pneumatic test ? 2

[Vladimir99]

What is the acceptable energy store in my system for a nitrogen test on a used (but empty) HTF line of 1 1/2'' dia, 3000 ft.

I have read 50000 (psig x cuft)

Is it alright ? I need reference

My client would like to do it in one run.

Thanks,
vladimir99

 

[Vladimir99]

I mean what is the maximum energy not to excede ?
Thanks
vladimir99

 

[SNORGY]

It's not quite as simple as that.

The NASA Glenn Research Centre used to publish salient information regarding safe stored energy and restricted distances involved with pneumatic testing, but it's hard to get that data or their methodology from their website now.

One thing that I did several years ago when I didn't have that data was calculate the total stored energy, divide it by the total mass of the pipe, assume instantaneous conversion of that energy per unit mass from potential energy to kinetic energy, assume a 45-degree launch angle into still air with an average ballistic number, and thereby calculate how far the pieces would fly if it blew up. Anything farther than 50 metres meant too much stored energy.

NASA Glenn's approach was to assume a 0.5 psi blast pressure (shatters windows) and back their restricted distance data and stored energy limits out from that.

Regards,

SNORGY.

 

[SNORGY]

As a follow-up to the above, I did develop a spreadsheet a few years back when I *did* have access to the NASA-Glenn Research Centre methods and data. The spreadsheet automatically computes the stored energy and restricted distance at a given test pressure. When I input 3000 feet of 1.5" XS pipe I get about 37 cubic feet. I then assume a test to 1.1 times 600# flange rating, or 11225 kPag. I get about 24,000 kJ and a restricted distance of 47 metres from the spreadsheet in that case. So, if you take 50000/37=1351 psi as your test pressure, you can see that your results would represent a similar scenario. Re-running the spreadsheet at a test pressure of 1350 psig (9305 kPag), I compute 18850 kJ and 43 metres.

So, at first glance, 50000 in terms of psi x ft^3 seems to be pretty reasonable to me.



Regards,

SNORGY.

 

[zdas04]

Calculate hoop stress at your test pressure. Divide that by SMYS. If the result is under 30% then there should be no restrictions on your pneumatic test. There is some good information on pneumatic testing in ASME B31.8. I've got a document on static testing on my web site that might help separate out some of the fear-and-superstition from real risks.

As to the "stored energy" discussion, most of it assumes that there is a point-failure and that the entire stored energy in a long line is dissipated instantaneously. What poppycock. A failed line at a point will release the energy in near proximity to the failure. The rest of the gas in the line will start rushing toward the opening (just as if you had opened a valve) and will make a lot of noise, but will not participate in the blast zone. Pneumatic tests require careful Engineering design, but the MegaTonne explosions that people calculate are so much nonsense--the stored energy is simply not in the proximity of the blast zone. It is like saying that if you light a firecracker, every grain of gunpowder in the world will sympathetically detonate.


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.

"Life is nature's way of preserving meat" The Master on Dr. Who

 

[rneill]

I've seen a similar rule to the one you mention where pressure in psig multiplied by volume in cubic feet should not exceed 10,000. I have no idea where this figure came from and I've never used this criteria myself but I do note that it is quite a bit more conservative than the figure you mentioned.

There is brief mention of a similar calculation or guideline in the ES&H Manual for Jefferson Labs ...

http://www.jlab.org/ehs/ehsmanual/6151T4.htm

 

[SNORGY]

zdas04 makes a good point.

I also realized and wrestled with the fact that you cannot concentrate all that energy into one local failure site; hence my energy per unit mass rationalization. But even then, there is no credit taken for fracture toughness and the energy lost due to fragmentation (tear-away energy, if you will).

That said...in my jurisdiction, the local Regulatory Body requires the application of the NASA-Glenn Research Centre methodology towards establishing restricted distances and safe work plans for pneumatic tests.




Regards,

SNORGY.

 

[SNORGY]

rneill:

I think I saw your number as well; might have been Syncrude maybe, circa 1986? But I seem to recall a plot like PV^3/1000...I forget what the abscissa was. I recall vaguely that if you were below the curve, you were safe; above the curve, they wouldn't let you do a pneumatic test.

Regards,

SNORGY.

 

[DVNCHIEF]

Snorgy, would you share your spreadsheet to calculate restricted distance?

 

[Cuemaster64]

Here is a sheet that I created with applicable data provided to me from my ABSA rep with references to a NASA document. Might help.