Volume of Nitrogen needed for boiler hydro 2

[gregg098]

My plant would like to hydro the boiler after an outage with extensive tube repairs and is throwing around the idea of using nitrogen instead of water. I have the volumes of water needed for each section, but am not sure how to convert this into the volume of nitrogen needed to perform the hydro at 1500 psi (not NOP).

Can anyone help me figure this out?

One example is the superheater section holds 9531 ft3 or 71,292 gallons of water.

Thank you

 

[gregg098]

Since this is most likely A LOT of N2, we could press up to about 130 psi with compressed air first.

 

[StoneCold]

That would be A LOT of stored energy if you used N2. Did you think about that? Go with water. Make sure you are designed to hold the weight of 72k gallons of water. That will be a lot safer. DON'T DO IT!

Regards
StoneCold

 

[rustbuster]

I agree with StoneCold, pneumatic testing would be insane and exteremly dangerous. Calculate the stored energy and determine the equivalent TNT weight.

I would bet this is in the hundreds of tons of TNT.

Don't even consider it.

 

[SNORGY]

Using a spreadsheet I wrote a while back, I end up with 2720 lbs of TNT and a restricted distance (minimum safe distance to stand back) of 315 yards.

I like water.


Regards,

SNORGY.

 

[gregg098]

Thanks for the feedback.

Out of curiosity, how is steam in the boiler when online less dangerous than Nitrogen while offline? The compressibility factor is about 10% more, but steam is still a pretty big energy source at 1000 F and 2500 psig.

 

[zdas04]

What dreck you guys are throwing around. Pneumatic testing is done safely every day with properly designed test procedures properly executed. It is allowed in the code. The TNT calculation makes the (mistaken) assumption that all stored energy is immediately available to participate in an explosion which is an outrageous assumption. The gas pressure in the immediate vicinity of a failure does explosively decompress, but the energy a small distance away simply flows into the void. The energy of an explosive decompression is usually less than 1% of the number SNORGY calculated and the exclusion zone (which I assume came from the NASA testing) is generally overstated by at least a factor of 15. The NASA document that everyone references had some real problems with similitude when they scaled it up.

There have been some notable equipment failures during pneumatic testing. I've investigated a few myself. Every one that I've investigated or read a competent engineering post-appraisal (as opposed to an hysterical "news" report) was caused by either an improper procedure or by improperly executing a procedure. Usually the problem was either that the gas was very cold or the ambient temperature was very cold. Either of these conditions can easily lead to brittle failure if you change pressure too rapidly.

Not being able to calculate the volume of nitrogen you need does not bode well for your being able to write a competent procedure, but the volume you need (in SCF) is the empty volume (your hydro volume) times test density divided by the density at standard conditions. Since nitrogen is approximately an ideal gas, if you deliver the nitrogen at 60F then everything excepte pressure in the density calculation cancels and the equation is V(SCF)=V(empty)*P(test)/P(std).

On my web page under "samples" there is a document describing the safety considerations of testing. You might want to take a look at it before you go a lot further.

David Simpson, PE
MuleShoe Engineering

http://www.muleshoe-eng.com

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

"It is always a poor idea to ask your Bridge Club for medical advice or a collection of geek engineers for legal advice"

 

[SNORGY]

zdas04:

You are correct. The calculation approach was based on the NASA Glenn Research Centre methodology, which our (up here) regulatory authority has dictated is the approach to be followed.

That aside, at the purely engineer-to-engineer level, I agree with you. I myself have tried to rationalize the amount of energy actually available to launch a fragment of steel into the atmosphere, once you deduct the energy stored at a location remote from the point of failure, and again after you further deduct the energy lost in the "tear-away" effect as the material breaks apart from what it was connected to / part of. You are right: nowhere near *all* of that energy can be concentrated for release at the point of failure. I have also pneumatically tested steam-methane reformer furnaces, PSA Units, big towers...safely...

In this instance, the OP has described that what is to be tested has been fraught with "extensive tube repairs". So, I do not think that inspires a lot of confidence. In this instance, I still prefer water.

Regards,

SNORGY.

 

[VEBill]

Make sure your pressure gauge has a stop pin.

http://www.discity.com/kc135/

 

[dcasto]

zadas
you are correct.

Since, in reality, there is no hydro test requirement and in fact there is only a leak test required and the pneumatic testing requires a lower test pressure.

I always test slowly with stop points to allow checking for leaks when using nitrogen