Pressure Problems with ASME VIII-1

Give your problem.I shall lllustrate your problem.

Thank you for yopur prompt reply. The problem concerns a solid drilled tapered thermowell in 316, ambient temperature, root diam 26.5, tip diam 18, bore 6.5 and tip thickness 5, all mm, welded [full pen] to a 2" 600 RF flange, immersion 226 mm. The requirement was to calculate max pressure that the stem would withstand without deformation, whilst remaining within the ASME limits for pressure vessels. I have looked at ASME PTC 19.3, together with a paper by J Brock [Stress analysis of thermowells] but the client seems to be looking at; " a calculation showing the relationship between the max applied pressure [149 Bar 2164 psi] and material stress in thermowell, eg ASME VIII UG 28".

I have problems associating a tapered thermowell with a pressure vessel.

Hope you can throw some light on my darkened world.

Ian Purdie

Try UG-36 to UG-42 and UW-16. This will allow for design of proper attachment of thermowell to the shell. Thread capability will depend on thread fit. Use proper class threads for your design.

Thank you. Your comments are interesting, but, unfortunately I do not have the parts of VIII to which you refer.

ASME PTC 19.3 Temperature measurement explains the maximum pressure that a thermowell can withstand for a given material at a given temperature.

Tapered thermowells made of solid barstock material rarely have a problem withstanding the pressure,at least in the normal industrial processes.

Fluid passing around the thermowell creates wake frequency.

The thermowell also has a natural frequency depending upon the geometry of the well.

This ASME recommends keeping the ratio of the wake frequency to the natural frequency to less than 0.8.

Preventing failure of themowells due to these vibration problems are equally important .

This can be done by reducing the wetted length of thermowell in the flow stream and or changing the geometry of the thermowell.

Thank you for your comments. I have looked at PTC 19.3 equation 4, but, as it requires a constant from table 1.5, figures, which no doubt were calculated via Imperial measurements, will give an error, albeit small with a ¼" sensing element.

You mentioned that the bore is 6.5 mm which is =0.2559"

Look at table 1.3 where the the d should be between 0.254 and 0.262 "

So you can use the constant k1,k2 and k3 shown on table 1.5 under

1/4" nominal size of SENSING element.

Thank you for your comments regarding PTC 19.3. I could also have used formulae from the 19.3 Appendix, but unfortunately my client is implying he would like calculation data based on ASME VIII UG 28. This, if I understand it, is mainly for pressure vessels and not for tapered, solid drilled thermowells. I'm not sure I can meet his requirements.

Ian,

I see this a lot. Some engineer working for your customer has prepared specifications without fully understanding the requirements. They have, no doubt, included information from previous specifications, "boiler plate" language, and references to codes without knowing what those codes state, or mean. Hence, you cannot meet the specifications as written. My approach is, armed with the information you have gained here, prepare the calculations as needed, then approach your customer, present the calculations and let him know these do not comply with the specifications as the specifications are incorrect in that Section UG-28 does not apply to the problem at hand. Use the argument as presented by srini as that is logical and correct. If your customer refuses to accept your calculations then he is a fool and does not understand what he is buying. Good luck. Welcome to the real world.

Thank you for your thoughts on this matter. Yes, you are probably quite correct in your assumptions as many individuals shelter behind a specification without really knowing what the spec. is really about.

In the end, to appease my client, I came up with some of Roark's calculations and produced axial stress, hoop stress and radial stress combined with a von Mises [equivalent] stress and game them a suitable margin of safety. Funnily enough, this was accepted