Why is a steam pressurized vessel more dangerous? 2

[radio1julie]

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!

 

[racookpe1978]

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. .

 

[EngAddict]

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

 

[radio1julie]

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?

 

[racookpe1978]

All of that.

 

[rmw]

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?

 

[saplanti]

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.

 

[radio1julie]

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

 

[1gibson]

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...

 

[rmw]

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

 

[gr2vessels]

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

 

[doct9960]

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?

 

[radio1julie]

Yes, thats correct! Saturated steam!

 

[doct9960]

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.

 

[radio1julie]

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??

 

[doct9960]

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

 

[radio1julie]

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!

 

[puigi]

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.

 

[radio1julie]

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

 

[puigi]

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.