MAWP requirement 4

[zjliang]

Hey folks,

Sometimes we noticed that the requirement of the MAWP limited by shells or heads instead of discontinuities like nozzles is stated either explicitely or indirectly.

What do you guys think of it? Please present whatever your thoughts from experiences.

Thanks in advance,

 

[Christine74]

I don't think "limiting component" analyses like this are very useful at all.

The purpose of the ASME Code is to establish safe guidelines for designing pressure vessels, not to determine the exact pressure at which a particular component will fail.

You can calculate all of the MAWP's of the various components using the formulas provided in the Code, but that doesn't mean the part with the lowest MAWP will fail first. Several assumptions are built into these formulas.

-Christine

 

[doct9960]

zjliang,

you'll get some good insights from the following threads:

thread292-12372
thread794-165400

 

[vesselguy]

Zjiang,

Put yourself in the position of a site engineer for an owner. The Plant Manageer (a tough guy who do not take NO for an answer) comes to you and tell you that it is very important to the viability of the plant to debottleneck this section and put more flow and at higher pressure through. He tells you the whole sha-bam depends on whether that old vessel out there can handle the new design condition. That vessel was designed for 100 psi and need to handle 250 psi.

So you go and look at the drawings for that vessel and see that Blowhead vessel shop, the fabricator for the vessel, states the MAWP is only 200 psi limited by a 12" and 16" nozzle, even though the shell and head has an MAWP of 260 psi.

What do you think?

 

[zjliang]

Thanks for your comments and especially the good insights in those two old threads.

Cheers.

 

[Christine74]

Another potential problem with requiring that the MAWP be limited by shells/heads is that of joint efficiency.

Suppose a vessel was made with good quality welds but was not radiographed. The calculated shell MAWP is found to be 700 psi. Then you decided to do 100% radiography and the shell MAWP is found to be 1,000 psi (43% higher) -- even though it's the exact same vessel.

-Christine

 

[JStephen]

Vesselguy- seems to me there's a flaw in that logic. Suppose you design the vessel, and find that by increasing the thickness of the head slightly, you can do away with a repad there, and come out with net lower cost. That could lead to exactly the scenario you describe. But if it's mandated that MAWP must not be limited by nozzles, then you'd just build the vessel with the thinner head and add reinforcing. Then when the PM wants to uprate the tank, the MAWP is limited by head design. Or, as pointed out in Christine's post, if you can't use the extra metal for reinforcing, it may be more economical to use more x-ray and thinner shell, and you're in the same situation.

It seems to me that in either case, if you anticipate that the pressure (or temperature) may be increased at some point, you'd be ahead to just specify it now, rather than taking an indirect approach that may or may not accomplish the goal later.

 

[metengr]

Suppose a vessel was made with good quality welds but was not radiographed. The calculated shell MAWP is found to be 700 psi. Then you decided to do 100% radiography and the shell MAWP is found to be 1,000 psi (43% higher) -- even though it's the exact same vessel.

The NBIC does not permit rerates of pressure retaining itmes by attempting to increase the joint efficiency factor. See RC3022 (a).

 

[LSThill]

TECHNICAL NOTE:

Regards to the set mode of the Pressure Vessel software application:

Design of Nozzles for Larger of MAWP OR MAP

Desing nozzle for Chamber MAWP where Chamber MAWP ignorse ASME B16.5/16.47 large rating.

 

[SnTMan]

In the context of shell & tube HX, I have seen requirements such as zjliang stated, or something like "MAWP shall not be limited by minor components". So I have a floating head exchanger. The MAWP can potentially be limited by shell, heads, dished or flat, nozzle reinforcements, pipes, flanges, tubesheets, tubes, body flanges and bolting. Which are the "minor components"? Only the customer knows.

I have had customers call wanting to re-rate based on say a cylinder MAWP, and asking if the limiting component, say a tubesheet, can be replaced and get that MAWP. I generally have to reply "No, the tubesheet may limit, but the floating head is 2 psi behind that, the body flange is 2 psi behind that, and the bolting is 1 psi behind that." Won't get there, with the amount of money anybody wants to spend.

I agree with JStephen, if you think you may want more temperature or pressure capacity in the future, specify it now, it's much cheaper.

Regards,

Mike

 

[vesselguy]

JStephen & SnTMan,
I agree with the position that if we know what future design conditions would be, we should just design for it now. In this context, if I knew 20 yrs ago Mickysoft stocks would get this high, I would've bought their stock back then and retired in Bora Bora now. LOL. 99% of the times in this business/life, we deal with the unexpected. In the past, I had to deal with Owners who wants to save money (who wouldn't) by reusing equipment in their plant that was in previous life for other application, or bought some used equipment from used-equipment dealers and use it for new projects (and there's a lot of this in upstream and storage facility projects).

Good talking to you guys. Have a nice day.

 

[rhg]

See UG-99(d), and UG-101 Proof test to establish maximum allowable working pressure

rhg