Burst Disc Vent Design - Pressure Transients

[stevepdl]

I am designing a vent for a burst disc from a process vessel. Downstream from the 1" burst disc the vent expands out to 4" and then takes two 90° turns to a blowdown seperator vessel (about 25" in dia). The 8" outlet from the blowdown seperator vessel discharges to atmosphere after taking another two 90° turns.

I assume the vent is a closed discharge installation as described in ASME B31.1 Appendix II.

Question :
Can anyone recommend literature or software to calculate reaction forces on the vent supports due to pressure transients following rupture of the burst disc.

 

[JohnBreen]

Hello,

Have you looked at API RP 520, and B31.1 (non-mandatory) Appendix II?

Regards, John.

 

[CHD01]

Steve:

I am not an expert structural engineer but

For a simple relief installation you can work with the maximum force. The support structure should withstand the maximum; this may result in some over-design - but at least it gives you an initial idea of what is required.

Reaction Force = 2 x Pressure x Area The maximum value will be the burst rating of the rupture disk x the rupture disk efective area; this force should then be applied at each change of direction for the discharge vent pipe. This force is increased by a factor of 2 to account for impulse loading. I have seen factors of as high as 3 applied to this formula. There are a variety of equations that are being used to account for this impulse loading - using a factor of 2 increase is simple to apply.

Next, you will ahve to convert this force to a pipe stress.

Stress = MC/I M = Calculated Force x moment Arm
c = distance from centroid of pipe
I = moment of interia of pipe

I = Pi x Mean Wall Radius^3 x Thickness of Wall

Once you have I, this will allow you to determine the size of the suport beam structure.

Hope this helps some. The more you learn, the less you are certain of.

 

[JohnBreen]

Stevepdl,

PLEASE look at API RP 520, and B31.1, Appendix II. There are dynamic force multiplyers that you must consiser.

Regards, John.

 

[CHD01]

I need to point out that my simple equation for reaction force is for compressible fluids - it is NOT for liquids. By the way, the 2 or 3 factor accounts for the dynamic multiplier. We currently use a factor of 3 (2 is what we used to use). For liquids, the formula is:

force = 0.000017 * velocity (fps) * SG * 62.4 lbs/ft3 * 0.1337 ft3/gal * relief valve capacity in pph

Note that a dynamic force factor is not included for liquids.

Another liquid formula (from Consolidated) is:

Force = 0.00176 (G) (GPM)^2 / D^2

where G is specific Gravity and D is diameter in inches


The API reference that John Breen refers to (for another method of analysis to determine in-compressible reaction force ) is API 520 Part II, Section 2. The version I have is not current, so the section numbers may have changed.

You will also find that various relief device vendors differ quite a bit on calculating reaction forces. I am not trying to defend any of the methods here, I'm just trying to pass along what I have seen and what is often being used.

Bottom line is the reaction force is basically equal to (per Consolidated): the mass flow rate times the exit velocity plus the outlet flange area times the difference between the exit pressure and the atmospheric pressure.
The more you learn, the less you are certain of.