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Author Topic: Over pressurizing wood boiler  (Read 6921 times)

notnim

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Over pressurizing wood boiler
« on: December 16, 2010, 12:48:31 PM »

I have begun using my home built wood boiler this season. It is a round design with a 36 in dia. x 48 in long water jacket, and a 24 in dia x 44 in long firebox. With a water cooled door also. All steel components are 1/4 in thick. The flue exits the top rear on the firebox thru the water jacket and then extends vertically 6 ft. I am using a 120 cfm ID fan. I have had no problems with a good clean burn. I have lined the lower third of the firebox with fire brick. I have made it a pressurized system with auto fill valve set at 12 lbs. I am using a plate heat exchanger for my hydronic propane fired boiler in the house (1600 sq ft) and a sidearm exchanger for the hot water tank. Also I am trying to heat my shop with a radiator type unit with a small fan behind it. I am running 1 in insulated pex 135 ft to the house and 150 ft to shop in opposite directions. The problem I am having is trying to get the water temp above 155 degrees ( at the boiler) with out the pressure relief valve set at 30 lbs popping off. When this happens the auto fill valve makes up the water drop with addition cold water and of course lowers the temp to about 120 degrees. I am using a 2 gallon expansion tank located close to the plate heat exchanger at the house furnace. I have a feeling this is not large enough. I would like to run at 180 degrees if possible. I would appreciate any ideas on this problem.
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willieG

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Re: Over pressurizing wood boiler
« Reply #1 on: December 16, 2010, 03:03:25 PM »

first things first, i am no pressure vessel engineer and pass on to you only what i find on the net (you can google items on expansion tanks yourself)

you need a tank for a 30 pound presssure system about 20 percent of your total capacity (lines, appliance and boiler) you also (i believe) need to put pressure into the tank while it is at room tempature, how much, this i am not certain of.

my concern is, why you would build a pressurized system, i sure hope that you ahve belled ends on your  tank (s) and that you have tested your system filled to capacity with water and added at least three times the pressure you will be running and have had a professional engineers stamp added to the bolier and follow all retesting dates stipulated, as all registered boiler require annual inspections

My real question is why you would not just build a nonpressurized system?

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notnim

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Re: Over pressurizing wood boiler
« Reply #2 on: December 16, 2010, 06:17:56 PM »

willieG, Thanks for your reply and input. The reason for a pressurized system is that I am trying to prevent corrosion problems. From what I have read this is an advantage of this type of system. Besides I tried to model it after the Burnham gas boiler I have used in my home the past 25 years. It has been trouble free all these years. It is set up in this manner. I am no vessel engineer myself but I do work In a coal fired power plant and am familiar with pressures and vessels. I did pressure test the boiler filled with water to 125 psi. while it was still in the shop. Actually I was wondering if I would be better off to convert it to a non-pressurized system.
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willieG

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Re: Over pressurizing wood boiler
« Reply #3 on: December 16, 2010, 06:42:43 PM »

not only for safety reasons but for ease of operation i think you should convert back to non presurized. you say you made it your self and all parts are 1/4 inch thick, well mine is made of old steel pipe about 7/16 thick and i am going into year 10 and never a problem yet.

like i said you can look up the info you need likely on the net, or perhaps you say you worked in a coal fired plant, there must be an engineer or two that could point you in the right direction. But i will tell you this, i am a retired welder and have held plate tickets and u69 (pressure welding ticket) and i would never build my own pressure boiler. i would sure be a sad day if one of my kids took over my property after i died and they have no clue about such things as pressurized boilers kept using it and blew it up. If it is not registered as a pressure vessel than i dought any yearly inspections would be done on it to insure it's safe operation.

i don't  know if anyone else will weigh in on this topic, and i am not telling you what to do, but i would ask you to reconsider, maybe (and maybe not) your stove corrodes out a year or two sooner than you think it should or perhaps (again perhaps not) your pressurized oiler lets go with a big bang and kills or scalds someone.

good luck in what ever your decison


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notnim

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Re: Over pressurizing wood boiler
« Reply #4 on: December 16, 2010, 07:07:58 PM »

You have made some good points and they are well taken, I am the only one in the family who understands this system as I have built it. could probably save myself some trouble by converting it. I have put a lot of work into this project and I just want to get it to perform properly and safely. Would you have any suggestions as to how I could handle the expansion of the heated water ( such as adding an expansion tank vented to the atmosphere) and the placement of it? Also do you use a water conditioner / rust inhibitor in your system?






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willieG

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Re: Over pressurizing wood boiler
« Reply #5 on: December 16, 2010, 07:26:58 PM »

many use conditioners (i never have but i do add an oxygen scavenger to help keep the air out of the tank-i am not sure they work and have missed a few years)

if you do not use pressure in your system, an expansion tank of 10 percent would be good. all i have is a piece of pipe on top of my stove with a peice of blue foam on top of it and about a 5 pound piece of steel plate to hold it there. i dont think it seals air tight 

I have heard of people using bladder tanks from home domestic pressure systems

i also use an inline filter to keep any rust out of my system (change it yearly)

i have had one guy tell me on the top of his expansion tank he just uses a funnel with an "indian rubber ball" in it. any pressure in the stove will just lift the ball. I wonder a bit about this one as if the ball seals tight after it lets the pressure off there is a chance there could be suction  on it at a later time. if the ball lifted with a little pressure at the high set point of the water then when it cooled down there would be (could be) suction on the ball?
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home made OWB (2012)
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