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Why is a steam pressurized vessel more dangerous?
2

Why is a steam pressurized vessel more dangerous?

Why is a steam pressurized vessel more dangerous?

(OP)
Hello,

   Could someone please explain why a pressure vessel filled with saturated steam at 150 psi would be much more dangerous than the same sized vessel filled with air at 150 psi, if it were to explode due to fracture? I am interested in an answer based on stored energy and I am guessing the key thing here would be the latent heat in the steam but how would this contribute more explosive power?

Thanks for anyone's help!
 

RE: Why is a steam pressurized vessel more dangerous?

Think about it!   

Sam epressure, right?

But with steam you have higher temperature, so you have lower strength in the metals resisting the same force.  

You have burns to worry about, plus condensation and expansion forces as the vessel is being filled.   If a air-filled vessel vents (breaks open), it releases its energy, nothing's left.  But the boiler will keep boiling off water as long as its pipes and walls and thermal mass is hot.  (We have to keep turbines turning for dozens of hours after the steam is shut off to remove residual heat and prevent damage from bowing and loss of lube oil to the bearings.  Lots of other "little" things like that.)   

Most important, after failure of a steam system, the metal and pipes is still hot, still burning people and things.    

Corrosion at high temps, chemical deposits, condensation and water traps and water hammer, thermal movement of the original container &  Movement of pipes and restraints cause more design problems -> More likely design errors and stress risers, so failure is "easier" to get, harder to prevent in a steam system.  .    

RE: Why is a steam pressurized vessel more dangerous?

Apart from the Hazardous contents of the steam, the water & vapour forms a propellant. Ever made a bottle rocket?

RE: Why is a steam pressurized vessel more dangerous?

(OP)
OK, I am thinking that the steam would also become superheated as the pressure dropped due to the fracture of the vessel and release of steam. Would this mean that the water droplets in the steam would themselves 'flash off' and cause expansion?
 Also, there would be an associated drop in pressure in the boiler supplying the steam and as the pipework connecting the boiler to the pressure vessel would be open, the water in the boiler would flash off to steam causing a rapid expansion of additional steam through the pipework and into the exploding pressure vessel? Is this correct?
 Any condensate initially in the vessel would also flash off to steam, causing more expansion?

RE: Why is a steam pressurized vessel more dangerous?

All of that.   

RE: Why is a steam pressurized vessel more dangerous?

And... if you were in an enclosed space of any kind, say boiler room or turbine hall, the steam released would asphyxiate anyone in the building where as the air filled vessel burst wouldn't have that issue.  If they survived the blast at least they could still breathe.

rmw

PS: when you asked your original question, it was a PV filled with saturated steam vs. air.  In a later post, you mentioned water in the boiler flashing off.  If this a PV or a boiler?

What changed?

RE: Why is a steam pressurized vessel more dangerous?

I guess due to the total energy carried by steam against the total energy carried by air at the same conditions. You need to look at h-s diagrams for comparison.

RE: Why is a steam pressurized vessel more dangerous?

(OP)
Thanks everyone!In answer to rmw's question, this would be a PV with steam supplied to it by a boiler. I am trying to present an argument for the safest condition in which to carry out a pressure test? i.e would air be safer? I have heard many people say that air is more dangerous because it is more compressible than steam? Even if this is true, there are other more compelling factors, such as those discussed, outweighing this that would suggest steam is more dangerous

RE: Why is a steam pressurized vessel more dangerous?

I think somebody overheard somebody who overheard somebody talking about a hydrostatic test with water being safer than a pneumatic test with air, because the water is not (as) compressible.

Whoever said that, ask them which they would prefer to stand next to during a pressure test...

RE: Why is a steam pressurized vessel more dangerous?

An anecdote:

Visitng a Hx fabricator recently, when I asked him about air testing a large vessel (at a fairly low pressure - about half of 100 PSIG) and his views on air testing, he recounted an interesting story.  Said he had to air test a Hx or vessel to 600 PSIG.  He stated that he installed a large faced pressure gage on the vessel and monitored it through binoculars.

rmw

RE: Why is a steam pressurized vessel more dangerous?

If it was designed by me, I would stand next to it. If I would have the chance to follow the fabrication, I would sit on it, during the pneumatic test. Geez, it's heart warming to see how confident is some people with their own engineering capabilities. Perhaps they should keep their day time job. It seems that it is afterall rocket science to design and fabricate high pressure air cylinders and air receivers.
Cheers,
gr2vessels

RE: Why is a steam pressurized vessel more dangerous?

Quote:

I am trying to present an argument for the safest condition in which to carry out a pressure test? i.e would air be safer? I have heard many people say that air is more dangerous because it is more compressible than steam? Even if this is true, there are other more compelling factors, such as those discussed, outweighing this that would suggest steam is more dangerous

radio1julie,

You mentioned pressure test. Are you considering a pressure test with saturated steam as the medium?  

RE: Why is a steam pressurized vessel more dangerous?

(OP)
Yes, thats correct! Saturated steam!

RE: Why is a steam pressurized vessel more dangerous?

Interesting. Why saturated steam? Why not water? Is the boiler new? What is the code of construction and in what part of the world would this boiler be installed?

Normally and preferably, pressure test is done with water. Water is almost incompressible so it stores less energy. If a hydrostatic test is not possible, then you consider other types of pressure test like a pneumatic test (air). But a pressure test with saturated steam, this one I've got to hear.

Let's say a pressure test with saturated steam is acceptable by your  local boiler authority, you need to make sure that your boiler is designed for external pressure. This is in case the steam condenses.

 

RE: Why is a steam pressurized vessel more dangerous?

(OP)
OK, I'll be totally specific about what this is! This is a paper dryer in a paper mill. If you're not familar with these, its basically a large rotating pressure vessel filled with saturated steam. Paper is transferred from the felt onto this dryer and the paper is carried around the surface and scraped off on the opposite side by a doctor blade before being wound onto a reel. The latent heat released from the condensing steam on the surface of the dryer bore inside dries the paper during the short time span that the paper sits on the outside surface (these dryers rotate quite fast).
As steam is the obviously convenient medium with which to pressure test these dryers (as and when this is required by local authorities and for whatever reason), then steam is what is used. However, from time to time one of these dryers may need to be tested in storage where steam is not available as it is not mounted, and piped up, in a paper machine. In this case, a conventional hydro test can be set up using water. However, from time to time people have suggested using air and this is when people have suggested, and in my opinion wrongly, during discussion that air is even more dangerous than steam if an explosion was to occur?? There HAVE been occasions when a dryer has been sat in a mill yard far from any water supplies and it has been suggested to use a portable air compressor to pressurize one of these dryers. This is where the arguments start. I just want to point out, and have the arguments/calculations to support this, that air is no more and possibly less dangerous than steam??

RE: Why is a steam pressurized vessel more dangerous?

radio1julie,

Okay now I understand what your equipment is. It is a pressure vessel with steam supplied to it by a boiler. Sorry I misread your previous post about it.

I've never heard of a pressure test with steam before. Like I said normally it's a hydrostatic test or else it is a pneumatic test. Have you considered acoustic emissions test?

This paper seems to tell some stuff about an equipment similar to yours... http://www.pulpandpapercanada.com/paptac/PDFs/Dec07/Equipment.pdf

RE: Why is a steam pressurized vessel more dangerous?

(OP)
Yes, we have been involved with acoustic emission tests many times around the world. Thanks for looking into this! We are familiar with the guy who wrote the paper you linked me to. Not seen the paper before though!

RE: Why is a steam pressurized vessel more dangerous?

Is the pressure test required as a result of a repair to the PV? If so, consult your AI, volumetric NDE coupled with an operational pressure test using the system's medium may be acceptable to the AI.  

RE: Why is a steam pressurized vessel more dangerous?

(OP)
Usually following a repair yes, but not always! As stated above the only time air testing would be a requirement is if its not sat in a machine and neither steam OR water are easy to supply i.e. sat in storage in a mill yard or shed, far from the main building

RE: Why is a steam pressurized vessel more dangerous?

radio1julie:

I realize a pressure test may not always be required for a PV repair. I was curious about why you were considering one and offering a possible alternate course of action.  

RE: Why is a steam pressurized vessel more dangerous?

Dead wrong to ever/even/under any case think about "testing" with saturated steam into a new or repaired pressure vessel.  (Illegal as well, as I understand the insurance req's - since the pressure test has to be complete before steam is re-admitted.

It's fundamentally "wrong" to use "air" as your pressure test medium if water can be used, regardless of operating pressure and operating fluid and temperature because of the risk of failure with air.

Water is a non-flamable, non-explosive, incompressible medium when used for hydrostatic testing.  That is, if (when!) the vessel or a component fails during the hydrostatic test, the pressure is released due to the leakage through the fitting or failure point.  When even a quart or liter (or even a single cup of water) is squirted out, the pressure goes down.  So your hydrostatic test is sensitive enough to show you leaks.   Your pressure test is safer, since, when a leak happens, the internal pressure goes down quickly.   

With air, you may not even notice (be able to display on the gage) the loss of pressure over time since the gas pressure goes down at a negligible value with the same loss of mass.  (Water is 1000x as dense as gas.)     

So -even if there were no heat energy (your original question was about testing with steam - remember!) - a failing pressure vessel under an air test has a MASSIVE amount of residual energy held inside that will continue the failure.   Continue the destruction and danger.  

Would I sit on top of pressure vessels I built when I test them?  Well, I have sat inside pressure vessels that I have built (technically, submarines that other people have built for me) under their design pressure and temperatures ....  

But, it is better to be prudent: If you can test without threatening your safety (Other people's safety) why threaten them and threaten your company?   And the Thresher DID fail its pressure test after being built and repaired.   As have hundreds of other submarines, all failing under various excessive pressures and shocks.    

RE: Why is a steam pressurized vessel more dangerous?

racookpe:

I don't believe anyone suggested testing a new PV with steam. Testing and examination for repairs and alterations of "in-service" (not new) ASME Sect I, IV, & VIII boilers and pressure vessels is governed under The NBIC Part 3 Sect 4. The type of testing and examination chosen will vary depending on the nature of the repair or alteration.      

RE: Why is a steam pressurized vessel more dangerous?

A vessel rested for extended period of time, possibly exposed to environment and out of sight / out of mind corrosion problems to be tested with compressed air or steam is like testing the barrel of gas with a match at the top opening...would there be any safeguarding the pneumatic test? Or the yard test can be remotely controlled from a safe distance? In the end the balance of hydrotest costs and the court expenses defending yourself have to be assessed by yourself, radio2julie. I vote with the hydrotest believers, because I don't know how safe could be the pneumatic test.
Cheers,
gr2vessels

RE: Why is a steam pressurized vessel more dangerous?

Radio1Julie,

Is a paper machine roll truly a pressure vessel in the sense of Sec VIII or otherwise?  Does it have an ASME (or PED) name plate and/or NB number?

I have bene around a lot of pressure vessels in paper mills and never considered that the paper machien rolls were one of them.  Was I just that blissfully ignorant?

And, what is the design operating pressure?  Most paper machine rolls that I have had any experience with (normally with the steam supply or condensate offtake systems) were at or very near atmospheric, and usually no more than 15 psig.  But I didn't check them all so I am a learner here.  I'd like to know how a pressure vessel can have a shaft down the middle of it or a rotating condensate shoe fitted to the rotation point.

I'd be interested in the details.

rmw

RE: Why is a steam pressurized vessel more dangerous?

Here are some  papers from TAPPI (Technical Association of the Pulp and Paper Industry).

http://www.tappi.org/Downloads/unsorted/UNTITLED---eng94125pdf.aspx

http://www.tappi.org/Downloads/unsorted/UNTITLED---0108040216pdf.aspx

On-site hydrostatic test for an existing dryer pressure vessel could be detrimental and dangerous. If a hydro is considered dangerous for this type of equipment, then a pneumatic test would be more hazardous.  

RE: Why is a steam pressurized vessel more dangerous?

(OP)
Hello,

  Yes these ARE ASME coded pressure vessels (up to 150 PSIG) and testing with steam OR air is accepted where situations dictate by the National Board of pressure vessel inspectors and has been accepted in the past by relevant authorities in other countries besides the States.

  The crux of the original question was basically; Does anyone know the difference in the expansion rate (due to difference in compressibility), between air versus saturated steam, if there was a sudden release of pressure through fracture of one of these vessels? Hence, which is more dangerous?

  Thanks for everyones input so far! Much appreciated!

RE: Why is a steam pressurized vessel more dangerous?

radio1,
Lets say steam is 10 times compressive than air  and it can clime up to 10 at the square.there is your answer.
 

RE: Why is a steam pressurized vessel more dangerous?

GenB, did you read your post? Give it another go.
Cheers,
gr2vessels

RE: Why is a steam pressurized vessel more dangerous?

This is as I understand a "Yankee" Dryer it will be constructed of most likely of Class 8 cast Iron.  It will be a Section VIII vessel.  Use of steam will require a warm-up period.  This equipment should be brought up to temperature no faster than 75-degrees per hour.  Thermal stresses are extreme, a Yankee failure typically takes out the machine room.  

RE: Why is a steam pressurized vessel more dangerous?

Bottom line:  use water if it is the slightest bit feasable [see racookpe's remarks].
Use compressed air or nitrogen if water is not even slightly feasable, and stand well away from the vessel during its first test, behind something sturdy.
If you are foolhardy enough to use steam for the initial testing, use remote viewing to run the test.  A catastrophic failure is extremely unlikely, but the consequences are HUGE.  Plan on all the equipment around the vessel being destroyed, and every person near the vessel dead, or dying from 3rd-degree burns.

Hydroing with steam is like walking a 6-inch wide beam 100-ft / 30m in the air.  You will almost alyways be OK doing it.  Almost.
But the consequences of a failure tend to be unacceptable.

RE: Why is a steam pressurized vessel more dangerous?

radio1julie (Industrial) 2 Dec 11 6:01  
Hello,

  Yes these ARE ASME coded pressure vessels (up to 150 PSIG) and testing with steam OR air is accepted where situations dictate by the National Board of pressure vessel inspectors and has been accepted in the past by relevant authorities in other countries besides the States.

  The crux of the original question was basically; Does anyone know the difference in the expansion rate (due to difference in compressibility), between air versus saturated steam, if there was a sudden release of pressure through fracture of one of these vessels? Hence, which is more dangerous?

  Thanks for everyones input so far! Much appreciated!  
------------------------------------------------------------------------------------------------------------------------------------

It is similar if asking people: Do you prefer dying from steam explosion of air explosion?
smile

RE: Why is a steam pressurized vessel more dangerous?

Don't forget to fulfill the requirements of UW-50.

RE: Why is a steam pressurized vessel more dangerous?

Rapidly expanded air is cool to cold.
Rapidly expanded 150# steam is still steam.

Gos 'boom' w/air - if the shrapnel misses you, you're OK
Gos 'BOOM' w/steam, due to better expansion energy of entrained condensate flashing off, the shrapnel will be a little more energetic.  And you get the pleasure of 2nd-degree burns over your body.

If I'm not smart enough to figure a way to use water, I'll take air or nitrogen, please.

RE: Why is a steam pressurized vessel more dangerous?

In terms of safety hazard analysis regulations such as PED, steam is more hazardous substance due to hot steam releasing massive amounts of energy onto a person's skin as it transforms to liquid.

In terms of an exploding pressure vessel with saturated steam I would expect the vessels shell to initially explode and be thrown apart in a similar way to an air explosion.  But then the steam would very quickly condense when it hits the cool atmosphere, so there would be a reduced pressure wave.

Testing an old vessel with steam just sounds crazy.  The cost of the setup.  All that extra safety hazard analysis. How do you get steady state conditions to assess the result of the test?

Pneumatic test for your vessel sounds crazy as well.  If it has an operating contents of steam then surely it can be hydrotested.

I work in cryogenics.  We only Pneumatically test vessels that we can't completely dry out because the tiniest traces of ice can be catastrophic.
 

RE: Why is a steam pressurized vessel more dangerous?

Tire companies who make Pneumatic pressure vessels that rotate at high speeds always burst test tires with Water.
As for the energy stored in Steam vs Air.  That is a no brainer.
We have had tires explode and send pieces through concrete barriers. What happens if one of your rolls explodes violently?  

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