Boiler Scale

Well, its being a pressure vessel it will be going through periodic insurance examination and testing. So there should be reports and an inspector, who has seen the equipment and I haven't, to talk to. I'd pick up the phone rather than trying CR4, but then I'm funny that way.

Well we need a detailed examination of the boiler tubes that were replaced....absent that I humbly offer this.....

http://www.powermag.com/boiler-chemical-cleaning-doing-it-correctly/

I say flush it, pressure test it and see what happens from there.

If it was able to be cleaned out and passes the pressure test and related technical requirements fire it up and go!

The situation that you describe requires much more investigation before proceeding. It appears that the feed water chemistry was way off spec, leading to the heavy scaling in what should be the coolest area of the combustion gas path. The opposite could also be the case; that the scaling led to high tube temperatures due to poor heat transfer to the feed water, perhaps due to low flow due to the scaling, in either case a degenerative process. In either case running acid through an obviously abused boiler is a poor idea without much more information on the overall condition of the water surfaces. You really need a boiler expert on site to guide you.

Replacing only those that absolutely need to be replaced is really a bit of false economy. You have the unit apart, now would be the time to do a complete replacement. Can those tubes not replaced be far behind those that have failed??

I don't think so. I believe 6 tubes were replaced and each one had about the same amount of scale, so you expect they would have the same amount of scale.

Well, you are the one who has seen the unit, so, hopefully, your decision is a good one.

My decision was to replace the whole economiser. Those higher up, was to replace the ones that sagged or burst and acid wash.

I lost.

Another engineering decision made by managers. Make sure you get them to document and put their decision in writing, otherwise it will be your problem in perpetuity. My condolences to you.

Well,I have decided to move on. Tired of picking up the pieces and senior management don't seem to get it. By the time it fails again, I will be gone.

You should (before you resign and leave):

(1) contact companies in your region that can clean the tubes. The should be able to remove most scale minerals without excessive additional corrosion of the tubes.

(2)document the complete condition of the tubes that were removed as to these: (a) average scale composition (carbonate based, sulfate based, phosphate based, or silicate based) and thickness (you already mentioned) with photographs; (b) perform some non-destructive testing of metal thickness (ultrasonic testing works); (c) and also document the appearance of internal pits to the tubes along with any external corrosion found. You might still be able to open the eyes of management (as long as forgiveness for taking the time overrides lack of permission).

(3) If none of these, the best thing to do is quietly distance yourself with appropriate insulation in terms of black and white documentation of what should have taken place for safety reasons. You did not mention the pressure of this boiler, just the power level, and that power level could have a pretty significant blast radius if general failure takes place.

I do agree with Post #4 where the Boiler feed water needs to be looked at. You need to take some scale samples and get them analysed. All scale samples with the exception of Silica deposists can be removed via mechanical cleaning. Only with silica , one needs to do acid cleaning. Personally I would try to refrain from acid cleaning since there is a tendency to remove base material as well, however well the acid cleaning is controlled.

Boiler feed water conditioning and also boiler water conditioning need to be looked at, otherwise the boiler tube replacement programme would not end here.

Silica deposits on boiler metal can be very irreversible by any means of removal. If you are OK with ammonium bifluoride that might work, but it will take an inordinate amount of time. Best practice calls for removal of silica from the source water in the first instance.

Methods for removing silica (efficiently):

(1) RO membranes can reduce the silica residual in feed water

(2) use of a special clarifier using alumina treated first with caustic soda, then with hydrochloric acid rinse. Efficiency is good but requires (a) long contact time up to 30 minutes to remove all but the last 1 ppm. (b) produces suspended fines in the water that are hard to filter, thus this also requires an added down stream clarifier that is being fed caustic soda, or lime, with iron chloride as floc agent. This produces a very fast floc on most waters, and carries down the hardness and alkalinity precipitated.

(3) demineralizer beds where the water first contacts cation resin in acid form (better results when this is fed RO permeate), followed by anion resin bed in hydroxide form. The resin must strong base resin type II (forms quaternary ammonium hydroxide when regenerated) in order to capture silica. Must regenerate beds when silica breakthrough is reached.

(4) Electrodeionization- never use this without an upstream RO, and it works best with soft water feed to the RO, and a two-pass RO with nearly all solutes totally removed from the permeate except small traces. Very high water quality is the final product here, and regeneration is continuous by electric current and water-splitting into ions. Periodic cleaning of the ion exchange membranes, electrodes, and media is required as a combined procedure.

The Boiler came from an area where there was a high silica content in the water. The Boiler has been acid washed with Hydrochloric acid, but the contruction of the economiser means there is no way of checking for scale inside them unless they are physically cut out for inspection.

As an aside, I have since been told by the Boiler Inspector who inspected the burst tubes, that he recommended to management to replace all the economiser tubes. I cannot fathom why this was not done. There were only 10 of this type of Boiler manufactured in Australia before the design was discontinued.

so now we get the REST of the story.

My first impression: If you have another job lined up, you might want to go there.

This company apparently has no regard for human safety, and is only interested in playing "CYA" games with the boiler inspector, but they are not even willing to follow the directions of a state authority. I wonder how that could affect any insurance claims made by said company with respect to equipment or production losses in the future?

It also does positively expose them to employee law suits when one or more employees is maimed/killed on site. By the way, have you determined the blase radius for this boiler? Are there highways inside the blase radius? Residential zone inside the blase radius? These are concerns to watch out for as well. I hope this company has deep pockets, because they are going to need them.