Are pads must for Platform clips???????

[ash008]

Hi all,

Are pads must for platform clips/ Pipe support clips?

I referred ASME Sec.VIII Div.1 App.G, under the sub topic G-2. What i understand is, that clips are not necessary until our shell is strong enough to withstand the load.....but when used it may serve us to reduce the secondary stresses over the vessel

Please provide ur view over my understanding.

Thanks in advance

 

[vesselfab]

Pads are not required.

Now if you have large loads, WRC-107 may force you to use pads.

Alloy shell with carbon steel clips you may want to use pads

PWHT vessel you may want to use pads in case clips need to be mooved due to being located incorrectly or platform not matching.

 

[jte]

First, keep in mind that Appendix G is a nonmandatory appendix. Thus, while what it says is good practice, nothing it says is actually mandatory. I find it interesting that it only mentions the reduction of secondary stresses - I'd say that repads also reduce primary stresses.

What is mandatory is that you, as the engineer/designer, consider and evaluate all of the loadings on the vessel. Based on my experience, if I were to put a 6' x 6' top platform on a vessel and I'm attaching it with vertical legs to a 4' diameter head/shell which is 3" thick and at 400°F - I will not need repads since it is a relatively light load and transferred into the shell through shear. Take the same platform with unusually high live loads (e.g. designed to set a heavy safety valve down on it for maintenance, etc.) and put it on a 20' diameter head which is 3/4" thick and I'll have to take a close look at putting repads on since it is a relatively heavy load transferred into a large diameter thin shell through radial loading.

As vesselfab pointed out, there may be other reasons to use repads such as changes in metallurgy or to allow a bit of margin for post-PWHT changes.

There are also reasons to avoid repads, such as on vessels subject to fatigue where a fillet weld is detrimental, vessels subject to thermal shock where the differential temperature between a shell and repad could be detrimental, and my personal favorite: It is impossible to determine wall thickness on the run via UT when a repad is blocking access to the head or shell.

jt

 

[robsalv]

When I'm reviewing a project manager's/engineer's spec for a new vessel, I always go in with a starting position of requesting compensation pads for major structural attachments to a vessel.

Frankly, I don't want my minimum shell thickness dictated by structural attachment loadings. And I don't want my primary pressure containment envelope having to deal with point loads.

If the manager/engineer doesn't like the dollars involved, then we have the arm wrestle - but as far as I'm concerned, it's a good practice.



To those who have responded, what do you view as the down side of mandating compensation pads?



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[gr2vessels]

The service sometimes precludes the use of reo pads (eg. Hydrogen service). External environment could also restrict the use of pads, like production separators on an offshore platform. As far as the normal loads for a pipe support or walkway, platform, the pads are not required. However, in contrast the lifting trunnions for a heavy vessel mandate reo pads anyway...
cheers,
gr2vessels

 

[doct9960]

Repads can be used on vessels in hydrogen service as long as you provide a vent hole on the repad or there is a "gap" on the weld (repad to shell joint). Client specs. dictate whether repads are allowed or not.

 

[ash008]

I thank u all for your precious reply.........i have included pads in my design........thanks again