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sethevans (Mechanical) (OP)
31 Mar 10 16:52
I have performed a calculation with Advanced Pressure Vessel software which says I can take the UCS-66(b) reduction for a lower MDMT. Is this correct (assuming 3/8" Tn and E* of 85%)? There are no calculations printed in order to verify that they are correct. The link is to the calculation sheet
hakkik (Mechanical)
31 Mar 10 18:10
Yes, UCS-66(b) allows some reduction on MDMT. The rationale behind this is simple. MDMT rating assumes you are fully stressing(Table 1A allowable stress) the part, but in reality generally you are not. so your part is stronge (and thicker) than the required thickness. Read UCS-66(b) carefully. You will find your answers :)

Sometimes using this reduction not allowed. Check eng specs also.

 
sethevans (Mechanical) (OP)
1 Apr 10 9:05
Hakkik,
Thanks for responding. The thing that has me concerned is the fact that SA-106 Gr. B is not listed in Fig. UCS-66 under curves "B", "C", or "D". Therefore, you would have to use curve "A". Unless I am using Fig. UCS-66.2 incorrectly, I'm coming up with a ratio of .9078. Using Fig. UCS-66.1, that ratio is subsequently not giving me the reduction I need for curve "A". I need this vessel to be designed for an MDMT of -49 deg. F. If this material won't work, can anyone lead me in the direction I need to be in?

Thanks in advance!!
doct9960 (Mechanical)
1 Apr 10 10:39
sethevans,

Curve "B" applies to SA-106-B.  I don't think you could rate SA-106-B to -49°F MDMT.   Try SA-333-6.
sethevans (Mechanical) (OP)
1 Apr 10 11:15
doct9960,
I did the calculation by hand, and I'm still not getting by with it. UsingFig. UCS-66.2, I'm coming up with the following:

tr E*/tn – c  = (.2598)(.85)/(.312-.125) = .22083/.187 = 1.18

If I'm calculating this ratio correctly, SA-333 Gr.6 won't work either. Can someone confirm that my calculations aren't totally out of whack?

Thanks again!
hakkik (Mechanical)
1 Apr 10 11:33
sethevans,

doct9960 is right, Curve B applies for pipes(Fig UCS-66 Note(2)(c) ) So you have -20°F without impact testing and UCS-66(b) reduction.

all values taken from your calc. rep.
ratio = (0,2881x0.7)/(0.375-0.125) = 0.80
with the ratio calculated above you can reduce MDMT without impact testing 20°F approx. so your MDMT=-40°F

looks like you have no chance to rate your pipe(SA-106-B) -49°F without impact testing. again doct9960 is right :)

you have two options.
1) impact tested SA-106-B. (see UG-84)
2) using SA-333Gr6(same allowable stress value at375°F)

I would prefer the second option.


 
hakkik (Mechanical)
1 Apr 10 12:10
sorry i miscalculated calculated ratio above. i forgot about pipe undertolerance 12.5% and tr in corroded condition.

Correct one

ratio = ((0.2881-0.125)x0.7)/((0.375*0,875)-0.125) = 0.562
so you get ~50°F reduction. you have no problem with SA-106-B.

again sorry for the mistake,
eliebl (Mechanical)
1 Apr 10 17:20
I wouldn't try impact testing SA-106 B.  It's very unlikely that it would pass.  SA-333 gr6 is acceptable under UCS-66(g).

EJL
doct9960 (Mechanical)
1 Apr 10 18:45
sethevans,

Keep in mind that the MDMT and MAWP are related to each other. As you decrease the MAWP, you decrease the coincident ratio, thus decreasing the MDMT.

Let's assume that the vessel MAWP is limited by the shell with the following conditions:
design pressure = 200 psi
design temp. = 375°F
design MDMT = -49°F
joint efficiency = 0.85
corrosion allowance = 0.125"
shell material = SA-106-B (14" STD pipe)
MAWP = S x E x t / (Ro - 0.4t) = 17100 x 0.85 x 0.203 / (7 - 0.4 x 0.203) = 426.46 psi

Now let's calculate for the rated MDMT for three different cases of MAWP.

Case 1: MAWP = 426.46 psi
tn = 0.375" x 0.875 = 0.328"
Unadjusted MDMT = -20°F (Curve B)
tr = MAWP x Ro / (S x E + 0.4 x MAWP) = 426.46 x 7 / (17100 x 0.85 + 0.4 x 426.46) = 0.203"
Coincident ratio = tr x E* / (tn - c) = 0.203 x 0.85 / (0.328 - 0.125) = 0.85
From Fig. UCS-66.1, temperature reduction = 15°F
Adjusted MDMT = -20 - 15 = -35°F
Since the adjusted MDMT of -35°F is warmer than the design MDMT of -49°F, you need to impact test or change material.

Case 2: MAWP = 350 psi
tn = 0.375" x 0.875 = 0.328"
Unadjusted MDMT = -20°F (Curve B)
tr = MAWP x Ro / (S x E + 0.4 x MAWP) = 350 x 7 / (17100 x 0.85 + 0.4 x 350) = 0.167"
Coincident ratio = tr x E* / (tn - c) = 0.167 x 0.85 / (0.328 - 0.125) = 0.699
From Fig. UCS-66.1, temperature reduction = 30°F
Adjusted MDMT = -20 - 30 = -50°F
Rated MDMT = -50°F (no impact test required)

Case 3: MAWP = design pressure = 200 psi
tn = 0.375" x 0.875 = 0.328"
Unadjusted MDMT = -20°F (Curve B)
tr = MAWP x Ro / (S x E + 0.4 x MAWP) = 200 x 7 / (17100 x 0.85 + 0.4 x 200) = 0.096"
Coincident ratio = tr x E* / (tn - c) = 0.096 x 0.85 / (0.328 - 0.125) = 0.402
From Fig. UCS-66.1, temperature reduction = 92°F
Adjusted MDMT = -20 - 92 = -112°F
Rated MDMT = -55°F (per UCS-66(b)(2))(no impact test required)

As you can see, MDMT decreases as MAWP decreases. If you still want to use SA-106-B without impact test, you need to rate your vessel with an MAWP that will satisfy the design MDMT of -49°F. This is assuming that your vessel MAWP and MDMT is governed by the SA-106-B shell.  
rneill (Mechanical)
2 Apr 10 10:57
Given the relatively small cost difference between A106-B and A333-6, I wouldn't even bother with A106-B below its normal Curve B temperature.  I have had A106-B material fail impact testing at temperatures well above -20F (for thick material) and for any use now below the normal regularly accepted MDMD on Curve B, I just default to A333-6.  myself, I don't bother with the stress ratio reduction.

Locally, A333-6 is readily available and has a very small cost premium over A106-B.
sethevans (Mechanical) (OP)
2 Apr 10 10:58
doct9960,
If I'm using the calculations with the new MAWP correctly, my adjusted MDMT would be as follows:

Case 4: MAWP = 395 psi
tn = 0.375" x 0.875 = 0.328"
Unadjusted MDMT = -20°F (Curve B)
tr = MAWP x Ro / (S x E + 0.4 x MAWP) = 395 x 7 / (17100 x 0.85 + 0.4 x 395) = 0.188"
Coincident ratio = tr x E* / (tn - c) = 0.188 x 0.85 / (0.328 - 0.125) = 0.787
From Fig. UCS-66.1, temperature reduction = 22°F
Adjusted MDMT = -20 - 22 = -42°F
Since the adjusted MDMT of -42°F is warmer than the design MDMT of -49°F, I need to impact test or change material.

The other materials that are on this vessel are SA-234 Gr. WPB and SA-105, so the design is governed by SA-106 Gr. B & SA-234 Gr. WPB which are both 17,100 at this temperature.

Thanks for Everyone's help on this!
rneill (Mechanical)
2 Apr 10 14:43
With the SA-105 you might want to consider SA-105N.  there is quite a bit of data out there that SA-105 in the as forged condition is not suitable for the MDMT's permitted by Code.

If it were me, I'd switch to SA333-6 (pipe), SA350-LF2-Class 1 (flanges), and SA420-WPL6 (fittings).

The cost of this would be less than conducting impact testing on alternative materials.
sethevans (Mechanical) (OP)
9 Apr 10 9:34
To all,
Would I be correct in assuming that by using SA-333 Gr. 6 pipe, SA-350 LF2 Class 1 flanges, and SA-420 WPL6 fittings for the vessel, then postweld heat treating the vessel, per UCS-68(c), I would get a 30 deg. F reduction which would put me @ -50 deg. F?

This seems like the easiest way to go if it is true.

Thanks in advance!
SnTMan (Mechanical)
9 Apr 10 11:09
sethevans, you should be able to get -55F without the PWHT using the materials in your post, see UCS-66(g). These materials are all tested at -50F.

Regards,

Mike

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