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]

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.

 

[puigi]

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.

 

[gr2vessels]

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

 

[rmw]

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

 

[doct9960]

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.

 

[radio1julie]

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!

 

[GenB]

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

 

[gr2vessels]

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

 

[psafety]

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.

 

[Duwe6]

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.

 

[curtis2004]

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

 

[weldtek]

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

 

[Duwe6]

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.

 

[Karloss12]

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.

 

[gadkinsj]

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?