Let's see....when does a boiler need to be replaced? Well, you have safety and economics concerns, and that is about it. Environmental regs will effect the economics of deciding to spend money to install or close the doors.
Power boilers can continue to run if an owner has deep enough pockets to evaluate and replace boiler components when they need to be replaced to assure personnel safety and boiler reliability for revenue.
Great concept except all of sudden, upper management complains that we do not have enough capital to spend because bonuses are low this year. O&M budgets need to be slashed so that upper execs have more money to please wall street.
So, capital and O&M expense budgets are reduced and now the boys in the trench need to decide on what targeted evaluations/inspections and pressure part replacements need to be performed to sustain performance.
Trying to run a fleet of aged boilers requires a knowledgeable and experienced team of specialists to conduct internal boiler inspections (based on best industry practices) during scheduled outages, and when necessary to recommend replacements of major boiler pressure parts that have limited creep life. Also, maintaining water cycle water chemistry to at least EPRI Guidelines is critical in assuring long life of water-wetted boiler components.
The company I work for finally wised up and recognized that spending capital dollars makes perfect sense to continue to safely operate AND maintain aged boilers. Revenue is a large motivator for IPP's and when upper management decides to avoid replacing boiler pressure parts, cut inspection programs and personnel, suddenly boiler reliability and personnel safety are compromised, it is time to close the doors.
the major problem (as i see it) is that the older base load plants were not meant to be cycled.
they were designed to run continuously and their life had a determined number of cycles calculated into their design.
some plants have gone through estensive "life extension" programs designed to increase the life of these boilers well beyond the original 30+/- years (financial).
this included replacement of headers, tubes, deareators, and on and on.
prior to the advent of cycling these plants these considerations were relatively easy. but with cycling you now have introduced a whole set of new design characteristics that were not originally considered.
and yes, complete boiler inspections are now a must to run safely beyond the limited calculated number of cycles.
eyec;
We don't cycle any of our Coal plants because power is needed, and is a premium. We used to cycle back in the 1980-1995. Not any more, we run base load.
Meteng is likely correct- other reports suggest a FAC attack of a feedwater or spray water line, or an econ drain line.
Boilers are not required to be retired, but 2 trends will push in the direction of repowering old sites. First , as being done in Japan and Europe, older 33% efficient coal fired boilers + turbines are being replaced with ultrasupercritical units at 43% LHV efficiency, for a significnat reduction in normalized CO2 emmissions. Second, the corut interpretations now suggest that signiificant modifications of old boilers will imply that billon dollar scrubber retrofits will also be required, pushing teh utility to mothball or retire or repower the site.
Also, ASME section I does not yet recognize fatigue damage for large coal fired boilers, so a 59 yr old boiler with 10,000 startups and which was never designed to manage thermal fatigue damage is probably ripe for replacement. While there are design features that can be retrofit to improve the plants operational flexibility and manage new fatigue damage, nobody can accurately confirm the extent of fatigue damage that had occurred over the last 59 yrs.
"nobody can accurately confirm the extent of fatigue damage that had occurred over the last 59 yrs"
I guess that was my point, nobody really knows the amount of cummulative creep damage that has occured, therefore nobody really knows how safe that the plant is nor do they know(exactly)how long to extend the life.
Of course there are "educated guesses" about replacement of superheater tubing, atemporator piping systems, places where FAC may occur etc. etc. are prudent, but to my way of thinking all of these "life extension" programs are still, fundamentally guesswork.
Because European accounting standards and environemntal laws are different than in the US, there is a tendency here to try to run older less-efficent plants virtually forever.
The European plants seem to have a plan for eventual retirement. The US accountants seem to be able to revive virtually any "zombies" boiler with a stroke of the pen
I cannot help to conclude that we will see more "Salem Harbor" type accidents in the future at older power plants....
Maybe some damage mechanisms are getting mixed up.
Creep damage in ferritic tubes occurs at metal temps above 800F, but the reports are the failed pipe was at 300-350 F. There are test procedures to estimate local creep damage.
FAC flow assisted corrosion usually can be detected by simple UT test for wall thickness. The many failures of the last 10 yrs and the many EPRI reports on this issue would suggest that a failure on an old boiler would be accompanied by the sound of many lawyers feet scrambling to get to the families.
Fatigue damage is trickier. Until the microcrack becomes visible, there is no physical test to indicate the amount of life consumed . And once the crack is visible, it is tempting to claim that a thermal stress induced crack is slef limiting, and ignore it until its lenght can predict a prompt failure due to primary stresses.
One thing about a boiler that old; when it was designed it was done with a slide rule and the factors of safety were a lot higher so it has a lot more metal in it. Design one the same rating to replace it using modern tools like FEA and that thing sure won't be there in 59 years from now.
I've seen some tough old beasts out there still ticking along just fine-although inefficient as he**. I have seen brandy new boilers split tubes and have piping let go. So who is to say?
All in all, I think it is criminal to be operating a 59 year old unit if it hasn't had modern pollution control added and that the power industry has brought a lot of the current hype about green house gases on themselves by hanging on to some of these dinosaurs.
To be honest, I am not qualified as a modern metallurgist to say with 100% assurance that, in the general case where a field sample is taken from a 59 yr old component, that one cannot with assurance demonstrate what the fatigue life consumption is, prior to the onset of a visible micro crack.
A quick review of the some recent literature indicates that , in a lab setting , where the sample's crystal structure is known, and the sample is deliberately cycled to a known strain value and known number of cycles, one can characterize the change in crystal microstucture vs amount of fatigue damage, using X-ray diffraction and magnetic Barkhausen emmission methods, prior to the onset of the first crack. But to proceed from that premise to determining the amount of fatigue damage consumed from a random sample sounds like a stretch at this time. Maybe in another 10 yrs.
Normally, when you are dealing with boiler pressure parts the concern is low cycle, thermal fatigue and from my standpoint, this damage mechanism is understood. Thermal fatigue damage is evaluated in lower drums and steam drums by monitoring crack growth every day. In my experiences, these cracks become limited in depth based on a reduction in the stress field as you go through-thickness.
The real concern is creep/fatigue. Here, things get a little dicey because there is no clean correlation to address the effects of creep interaction with fatigue. One can only deal with periodic inspections and monitoring crack depth with the premise to change out the component at some future scheduled outage to reduce risk of failure.
Regarding critical components like piping and high temperature headers, one has to continue to monitor these items and when cracks or flaws are found to replace them to avoid risk of failure.
Age of the boiler has nothing to do with this, It is design and selection of material.
Salem Harbor failure sounds like a potential corrosion fatigue of a boiler external supply tube or riser. I have seen failures like that and normally they are un detected until failure. The best detection is eather internal boroscope or digital xray looking for ID initiated pitting forming a line and eventually a crack. However if the temperature of 350F and 200PSi are correct it my not be a boiler riser.
Corrosion fatigue failures at several Utilities have become more common due to the age, number of cycles and the corrosion from multiple chemical cleanings and less then perfect water chemistry.
Anyone heard what where the failure ocurred?
I've read a post on the Yahoo finance message boards that it was a water wall tube rupture on the cold side of boiler around the 16' to 20' el. The 3 men were working on a fan at 16' el. It definitely sounds like it could be corrosion fatigue.
