Can a steam generation boiler be de-bottlenecked????

[robsalv]

I suspect I know the answer, but I thought I'd ask.

I was wondering whether there might be any methods, suggestions, technologies etc., that might be able to result in an increased steaming rate for the same nominal operating conditions. I guess I'm ultimately asking if there's a way to retrofit an existing boiler to improve it's efficiency so that more steam is generated for the same heat input. e.g. High flux tubes?? forced cirulation? etc

I want to consider options which don't involve uprerating the boilers or adding additional burners.


We have several gas fired, D type marine package, natural circulation, forced draft boilers. By D type I'm implying a steam and mud drum. The boilers are fitted with an economiser/waste heat exchanger to preheat incoming water. The package boiler includes a superheater and so pumps out 410degC, 4200 kPa steam. Nominal MCR is 66,000 kg/hour.


One idea I'm about to play with is to investigate the use of pilot operated safety valves set to design pressure with conventional SV's backing up the pilots at say 115% of set pressure. This would allow the boilers working pressure to be closer to the SV set pressure and therefore get a marginal increase.


The boilers haven't been descaled in a while, so this is also on the cards.


Any discussion, suggestions, opinions would be valued.


Thanks in advance.

Rob

 

[gerhardl]

I am no expert directly on the main equipment you describe, but have some knowledge about steam equipment and processes in general.

Following thoughts:

Steam generating in medium = (heat generated from burner) -(loss in transfer to medium to be steamed)

Answer: as you indicate, the only effect improvement you can obtain is improving by searching for less loss in the total process, trying to obtain ideal conditions.

One part would be 'normal maintenence and operating procedures': descaling /desooting inside - outside to improve heat transfer, best possible control (pilot operated PSV, best operation of burner and fuel).

The other would be possible changes for improving heat transfer and energy /steam / condensate reclaim and re - utilisation.

In addition to the obvious perhaps:

Steam traps (if any) correct type, installed and maintained?
Isolation?
Better heat distribution /agitation possible of medium to be steamed?
Steam temperature and amount best possible choosen for medium and equipment?
Extra loops, correct temperature for pre-heating water?
Preheating of media?

 

[davefitz]

regarding modification of relief valve set pressures- read ASME sect I; I do not think your proposal meets ASME rules or whatever insurance inspection agency is used by marine boilers. Usually max permitted overpressure of drum boilers is 7% over MAWP, including acumulation and errors in set pressure.

 

[rmw]

What is your flue gas O2 percentage. If you are pushing too much air through the boiler it can affect the firing temperatues which then drop the delta t across the tube wall and can have a negative impact on steam production.

Your fuel needs to be heating water to make steam, not air.

rmw

 

[robsalv]

rmw, oxygen percentage in flue gas is between 1 - 2%.

Davefitz, which code applies is an intersting question. The boilers/steam generators are 47 years old and located in Australia. The original documents refer both to ASME and Australian codes. I'm aware there are no section 1 approved pilot operated SV's, but I haven't yet checked how flexible Australian boiler codes are on this topic.

Gerhadl, thanks for those thoughts - the steam trap part is a non issue, since services that use trapping consume intermediate and low pressure steam, which essentially is generated by the let down from our steam driven process / refrig compressors. Unfortunately we have a big excess of this sort of steam.

The condensate return side would improve overall efficiency and save money, but wont generate more steam...

 

[MJCronin]

rob..

Lets see....

You have a variety of ~47 year old boilers, that haven't been maintained (internal scaling) probably have poor water treatment and you want to run them up to and possibly past MCR ?

Many bridges/buildings governments and certainly, relationships do not last 47 years and perform properly.

At what point do the MBAs in your organization decide to replace or add new boilers to the existing plant ?

At what point in time (60,70 or 100 years of service) do you plant on replacing these old beasts ?

My opinion only

-MJC

 

[robsalv]

You have a variety of ~47 year old boilers, that haven't been maintained (internal scaling) probably have poor water treatment and you want to run them up to and possibly past MCR ?

Actually it's not that bad.

The boilers are in pretty fair shape considering their age. They've been well looked after, though they haven't been descaled in about 15yrs, so that's probably due. Our on site demin plant produces pretty good quality water.

The cynicism, probably well earned over the years, is not deserved in this instance.

The question is a simple one. Just wanted to canvas my peers as to whether there was some available technology that I wasn't aware of that makes steam generators more efficient and that can either liberate more steam or produce the same steam whilst running less hard.


As to the replacement question, there's no need to replace something that is doing its job well. If reliability/safety issues start rearing their ugly head and/or maintenance costs sky rocket, then the CBA becomes pretty straight forward.

Thanks for your contribution.

 

[MJCronin]

rob...

Cynicism is a tool for attention....

Sooner or later, almost every machine must be replaced. The zillions of thermal cycles endured by these boilers cause incremental damage. This type of fatigue damage is not easily detected and frequently can lead to catastropic boiler failure.

You state: "If reliability/safety issues start rearing their ugly head and/or maintenance costs sky rocket, then the CBA becomes pretty straight forward."

Yup...... if your system fails in an incremental, non-lethal manner. But, wait,... will you be operating this system BEYOND MCR?. What are the chances of a spectacular failure then ?

Again, my opinion only

-MJC

 

[chicopee]

Are the boilers actually in need of being descalled? If yes then obviously steaming rate will increase once descalling is done.
Since at the time of installation, the boilers were at their optimum condition, compare the original performance test results with today's results, particularly with those dealing with flue and return water temps at different firing rates and loads. It is unlikely you can improve on the original performance.
Also do an inspection on the fireside for sign of baffle failure, heavily sooted tubes, sootblowers in good condition and check out the external casings for corrosion and leakage of flue gases.
Any improvement will probably be with the burners whereby turn down ratios have increased for better modulation of firing rates. This is the area you should explore with the boiler manufacturer. You may also want to look at the cycling of the burners and try to reduce that if possible.

 

[robsalv]

MJC - you're stating that the sky is blue.

I don't know about your work situation, but I think almost every where else you don't replace something "old" if it's not costing you money, is still performing its duty satisfactorily, is still maintainable and it's integrity is not in question.



Chicopee, the boilers are down on their nominal MCR by only a few percent. We're bumping up against a conservative superheater temperature alarm, hence the OP question. I'm disuading the process department from raising the alarm. FTR, these are gas fired multifuel boilers, so external soot is not an issue. Water side descaling is probably required, even though our demin plant produces pretty good quality water.

Thanks for your contris.

Cheers