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Furnace corrosion

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Posted by Rod Nissen on June 28, 2002 at 08:06:16:

I am a combustion engineer in Australia with my own business but spent many years previously operating, and maintaining boilers at sea in the Merchant Navy and onshore for package boiler companies. Today a previous customer of mine (from a number of years ago) contacted me regarding a corrosion phenomenum in his D type watertube package boiler which has been ongoing for the last few years. Fuel is natural gas.

I made a very brief inspection of the furnace and am due to return next week. This is the history of the problem as reported to me. The first indication of a problem a few years ago, was the appearance of what can best be described as a "high tide mark" roughly just below half way down the vertical furnace tubes (fireside). It was a whiteish powdery deposit making a well defined line around the furnace, easily removed . No explanation was found and the powder deposit was not analysed. Over the last few years this has reappeared. Supposedly the tubes also exhibited some oxide coating, again on the fireside but this year the increase in this "rust??" caused them to seek further investigation.

When I saw the state of the tubes in the furnace it appeared to me as if the boiler had been abandoned in a field in the open somewhere for many years, not as is the case a working boiler. The well defined tide mark is not so clear and appears to have lowered into a wider band (say 150mm wide) from its original position. Its level is not constant around the furnace and stops and starts. What is consistent above and below this white stain, as well as on the roof tubes is that they are heavily scaled with a rust like deposit (magnetic). The scale is loose and can be wire brushed off. The metal under is not perfect (considering this is a 35 year old boiler) but not particularly pitted. Uniform wasting has not yet been identified as no thicknesses have been taken. This scaling appears to become thicker on tubes at the entry to the convection bank and next week I will be checking the tubes at the flue exit to see if they are worse the colder the gases get.

This boiler is a Trevor D-Type made in 1967and rated at 250psi saturated steam. I have to confirm but the capacity is approx. 600HP. The boiler is operated in the 130 to 175psi range. Its use is alternated with another adjacent watertube of different make but operating under same feed and steam conditions. When either boiler is on standby they are maintained hot at a pressure of 85psi.

Past combustion reports show it operating at 8.5% O2 at low, 4.8% at mid and 3.2% at high fire. The CO's are low but the flue temps seem high at 200degC at low to 300degC at high. Having this scale on the fireside would not help the heat transfer. The waterside condition is reported to be "acceptable" though not pristine! The burner generally operates around half rate when the boiler is on line.

I have noted that the combustion air fan for this boiler has a suction which is close to an open to the outside alleyway so the intake air would be relatively cooler compared to the other boiler. Also there is a ammonia refrigeration compressor room closer to this boiler. However the operators assure me that any "smellable" leak is fixed.

I have taked some photos but have yet to have them developed. If anyone can shed some light on this initial appraisal I would appreciate it, and I can scan and send the photo images by E-mail if required. I also took some scrapings of various deposits and hope to have them further analysed if necessary.

Thanks in advance,
Rod Nissen.

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