Thermowells subjected to external pressure

[ME231]

Would the external pressure calculations of UG-28 apply to thermowells? I am concerned with ones that are essentially suspended (hung) inside a pressure vessel with one side free while the other side is part of a pressure retaining seal.
As I read through the "Basis for external pressure calculation charts" given in Section II Part D, I understand that it assumes that both the ends are simply supported (and therefore suggests could therefore lead to buckling failure). This is clearly not the case for the case of a thermowell whose one side is completely free to undergo any longitudinal dimensional changes.

 

[DSB123]

ME231,
I think you are getting in a tizzy here. UG28 is for radial bucklin of shells whereby the shells buckle radially. UG 28 does not check for Euler type buckling. Why are you concerned about a thermowell? Normally thermowells have a low D/t ratio and checking for external pressure is not performed. (i.e. to buckle a thermowell you would need a very very high pressure)

 

[ME231]

DSB123,
UG-31 specifically does say that UG-28 is to be used for external pressure calculations when tubes and pipes are used as tubes or shells. Agreed that t/w bucking or failure under external pressure would be highly unlikely. However, how else would one calculate their external pressure ratings? Also, please state a quantitative value for "very very high pressure" (and temperature?).

 

[hacksaw]

the pressure rating described in ptc-19.3 relies on UG-28, and is more than adequate. you do not need to worry about bucking. you can fold on over if you have severe windage or an internal agitator to worry about.

thermowells do collapse though it is usually the fabricated tube design not the drilled barstock type

good luck

 

[ME231]

The tube design is what I worry about myself. However, the premise of simply supported ends as stated in Section II Part D seems not very applicable in this case. Hence my question regarding alternative methods. Do you think CC 2286 might help? I do not have a copy of the same but have read about it.

 

[JohnGP]

ME231,

The thermowell is closed on the end so that provides one support for the "shell", in a similar way that a dished end of a pressure vessel shell provides support. Check out FIG UG-28.1(d).

Many pressure vessels are free to extend in a longitudinal direction, when fitted with a sliding support, so I'm not sure what your concern is.

Cheers,
John

 

[GenB]

posting the size would hel.

 

[hacksaw]

there is design guidance specific to thermowells, that relies on UG-28 for the pressure rating and otherwise covered by the performance test codes(ASME).

are you attempting to redefine available design criteria or develop a new one?