MAWP Limited by Bottom Head Vs. Nozzle falnge 2

[Meck91]

Folks,

I recently received pressure vessel calculations for review per ASME Section VIII Div. 1, from fabricator. Design data shows that the MAWP is limited by vessel bottom head (i-e. 170 psi @ design temp.)
However, when I looked at pressure summary report (available in detail calculations) I found out that all of the vessel nozzles including manhole nozzle flanges has the same MAWP (I-e. 170 psi @ design temp.). In this case why the design data shows that MAWP is limited by bottom head ONLY and why not by bottom head & all nozzles flanges.
Is there any specific reason for not mentioning nozzles flanges as a weakest component of vessel with respect to MAWP?

Your feedback would be highly appreciated.

Regards,

Meck91

 

[vesselfab]

nozzle reinforcement is done last and will not limit vessel (normally)

the nozzles are designed to reinforce at the MAWP of the chamber, no need for anything more.

 

[rneill]

It does seem a bit strange and it is possible that they have just assigned the lower rating as the "design pressure" for all the nozzle and nozzle reinforcement calculations.

Myself, I'm okay with the nozzle ratings as long as they are not lower than the basic rating of the heads or shells (I never want a nozzle to be the limiting component - personal preference) but I do require that the manufacturer show me the actual MAWP rating for each component including the nozzles. It would be very unusual for these numbers to exactly match if they are actually listing the MAWP.

 

[vesselfab]

the mawp is found by checking the heads, shells, transitions, etc of the vessel chambers.

the mawp is reduced by the amount of static head on the lowest rated part.

the mawp has been found at the top of the vessel for the main vessel components.

then each nozzle is checked for reinforcement for the mawp + static head for each nozzle at it's coresponding elevation so that a nozzle may not limit.

software normally does not find the mawp for each nozzle, but rather, designs for the MAWP or MAP or both and lists the mawp designed for.

I had an engineer here once that wanted to check design for each component of nozzles and then thicken the shell to match those MAWP. He did not last long here.

 

[rneill]

I agree that the normal practice is size the heads and shell, and then ensure that the nozzles are at least as strong or stronger than the MAWP of the vessel body.

What I don't permit is a situation where the nozzle rating (e.g., the ASME B16.5 rating for a standard flange) is the limiting value for determining the overall vessel MAWP. Example, I would not accept a situation where the heads and shell had an MAWP of 1900 psig but the Class 600 flanges had a MAWP of 1480 thus limiting the vessel to 1480 psig, In this case, the nozzles should be bumped up a class so that they are not the limiting components - to Class 9000 or 2220 psig. There is no reason to require the vessel heads and shell to be resized to the 2220 psig. They would now be limiting and the vessel MAWP would be 1900 psig in this example.

I have had vessel designs submitted to me for review in the past where the nozzles are the limiting component and I have normally pushed the nozzles up a class.

 

[vesselfab]

if you did that on one of my vessels....you would be facing a hefty change order.

the vessel fabricator may have had thick plate in stock or needed it for vacuumn or a local large nozzle or wind or for zick trying to get rid of stiffeners and based his reinforcement on excess plate.

you specified design pressure and nozzle ratings.

he met that and the ASME Code.

but then again....I don't know....maybe he was just a dumb fabricator

 

[TomBarsh]

Commercial software may provide an option to design the nozzles to the pressure chamber's MAWP. This might match your case where the nozzle MAWPs "coincidentally" match the bottom head (limiting component) MAWP exactly. In this case the nozzles most likely have additional pressure capacity above the bottom head's MAWP but this is not reported due to the software option settings.

Most commercial software probably provides an option to determine and report the MAWP of each nozzle separately. This option would give you the nozzle MAWPs precisely for each nozzle.

This is true of our software and probably most others.

 

[Meck91]

TomBarsh,

For your info. "Compress model" is being used for calculation purposes. Is your comment valid, with respect to this program.

Thanks,

Meck91

 

[TomBarsh]

Meck,

Yes.

You may be interested in these FAQ documents.

1- the four nozzle design/reporting modes available:

http://www.codeware.com/support/faq/1013.html

2- how the MAWP of a nozzle is determined:

http://www.codeware.com/support/faq/1015.html

Tom Barsh
Codeware Technical Support

http://www.codeware.com

 

[rneill]

The specs I send out to bid for vessels clearly state my requirement that the nozzles not be the limiting component. This is a fairly standard requirement amongst the "Majors" (e.g., Chevron, Exxon, etc) and I know that least one vessel design package that has a checkbox option to impose this additional requirement (PV Elite)

In my case, since this is specified in the original bid package, unless an exception was taken and agreed upon during final negotiation of the contract, I am within my rights to force the nozzles up a class if they are the limiting component.

As an end user, I want the maximum flexibility I can get and don't see the benefit of limiting a vessels maximum rating for the sake of some relatively inexpensive nozzles.

 

[roca]

rneill
Probably stating the obvious - but don't forget to change the connecting flange ratings as well if you do go up a flange rating on the vessel.

Meck91
In places where I have worked before if the nozzles are not allowed to limit the MAWP a prelim check is done for this before the vessel goes out to tender with it's datasheet, specs, etc as the design of the plant (e.g. piping design) is continuing in parallel.

 

[EngAddict]

There is nothing wrong with the flanges limiting the MAWP, it is quite common. I would be skeptical of the calculations if they are the same. Quite often the MAWP calculations are not calculated correctly, I would double check the calcs but either way the head or flange will not be a problem.

@rneill
I would not say it is a standard requirement by the "majors" to disallow the flange to be the limiting component. I am currently working on the Gorgon Gas Project for Chevron and in their specification it clearly states that the shell, head or flanges may limit the MAWP but nothing else.

Just my 2 cents..

 

[vesselfab]

generally, the flanges pressure/temperature ratings are allowed to limit vessel.

going up a flange rating makes it hard on pipers going with out of spec flanges at vessel, more gasket spares, bigger bolts and future problems when tieing in new pipe.

uhhh,,I thought that was a 600# spec line.

But it aint my plant, so more power to the company

 

[rneill]

EngAddict, I wonder if your project spec might have diverged from the Corporate specs (or it's possible the corporate specs have changed) as I worked with their Corporate Vessel Specs (both onshore and offshore) as recently as 2008 and they both contained wording something along the lines of:

...the maximum allowable pressure shall be limited by the shell or heads and not by minor components such as nozzles or manways ...

I know the Major Capital Projects do tend to develop their own project based specs and I have no familiarity with your project so it wouldn't surprise me if they might have some differences to the established Corporate specs.

It's been about 10 years since I had to work with the Exxon/Mobil vessel specs but at that time they also had the same general requirement that the MAWP was to be based on the shell or heads and not the nozzles.

Having typically worked for the Major's, in the past 20 years I have had to work with this requirement more often than not. However, I know that EPC's and Vessel Manufacturers will not do this unless mandated by the end user and most end users do not require it.

 

[jamesl]

As a general rule, nozzles shall not limit vessel MAWP. However, changing flange class to meet this rule is a little extreme.

This rule gives end user some flexibility. However fabricators are often punished for using thicker plates. For low pressure thin wall vessel governed by external pressure, calculated MAWP is often several times of design pressure. Many fabricators will specify minimum head thickness barely greater than the required to limit MAWP close to design pressure. There is not much the client and their agent can do about this practice.

 

[vesselfab]

yep sure can

top head to decimal minimu for design, bottom thicker for static head and shell whatever you need.

funny how some engineers then try to say that both heads need to be equal thickness.

 

[superdaigle]

Rneill, You are correct in stating that many majors request that the design pressure not be limited by nozzles or the like; however, many specs also state that spec sheet takes precedence over the company pressure vessel specification.

When push comes to shove if the vessel is going into say... 600# class carbon steel piping, you're limited to 1480 psig @ 100 F. I haven't found many operators willing to spend the extra money on higher pressure fittings that are over and above the requirements of the service, no matter what the shell/head thickness comes out to be as determined by the calcs.

 

[rneill]

superdaigle, you're right in that the project specific data sheet takes precedence over the general specification and often the project engineers will take the liberty of relaxing this requirement.