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low and high pH in demineralized water for BFW
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pH is somewhat of a meaningless parameter with highly demineralized water. The biggest problem is how to measure it without special electrodes. Highly demineralized water is a very good resistor to current flow.
The demineralized water will be within the pH range you mention as long as it doesn't absorb CO2 from the atmosphere, in which case carbonic acid is formed and the pH drops.
If there is mild steel in the system, corrosion will occur and iron will go into solution. If the iron is allowed into the boiler it will eventually precipitate as hematite (bad juju) or magnatite. While the magnatite is desirable, too much causes all sorts of problems with under-deposit corrosion, caustic gouging, hydrogen damage, etc. All ultimately lead to a tube failures especially in high heat flux and low flow regions.
Ammonia or some other amine is generally added to raise the pH of the BFW to 8.5-9.5 range. If your system is all ferrous the 9.0 - 9.5 range is better. If you have any copper in the system keep the pH in the 8.5 - 8.8 range.
The idea is to minimize Fe corrosion in the system and the 9.0 - 9.5 range is the minimum of the Fe solubility curve.
Keeping demineralized water in the 6.5 - 7.5 range will require a N2 blanket on the storage tank and a whole lot of effort to prevent air inleakage.
The demineralized water will be within the pH range you mention as long as it doesn't absorb CO2 from the atmosphere, in which case carbonic acid is formed and the pH drops.
If there is mild steel in the system, corrosion will occur and iron will go into solution. If the iron is allowed into the boiler it will eventually precipitate as hematite (bad juju) or magnatite. While the magnatite is desirable, too much causes all sorts of problems with under-deposit corrosion, caustic gouging, hydrogen damage, etc. All ultimately lead to a tube failures especially in high heat flux and low flow regions.
Ammonia or some other amine is generally added to raise the pH of the BFW to 8.5-9.5 range. If your system is all ferrous the 9.0 - 9.5 range is better. If you have any copper in the system keep the pH in the 8.5 - 8.8 range.
The idea is to minimize Fe corrosion in the system and the 9.0 - 9.5 range is the minimum of the Fe solubility curve.
Keeping demineralized water in the 6.5 - 7.5 range will require a N2 blanket on the storage tank and a whole lot of effort to prevent air inleakage.
cub3Bead hits on some interesting points: When you take a grab-sample from the system you are measuring what's in the beaker, not what's in the pipe. As soon as the sample is exposed to air, it absorbs contaminants and its chemistry changes. If you grab a sample and drag it back to the instrument shop to measure it, you might as well just be sampling the water fountain instead of the process. The only way to get a reasonably close measurement is to measure the pH in-line, or at least in the beaker with the sample stream pouring in as it leaves the pipeline.
As you heat water, the pH increases anyway. At above 250WSP the pH is so high that condensate attacks glass. That's why sight-glasses have mica shields at high pressures. InSitu pH monitors are temperature-compensated back to a reference standard.
If you have true demin water it will have a pH of 7 at ambient temperatures, and it will be one of the most agressive solvents you ever experienced. KEEPING it demineralized is a chore. Storing it in stainless steel tanks is a good idea, as is the inert gas blanket mentioned earlier. SOme utilities have a condensate storage tank with a rubber bladder in it, so the condensate will have no gas contact at all.
As you heat water, the pH increases anyway. At above 250WSP the pH is so high that condensate attacks glass. That's why sight-glasses have mica shields at high pressures. InSitu pH monitors are temperature-compensated back to a reference standard.
If you have true demin water it will have a pH of 7 at ambient temperatures, and it will be one of the most agressive solvents you ever experienced. KEEPING it demineralized is a chore. Storing it in stainless steel tanks is a good idea, as is the inert gas blanket mentioned earlier. SOme utilities have a condensate storage tank with a rubber bladder in it, so the condensate will have no gas contact at all.
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