Boiler corrosion

[op9]

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 mainly above and some 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 about the same at the tube exit into the flue.
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. Both boilers go into a common flue but initially separated for about 6mtrs. There is a draft in the standby boiler when the other is on line. Changeover between boilers is not on a definite schedule but left up to the "whim" of the operators. Could be once a shift, once a day, etc.

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! This 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 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.
Rod Nissen.
Combustion & Engineering Diagnostics
nissenr@iprimus.com.au

http://www.canded.com.au

 

[ChasBean1]

op9, very interesting problem. I once had a project with an air handler that had too high an intake velocity, causing it to take in snow at times. The snow wetted the filters. Immediately downstream of the filters was a heating coil, which was becomming caked with little white crystals. These little flakes of aluminum oxide (in my case) were releasing and traveling through the distribution duct work and showing up as a grit on on horizontal surfaces in all of the areas served by the unit.

The phenomenon was that tiny water droplets in the wetted filters were landing on the heating coil. Due to the filter material, the droplets had extremely high concentrations of chloride and other ionic impurities. When the droplets impacted the heating coil, the water would instantly flash or evaporate and leave minute areas of concentrated residue. This caused accelerated corrosion of the coil as indicated by a white, powdery appearance on the coil fins

I would expect something similar is occurring in your application. You mention the nearby ammonia chiller plant that sounds like it had leaked in the past. Ammonia itself is volatile. I would expect the problem to be more related to a non-volatile impurity. Is this facility near the ocean? Air containing salt or any elevated level of impurity that is non-volatile could immediately concentrate in impacted fireside surfaces which might be causing this.

The problem is very interesting - I look forward to hearing more as it is uncovered.
-CB

 

Good Problem:

1.Does it happen in the other boiler?.
2.During idle storage will the water level be normal in drum or is it likely that the waterlevel is the zone of 150 mm band. Can we be sure of idle storage pressure above 85 psi. I have observed whitish deposits in surfaces where the heat transfer is high and the gas temperatures are lower than dew point. The deposit formation may be due to condensation of the flue gas which is filled in the furnace from the operating boiler. That is where the steam condenses to water due to heat loss by convection.

 

[op9]

Thanks to both of you for the responses.
Firstly,it only happens on one boiler, the one nearest the outside air and the ammonia compressors. The plant is near a river (Brisbane River, Brisbane Australia). There is a considerable draft in the idle boiler due to the other being on line, so unlikely flue gases are entering. The boiler level is normal in the drum and the burner automatically maintains the 85psi.

From another forum a US boiler inspector says the white band is the mean path of the draft drawing in cold air from the outside. This line is right through the boiler up to the tubes on exit into the flue so this theory looks feasible.

What has been done so far, is the tubes have been cleaned down and tube thickness readings taken at pertinent spots...they look OK. The burner auto standby pressure is to be raised to above 100psi. The burner air dampers are to be arranged to shut completely on shutdown, not just remain at the low fire pos'n. Boiler will be put back into service/ stand-by modes and the furnace then will be re-inspected in about 2 months time.

Will keep you posted.
Rod. Rod Nissen.
Combustion & Engineering Diagnostics
nissenr@iprimus.com.au

http://www.canded.com.au