Boiler Safety

[rake60]

I've seen several threads about building boilers.
I've never built a boiler myself because I do not have enough knowledge
on the subject to be positive I could do it safely.
I'm certainly not qualified to discuss boiler design safety.

I do know what runaway steam pressure can do.

This for instance.

A few minutes before then, that was a locomotive engine.

Water is a pretty benign compound.
It boils at 212F, 100C at sea level and turns to steam.
When water turns to steam it's volume increases by a factor of 1600.

If you have a small model boiler that holds 1/4 cup (4 ounces) of water, and
it gets far to hot too quickly, that 1/4 cup becomes 50 gallons by volume.
If your pressure vessel isn't properly vented something is going to break.
The first thing will obviously be the pressure vessel.
It's the things that it takes with it that concern me.

A little knowledge can be a big hazard.
Let's be careful out there!!!

Rick

 

[Mcgyver]

good advice Rick, you don't want to mess with boilers. Unless you are very comfortable with the engineering, build to published designs.

In fact its not the pressure that you need to be afraid of, its the drop in pressure - that's what causes the explosion.

At say 100 psi, the water is liquid at 328F, which of course it can only be because of the pressure. If you drop this pressure quickly by lets say a rupture in the boiler, you have 328F water at say atmospheric pressure, in other words you have a bomb. It was only the pressure that kept the water liquid, at atmosphere, that liquid needs to instantly flash to steam - explosion results. A lot Newbs don't appreciate this and erroneously assume a boiler letting go is about the same as an air compressor - WRONG! There is many times more energy stored in the boiler.

The boiler safety release valves keep the pressure well under the engineered level where a rupture can occur....but there are still other ways to blow oneself up! Drawing off steam too quickly with too large a pipe can pressure to drop too quickly and an explosion results.....I've been told that valves on large boilers must be opened slowly to avoid this.....or letting the water level get to low, the crown sheet gets exposed and overheats, silver solder melts and there's a big pressure drop.

 

[rangerssteamtoys]

So if one was building a steam buggy, you would NOT want to hit the throttle super fast. Boilers are dangerous, but with saftey precaustions everything can be less dangerous. But with a stupid operator................Well. thats when things go wrong.

 

[itowbig]

WOW!!!

 

[rake60]

Ranger no one is tryinf to DO anything other then point out the dangers of a
boiler.

The fireman responsible for the boiler on that exploded loco engine was an
an experienced man.
A sudden change in the rail bed grade caused the water level in the boiler to
shift exposing the metal to an unprotected heat spike.
By the time he would have known something was wrong he was too dead to
talk about it.

It's that REAL!

Rick

 

[chuck foster]

ranger
i'm not trying to side with anyone or pick a fight with you but i think the last to posts were meant to give you some in site as to what MAY happen.
we all want you to have fun and be successful with you engine,boiler and buggy. we all want you to live to tell us all about your adventure with steam.
hell i'm envious of such a young guy doing what you are doing, when i was your age i new what a steam engine was but had no idea how it worked and certainly couldn't built one!
you know how an engine works (you proved that) you are well on your way to having a boiler and as far as the buggy part............i'm very certain it is just a matter of you building it.
at you age the sky is the limit, so i for one just want to make sure you are safe and live to be old.

if i'm out of line with this post or too blunt, i apologize and please remove it.

chuck

 

[Cedge]

Chuck
Remove it my great aunt Fannie.... You've just eloquently expressed the thoughts of many of us. Ranger's freedom to explore his imagination is a thing to be envied. In just a few posts, he's managed to assert his bona fides and to scare the crap out of a number of us, but few of us would hesitate very much before trading places with him...LOL He's quickly becoming worthy of the honored title of "House Mouse" on this board.

Steve

 

[tmuir]

Wow that loco photo.

I remember a few years ago going to the 'Beamish Museum' in England and they had a portable engine that had suffered a boiler expolsion there. The metal was a good half inch thick and had been peeled back like an orange.
Wish I had taken a photo of it. It's scary to think about the power inside a large boiler.

 

[ksouers]

What Chuck said!

Double!

 

[Mcgyver]

Mcgyver, I know what your doing.

??? then that makes one of us - enlighten me

 

[1Kenny]

A quick Google search of steam boiler operation gave me quite an in site.
http://www.doli.state.mn.us/pdf/guide2saferboiler.pdf

http://www.licensedelectrician.com/Store/AT/Boiler_Oper_Wbk.htm

It seems like it takes around 2000 hours of training to be able to take the test get a license and run a boiler system in a trust worthy manner.

When I taught aviation maintenance it was a 1950 hour course to qualify to test for the license. We would tell our students after testing they were now qualified to learn.

Kenny

 

[PTsideshow]

Here is the investigation results for the Medina Ohio traction engine boiler failure from a couple of years ago,with pictures.
http://www.doli.state.mn.us/boilerohio.html
and here is the state of Michigan's boiler licensing info for all types of boilers model small or full size.
http://www.legislature.mi.gov/(S(tpo0ip55foguqs55iqchdt55))/mileg.aspx?page=getobject&objectname=mcl-act-290-of-1965&queryid=39491&highlight=
Called the boiler act of 1965. the sections are broken down into type etc.
glen

 

[Alphawolf45]

.

Lets not pass quickly by this point..What about the pressure relief valves- if they let go as they supposed to do, does letting off the extra pressure exacerbate the situation?
.

 

[PTsideshow]

Lets not pass quickly by this point..What about the pressure relief valves- if they let go as they supposed to do, does letting off the extra pressure exacerbate the situation?

When the safety valves operate as they should whether there is one or more they reduce the pressure at a controlled rate. Makes no difference if a model or full size. Most of the older toy and plans for model boiler safety valves are spiring loaded and also acted as as the fill valve for the boiler. So they would be assured of opening upward. Since they would have to be pulled up against the spring inside the boiler to fill. Unless they were of the knurled threaded type that could be removed for filling and could be visually checked.
The pull up style would leak a little, if they received heavy use for filling. As the springs were compressed.

 

[Mcgyver]

Alpha, the relief valves set an upper limit on pressure and that protects the structure of the boiler from undergoing more force than it was engineered to handle. The water, heated continuously will continue to rise in temp which correspondingly increases pressure. Safety relief valves create a pressure ceiling, but i sense you know all that.

To your question, the safety relief valve would only exacerbate the situation if it was so large in diameter that it dropped the pressure too rapidly. So don't build a 1" dia relief value . As to what rates and sizes for given boiler lead to instability, not something I’d know offhand, but every safety relief valve I’ve seen is quite small compared to the steam lines so i hardly think it an issue.

For the poor victim of an explosion, that its not the rupture of the vessel itself that causes the explosion but the resulting sudden drop in pressure likely would seem painfully academic. However in trying to understand where the danger lies and in preventing a boiler explosion it is important to properly understand this

Here's another way of coming to understanding it. Situation 1, hydrostatically testing a boiler to 150 lbs and the end lets go. What happens? nothing other than a bit of mopping up and sheepish looks from the builder. Situation 2, same boiler, same pressure, same failure but this time the pressure is create by the water being heated instead of a hand pump. the end lets go there's a major explosion - all the water (which is at 365F) is brought to atmospheric almost at once and as water doesn't want to be a liquid at 365, flashes into steam.

Point is, that as shown in situation one, the hydrostatic test, the boiler being under too much pressure and lets go is in itself fairly benign - the boiler 'exploded' under pressure but there's not a lot of energy in what was in the boiler. In situation 2, when that energy is stored as water heated over 212, there can be a lot of stored energy that wants to express its self by suddenly occupying a very large volume, kind of the definition of a bomb.

Ranger, still wondering what were talking about with the crack-off about knowing what I’m doing, are you peaking in my windows or something? seriously, wtf were you talking about?

 

[SandyC]

Hi Guys,

Lets get this a bit more in context....... Water reaches boiling point at 212deg F...however, it does not turn to steam...... this requires the input of additional heat energy to change it from one state to the other, however...there would be no increase in temperature during this process.
This is known as 'the specific enthalpy of evaporation' and for water at 0psi.. this is 970Btu/lb.

1lb of water at 4deg F would occupy 27.7cu ins of space... the same 1lb of water, when turned to steam @ 0psi would occupy 26.8 cu ft (46,324 cu in) and it's temperature would be 212deg F.

It is only when restricted from free expansion, that steam can increase in pressure.

It is not a pressure drop that creates an explosion.... it is the sudden, 'UNCONTROLLED', release of the contained energy, via E.G a failed stay, or a corrosion hole in the shell.

Any release of this energy will result in a pressure drop, if the rate of release is greater than the rate of evaporation.

In the first case posted by ptslideshow... 'The michigan traction engine boiler'...... the problem was caused by poor maintenance/inspection procedure...... when the first stay gave way, at the very much weakened plate juncture, the load was immediately transfered to the remaining stays, thus increasing their load.... which then gave way one after the other, resulting in a massive, and almost sudden release of the boilers stored energy..... the pressure drop to zero was a direct result of this action..... it was not the pressure drop which caused the explosion.

Safety valves are designed to give a controlled release of unwanted pressure and their throughput/capacity is carefully calculated to do this....... when they do open, it is usually a sudden action (pop action) and they open into what is probably the biggest dia pipe you can get...I.E the atmosphere.... so pipe dia has very little to do with it..... if this were not the case, and things happened as has been suggested... then the sudden opening of a safety valve would instigate an explosion...... NOT SO.

The same applies to boiler steam valves.....they are controlled release devices and if correctly matched to the service they are connected to will operate safely.

The reason boiler valves are generally opened slowly is to eliminate/minimise thermal shock, not only on the valve, but also on the pipework leading from the valve to the receiving service device.
There were many maritime cases of CAST IRON steam deck valves (such as would be used for deck winches), especially in cold climates, being opened to quickly with the result that the valve, or the pipework leading from it, cracked, due to thermal shock, and subsequently broke off, which then allowed a huge amount of steam to escape uncontrollably...resulting in a nasty accident....... certainly the pressure in the ships boilers dropped pretty quickly, however, the boilers did not explode.

A few years back it was necessary to change all UK boiler and deck service valves from cast iron types to cast steel types, since these have a higher shock load strength and are deemed less prone to failure.

So...yes steam can be dangerous, if not used with proper caution.... But so can driving your car.

Treat it with respect, learn about it's peculiarities and you should have little to worry about.
Above all, if you are not comfortable designing a boiler yourself, then build from a published design.

Best regards.

SandyC   ;D ;D

 

[te_gui]

Can someone let me know how to post pictures. I wanted to tag a couple on here and can't figure out how to upload them. I'm certainly no boiler expert but have been involved in rebuilding a couple full sized ones. I also ran the calcs and presented my findings to my state boiler board on a non ASME code boiler in order to receive a certificate to operate it.

While we are on the subject of devices to drop pressure quickly, let's not overlook whistles. They can be real steam hogs and some overzealous folks like to blow the hell out of them. It makes me cringe at some of our local steam shows when I see a big mill whistle up on the dome of a traction engine, usually with several reduceres to get it down to the engine plumbing. Between the volume of steam and the large unsupported mass hanging off the small pipe, Yikes.

Brian

 

[PTsideshow]

Can someone let me know how to post pictures. I wanted to tag a couple on here and can't figure out how to upload them.
Brian

For some reason on this thread The attachment window below this message window doesn't show. With the additional options a plus or minus.
I'm having the same problem when I went to add pictures of the two safety valves in my other post. For me at least it seems to work in other threads!

 

[Mcgyver]

It is not a pressure drop that creates an explosion.... it is the sudden, 'UNCONTROLLED', release of the contained energy, via E.G a failed stay, or a corrosion hole in the shell.

and

.... which then gave way one after the other, reulting in a massive, and almost sudden release of the boilers stored energy..... the pressure drop to zero was a direct result of this action..... it was not the pressure drop which caused the explosion.

Sandy, i think we're basically saying the same thing, but it sounds like disagreeing on the chicken and egg. With the greatest respect to you, It is the sudden pressure drop that causes the instability and hence the uncontrolled release of energy - I perhaps should have been more specific in saying the SUDDEN pressure drop. At least everything I’ve ever read or been able to find on the engineering of what's going on says so. The two are of course dependent, but it doesn't matter how the sudden pressure drop occurs. Put some ridiculously oversized valve on the boiler and you can create the same explosion by cracking it open. I agree with you open valves slowly is to prevent thermal shock, as well as gently clearing out any condensate in the line but it is also to prevent sudden pressure drops.....granted in a balanced design the later might not be paramount but its not just in my imagination ;D

when they do open, it is usually a sudden action (pop action) and they open into what is probably the biggest dia pipe you can get...I.E the atmosphere

do they? I think they open up a small dia passage that creates a restriction to how suddenly the pressure is dropped

 

[doubleboost]

Hi
I am in total agreement with the safety issues brought up hear.
I am busy with a large vertical steel boiler for my marine engine at the minute
The exploded boiler at Beamish Museum is just that ,the army put a charge in to a worn out boiler to simulate a boiler explosion ,next time i go i will photograph it for you and post it hear

boiler shell 8 inch diameter 20 inches high

Tube plates 10 mm thick 19 1 inch tubes
The boiler will be tig welded together hydraulic ed to 250 psi the working pressure will be 100 psi it will be fitted with 2 safety valves of good size
Regards
John