Soot Blowing Basics 5

[dave472005]

I am a seasoned Mechanical Engineer, however, most of my 15 years experience has been on the nuclear side of power generation. I am leading a very small team to evaluate and improve soot blowing performance on a boiler with PRB coal. As you might imagin, I am on a steep learing curve to understand soot blowing, performance measurements and practices. I am also going to a soot blowing symposium by an equipment manufacturer in a couple of weeks.

I am looking for advice on sources of information to help me get up to speed on keeping the boiler tubes clear of ash build-up.

Thanks in advance - Dave

 

[rmw]

dave472005,

Also, to your point above, 99% of the components on a repaired board, irrespective of the costs involved are the same age as the component that failed. Might not matter, but it is all the more reason to trash it if reliability is paramont.

rmw

 

[dave472005]

I understand your point of view - and think you are making a common assumption - the failure is age related. This is not unreasonable - however, it is still an assumption.

While there is room for discussion on this point, we don't typically get there due to the cost of labor to do the work. To get to the point where there is a reasonable expectation that changing one component on a computer board will restore the board to service, the labor/engineering time cost is significantly more than the cost of procuring a replacement board.

It rubs me the wrong way, but it is a business reality.

 

[marada93]

Dave,

Sorry to get in on this thread so late (I've been busy) but in all the replies I haven't seen what I feel to be the most significant points. At least in my humble opinion anyway.

If you want to maximize sootblowing, regardless of the motive source, here is what I have found, and have taught operators at various plants over the past 25 years.

1- Monitor tube metal temps. They are the most significant factor in determining the need to blow soot. After soot is blown, you'll find tube metal temps (usually superheaters) are at or near the gas temperature for that particular zone.

As time goes by, tube metal temps will decrease, indicating poor heat transfer (fouling) and the furnace exit gas temperature will increase accordingly. The time element involved will vary depending on the fuel and is learned through trial and error. Some furnaces may experience fouling in one superheater, and maybe not in another. This is where selective sootblowing is utilized. No need blowing soot in an area of the furnace which is showing itself to be clean.

2- Monitor draft across superheater banks. Most of the places I've seen have the ablitily to read the raft around particular elements of a furnce (superheater, air heaters, etc). If these indicate pluggage, its time to blow soot.

3-Time. Once you have a grip on the 1st two items, you will notice that the frequecny of sootblowing is reasonably the same from time to time. Do not overlook this aspect as it is as important as the 1st two. Lets say, for example, that a boiler firing rate is reduced say 50%. All indications may be that sotblowing isn't required due to lower than normal furnce temperatures and flows. However, the boiler is still fouling normally. Just because it would seem a boiler at half load should be able to go twice as long between sootblows is false. If you wait twice as long, boiler circuits will be fouled alot more than usual, and when the sootblowers begin blowing, they begin removing extremely heavy ash load. These ash loads will cause extremely high spikes in exit gas temperature, but even more importantly, furnace draft swings. This is due to the fact that what used to be occupied by exit gas, is now being occupied by ash. Much harder for a fan to control. In some cases I've seen plants tripped due to this very sequence of events.

Use these items as your guide. Other things that will influence the above mentioned items are outside air temp, and fuel quality. Other items to look at are attemperator sprays. As tubes become progressively fouled, the steam temperature within begins to decrease, so the spray valve positons will gradually decrease to the point of becoming full closed. Once this happens, then the steam temperature itself will begin to fall. Exellent indication of fouling.

Used to be that operators at plants I've worked at would give the boilers a "courtesy blow" before the next shift arrived to relieve them. Thinkg they were somehow doing them a favor. This is far from the truth. What they are doig by blowing soot when it is not needed, is putting the boiler that much closer to a tube leak. This is a much bigger pain in the arse for everyone involved. They took to my meaning quite readily once I explained this to them.

I have my operators fill out a log with some of the pertinent data I have mentioned each and every time they blow soot. I also have them print out a tube metal temperature profile each time so that they become familiar with the spread of temperatures across a tube bank before and after a sootblow. Not only does this give you a clear indication of fouling followed by effective cleaning. It give you something else of value as well. If you clean a section of boiler, and tube metal temperaures in that area don't increase as expected, it could indicate a malfunctioning sootblower, or worse yet, a clinker beginning to grow...like some of those mentioned in previous posts. Could mean you have a tube leak in that area causing the ash to build up around the leak. Most importantly, an early warning that something is amiss so you can begin diagnosing before some sort of catastrophic failure. A large slag fall for instance.

By having the guys perform this type of log recording, it has become sort of competitive between control room operators to show the most effective sootblows. When they start doing that, you'll have it made.

Hope this helps!