't take a rocket scientist to read a Guage 

I was told by a manufacturer to set the bladder pressure to half of the working press of the pump. Not the static on the system. Yet by a another I was told not to exceed 12 psig on the bladder tank.

triggerhappy24

Just so we can end this 

STEVEusaPA

Calm down...don't be so 'triggerhappy'... @triggerhappy24
Now that we have all the background information, we can advise you better.
The 'reading the gauge properly' has come up many times because of this...

If you look at the blue dial and the gauge was pointed at the bottom blue number of 50 (feet of H2O), homeowners will say they are running at 50 psi (bottom blue numbers) instead of properly reading the pressure at a little over 20 psi (upper blue numbers).

Jamie Hall

I am not at all certain that a bladder tank is quite the correct solution here (and no, I don't think you reading the gauge wrong -- just that folks are so accustomed to residential systems it can be hard to believe!).

You have two quite different problems. One is to handle the thermal expansion of the water in your system. The other, of course, is what happens when you fire up the pump.

The former will be the determining factor in what tank you use. Honestly in this application I would not use a bladder tank. Other than the minor problem of one being available which can handle the working temperature, I'm not at all sure that you could find one which was big enough. My choice in this application would be a tank (obviously rated for the pressure) equipped with air and water release valves to maintain around half the tank air and half water, and a compressor to maintain the air pressure at the desired level. The tank should be large enough to provide ample room for expansion, so as to minimize the pressure change. This is not all that hard to calculate, though it would be well to have an registered engineer do it (not me; I'm retired). Old fashioned? Perhaps. A little more complicated and something else for you to maintain? Indeed. Reliable? If you take care of it, certainly.

I might observe that if your building is tall enough -- at least 50 feet -- you could accomplish much the same thing more easily with a simple elevated vented gravity tank to maintain your pressure. If equipped with an overflow to drain and a float valve, it would also double as your makeup water location.

The other problem us where do you connect the tank. If ever there was an argument for pumping away, you've just made it. The tank should be connected at a location where there is a minimum pressure change in the system with pump operation or, thinking about it another way, where you want to be able to maintain a stable pressure. That location is near the inlet (I'd say about 10 pipe diameters, not less, of straight pipe). What you are trying to do is to ensure that the pump has a minimum pressure at the inlet; you really don't care what happens at the outlet, so long as it works!

triggerhappy24

triggerhappy24

I'm sorry didn't mean to get testie about the gauge. At the same time I'm fixing another issue that is becoming more of a head ache than it should. 

System was engineered this way. I agree I hate bladder tanks and would rather another tank with a sight glass.....but this is what I got. And I have to make it work. This all started before I got this building another engineer messed it up really bad. He lost prints and most of the specs for all the equipment here. He didn't add make up to the system for idk how long and let the system run. It worked, but not well. Pressure would just on the gage from 20 to 90lbs from air pockets in the system. After I found it I brought the system up to proper static pressure, around 20 no more. Turned on pump and hit my 60psi. 

Problem was after it running at the right temp and pressure for a few weeks, the expansion boot on the discharge of the pump blew. After fixing it another blew. So after that I started looking more into the system. The old engineer had the air in the bladder up to 90lbs of air. I said well here's the issue. Got everything back to normal, drained the system and started with 12lbs of air in bladder. Work good for about 3 month and the popped the other 4 expansion boots. Now thet everything is fixed I'm wondering if the air in the bladder is correct and how to know how much should be in there? Exactly!! 

It has been running good with 15lbs for the most part but I still don't know if the pressure is correct in the expansion tank 

Jamie Hall

Short answer: IF the bladder tank is big enough and IF the bladder tank is connected to the pump inlet then 15 to 20 psig is probably what you want to maintain -- and thus the bladder tank should be precharged to the same pressure. If the pressure at the bladder tank changes more than 20% gauge from cold to hot, the tank isn't big enough -- or it has failed.

The bladder tank is there for one purpose and one purpose only: to provide a volume of compressible material -- air -- to absorb thermal expansion of your water.

It is NOT there to absorb any other pressure changes or spikes from whatever cause in your system. If you are getting large spikes on the outlet side of the pumps, you have a different problem and the bladder tank has nothing to do with it.

You may have a rather serious water hammer problem contributing to both the spikes on the output side and to the failures of the "expansion boots" (what fitting, exactly, are you referring to as an "expansion boot"? Flexible joints to absorb thermal expansion in the piping (not the water)? I think I see metal expansion joints on the vertical piping in the photo -- is to which you are referring?). Are there quick opening or closing valves anywhere on the system? What are flow velocities like (particularly in the pump outlets)? How big are these pumps?

triggerhappy24

Yes the spikes where being caused from air pockets in the system from not having it filled properly. That has been fixed 

EBEBRATT-Ed

@triggerhappy24

Ok. Measure from the boiler room floor to the highest pipe in the system and find that footage.


Example: you measure and get 40' so 40/2.31 ft/psi=17.3 psi. Now add 5 psi to your # so 22psi is the static pressure you should read in the boiler room with the pumps off.

The air pressure in the bladder tank should be set to 22 psi (in this example)

Don't confuse pump discharge pressure with the expansion tank bladder pressure/static pressure/fill pressure (those 3 should all match)

If the highest pipe is 23' or less your fine at 15 psi standard fill pressure

GroundUp

Jeepers guys.... No need to reinvent the wheel here. Whatever the static fill pressure is with 3-5 psi at the highest point, that's what you want the air pressure to be in the bladder while unloaded on the water side. You cannot accurately adjust air pressure while there is any water pressure against it. SO once you have the static pressure figured out, you must isolate and drain the water side of the tank, then adjust the air pressure to whatever that aforementioned static fill pressure is. Reintroduce the water side to the tank, and you have a proper setup. With that done, if the pressure still varies with temp, the tank is too small. The static pressure should remain constant regardless of the water temp. Also, the discharge pressure is irrelevant to this subject- all the tank cares about is the static/suction pressure. The fact that you did not know this, confirms why everybody was asking about the gauge.

Jamie Hall

" Also, the discharge pressure is irrelevant to this subject- all the tank cares about is the static/suction pressure"

This is true only if the tank is on the suction side.

JUGHNE

Is that your tank hanging high in the air? It looks to be a simple compression tank.

If so then the fill valve pressure is the only variable you have to work with, it would have to match the height of the highest water pipe plus a few pounds.
The only variable of the tank would be it's physical size......IIUC.

EBEBRATT-Ed

@JUGHNE

Hard to see but I am thinking a bladder tank. Looks like a flange on the RH end of the tank where you change the bladder

Zman

The tank should be pressurized to the static fill pressure with the tank isolated from the system. Since you are "pumping away", the pressure after the circulator is not relevant.

triggerhappy24

For some reason I can not reply to personal people on my phone. But I will answer all in here. Sorry 

Yes it is a bladder tank. No the bladder is not blown. I have changed one out before and hope to not do another in my carrier 😆

Yes the tank is up there 18ft high. So the 20 static pressure I have, would mean the 20 in the bladder would be about 3 lbs or so little??? Which I'm sure is not that bad.

Yes, any pressure change to my tank, changes the pressure on my discharge of the pump, because the tank is on the suction line of the pump. So setting it at 20psi I get 16 to 18psi on suction 

Measuring the ceiling hight, or static sounds about right. I have hear that one 3 times now so I would have to take majority on that assessment. That is where I am at now so I think I will fly with that and hope I don't see any problems......if so I'll be back for more **** happened now lol 

Thanks for all the responses 

EBEBRATT-Ed

@triggerhappy24

So if the tank is 18' up measure from the tank to the highest pipe in the system. If it's less than 23' your safe with the standard 15 psi fill pressure.

In that case you should have 15 psi static air pressure in the tank with no water in the tank.
Your static fill pressure should be 15 psi as well.

With the expansion tank on the suction side of the pump and with the pump running the pressure may drop 2-3 psi...that's normal.


If the highest pipe is more than 23' above the expansion tank calculate the bladder pressure and fill pressure as in my previous post.