API 521: Tw (vessel wall temperature) 2

[foxymophandlpapa]

I'm using equation 5 in API 521 to calculate the relief orifice size of a gas filled vessel. The explanation for the Tw value isn't very specific. Am I supposed to use the maximum wall temperature? Otherwise it would seem that Tw would just be equal to Tn, the normal operating gas temperature. Thanks for the help.

 

[pleckner]

Tw is the maximum recommended wall temperature for your particular vessel. For steel plate, this would be about 593 C or 1100 F. It is material dependent.

By the way, it appears you are still working from the older version of API RP521. See if you can get the update API Standard 521, 5th Edition, January 2007. However for this purpose, you can still use the 4th edition.

 

[someguy79]

If Tw is the maximum rated temperature for a vessel, would it be accurate to say that the high "Design Temperature" could/should be used for Tw? (Assume that the vessel under consideration must comply with ASME BPVC code.)

 

[JoeWong88]

There are many factors affecting fire type, heat flux, etc and wall temperature would be varies.

Heat Flux Varies
Pressure containing equipment exposing to fire, the wall temperature is very much subject to type of fire, relative distance, momentum of fire, how a fire flame impinging equipment, what type of material on fire, etc and the results is heat flux would be localized.

Low Heat Transfer
Low heat transfer between wall and vapor would leads to wall temperature far higher than the gas temperature.

Environment
Environment factor such as humidity, wind condition, etc affect the heat transmission.

As above factors are difficult to quantify, thus wall temperature of equipment would be difficult to be estimated. Personally i would always recommend to put extra efforts in fire detection, prevention and evacuation of risk instead concentrating in heat flux, wall temperature for gas expansion case. Read more HERE

However, for PSV sizing, i would consider to reduce the set pressure as low as reasonable possible and use the conservative approach such as taking the gas temperature at relieving condition even though it is higher than the nominated design temperature, worst F factor, etc.

It is not cost effective to consider maximum recommended wall temperature (Tw) as design temperature of the system. Otherwise 90% (if not 100%) of plant design temperature would be maximum recommended wall temperature (Tw).

JoeWong

 

[pleckner]

It is not customary to design for fire for the reasons Joe gives in his last paragraph. You should however consider other non-fire relieving scenarios in determining design temperature, i.e. the relieving temperature coincident with the relieving pressure.

 

[someguy79]

The item I thought was interesting is that Tw has a recommended maximum for carbon steel, but I have found no such thing for other material types. The temperature given (1100F) does not correlate with any other design condition I know of.

If there was some guiding principal behind that recommendation, I'm curious to know.

This is starting to sound like a question to be sent off to the API committee in charge of the standard.

 

[pleckner]

I think API Standard 521 covers its bases with the following three paragraphs,


"5.15.1.2.2 A characteristic of a vessel with an unwetted internal wall is that heat flow from the wall to the contained fluid is low as a result of the resistance of the contained fluid or any internal insulating material. Heat input from an open fire to the bare outside surface of an unwetted or internally insulated vessel can, in time, be sufficient to heat the vessel wall to a temperature high enough to rupture the vessel. Figures 1 and 2 indicate how quickly an unwetted bare vessel wall can be heated to rupture conditions. Figure 1 illustrates the rise in temperature that occurs with time in the unwetted plates of various thicknesses exposed to open fire.For example, an unwetted steel plate 25 mm (1 in) thick takes about 12 min to reach 593 °C (1 100 °F) and 17 min to reach 704 °C (1 300 °F) when the plate is exposed to an open fire. Recent calculations indicate that the heat flux of the fire is in the range of approximately 80 kW/m2 to 100 kW/m2 (25 200 Btu/ft2h to 31 500 Btu/ft2h)."

"Figure 2 shows the effect of overheating ASTM A515 Grade 70 carbon steel, from data published in Reference [48]. The figure indicates that at a stress of 138 MPa (20 000 psi), an unwetted steel vessel ruptures in about 0,1 h at 649 °C (1 200 °F). A source for time-dependent rupture stress for different metals is ASTM Data Series DS 11S1 [29], which contains stress rupture and other elevated temperature property data for wrought carbon steel. This work was performed by the Materials Properties Council but is available through ASTM. A more recent source is Guidelines for the Protection of Pressurised Systems exposed toFire [30]."

"5.15.2.2.2 Vessels containing only gases, vapours or super-critical fluids

The recommended maximum vessel wall temperature, Tw, for the usual carbon steel plate materials is 593 °C (1100°F). If vessels are fabricated from alloy materials, the value for Tw should be changed to a more appropriate recommended maximum. See 5.15.4 for guidance on the potential for vessel failure from overtemperature due to fire exposure."

This last paragraph is given in an example calculation.

 

[someguy79]

Thanks for digging up those references!

 

[pleckner]

My pleasure.