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MAOP of a 6" SS Pipe

MAOP of a 6" SS Pipe

MAOP of a 6" SS Pipe

So I've done this calc before for CS pipe but SS is throwing me for a loop, I guess. If someone would check my calcs/results I'd be quite grateful.

I'm putting a 6" SS line into 900# service, covered by B31.3 (I have the 2012 edition). According to my 12th edition GPSA, SS falls under Material Group 2.1 and the flange pressure limit for 900# ANSI is 2160 psig. I'm assuming seamless pipe.

From B31.3 and given above, I get the following values:
S=25000 (Table A-1 B31.3)
Y=0.4 (service will never exceed 100F)

Based on this, I get a resulting minimum pipe thickness of 0.277. So based on ASME B36.19M, the wall thickness of 40S 6" is 0.280 wall, so 40S pipe should be fine for the service, correct?

So, moving on to test pressure and MAOP, tell me if I get this right because I don't know if there are various grades of SS pipe like there are CS pipe (i.e. Gr. B, x42, x46, etc.)

For a 0.280 wall SS pipe, the max allowable working pressure is 2187 psig. So that puts us at 100% of SMYS for a design factor of 0.5 of 4374 psi. 90% of that for testing is 3937 psi, and the resulting MAOP of the pipe is therefore 2625, which gives me pause because 2625 > 2187. Is this "normal"?

So I would pressure test the 40S 6" pipe for one hour at 3937 psig, and the MAOP of the line would be 2625.

Just, general comments, tell me if I'm off base or clueless, etc. Just want to make sure I don't buy the wrong pipe!



RE: MAOP of a 6" SS Pipe


Be careful - you're mixing codes here.

I can only assume from what you've said that your design is to ASME (not ANSI) B 31.3 and your design is to a ASME class 900 rated maximum design.

You need to follow the design code for hydrotest and maximum pressures, not make something up like you have done.

B31.3 provides an allowable stress figure. This commonly comes from the lower of a fration of SMYS or a fraction of UTS. This is not relevant, but means you don't know what the figure is or need to do anything else.

Pressure test is (from memory) 1.5 times MOP. MOP can't be higher than the maximum working pressure of the flange - See ASME B 16.5 to find that for your material and flange class rating. Your pressure test is (from your data which I haven't checked) is 1.5 x 2160 = 3240 psig.

That's it. The second half of your post ( from So, moving....) is totally incorrect and should be hastily forgotten.

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

RE: MAOP of a 6" SS Pipe


I agree with LI that you are mixing codes and also selecting wrong values from B31.3 table A-1. You do not state what actual grade of SS you are using but I will assume it is A312-304L or 316L which are very common and I am basing this also on your S=25000 which if I am correct you are quoting the value for Min Yield Strength and if so this is incorrect as you should select value to the right of this corresponding with your design temperature e.g. up to 300 DegF 16.7 KSI and 400 DegF 15.8KSI.

Your design pressure if not set by process but by maximum flange rating will be to ASME B16.5 and group will depend on grade of SS which in assumption above is 2.3, again my guess is due to your S=25KSI (Min Yield) which matches with 304L/316L as 304 and 316 are 30KSI. Again you have to take into consideration your design temperature when selecting from P/T Group Table.

Finally you also have to allow for mill tolerance in your thickness calculation which for seamless pipe is 12.5% and any corrosion allowance, not common for SS.

Based on 316L/304L with no corrosion allowance and design temp of 100 DegF to Class 900 flange rating I calculate wall thickness to be 8.69 mm and with MT of 12.5% 9.78mm (0.385").

Your hydrotest pressure is simply 1.5 x DP which in hypothetical case above is 1.5 x Flange Rating @ 100 DegF which is 2700 PSI. (Group 2.3)

Hope this helps.

RE: MAOP of a 6" SS Pipe

Both of you,

Thanks so much for responding.

This is 316L, produced water, S.G. 1.2, briny solution being put into an injection well.

MickMc, I was wondering about those values, and indeed I thought I should have been using 16.7 ksi but I wasn't sure. Thank you for clarifying.

Are there separate ANSI and ASME B31.3 codes covering Process Piping?

MicMc, thanks for pointing out the Material Group for this; I see in B16.5 that the working pressure for a 316L SS flange would be 1800 psig. Checking my calc against yours, I get 0.385 also. Sweet. So this corresponds to a schedule 80S pipe.

Where are the mill tolerances listed? Is that for all seamless pipe or is that specific to 316L? There's always so much more to know!

So a follow up question, they've got a PSV on the line set at 1400 psig, with a max pressure of 1600 psig. In order to save money on the pipe, it seems to make sense to rate the pipe for the PSV set pressure as that's what it's permitted to run at. Would you all agree? But re-running the calc it doesn't get us below 40S so we're stuck where we are.

Again, both of you, thank you for your help!


RE: MAOP of a 6" SS Pipe


To give you better values you can specify dual graded pipe and flanges i.e. 316/316L which give the mechanical properties of 316 and the weldability of 316L. In our piping specs we specify all dual grade and no problems with supply.

There are no separate standards ANSI & ASME B31.3 codes although in the past I believe many codes/standards were termed ANSI/ASME. I suggest doing a search for better explanation.

for mill tolerance refer to the particular ASTM code which in your case will probably be ASTM A312 which is 12.5% for both seamless and welded. For welded SS pipe in our project (8" and above) we would use ASTM A358 316/316L as welded A312 does not allow filler metal and therefore is limited in wall thickness and Mill Tolerance for A358 welded pipe is 0.3mm which is typical for welded plate.

RE: MAOP of a 6" SS Pipe


I'm kind of getting a bit worried by this thread that you're just shooting ideas off all over the place clearly without too much supervision and that equals danger.

The code is ASME - look at the title page. B 31.3 latest edition is 2014 - Get a copy and use it.

You need to understand that the design pressure / MOP is what you work to. That's what everyone else will use when you're gone. Use the change procedure applicable if you want to change something, not just switching thicknesses and pressures around at random.

To have a PSV with "set at 1400 psig, with a max pressure of 1600 psig" is well outside the range normally permitted for a relief system unless you've misunderstood the figures. A PSV has a set point, an under tolerance and then a maximum over pressure (termed accumulation) of no more than 6% of the set pressure. What do you mean??

Mill tolerances are set by the material specification. There are set standard figures which can be amended by the user in conjunction with the vendor.

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

RE: MAOP of a 6" SS Pipe


I see where I mixed my ASME/ANSI terms in my first post. I'd always called them "ANSI flanges" out of habit as I started in this business using the GPSA handbook and they refer to B31.8 and B31.3 as "ANSI B31.3", etc.

The PSV has a stamped range on the nameplate, which is weird, all I can think of is that there's a spring range which I would normally associate with a regulator. I was curious so I got a picture of it. Sure enough, it says the set pressure is 1400 psig with a max pressure of 1600. I'm assuming it's just a way of letting someone know it can be set to a higher pressure if desired, with the appropriate testing documentation of course.

When you mention the limitations on a relief system, I assume you're referring to the range that I mentioned. I don't believe that the nameplate is referencing an accumulation term. This was the first time I'd seen a "max pressure" stamped on a relief valve, in addition to the set pressure. Unusual.

MickMc, let me know if I have this correct. In B31.3, A358 is the spec referring to the pipe. In B16.5, A182 is the spec referring to fittings. And by "spec", I assume there's another document somewhere that gives information on the fabrication requirements and molecular content? (forgive my n00bish descriptions!)

And please don't worry. This is something I will submit for review by others to confirm my (our) findings based on the given data and assumptions.

Thanks again to you both!!


RE: MAOP of a 6" SS Pipe

Ok, that makes more sense. Many relief valves are able to be set anywhere in a certain range by adjusting the pre-load on the spring, which is what your range 1400 to 1600 means. Once set by the process conditions and MOP, the maximum pressure seen by the pipe / vessel it is protecting should be no higher than 6% above the set point.

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

RE: MAOP of a 6" SS Pipe


ASTM A358 is a specification for SS pipe as is ASTM A312 with differences in manufacturing and testing of similar grades e.g 316L and A182 is forging which in B16.5 refers to flanges. In B13.3 refer to Specification Index for Appendix A which precedes Notes for Table A-1 for title of referenced specifications and note that for A182 it is for flanges, fittings, valves etc.

The specification (A358/A312/A182) will give you manufacturing process, chemical composition, testing etc which are all minimum requirements and typically additional requirements if any stated by purchaser in material requisition.

RE: MAOP of a 6" SS Pipe

MickMc, thanks I understand.

Would you be able to elaborate a bit on the dual grade materials? I think the reason I was told to use 316L was due to it having better corrosion resistance around the weld. I would think this would have something to do with the choice of welding rod, no? Should I start a new thread for this question?


RE: MAOP of a 6" SS Pipe


316L base metal is less prone to "sensitization" around the weld area when compared to 316 or 316H base metal.

The L designates low carbon, and as such, has less tendency to form chromium carbides in the welded area.

The filler metal also plays a role in sensitization, and should be chosen to match the base metal.

In your case, 316L or dual grade 316/316L filler metal should be used for 316L or dual grade 316/316L base metal.

Hope that helps.

RE: MAOP of a 6" SS Pipe

DVWE, thanks!

So, following the thread here, what are the consequences of having chromium carbides form?

RE: MAOP of a 6" SS Pipe

When the carbides form, they deplete the chromium in the welded area. Chromium is the main alloying element in stainless steel, and when it is removed or depleted from the welded area, you will get accelerated corrosion.

RE: MAOP of a 6" SS Pipe


DVWE is more qualified I think than I am to elaborate on specific elements.

I would think that the better corrosion resistance would be selection of 316 rather than 304, both are commonly used in process piping. When components are being welded our preference is for 316L or 304L for reasons stated by DVWE but by stipulating 316/316L we get benefits of 316 higher strength with 316L better weldability.

RE: MAOP of a 6" SS Pipe

Thanks MickMc, sorry I didn't notice the reply before today. That is indeed why we're going with 316L; better corrosion resistance.

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