Parris Island Marine Corp Recurit Depot has long been the starting point where Marines are born and obviously a very important facility for our armed forces on the coast of South Carolina. It was also an eye-opening experience for me on our topic "Steam Quality," which, before this experience, was a term not generally talked about very much. Seems sometimes things are not important until they come home to roost at your home!

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Thermaflo had sold two TH750 steam-fired domestic water heaters to the base heating water for showers, and a kitchen facility. The phone call came from our mechanical contractor, who did the install, saying they were having capacity issues and swings in temperature from our heaters, so I made the drive to the Low Country to visit the first party. Upon arrival, we saw the two TH750 heaters perfectly piped in with 15 psig steam from the reducing station and condensate from the heaters piped to a gravity drain duplex condensate return receiver, all to textbook.

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However, as a load was introduced, the heaters performed perfectly until the load reached a certain point, then the 140F temperature outlet dropped drastically down to 100F. The situation got worse the longer we ran the units, making us question control valve sizing, heater sizing, and so forth, so the examination began. After a few hours of experimenting with flows, controller tuning, and reviewing our performance data, all was within the specs, so a befuddling situation was at hand, and by that time, many people were looking over our shoulders. One thing that stood out was that at low demand, the units performed perfectly.

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As it is in the late fall in South Carolina, the temperature at night was cool, but as the day went on, the temperature gained strength, sending us to open the back mechanical room door for some air. At Parris Island, the steam lines in the old part of the grounds are above ground, and the main drip condensate drain traps discharge to visual French gravel drains. With the back door open, I looked out at one of these close to the building on a high-pressure supply line, thinking what a waste these were. About that time, the main kitchen equipment after lunch started the need for hot water, and our steam control valve started to open fully, attempting to keep up with the load. Looking out, I noticed the steam trap that was operating at a gentle on and off at first glance was wide open, pouring hot condensate fully to drain, and the duplex condensate pump started running both pumps instantly. At the same time, the temperature of our hot water outlet began to fall quickly.

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At that point, is where the "Blind Flash of the Obvious" took over, and I realized that the steam line was not steam at all but a hot condensate line feeding "Low BTU Poor Quality Steam" along with condensate into our heaters and at a very high rate. Without high-quality steam, the TH750s could not meet high demand. Further investigation of the supply line, we found several long dips in the 4" high-pressure main steam supply, causing lakes of condensate to form at the bottom of the pipe, and a few cold traps were no help. Remember, water "Seeks its Own Level." After everyone realized what we had, repairs were made, and the steam quality was restored to a high level. The TH750s performed beautifully. Most people think Steam is Steam and fail to realize that steam at a given pressure saturated has a certain BTU amount per lb. For example: 100 psig steam, like we were dealing with here, should have 880 BTUs per lb of Latent heat - "That’s the Heat That Does the Heat Transfer Work" if its 100% quality. Now, when that steam pressure is reduced to 15 psig, the Latent Heat vapor BTUs is increased to 945 BTUs Latent Heat per LB because the density is less and it can hold the heat, so as you see, low-pressure steam has more BTUs per lb. Steam at lower pressures also requires a larger pipe size to deliver it to a user like our TH750 heaters. What we were being supplied amounted to a hot water/steam mixture with a BTU content per lb in the 180-200 range, so it was like pouring water in our gas tank.

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In the real world, you can expect steam to be generated at the boiler at 98-99% Vapor (.98 to .99 % Dryness Fraction) so that 100 psig steam will have 886 BTUs per lb in reality. As the 100 psig steam travels out to the users through the main supply pipeline, it transfers small portions of its latent heat through the pipe walls, and moisture (condensate) entrains with the steam vapor flowing along. Without steam trap drip stations located at strategic points and dips in main supply piping, this entrained moisture increases, dropping the Steam Quality to the 80 to 85% range (.80 to .85 Dryness Fraction), even with proper trapping, 90% is a top number in most facilities. What this all boils down to is that wet steam does not contain the "Latent Heat BTUs Per Lb" to do the work for you.

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A steam supply with a high dryness fraction is very important when heat transfer equipment like the TH750 water heaters is being used.

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As we drove out of the base that day we were discussing what a huge lesson that was and it had been a problem on many buildings for years, but no one really understood what was happening until that "Blind Flash of the Obivous" set into motion the expedition of now educated steam maintenance looking for the culprits and eliminating them one by one so that high steam quality was order of the day.