Editor's note: This is the final installment of a multiple part article chronicling the true geothermal district heating system at the Breitenbush Hot Springs Resort, Detroit, Ore.
All during my tour of the massive district heating system serving these facilities, I kept thinking to myself, "What a great free natural resource!" I happened to mention this fact during one of our numerous meetings with staff and management, and it was pointed out to me, that although the per kW cost of these systems were close to zero, these resources were considered precious, and should not be squandered. There are situations that occur with these systems that when you are dealing with conventional systems, that you don't always take into consideration. Like, what happens if you do have to shut down flow? What are the consequences and how are these consequences controlled?
For example, we determined through our discussions that it would be a good idea to flush the down hole heat exchanger out to eliminate any accumulations of rust, scale, etc.
First off, we are talking about a pipe that is about a third of a mile long. It is also about 750 feet below the surface of the earth, and it is immersed in a fluid that can and will go geyser in short order without flow. What this means, is that you can’t keep the DHE off line for a long period of time without creating a whole other set of problems. It’s not as simple as flipping a switch to the off position on a conventional heat source. It requires significant coordination and preparation. Plus, how does one perform a good flush of a heat exchanger that is below you, and maintain any kind of a reasonable fluid velocity to scour the accumulations of rust, sludge, etc., from the pipe. Well, this is where the Breitenbush Wild Lands Fire truck comes into the picture. I've seen some flush carts in my day, but never had the opportunity to work with one that had a 700 horse power gasoline engine on it.
The near heat source piping was conducive to being connected to in parallel, so we were able to connect the "flush cart" up to the heat exchanger with the distribution system still running. Once we had confirmed the flush cart was purged and ready to push a lot of water, we were able to take the heat exchanger off line from the distribution system and bring the flush cart, with its significant fluid pushing capabilities, on line and begin the process of flushing the DHE out. We were pumping straight river water from the river into the pumper trucks reservoir, and then pumping that water under serious gallons per minute and pressure through the DHE, and onto the ground.
We tried to capture some of the hot water coming from the DHE, so we could get an idea of what was coming out of the heat exchanger, but the flow was so significant in gallons per minute and so hot, that the barrel we were pumping into collapsed from the heat. So much for trying to get a look at what was coming out of the heat exchanger.
During the flushing process, I found myself hanging on to the live end of the discharge hose. There is nothing quite like hanging on to a 2-in. fire hose discharging extremely hot water at a velocity that can actually give you the feeling of being on the brink of being flung around like a rag doll to get your heart pumping. We performed a forward flush for about 15 minutes, and the water began coming out of the DHE fairly clear, so we decided that it would be a good idea to flip flop the supply and discharge hoses from the flush cart to the DHE in order to make certain that our efforts were worthwhile.
Again, remember that you can’t simply flip a switch and shut off the heat source. We stopped the flush cart, relieved the residual pressure and began flip flopping the lines. In short order, the DHE started geysering, kicking large bubbles of steam out, and making the task of switching hoses a lot more difficult than I am used to. I had to close a ball valve to keep the hot water from belching on me, and got my end of the hose off.
My associate, who was fighting the same situation on the other side of the heat source asked if it was OK for her to close her valve so she could get her hose flipped around, and I quickly thought about it for a microsecond and said, "Sure". She closed her valve, and quickly removed and reconnected the hoses. When I went to open the ball valves, it suddenly dawned on me that I had made a big mistake. The ball valves would not budge — hydraulic lock up. A 1,500-ft. long pipe, filled with cold water, and immersed in a boiling bath. When all valves are closed, and the cold water is heated, the water will expand, and if there is no where for the expanding water to flow to, it will generate static pressures that will cause ball valves to lock up.
I grabbed the closest thing that would work as a hammer (crescent wrench) and started tapping on the handle of the ball valve in an effort to open it up and relieve the building pressure. It would not budge. I kept tapping on the valve handle, in hopes that it would open, when all of a sudden, there was a quick explosion of water and steam right in front of me. Fortunately or unfortunately, depending upon your perspective, a black iron pipe tee had split in two at my chest level. I was blown backwards by the force of the initial pressure release, which was short in duration because it was strictly static energy, which once released stops immediately. I was showered with some fairly hot water, but was so well clothed (it was cold and rainy) that I fortunately didn’t get burned or scalded. I felt numerous waves of relief wash over me. One, that the static pressure had been relieved, and secondly, I had not been seriously burned, and more importantly, no one else had been seriously injured.
Once the dust and steam had settled, we quickly assessed the situation and determined that we needed to rebuild the near heat source piping to replace the cracked tee, and also to incorporate some means of temporary pressure relief that could be opened when other valves were closed, to avoid the possibility of this situation ever occurring again. Fortunately, the well stocked shop facilities had everything we needed to rebuild the well head, and everything was back on line within a couple of hours of the time we had shut it down. The back flushing process was postponed until a later time, and with the newly gained knowledge, was much less eventful than the first try.
Lesson learned: Always make sure that whatever situation you find yourself in, that you have a means of control, escape and pressure release, regardless of the heat source. Also make certain that any snap decisions are thoroughly thought out before making them.
Tune in next month as we look at some tools designed to increase employees’ efficiency and production. Until then, happy hot tub hydronicing!
Mark Eatherton is a Denver-based hydronics contractor. He can be reached via e-mail at [email protected] or by phone at 303-936-7606.
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