Boiler Safety

[rake60]

The MAIN point is a boiler is a pressure producing unit.

Pressure wants to be find a release point and it WILL!
If that weakest point happens to be someting other than the outlet or
the safety relief valve, you have a grenade in front of you.

That is also a very useful tool in it's place, but NOT one you want in
a hobby environment.

Rick

 

[Hilmar]

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.

Hi Sandy are you sure about the 4° F ? or is it 4° C

 

[PTsideshow]

here is one style of the Jensen external spring/fill plug safety valve

here is the safety valve with the spring compressed so it was in the open or relief position

here is the PM research internal safety valve taken apart. there are three holes in the top of the cap for the steam to come out when the valve is open. this valve has a spring pressure of 60 to 80 pounds.

No o-rings,or other type packing that can melt or impede the operation of the relief valve.

 

[Cedge]

You guys seem to be getting trapped in a terminology vortex. To a point, both of you are right. The controlled release of the steam at any pressure or temp is not likely to create any problems at all. . ie ... whistles, throttles, blow downs or even a safety plug drop. Where the argued problem rears its fatal head is when there is an instantaneous event where the super heated liquid is suddenly exposed to normal atmospheric conditions such as through a catastrophic vessel failure. Once this type of event occurs, the damage to the pressure vessel is rapidly progressive as additional components can't withstand the added stress. The fluid state which has been maintained due to the high atmospheric pressure within the vessel is suddenly unleashed, "accompanied" by an extreme drop in pressure which results in a super expansion of the now highly unstable fluid. The flash to steam results in the stored energy seeking equilibrium in a normal atmospheric pressure environment. Results.... Big boom with a huge speedily expanding cloud of very angry searing hot water vapor that happens to be hiding rather large chucks of high velocity metal.

You'd see a similar effect if a decompression chamber failed, as the very air inside sought equilibrium through rapid expansion. The results would be exhibited as intense heat. One of the most unsightly deaths I've seen was from a truck tire failure, while it was being mounted back on the truck. The man was never struck by the tire at all, but the super heated air generated during the sudden loss of "containment" burned his flesh and seared his lungs beyond function. He was a dead man before the rest of us even heard the explosion.

Boiler size is not much protection either, as even a mistreated toy boiler can become a grenade, in a worst case scenario.

Steve

 

[SandyC]

Hi Guys,

You are quite correct here Steve it is a terminology thing…… and Yes Mcgyver we are indeed talking about the same thing in our own, NOT SO CORRECT WAY.

Notwithstanding this, I feel it is important, for the newbies who may be reading this, to get the actual cause correct.

As Steve has pointed out…  “Where the argued problem rears its fatal head is when there is an instantaneous event where the super heated liquid is suddenly exposed to normal atmospheric conditions such as through a catastrophic vessel failure”

Not so much an argument, but rather a poor explanation by both Myself, and Mcgyver of what actually occurs.
so lets take another quick look at the thing: -

Since PRESSURE is a MEASURE of energy density expressed as: -

P = force/area which, when related to fluids (and yes, steam is actually classified as a fluid, not a gas),  can also be expressed as P = energy/volume (Bernoulli equation).

It follows then that you cannot have a sudden change in PRESSURE, since this is determined by the other two parameters.

By definition, it cannot, therefore, be a SUDDEN DROP IN PRESSURE that instigates the problem…. a change I one of the other two parameters MUST occur first.

In the case of a boiler rupture the sequence will be as follows: -

Boiler shell fractures (cause irrelevant) which initially connects two separate VOLUMES together (in other words we have a change in the VOLUME parameter.

If the added volume is at a different PRESSURE (in this case a lower one (atmospheric))

We now have a situation where there is a PRESSURE DIFFERENTIAL, and also, by definition, an ENERGY DIFFERENTIAL, between the two connected (via the fracture) volumes….
What follows is the rapid shift of the ENERGY content, in order to eliminate/equalise the ENERGY DIFFERENTIAL…… the high energy content within the boiler volume will move towards the lower energy content of the connected, increased, volume.

As a result of this exchange/movement of ENERGY the resulting FINAL PRESSURE as a whole will drop and is in fact the LAST thing to stabilise.

(if the added volume pressure was higher (meaning there was higher energy within it), then the ENERGY exchange would occur in the opposite direction, and the resulting final pressure within the boiler would rise)

That is not to say that this happens slowly, in fact it happens extremely quickly, however, the sequence of events does not change…… just don’t be anywhere near the thing when it happens.

So you see guys, we all are wrong, yet right at the same time…. It is a complex subject, and one where terminology can often become miss-used; I for one hold my hand up to that one.

Ok chaps, lets put this to bed now, Else we will get told off for arguing by the MOD’s ; 8) ??? ???
'Spoilsports.....Can't have any fun these days' ;D ;D ;D ;D ;D

Best regards.

SandyC.

 

[raym 11]

One bit of fun before bedtime;

Popped corn is a result of a steam explosion.
I wonder what the average pressure is at rupture?

Ray M

 

[SandyC]

Hi Sandy are you sure about the 4° F ? or is it 4° C

Hi Hilmar,

OOPS.....You are quite correct..... that should have read 4deg C (39.2deg F)..... my mistake when typing.

SandyC

 

[Mcgyver]

Oh jeez, this may be long ;D

Ok chaps, lets put this to bed now, Else we will get told off for arguing by the MOD’s

If you don’t want to debate further, peace, to me its a debate not arguing, but don’t hold off for the wrong reasons…imo, so long as no one is using foul language or being mean spirited/malicious, these discussions should carry on. If for some reader it becomes too esoteric, they can walk on by.

They should go on because they bring about learning. It’s not about having to be right, I can and do admit when I’m wrong – but in having to debate a point forces a more complete understanding and often enlightenment when you learned something new or even the flaws in your existing thinking. Imo it’s a cop-out to the second there are differing views on something quickly proclaim there’s more than one way to do something and stop the discussion….provided everyone is being polite that just creates mush not learning and is not as interesting/engaging.

So in the friendly spirit of lively discussion…..

As Steve has pointed out… “Where the argued problem rears its fatal head is when there is an instantaneous event where the super heated liquid is suddenly exposed to normal atmospheric conditions such as through a catastrophic vessel failure”

Steve phrased this very well but missed a crucial point – its not that the superheated water is exposed to atmosphere, its that its exposed to a lower pressure….there would just as readily be an explosion if the pressure suddenly dropped from say 100 psi to 1 psi or 10psi etc over atmosphere – while the water is still at an elevated, superheated temp. Where he did nail it is that it is the fact that the water is now super heated the causes the trouble.

(and yes, steam is actually classified as a fluid, not a gas),

All liquids and gasses are fluids.

Since PRESSURE is a MEASURE of energy density expressed as: - P = force/area which, when related to fluids can also be expressed as P = energy/volume (Bernoulli equation).

I understand P=F/A but can you walk me through how you go from Bernoulli’s equation to P=E/V?? Bernoulli’s equations deal with flow, right? I can see that for compressed air where the added potential energy all shows up as pressure, but this equation P=E/V? for a boiler? Rearranged that’s E=P*V. That’s saying then that a 100psi 1 gallon boiler with .7 gallons of water at room temp has the same energy as a 100psi 1 gallon boiler with .7 gallons of water at 328F which is obviously false.

The potential energy of water/steam is function of its temp and mass not volume– iirc it’s same energy whether its held within a boiler or as steam occupying 1600x’s the area (the explosion comes from the rapid transition from one to the other).

It follows then that you cannot have a sudden change in PRESSURE, since this is determined by the other two parameters.

But you can – the explosion is the violent attempt of the super heated water to again reach equilibrium with the sudden drop in pressure – the equation balances because of the increase in volume.

By definition, it cannot, therefore, be a SUDDEN DROP IN PRESSURE that instigates the problem…. a change I one of the other two parameters MUST occur first.

I guess we’re just contradicting ourselves on this point – It is the sudden drop in pressure that instigates the problem, however the pressure drop is achieved. I don’t want you to actually try this, but if you build a model engine sized boiler and put a large dia quick throw valve on it, opening up that valve would bring about an explosion just as a rupture would. Nothing magical about the rupture or going to atmosphere, the cause is the sudden drop in pressure that forces the water flash to steam to maintain equilibrium. To much water flashing to steam to quickly is what a boiler explosion is

Anyway, I’m just repeating myself on that point, here’s some additional info, an excerpt from Johnson Control’s intro to boilers (it was a google book, this was the only way I knew to capture it)

http://i20.photobucket.com/albums/b201/michael0100/causeofboilerexposion.jpg

here’s another detail piece from the National Board of Boiler and Pressure Vessel Inspectors. It goes through the sequence of events and clearly says it is the sudden drop in pressure that instigates events.

http://www.hsengco.com/nbexplosion/nbx.htm

Boiler shell fractures (cause irrelevant) which initially connects two separate VOLUMES together (in other words we have a change in the VOLUME parameter.

If the added volume is at a different PRESSURE (in this case a lower one (atmospheric))

We now have a situation where there is a PRESSURE DIFFERENTIAL, and also, by definition, an ENERGY DIFFERENTIAL, between the two connected (via the fracture) volumes….
What follows is the rapid shift of the ENERGY content, in order to eliminate/equalise the ENERGY DIFFERENTIAL…… the high energy content within the boiler volume will move towards the lower energy content of the connected, increased, volume.

As a result of this exchange/movement of ENERGY the resulting FINAL PRESSURE as a whole will drop and is in fact the LAST thing to stabilise.

What you have described well is what happens when an air compressor lets go. There is potential energy and the there’s a rush to equalize the pressure differential. In a boiler, the resulting explosion is many magnitudes larger than say an air compressor because of the of enthalpy – the massive amount of energy in the super heated water.

The rush of the tiny bit of steam and big bit of water in a boiler to equalize pressure isn’t a big deal, think about what happens when a model boiler lets go under hydrostatic testing – almost nothing, yet that scenario would perfectly fit your description of the rush to equalize pressure. It’s not equalizing pressures that is a boiler explosion its the super heated water and all of a sudden, because of the lower pressure, needing to occupy 1600x the volume.

(if the added volume pressure was higher (meaning there was higher energy within it), then the ENERGY exchange would occur in the opposite direction, and the resulting final pressure within the boiler would rise) .

I think again what you are saying is accurate for pressure vessel like a compressor where only have the idea gas law to deal with. Energy in a boiler is temp * mass, sum of the enthalpies, or basically how much stuff is there at what temperature.

thanks for challenging my thinking on this – I enjoy the lively debate

 

[rake60]

thanks for challenging my thinking on this – I enjoy the lively debate

And a very valuable debate it is!

There has been more information brought to light in this thread than I had even
hoped for. I'm very pleased to see it!

Rick

 

[tmuir]

A little video that was brought to my attention a little while ago showing what can happen even with small 'toy' or 'model' boilers that is fired with the incorrect heat source and has either a non functioning SV or one set too high.

Watch it to the end.
[youtube=425,350]<object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/0R348vGkjyw&hl=en"></param><embed src="http://www.youtube.com/v/0R348vGkjyw&hl=en" type="application/x-shockwave-flash" width="425" height="344"></embed></object>[/youtube]

 

[1Kenny]

Thank you,Tmuir :bow:

A very good point is made. Everyone should watch this.

Kenny

 

[Bogstandard]

Very good video T. Imagine what would have happened with something a little larger.
A good case of not taking safety seriously.

I think on this one, everyone there needed a pants change.

John

 

[Divided He ad]

HOLY SH@% That was just damn scary.... People should just not play with things that they know nothing of!

Did you hear.... 'We've tweaked the pressure release valve' ... 'the valve of death as it is also known' They really do/did have no idea what they were doing!!!!!!

I'd be amazed if no one got injured.


This should be shown to every newbie from day one.



Ralph.

 

[PTsideshow]

From the abrupt ending one or more got a least scalded or hit with the boiler when the end closet to the torch came off or let loose and the rest took off like a rocket.
Looked like a school, or community hall of some kind. I hope no one was injured.
The draino bombs are currently making the rounds in the area with graduation. Somebody will get hurt or screw their life when they take out a mail box. And video tape it, a couple of geniuses did that last year and made sure their faces were nice and clear for the close ups.
As has been said time and time again. A little knowledge, is a dangerous thing.

 

[raym 11]

On another thread ...I said (Don't forget the most important things.... Hydrostat finished boiler....calculate the proper size and pressure of the PRV (Pressure Relief Valve).

There were no replys or comments. Hopefully it was read and understood.
Ray M

 

[LCT]

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.

Thank you for this!
I assumed boiler explosions were from over pressure (too much heat, too little steam drawn off). I was gonna fix that with a simple pop off. Now I see that may not be enough. You may have kept my children off welfare.

 

[kvom]

WRT to the question about popcorn, I found the following via Google:

"Before you start cooking popcorn, the pressure inside and outside the kernel is the same.

As the kernel heats, the moisture turns to steam, and the internal pressure of the kernel rises. When the temperature inside the kernel climbs above 100 degrees C., you might expect that all the water would turn to steam.In fact, only a small amount vaporizes, because the tough pericarp acts like a pressure cooker.

The high-pressure steam penetrates the starch granules and transforms them into hot, gelatinized globules.

Finally at about 175 degrees C., when the pressure inside the kernel is about 9 atm, the pericarp explodes. "

So popcorn pressure is ~130 psi.

 

[Mcgyver]

LCT, its a fine point which some might consider academic - as the most common reason for a sudden drop in pressure is a rupture brought about by high pressure or other mechanical fault many take the rupture and the explosion one.....and the safety valve is a line of defence against this happening so for sure you want one (or two). I don't think its academic because if get it, you understand why they are dangerous and how dangerous they are, the sudden release of all the latent heat blah blah blah. If I've kept your kids off welfare its a good days work (tongue planted in cheek) but please read read read before building a boiler. irrc the standard is a 8x safety factor and everything has to be engineered, thickness, construction method (rolled, tube, riveted etc) stays etc plus right material for fittings, pressure relief valves, site glass etc

 

[raym 11]

kvom;
thanks for digging into the popcorn 8) ;D
Ray M

 

[Bernd]

Dose anybody here remember the traction engine explosion that happened in Ohio a couple of years ago?

I believe a write up was in Live Steam giving the cause of the explosion. There were several factors. I don't recall all of them. After reading this thread I've only come across the fact that a rupture can cause an explosion. From what I remember of that write up there are several factors that need to happen for such a fracture to occur. One is corrosion of the stays over the fire box. Another is water below the crown sheet. When the water sloshes around in the boiler and hits the hot crown sheet, instant steam and a sudden rise in pressure. These I believe were only a couple of the things that caused the explosion.

I'll try and find a link to that if I can.

Bernd