Time to Rupture for SS316 Head of a S&T HX for External Fire Scenario.

[Pavan Kumar]

Hi All,

I want to calculate the time to rupture for a ASME F&D Head of a Shell and tube Heat Exchanger when exposed to external fire. The shell side fluid is Boiler Feed water while the tube side is Process gas. I am performing an external fire scenario on the tube side to determine the time to rupture, wall temperature at rupture stress and the relief load if a pressure relief device needs to be installed to depressurize the tube side to delay the rupture. API 521 7th Edn Section 4.4.13.2.4.3 provides the calculation methodology for calculating the relief given the exposed surface area, wall temperature and the relief temperature.

The dimensions of the heat exchanger indicates that I need to consider only the bottom head and the shell exposed surface area to calculate the relief load(using conservative approach). The heads are made from 1/2" thick SA-240 316 plate (UNS S31600). From ASME Sec 2 Part D from the Allowable Stress tables for this material the maximum temperature up to which the stress values are given is at 825 Deg C(1517 Deg F) at which the allowable stress value is 8.05 MPa(1167.87) psi, see pages attached. So 1167.87 psi is the rupture stress and 1517 Deg F is the wall temperature at rupture. My questions are:

1. Does this way of evaluating the rupture stress make sense and if yes can any one share the rupture stress vs. time to rupture curve for this material.

2. Can I use the wall temperature calculated above to calculate the relief load.

Your guidance will be very helpful to me.

Thanks and Regards,
Pavan Kumar

 

[shvet]

What is a weakest element of the system being overpressurized? Head gasket, flange gasket, valve packing/seal, thread connection, field instrument, elbow, nozzle, shell? Note that after gasket rupture or similar shell side will depressurize with no consequences, therefore what is the reason to protect shell side if no consequences will arise?

Note that water critical temperature is 375°C.

Max limit of temperature might become a challenging issue

https://www.cheresources.com/invisi...eding-vessel-failure-temperature/#entry129900

Doublecheck wetted area

https://www.eng-tips.com/viewthread.cfm?qid=495159

 

[georgeverghese]

What is the TEMA configuration of this HX ?

 

[Pavan Kumar]

Hi georgeverghese,

What is the TEMA configuration of this HX ?

TEMA Type "NEN".

Thanks and Regards,
Pavan Kumar

 

[georgeverghese]

A TEMA NEN HX does require a PSV for firecase on the tubeside, even if blocked in inventory is <500litres. I cannot comment on the specifics of this calculation.

 

[Pavan Kumar]

Hi georgeverghese,

The tube side is process gas side. Shell side is BFW. With low heat transfer to gas side the chances of flange gasket rupturing or head rupturing are higher compared to a relief event.

Thanks and Regards,
Pavan Kumar

 

[georgeverghese]

Agreed, since this is all gas on the tubeside, there is no need for firecase RV. My previous comment was for tubeside with liquid inventory. Moreover, a lot of the heat input on the HX head will dissipate out to the tubes which are immersed in boiling BFW at the time of relief.