Instrument Nozzle flange rating 1

[Steve.sw90]

Dear all,
Our client is asking for 300# flanges for instrument connections on vessel, on which all process connections are of 150#.

What is the technical reason behind this?
I checked with few people they say this is to prevent inventory issues.
This response couldn't satisfy me because on project which we are working there are many 150# flanges, so they must have 150# flanges in their stock for future maintainance.

Regards,
Stephen

 

[XL83NL]

Refer to moltenmetal's reply in this thread (dated 25-Aug-14);

http://www.eng-tips.com/viewthread.cfm?qid=369092

Perhaps this one alos helps;

http://www.eng-tips.com/viewthread.cfm?qid=293246

Im sure others can contribute to this thread in a helpful way

 

[marty007]

Often where I see this is in CS vessels using SS instruments. If the vessel is rated right to the limit of a 150# CS flange, then you'll often have problems with the stainless steel instrumentation flanges.

Here's an example:
Carbon steel vessel MAWP: 230psi @ 300°F
This matches a 150# A-105 flange rating in B16.5, so external piping should be okay.
Now, the user wants to hook up an instrument that is normally supplied with stainless steel flanges.
The rating of an A-182-F304 150# flange at 300°F is only 205psi.
Now the user has a problem, and they have to find a work-around (CS instruments / intermediate flange / ...)
It's easier to just add a 300# CS flange for instrument nozzles, allowing a 300# SS instrument to be connected directly.

Cheers,
Marty

 

[Steve.sw90]

Thanx Marty for ur response, that could be one reason.

Regards,
Stephen

 

[moltenmetal]

This is a clear example of the downside of flange-limited pipe specs. Designing something by spec is always sub-optimal. The smarter thing to do in this case is to de-rate the entire system to the limitation of the 150# 304/L SS flange, since it is a virtual certainty that the MAWP arises only from this "design by spec" approach and is higher than necessary for the application.

 

[marty007]

moltenmetal - I couldn't agree more. This approach drives me bonkers sometimes.

One thing that constantly happens is that a customer will set a vessel design pressure/temperature exactly equal to the maximum rating of a B16.5 flange. Now they add a design liquid level to the vessel that adds 1-5 psi to the bottom nozzles. So, that perfectly rated flange is no longer adequate for the nozzle design pressure, and we're forced to ask the customer to either lower the design pressure by 5-10psi, or swap out all of the flanges that are below the design liquid level to one class higher.

So to add to your comment, the best thing to do would actually be to set the design pressure to 10psi under the 304ss flange rating.

This conversation happens at least a half-dozen times per year, you'd think I would have a copy/paste form letter by now...