temperature limit for leg supported vessel 4

[Rahulvaland]

In almost every specification and book,the temperature limitation has been specified as 230°c.and if the temperature is above 230° leg support shall not used.

can anybody provide the reson behind this limiting temp.
is it for thermal expansion?how it affects to leg?

 

[MJCronin]

I have dealt with leg supported pressure vessels for 30 years and own nearly every book on the mechanical design of pressure vessels.

I have never heard of such a restriction limitation.

Show me an authoritative text or report with this restriction.

Was this temperature restriction something that a customer or consultant stated at a meeting ?

Was the customer/consultant an MBA or Certified Project Management Professional ?

In these mysterious "books and reports", what type of support is suggested for vertical axis vessels operating above this temperature limit ?

 

[jamesl]

This is not something made up by a MBA. It is in PIP VECV 1001, and many companies follow PIP standards.

 

[Rahulvaland]

Thanks you james,
Exactly, it is written in PIP standards and also if you refer any handbook directly or indirectly it is limited by temp.

What are the probable reason behind it??

 

[Rahulvaland]

In henry n bedner book...it is limited to 650 F.

 

[Duwe6]

Probable reason is structural stability and integrity during a fire.

 

[jamesl]

I think this is about thermal stress at the junction between cold legs and hot vessel. If there is skirt support at this temperature, a hot box is typically required. You cannot do hot box at legs.

 

[MJCronin]

The responsibility for the proper design and support of an ASME or other certified pressure vessel lies with the competent vessel deigner.

In accordance with the code of record, he has at his disposal a multitude of methods, designs and materials available to produce a stable and code compliant design. He must check for strength as well as elastic stability under all loading combinations

He can use a variety of leg/skirt designs, select from a variety of materials, structural shapes etc.

He may use a detailed finite element analysis method of the legs and shell juncture to justify his design.

He may require fireproofing on the supporting legs or skirt of a vessel. He will note this on his design drawings

I don't care that recommendations of the PVP has somehow (subjectively) limited any and all legged vessel designs to 446F.
(230C).

There is no justification .... without a valid research paper/study/peer reviewed report....its just an opinion and its asinine

 

[Rahulvaland]

Thanks james and mj cronin for your reply...

I have asked to many of my frnds the same and maximum of them has given their opinion same as what james has given above.

 

[jamesl]

With all due respect, the 650 deg F limit for use of leg supports is in vessel specs for many major US oil companies. Of course this makes good engineering sense (at least in my opinion). ASME code has no limitation on a lot of design or fabrication details; however company specs can make good engineering practice mandatory requirements. In these days, you can no longer design and fabricate a vessel based on judgment of a competent designer. After all, his judgment is just his own opinion, not necessarily his client's, not even his fellow engineer's. Fabricator always have to follow the requirements in a 50 page (or thicker) client vessel spec. They may disagree with a lot of requirements in the spec, but they have to comply with all of them.

 

[MJCronin]

You state:

"Fabricator always have to follow the requirements in a 50 page (or thicker) client vessel spec."

Yes, of course, but this is beside the point. If the client wants the vessel painted purple with yellow dots, you paint it purple with yellow dots.

The original question was "why do we have a 230C limit on legged vessel designs ?"

The answer to the question is "Because sombody said so.."

There is no reason based on any code, or stress analysis, or elastic stability study.

I will repeat ....... This is an arbitrary restriction placed on the fabricator by the client.

I also believe that 230C (446F not 650F) is a very low threshold

 

[jamesl]

This issue has two layers:

First, is there engineering basis to limit the use of legs on hot vessels. My answer is yes based on my education and stress analysis experiences.

Second, is the threshold 450 F or 650 F? I agree 650 F is a better number. PIP may have gone extra conservative on this one. This is just like steam out case: some vessel specs do not care; some spec call for 0.25 X full vacuum; some call for half vacuum; and some ask for design for full vacuum. They all have their reasond behind these choice.