UCS-66b3 MDMT 1

[loilfan]

I am doing calculations on a fitting in ASME VIII.

If I have a Curve C, P1 Gr. 2 material that is 4" thick, the MDMT per Fig UCS-66 is 52F. When I go to calculate the ratio in Figure UCS-66.1, it comes out to be less than 0.35.

As per UCS-66(b)(3), this component is impact test exempt if the MDMT is between -55F and -155F.

The other components as part of this design fall under UG-20f, but their MDMT can only be -20F.

Why is it that UCS-66(b)(3) lets the thicker component have a significantly lower MDMT than the thinner components in the same piece? Why does UCS-66(b)(3) have that range specified? It's limiting me from having a warmer MDMT.

 

[SnTMan]

loilfan, UCS-66(b)(3) is not preventing you from having an MDMT warmer than -55 F. UCS-66(b) is giving you a means to have an MDMT lower than 52 F.

Sounds to me like you need read UCS-66 carefully in its' entirety. Then read it some more.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[loilfan]

SnTMan, UCS-66(b)(3) says "When the minimum design metal temperature is colder than -55F and no colder than -155F, and the coincident ratio defined in Figure UCS-66.1 is less than or equal to 0.35, impact testing is not required." This lets me have an MDMT only between -55F and -155F. However, I don't see elsewhere in the Code where I can have a MDMT between 52F and -55F when I have a coincident ratio less than 0.35.

This obviously doesn't make sense, which is why I am hoping that someone on this forum can point me to the correct interpretation/Code Reference.

 

[SnTMan]

loilfan, details of your proposed construction and desired MDMT would be most helpful to the discussion, however, pressing on:

First, it appears you are trying to apply both UG-20(f) and UCS-66 to a single welded assembly. I personally would be dubious of this approach, especially so as your fitting seems to exceed the 1" thickness limit per UG-20(f).

Second, if welded to a thinner component, the governing thickness of your fitting is based on the thinner of the two parts joined, see UCS-66(a)(1)(-a)(-1) and (-2). See also (1)(-a)(-5).

But, supposing your governing thickness is 4", then the lowest permitted MDMT for a curve C material is 52 F, as you stated, before any permitted reductions, either per UCS-66(b) or UCS-68(c).

UCS-66(b) permits a reduction to the UCS-66(a) MDMT on the basis of excess thickness / reduced stress in the subject component. This is why your very thick component can achieve a greater reduction than your thinner component.

UCS-66(b)(1)(-a) permits reductions up to -55 F. Note also that Fig. UCS-66.1 permits reductions of up to 140 F. UCS-66(b) requires impact testing for MDMT below -55 F except as permitted by UCS-66(b)(3) or UCS-68(c).

Reading your posts it appears to me you are unclear between required and permitted MDMT's. The Code rules are used to establish the lowest permitted MDMT. They do not mandate any particular MDMT.

Recommend again you read UCS-66 completely and thoroughly. It is not the easiest thing to absorb.

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[loilfan]

SnTMan, unfortunately, I can't share the design because it is owned by my client.

I am looking at each thickness in the design so I can make sure that I actually get the minimum permitted MDMT since the rules in UG-20f and UCS-66 are completely different depending on thicknesses, ratios, etc.

I don't disagree with anything that you wrote, however, I still don't see in the Code how it lets me have a Curve C material with an MDMT between 52F and -55F with a coincident ratio less than 3. It's strange to me that UCS-66(b)(3) says that the MDMT must be colder than -55F and no colder than -155F for it to apply. If it had said just that it must not be colder than -155F, it would be clear.

 

[metengr]

loilfan
First, when you read through UCS-66, what applies to your case regarding Figure UCS-66.1 Reduction in Minimum Design Metal Temperature Without Impact Testing using the coincident ratio is UCS-66 (b)1 a and b below before you apply any reduction;

(b) When the coincident ratio defined in
Figure UCS-66.1 is less than one, Figure UCS-66.1 provides
a basis for the use of components made of Part UCS materials
to have a colder MDMT than that derived from (a)
above without impact testing.
(1) See below.
(-a) For such components, and for a MDMT of
−55°F (−48°C) and warmer, the MDMT without impact
testing determined in (a) above for the given material
and thickness may be reduced as determined from
Figure UCS-66.2. If the resulting temperature is colder
than the required MDMT, impact testing of the material
is not required.

(-b) Figure UCS-66.1 may also be used for components
not stressed in general primary membrane tensile
stress, such as flat heads, covers, tubesheets, and flanges
(including bolts and nuts). The MDMT of these components
without impact testing as determined in (a) or (c)
may be reduced as determined from Figure UCS-66.2.
The ratio used in Step 3 of Figure UCS-66.2 shall be the ratio
of maximum design pressure at the MDMT to the maximum allowable pressure (MAP) of the component at
the MDMT. If the resulting temperature is colder than the
required MDMT, impact testing of the material is not required,
provided the MDMT is not colder than −55°F
(−48°C).

Looking at UCS-66 (b)2 or even (b)3 are not applicable because your MDMT is not colder than -55 deg F or between -55 deg F and -155 deg F before any reduction.

 

[loilfan]

"Looking at UCS-66 (b)2 or even (b)3 are not applicable because your MDMT is not colder than -55 deg F or between -55 deg F and -155 deg F before any reduction"

Agreed. Which is why this is causing me trouble. The way I read UCS-66 seems like I am only permitted to have an MDMT above 52F or between -55F and -155F. UCS-66 (b)1 only applies if my coincident ratio had been above 0.35. Since it isn't, Figure UCS-66.2 directs me to UCS-66 (b)3, which also doesn't apply because our required MDMT is -20F.

 

[GenB]

Thank you... Great and interested answer by Metengr.