Can this happen if the pressure in boiler too low?

Joseph_4

I had a customer recently where their expansion tank clearly had ruptured but it didnt cause the prv to leak. The pressure gauge ,which i tested against another gauge( which i carry around) read 20Lb while boiler was hot. When I removed expansion tank to replace..it clearly was shot. I want to understand why prv didnt leak. Here is the question…… If the Feeder was broken or say erroneously set at a fill pressure of 7 to 8 Lb instead of 12 Lb… could that explain that even when the added pressure when boiler heats up and pumps are on, that it will stay under the 30lb and thats why the PRV didnt leak when boiler heated up?

Another way to ask question i guess is- is there a maximum pressure that a system expands when boiler and zones heat up.. lets say 7LBs and no more?

Thanks

Joe

WMno57

@Joseph_4 asked: is there a maximum pressure that a system expands when boiler and zones heat up.. lets say 7LBs and no more?

No. When water heats, it expands. Water is not compressible. Pressure can rise to thousands of pounds if the water does not have somewhere to expand into.

Air is compressible. What you observed indicates there was air somewhere in the system. Top floor radiators, air still trapped in the failed ex tank, etc. If the system has good air eliminator vents, eventually all the air will be eliminated, and then (hopefully) the PRV will release.

Unlike a steam boiler, if the PRV doesn't release, there won't be an explosion (assuming less than 212F). Just a break or failure in the weakest part of the system. Again liquids are not compressible, gasses are. Failure of a pressure vessel containing compressed gas will continue to expand (blowing shrapnel across the room) until the gas returns to atmospheric pressure.

This is why hydrostatic testing of pressure vessels (like scuba tanks) is done with water, not air.

https://en.wikipedia.org/wiki/Boiling_liquid_expanding_vapor_explosion

Jamie Hall

Too many variables. For one thing, it depends a lot on exactly how the expansion tank had failed. if it had failed in such a way as to trap some air, then it would still function to absorb the volume changes. Then there is the overall question of total system volume vs. temperature change… elasticity of some of the pipes (in particular: any PEX in there?). And so on. It also does depend on how low a starting pressure you are working with — so yes, low system pressure cold pressure could help explain it.

EdTheHeaterMan

To give a example of how water expands, if you have 100 gallons of 60° water, and you heat that water to 240° you will have 105 gallons of water.

• But water boils at 212°F. How can you have 240° water?
• Water boils at 212°F at sea level where the pressure on that water from the atmosphere is about 14.7 PSI absolute. If that pressure is increased (like inside a closed tank or closed heating system) then the water boils at a higher temperature. At a pressure of 12 PSI gauge or 29.7 PSI absolute water will not boil until it reaches over 247°F.

Back to the 100 gallons of water 60° water and that same water is 105 gallons at 240°. The expansipn than is a place for that extra water to go when the water heats up and cools down, in a closed system. in this drawinf you can see that the location of the air cushion does not matter, it just needs to be inside the closed system somewhere. as shown in the lower left illustration.

The problem arises when the failed tank lets the air into the system and the air is vented to the atmosphere. When that air leaves, and the boiler water cools, the auto fill will replace that missing air with water that will not compress. When heated, pressure will increase past the relief valve setting, as shown in the lower right illustration.

Zoom in to see the expansion tank details.

expansion tank1.jpg

EdTheHeaterMan

@WMno57, like that you referred to a "BLEVE"

• 🎼 So Bye, Bye, Miss American Pie,
• 🎶 Drove my chevy to the BLEVE
• 🎶 but the BLEVE blew up.
• End of song

hot_rod

The Amtrol handbook has tables to show how much the volume expands.

Lets say the water goes from 50- 180°F. The factor .02757 time the volume of the system

If the system hold 70 gallons X .02757 , you need to accomodate 1.9 gallons somewhere.

Joseph_4

I appreciate all the responses. However, the reason i was lead to this question was that i also I changed a pump on customers indirect and bled out air the same day i did his expansion tank. water was hot for a day then cold.. came next day again more air bubbles. rebled indirect.. this happened 3 x then i installed a spirovent in indirect zone alone still air came back. I called Htp tech support they said check fill pressure…. it was about 3 lbs less than normal.. so i manually fed it to 20lb….it now gets hot and its now reading 20 to 25 psi and for a few days (first time in 8 days) so far no call back and i verified with customer all hot. It would seem the higher pressure prevents the air bubbles from coming back.. I never had this before when put the 2 things that happen together seems

lmk what you think

Joe

hot_rod

hmmm, I didn’t know there was a normal fill pressure. The pressure should be based on the height or lift the water sees.

12 psi seems to be a number that some fill valves are pre-set to, as well as expansion tank pre-charge. Although I would check both with an accurate gauge to confirm this.

Also if you boost fill pressure, you should also re adjust the expansion tank precharge to match. If the tank has a 12 psi charge and you fill to 20psi you reduce the expansion tanks ability to absorb the expanding volume. You will know this by watching the pressure increase as the boiler heats to max temperature. 27 psi is about as high as you should comfortably run a system with a 30 psi relief.

Increasing fill pressure squeezes the air bubbles smaller and helps them pass through the piping to the air sep. In some cases you can boost the fill while you do the troubleshoot, then reduce it to the tank pre charge before you leave.

The relationship of the expansion tank connection to the circulator can make a big difference in air removal. Always pump away from the tank.

A copy of Pumping Away goes into more detail about air removal and system components.

EdTheHeaterMan

I remember running into a modulation boiler that required 16 PSI to operate. there was a pressure sensor inside the boiler that was wired to the main boiler control. Of course after plumbing in the Watts 911S that comes with 12 PSI and piping in an Extrol 30 that comes pre-charged with 12 PSI, I could not understand why the system failed to operate. Looking at the Diagnostics, I found the minimum 16 PSI requirement and adjusted the tank and fill pressure. All good after that. I cant remember what boiler is was, but that boiler has a "Normal Fill Pressure" of 16 PSI regardless of how high the radiators go up.

Except for that particular boiler, I agree with @hot_rod. It is kind of like "What is room temperature?" Well it is what ever the room temperature happens to be! And if the heater ain't working , that room temperature can be pretty cold at times!

Joseph_4

I appreciate the comments, i remember hearing what Hot Rod said to do to expansion tank a long time ago… of course I forgot to add pressure. i will be going back to do it

thx to all

Joseph