There are typically supply tubes 4-5inch in diameter that are external of the boiler walls. They will come off the lower mud drum and these are the ones that are most susseptale to corosion fatigue and rupture rather then leak before break. The reason is they are thick wall and can have some elevated stress level. The age (59 years) is also a clue. CorrosionnFatigue is showing up more now due to the aging boiler fleets. It is a combination of corrosion and stress.
On Friday November 16, 2007 a Dominion Boiler Inspector(?)held three meetings for plant employees at 0930, 1000 and 1030 hours to give a slide show presentation covering the explosion that killed three men.
The job these men they where doing had nothing to do with the failure. They where "at the wrong place at the wrong time"
They where on Elevation 16 (the ground floor) and the explosion occurred directly over there heads and behind them.
Up to about two feet of ash covers the floor were they where working. All the units are off at Salem Harbor and the building is sealed while an asbestos clean up plan is made for the entire(?) building.
There was a tube failure of division wall tubes at the header (16") which was in a "dead air space". These were riser tubes, that is they supplied water to the boiler which as the tubes existed the dead air space would be exposed to products of combustion, and turned to a steam and water mixture.
The water temp. out of the economiser which supplied the header was possibly 500 to 600 degrees F and about 2000 PSI(about boiler pressure).
The header had counter bored(?) recesses in the holes where the tubes meet. A short sleeve is welded into the hole for the tube (Maybe 2" OD- not sure) the tube metal is reportedly .240" thick(when new)).
Two tube where sheared flat across just above the welded sleeve. There was evidence of other leaks cutting tubes in this area. And other tube damage "dimpled?"
I-Beams in the area about the size or bigger than the header were bent or buckled. Some tubes that did not fail had moved 12" from original position. (before the explosion?)
The dead air space starts 19' up from the bottom of the boiler (not the floor) and slopes in 5' from the east wall to the "throat"). the boiler has dead air space running north to south on the east and west side of the unit.
This dead air space was "full of ash and asbestos". The last time it was cleaned out and entered may have been as long as 12 years ago. The "guys" at the plant are trying to remember. The Dominion "inspector" said he did not know what B&W recommends for cleaning and inspection times, and he did not know when the last time it was looked at. Dominion has owned the plant for about 3 years.
Reportedly 1 to 2 years ago this area was suspected to have a leak but the inspection was called off because when the access cover was opened and the "Ash and asbestos mixture" was seen to be in the way it was decided to put the unit online rather than delay the start up and pay for the asbestos cleanup.
Earlier this year, before the scheduled major outage which included a major turbine overhaul, (6 weeks long ?), it is said that this boiler was operated at reduced pressure because of concern about reoccurring tube failures. Dominions "engineering department" had calculated a reduced safe operating pressure based on reduced tube thickness which was about 100 psi below normal.
Some employees believe Dominion will remove these Emails and other Emails that might be useful to investigators trying to determine the level of negligence, if any, that exist at this plant.
Unit 3 has had several serious tube failures in the last couple years, at least 1 of which "would have killed any one who happened to be near it". According to some people at the plant.
Other units at Salem Harbor have had similar serious tube failure that "would have killed any body who happened to be near that area". This is according to plant workers.
These same workers say that the only reason that the plant is closed for asbestos cleanup and "examination" is because "people died" and the other asbestos releases from these other tube failures were just"cleaned up" without closing the whole plant to do it, and that other tube failures which "could have killed someone" where not investigated to the degree that this failure will be.
Dominion Management is alleged to have pushed operators and maintenance workers to ignore standards that may be required in order to get units online quickly. This includes not doing hydrostatic testing after welding repairs on pressurised parts of the boiler . Unit 3 was reported to have been put online recently without a "hydro" after a welded repair. (This if true should be able to be confirmed through interviews, log book entry's and examination of the "PI system" (sic?)".
The ash in this boiler when mixed with water is reported to go to a PH of 2 or less. This dead air space where the failure occurred is close to the water seal, and the water used to remove the bottom ash.
Allegedly, the acidity of the ash water mixture is common knowledge in the plant and procedure are in place to raise the pH of the water used to wash ash before it is dumped in the ocean in the harbor.
Units 1,2 and 3 use a selective non catalytic reduction system (SNCR)to reduce oxides of nitrogen (Nox). The station
does not have a permit requirement for Urea "slip". This means that Salem Harbor could spray excess urea in order to reduces nox. The effect of the excess urea, if any, that may have have collected in the ash and have an effect on the tube failures, is not reported at this time.
Salem Harbor units 1,2 and 3 run "out of compliance" in regards to oxides of sulphur or Sox permit requirements. There are conflicting reports to how this is legal but this may be a "grand-fathered" plant.
Dominion is understood to have said publicly in newspaper articles, that when they bought the plant that they where going to spend the money necessary to bring the plant up to modern environmental standards, so that it would not be among the "Filthy Five" worst poluters in Mass..
Salem Harbor Station is said to have between 5,000 to 8,000 outstanding work order request, while Brayton Point, another Dominion owned coal plant in Massachusetts, is said to have over 10,000. Work orders are requested by workers when a piece of equipment is not working properly.
So far dominion has not built the spray dry atomization (SDA) vessels required to remove sulphur from flue gas. Some critics believe Dominion was planning to "run the plant into the ground" and was not serious about ever bring the plant up to modern standards, this would explain the large number of outstanding work request. (Why spend money on a plant you are really planning on closing when regulators finally "get sick of you stonewalling").
This year dominion experimented with injecting lime slurry into the furnace area of the boiler, in an area probably hotter than than 1,700 Deg f., in unit 3 to reduces Sox pollution from going to the atmosphere, These test were not successful.
This experimant is alleged to be based on succesfull labortory test, however succesfully reducing Sox by injecting lime slurry into temperature zones this hot in a coal fired furnace is allegaded to be uncommon and speculated by some to have been a false attemp to buy time by showing regulators et al that Dominion was moveing forward to reduce pollution.
The sulphur in the coal, some people say, combines with water to create sulphuric acid. What difference the fact that Salem Harbor may not have been burning the lowest sulphur coal available and that they appear to have been running out of compliance with regard to sulphur oxides pollutants, and what difference this may have made if any to the increased corrosiveness of the ash or the increased build up of sulphuric acid and its relationship to the tube failures has yet to be proven, if it exist at all.
The availability of low sulphur coal in the United States is limited in part because President Clinton prevented mining of the worlds largest supply of low sulphur coal when through executive order(?) he created a "National Monument" on the hundreds of thousand of acres that held the low sulphur coal.
The next largest supply of low sulphur coal is in China. Salem Harbor ususually burns coal from South America in units 1,2 and 3. Unit 4 for burns Oil
Allegedly ISO New England which (operates?) the grid which Salem Harbor supplies electricity to is, according to some people understanding of newspaper articles, is under the impression that, in essence, Salem Harbor must be allowed to run because there is insufficient distribution infrastructure (electric lines) into the area around Salem Harbor.
This may be one reason Salem Harbor has been allowed to run out of compliance to its sulphur environmental permit and without a permit limit on urea that leaves that stack.
The "plume" from the stack at Salem Harbor which was sometimes visible 5-10 mile away from the plant has been attributed to excess urea "slip".
At some times Salem Harbor would reduce the urea spray for PR reasons so as not to upset "activist". This was thought to reduce the plume.
The Dominion Boiler Inspector(?) said Dominion owns other boiler with similar design and that as of Friday, november 16, 2007,( 10 days? after the explosion), they are still online(not an exact quote).
In newspaper articles and on TV news, some IBEW Union representatives and the Salem Fire Department have publicly said that this is a very safe plant.(not an exact quote)
The president of Dominion spoke at the Plant Friday, 11/16/07 at 1:00 pm at the North Stores warehouse. He did not address the outstanding work order problem but he did say that for Dominion Safety was the #1 priority. (not an exact quote). Nor did he say how this accident might effect Dominions plans to open new nuclear plants.
He did not talk about bonuses paid to himself or plant managers either nor did he say if he received bonuses paid for meeting online percentage goals.
Some workers believe that "corruption" will allow for criminal responsibility, if any, to be covered up.
According to news paper articles( see Boston Herald, web sight FYI1301.com and others) FBI and Massachusetts State Police have arrested or investigated some Massachusetts Dept of Public Safety Boiler Inspectors in the the last 10 years for corruption.
The District Attorney, OSHA and Massachusetts Department of Public Safety and others are investigating this accident.
Posting this with the hope that nobody else dies in our plants.
A good write-up on the failure. But, I'm still not clear on the cause. Was the riser tube failure described as corrosion fatigue or was it flow accelerated corrosion? Also, it sounds like the dead air space was pressurized by the tube failure and resulted in an "explosion" which could have caused the buckling of the I-beams and bulging of the tube panels. Is this correct?
From the write-up above it was neither, to me it appears to be external corrosion attack on one or more terminal tubes coming off the the lower supply header. The external corrosion was attributed to wet boiler ash (during off-line conditions) that accumulated in the dead air space.
Could be external corrosion, could be a small leak had washed and cut away the tubes prior to failure. Could also have been a slag fall- such a large slag fall had killed 21 operators at a boiler in China 10 yrs ago, tubes ahd failed in a similar boiler section. Let the boiler CSI team make their report- we'll hear about it later.
The report is contained in a large .pdf with an extensive collection of pictures.
IMHO, It is important to note that we seem to have a "perfect storm" for similar boiler accidents in the future as we have:
1) Many other 40-60 year old boilers designed by B&W (the USA GM of boilers) still in operation, at or beyond thier rated capacities and performing in a cyclic service that they were never designed for
2) A US accounting system that is configured to keep these "Zombies" in operation until people die....
3) A reactive, punishing state regulatory agency that only becomes functional after an accident, only seems to have a limited understanding of boiler operations and can only shut things down and punish individuals.
Please tell me or prove me wrong here....Why cant a very similar accident occur in another state with another of these ancient boilers..??
Well, on the one hand any poorly maintained 50 year old unit doing cyclilng service is probably a ticking time bomb, but on the other, this reminds me of the current mortgage crisis-featured daily are the 4-6% of the mortgages that are in trouble and nothing is ever said about the 94-98% that aren't. There are hundreds of these old units, (adding in the FW's, CE's, etc that were also built during that time) that didn't (or haven't yet) explode(d).
I suspect that there are some serious inspections being conducted by the other 399 of these old B&W units (as well as the other makes that I mentioned) that were built. The insurance carriers are seeing to that.
And, I have seen furnace bottom slope explosions in boilers that weren't even old enough to be termed teenagers yet, so I don't think the problem is isolated to just these old boat anchors. Poor maintenance will get you every time.
This all makes for very interesting reading. Unfortunately, nowhere did I see anything mentioned about the boiler supports. Some would say that the North American "bang and clang" method of setting and monitoring hanger loads may have had some impact here.
Significant effort is taken during the design phase to properly spec the load of each hanger so that headers are accurately balanced in an effort to prevent undue stresses (which can lead to tube failure - exacerbated by creep). Alas, when it comes time for construction, the extent of QA in this critical application is the use of the "Magic Hammer": If the rod "pings" when struck by this high-tech instrument, all is fine. If it "pongs", the nut must be driven further down the rod, against the bearing plate. This same "complex and highly controlled technique" (hah!) applies a year or so later when it's decided that it might be time to determine if the loads have migrated.
Come now: Is this "Mechanical Voo-doo" really how we want to entrust the safety of our people and the availability of our plants? Following this I wouldn't be surprised if somebody proposes checking rad levels in vaults by measuring the viscosity of a 'possum's spit after being inside for two turns of a purple egg timer :-(
