ASME B 16.5 FLANGE RATING - WORKING PRESSURE OR DESIGN PRESSURE? 6

[RAJPT89]

Dear members,

I have been thinking for a while now, For ASME B16.5 flanges, Pressure indicated in Flange rating tables are OPERATING PRESSURE or it is Design Pressure ??

The question arises as the Software (PVElite) considers this pressure as Design Pressure, while some the clients has designed there piping based on this table considering the pressure as Operating pressure and levied there flange rating on our equipment as well, though as per software calculation their flange rating fails, stating that ASME B16.5 allows flange to work on those pressure tabulated, Apparently Design pressure can be high.

If this get clarified anywhere in Code?

Thanks,

Rajput

 

[LittleInch]

The phrase in B16.5 is that the pressures stated in there are "Maximum allowable working pressures" listed in gage (guage) pressure.

After "operating pressure" in your post I'm afraid I can't work out what you're trying to say. Please elaborate.

Some numbers as an example are always useful.

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

 

[david339933]

Design pressure is the pressure you design the system for....if you base your flanges on the operating pressure...that now becomes your design pressure. What happens down the road if some brilliant engineer wants to increase the operating pressure close to that "high design pressure" thinking it is ok?

B31.3 - 302.2.1

 

[moltenmetal]

Here's how you sort this out. MAWP is the highest pressure to which you may set your relief valve. Maximum operating pressure must obviously be below that, or your relief device will be active. All relief devices have a margin between their set pressure and the lowest pressure at which they will reliably remain sealed.

B16.5's pressures are MAWPs and hence RELIEF pressures. Maximum operating pressure will be a pressure BELOW that MAWP by a margin determined by the relief device.

 

[LittleInch]

You need to be careful about wording and the difference between piping and pressure vessels.

PV's use MAWP as the max pressure the vessel can see and , IIRC, there is no allowance for overpressure in the same way there is for piping systems.

In theory you can operate a flange at its MAWP, but in practice this is difficult.

Most people, if they want to max out the pressure rating of their system will use the MAWP of the flange as the design pressure of their piping system.

I still don't really follow your post as to what the client is doing or not doing, but as you haven't responded yet I may never know....

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

 

[moltenmetal]

LittleInch is right that piping codes tolerate a little more pressure (and temperature) above MAWP/MAWT for brief periods than vessel codes do. But the intent is the same- piping is either protected by its own relief or by the relief of a vessel it's attached to. Treat MAWP as maximum relief pressure and you won't go far wrong. Select maximum operating pressure based on relief device operating margin and MAWP. MAWP can be limited by any component in piping or vessels. Choosing B16.5 flange ratings as the MAWP limit for piping is a commonly done shortcut, with the theory being that such piping will leak at flanges rather than failing elsewhere and hence is safer. However, assuming that all piping in a line is good to the MAWP of the piping flanges can lead to problems in piping systems containing mixtures of components- stainless steel flanged instruments in carbon steel lines for instance. Again any component- including an instrument- can limit MAWP and hence set the relief requirement in a line. What you'll see as a result is that sometimes, totally needlessly, piping systems will go up a flange class on all instruments as a result. That drives up cost for an assembled system without actually a process need to drive it, and hence is generally bad practice.

 

[RAJPT89]

Thank you so much Littleinch, david339933 & moltenmetal for your responses,

Your reply resolved my doubt, when you mentioned that B16.5 states "Max. allowable working pressure" in gauge pressure.

I had ASME certification audit on 11th and 12th, thats why couldn't respond on your reply.

I would like to share a past project with a well-known engineering firm, where we have to design and manufacture a Double pipe heat exchanger (NH3 evaporator), where in the Inner pipe, Liquid amonia is getting superheated from 18 degree C to 225 degree C, operating pressure is 127.5 kg/cm2g, MOC is SA-182 F316 (Group 2.2), Now in this case, at Inlet nozzle Flange rating is 1500# and at outlet flange rating is 2500#. For Equipment design, Design pressure is 165 kg/cm2g & Design temperature is 300 degree C
As per table 2-2.2, 1500# is allowed upto 160.15 kg/cm2g for design temperature 300 degree C, Hence for Design pressure and temperature, Flange rating of Inlet Nozzle fails, but we can't change as the further connected piping is in customer scope with 1500# flange rating.

From your replies above, i can asses that customer has considered this equipment as piping and an equipment.

Eventually after installation at site, the equipment does not stop leaking, and the fault was found to be on the licensor part as to provide such complex construction , The exact case i can not share publicly.

Thank you again for your valuable feedback.





Thanks,

Rajput

 

[LittleInch]

Ah, now I get you.

The issue here is how to separate coincident pressure and temperature.

The design has worked on the basis of flow being present to allow the inlet flange to be designed to a lower temperature than the outlet flange. A little strange, but I can see why they've done it, especially as you are very close to the limit.

In the event of no flow and the design pressure and temperature of the inlet flange rises to your vessel design temperature, the piping is probably using the overpressure allowances in the piping code B31.3 which allows excess pressure for certain periods of time.

So your vessel analysis probably needs to stop at the nozzle/flange interface or at least note that this has been required due to the requirement to match the incoming pipe specification.



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

 

[marty007]

There needs to be a change in understanding of B16.5 pressure ratings at the large engineering firms.

Recent ASME interpretation VIII-1-16-85 highlighted the need for designers to take nozzle loadings into consideration for flange pressure ratings. This has now been codified in ASME VIII-1(2019) in paragraph UG-44(b).

This often results in flange ratings being de-rated from the tables listed in ASME B16.5. Unfortunately some large engineering firms are still using these B16.5 tables as-is. Many times I see equipment being requested with design pressure/temperature exactly matching the B16.5 table values.

We need to somehow get the message across to end users that this practice is no longer appropriate. Instead of basing the design pressure/temperature on flange classes, they should look at their process to determine a suitable design pressure, only then should they select the appropriate flange.

/rant

 

[krb]

Marty007 - thanks for the tip. I just looked at the recent and previous version of Sec Viii Div 1 and see the difference.