Safety distances between boilers? 1

[jonesbk]

How safe are adjacent boilers, if the boiler in the middle had to explode? The boilers of concern are coal fired (55 tons/hour) and how far should these boilers be build from each other? Is there a scientific method of calculating the distance/explosion pressure circles or a rule of thumb that is applied?

 

[091123]

well jonesbk, in our practice at least a meter between adjacent boilers for firetubes for the reason of the availability of space. And u must have sufficient space for pull-tube area. If you design a boiler you must consider the stress value of the material, maximum allowable pressure, bursting pressure, at least two safety valve, and factor of safety. If you are not sure, check the ASME code sec. 1, 8, 9

 

[jonesbk]

Thanks for the reply. I currently doing a risk assessment and I am trying to predict the worst case scenario. These boilers are existing and are about 8m apart. I have assume that one boiler will be total destroyed and the boilers on either side will have a downtime for approximately 3 months for repairs. (Including inspection etc). Do you think this is realistic?

 

[chicopee]

I dont ever remember safe spacing requirement for boilers in tandem. There are clearance requirement for repairs maintenance and inspection of boilers and these will be found in the boiler codes that each state in this Nation maintains.

 

[091123]

well jonesbk, that will depend, for example in the food manufacturing industries, your downtime must be maximum of 1 - 2 days or else the huge amount of losses including the repairs and inspections.

 

[jonesbk]

It is our jobs as engineers to keep the down time to a minimum, but the question is what if? What if all safety system fails / could we loss all three boilers / what are our loss in profits / how much of this risk should we carry and how much should we transfer to insurance companies?

 

[TBP]

Depending upon what fails, and how, there's no way to predict how big pieces can be, or how far they'll travel. If you have a massive failure, you'll be down for a while anyway, as the inspection authorities will very likely insist on carefully inspecting any undamaged boilers. The steam header, feedpumps, DA, stack, etc could all be damaged.

I've never heard of industrial boilers being spaced to allow for explosions. Usually, things get crammed in, as close as codes will permit to minimize building costs. If the steam source is considered very critical, then separate steam plants at each end of the production facility are the answer. This is very rare as well, since now you have to have two of everything.

A properly designed, installed, operated and maintained powerhouse will be safe and reliable. Build it right, and staff it with properly trained people who have the resources to do their jobs, and you won't have much to worry about.

 

[davefitz]

There were some recent posts in other forums that described the safe distance based on energy of the blast zone.

The 2 components of most concern are :

a) steam drum - due to steam overpressure. This could only happen due to :
1: gagging of safety valves
2: drum metal fatigue cracks ( or cracks for hydrotest or corrosion fatigue) in the longitudinal direction
3: backflow from the main steam from a higher pressure system and lack of protection from this other system

b) furnace combustion puff- due to feeding coal fines into a hot furnace without proper flame safety systems in place ( or manually wired out using alligator clips).

Although you can make estimates using the safe blast zone methods, it would be better to operate and design the system so such events do not occur. Each of the above issues can be addressed by sound design, operating, and maintenance practices.

 

[MJCronin]

jonesbk,

I have seen many multiple boiler installations and read many installation guidelines by boiler manufacturers. There is no "explosion spacing" typically used in the layout phase of plant design.

Most boiler plants I have seen have about four to eight feet between casings. The spacing is usually set by the building limitations as well as nominal workspace condiderations. This is true for firetube as well as watertube boilers.

It seems like you are using a technique used for process plant evaluation (say for a "fired heater&quot on an area of the plant that does not see this kind of evaluation.

The safety of boilers is a function of proper operation, regular maintenance and the design of fuel and water cut-off devices. Insurance inspectors and local boiler inspection requirements reflect the importance of these features.

My thoughts only

MJC

 

[Spearsman]

jonesbk,

It seems that your concern is boiler explosion (as the worst scenario). There is a saying that "prevention is better than cure". Instead of accessing what happens to the other boilers in case one boiler explodes, for me, it is better to access how to prevent this explosion. Check your control systems whether they comply to relevant standards. You mentioned that you have a coal fired boiler, it maybe working on a balance draft system, therefore, there is also a possibility not only of explosion but also implosion. I recommend you check these standards for reference in accessing the safety of your boilers. (1) NFPA 8501, Standard for Single Burner Boiler Operation (2) NFPA 8502, Standard for the Prevention of Furnace Explosions/Implosions in Multiple Burner Boilers (3) NFPA 8503, Standard for Pulverized Fuel Systems. If your control systems comply with these standards and are operating properly, you don't need to worry about explosions.

Only my idea. Hope this will help.

 

[jonesbk]

I do appreciate the information for the up keep of boilers, prevention is better then cure. But if you dig deep into your insurance cover under assets, you will definitely be covered for boiler explosion. jonesbk

 

[hrsgguru]

If we're worst-case-scenario-ing the question, then I would say you'd have to assume a total loss for the one and significant damage to the other two, given the boiler size and proximity. For a downtime estimate, I'd call the boiler vendor or good local repair company and find out how long it would take to basically rebuild all of the boilers. While you could probably reuse the drums, headers, and some of the water walls, you'll likely have some significant structural damage to the boiler support steel. I've seen the results of a gas-fired boiler that exploded when someone tried to light off the burner with a furnace full of gas. The boiler was apparently unrecognizable. But the building that housed the boiler suffered significant damage also. The main building columns were wide flange shapes about 50 cm x 50 cm and x 4 cm thick. They were twisted and bowed quite alarmingly. If they were part of the support structure for an adjacent boiler, that boiler would have had to have been rebuilt. I think that in the scenario currently under consideration, that building steel is analagous to the adjacent boilers.

 

[htb]

As far as my knoledge goes, I don't think there are any guidelines on the min. distance required between two adjacent boilers. The major criteria would be the availbility of space, area required for regular maintaince and free movement of boiler personnels, id retractable soot blowers are provided than their lengths have to be considered.

As far as predicting boiler explosions you cannot be sure of what and how it may happen, I have seen quite a few boiler explosions and let me tell you there was nothing similiar about them, in some cases the tube shetts had flown about 500 mtrs after going thru a wall, in one case the safety valve was located about 0.8 to 1 k.m from the boiler. Also there was an instance where a non certified boiler (vertical) had actually taken off like a rocket from its foundation and landed on an adjacent tree about 100 mtrs away taking three lives.

What I would suggest is first identify the weak points in your boiler, according to me you should throughly ckeck the welding seams look for under cuts in the welding, have an ultrasonic thk. gauging of the pressure parts look for lamination in the plates check the thk. at places corroded/pitted, look for signs of flue gas short circuits or secondary combustions etc....

 

[MJCronin]

htb,

These boiler explosions that you describe are unreal......do you have any websites or other references that describe these failures and the root causes ?

Do you have any other references, books, reports etc that you could point to about these kind of failures ?

MJC

 

[htb]

mjcronin,
Hi, I have not seen any sites on boiler explosions, the incidents that I have mentioned are true and have seen them, if you want I can give the address where the boilers were installed and here are a few more,
The one about safety valve: well after investigations it was found that the L.S of the shell had opened due to undercut in the seam, and the explosion also caused damage to the adjacent boiler.

One more, lets see if u can explain this one:

Shell type external furnace boiler:
The F.E tube sheet bludged due to low water conditions, after inspection it was found that the shell which was not exposed to the flames or gases that multiple buldges on the top side i.e steam space say from 11 o'clock to 1 o'clock, could u explain why were there buldges in the seam, tell me your opinion then I shall share mine. I have seen these accidents and have my own opinions about them, lets discuss one at atime, even if you have seen any incidents pl. write.

 

[TBP]

MJC - I have seen websites (sorry - I don't recall exactly where) where there are accident reports for boiler explosions in developing countries, like India. Boiler accidents involving loss of life seem to be a pretty regular occurance, as it was in North America 100 years ago. I would suspect that the experience in places like China would be largely the same, as they become more industrialized.

 

[davefitz]

There have been recent USA boiler explosions.

Circa 1999 the River Rouge ( Ford auto plant) boiler exploded and suffered some fatalities. It was origianlly a coal fired boiler built circa 1948, and converted to natural gas in the 90's. Furnace explosion due to inadequate flame safety /purge/ jumpered flame safety opanel, I assume.

Circa 1979 Arizona Power or Nevada Power ( near four corners, new mexico) had a furnace explosion. Coal fired boiler was recently converted to lo-nox burners, allegedly the flame saftey panel was jumpered to always show a flame ( via alligator clips) , poured coal into a hot furnace and no supporting flame led to the accident. Parts were found 1/4 mile away.

Circa 1988 Mohave station ( near Las Vegas) had a hot reheat transfer pipe fail outside of the cafeteria. 11 workers killed. Caused by type IV creep cracking of a P11 longitudinally welded pipe- creep daamge accelerated by operation of the plant for about 1 year at 100F above design metal temperature.

Circa 1993 , BeiLungang tower type coal fired plant ( China) brand new unit by US mfr , just 1 day past commercial acceptance , 50 ton coal slag piece dropped off upper platen superheater onto hopper- caused a 6" feeder ( from downcomer to waterwall inlet header) to fail, killed 23 workers who were trying to rod out slag from hopper throat.

 

[htb]

Failures due to flame / electrical / sensor failure is a common feature, from my exp. and what I have seen I would say that majority of accidents take place on auto controlled boilers, because of automisation the attends tend to relax and don't often check / verify the controls.
The accidents that are interesting to study are the ones caused due to material failure or faulty workmanships, these are the things that give an engineer the insight in failures that no book can provide, there are some instances that no engineer while designing might have dreamt of.