Opening reinforcement in pressure vessel 3

[milan khatri]

To reinforce a opening in head / shell we can either increase nozzle schedule or instead we can use repad keeping schedule lower. How to determine which choice is more suitable from both economic and strength point of view?

 

[SnTMan]

milan khatri, kind of an open-ended question, depends on, among other things, class of work, shop practice and preference, weld details, external loadings, etc.

Repads are difficult on heads and usually avoided. If a moderate nozzle thickness increase provides the needed reinforcement, that is usually least cost. However some thick schedules may be hard or impossible to get. Avoiding unneeded welding is always preferred.

Some classes of work may require integral reinforcement, so that a thick pipe or forged nozzle is required. For a short cylinder such as a heat exchanger channel it might make sense to increase the cylinder thickness rather than add a pad.

Nozzle reinforcement is one aspect of strength. If nozzle loads are present, even an otherwise acceptable integrally reinforced nozzle may need a pad or thicker shell.

A lot of the time though it makes sense to just add a pad on a cylinder.

Regards,

Mike



The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[YuJie_PV]

It is complex.
First of all, you should tell if the repad is suitable for your pressure vessel. Generally, our company has some recommended practices which are not mandatory but practical, here listed for your reference.
Repad may be used only if :
1. design pressure less than 6.4MPa;
2. design temperature less than 350 C;
3. thickness of the shell to which the repad is attached less than 38mm;
4. min. tensile strength of the shell material not exceed 540MPa.

If you vessel meets all the requirements above, the repad may be used.

secondly, if possible i would rather choose to increase thickness of the pipe to meet the requirement of reinforcement. another recommended practice to be kept in mind that the thickness of pipe shall have no sharp contrast to that of shell. the thickness ratio around 1 may be perfect, 0.5~2 is acceptable as well. if no available schedule of the pipe satisfies the reinforcement, i suggest forgings.

Regards

JIe

 

[LittleInch]

The cost of the metal difference between each should be negligible, so it goes down to the cost and amount of welding and fitting.

Each one will be different and as noted above , the requirement should come from the design, not the installation. Re pads are not always the right answer for reasons other than cost.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Christine74]

Every situation is unique but in general I would say that it is often less costly to use a thicker nozzle schedule than to add a reinforcing pad, and for smaller nozzles it is often cheaper to use a Long Welding Neck flange than to install a reinforcing pad on a conventional nozzle.

For small vessels with several nozzles it may be more economical to make the shell thicker then to add reinforcing pads. And in some cases you may be increase the size of the nozzle attachment welds in order to avoid a reinforcing pad, but this is seldom economical.


-Christine.