ASME VIII MDMT and Impact test exemption

ASME Code Section VIII Div 1 exemption rules for ASME Impact Test Requirements:

There are specific rules in the ASME Code for exemption from ASME Impact Test Requirement. This test is very expensive, so pressure vessel manufacturers are trying to be exempted from this costly test.

You need to follow the following clauses to make exemption assessment for ASME impact test requirement:

UG-20(f) →→ UCS-66(A) →→ UCS-66(b) →→→UCS-68(c)

First, you have to keep your pressure vessel design data available and then refer to UG-20 (f). If you are exempted from this clause, you do not need to proceed further.

But if you are not exempted by UG-20 (f), you have to proceed to UCS-66(a), but again if you are exempted, there is no need for more assessment.

But if not, you have to proceed to UCS-66(b). If you are exempt now, there is no need for more assessment; otherwise, you have to proceed to UCS-68(c), and again if you are still not exempt, you have to carry out impact testing.

In some cases, you might be exempted from the ASME impact test requirement in the first stage of UG-20 (f). In others, you may be exempted from UCS-66(a) or UCS-66(b) or UCS-68(c). If you are not exempted, you must prepare yourself for doing this costly test.

This test would be more costly out of the US because of Laboratory Accreditation requirements. Also, there are fewer accredited labs in Europe and the Middle East, and their price is high as well.

UG-20(f)

We will start with UG-20(f) for the ASME impact test requirement. If your MOC (Material of Construction) is categorized in P-No. 1 or 2 (Refer to ASME Code Section IX for P-No Definition) and your MOC thickness has the limited value defined in this clause, then you might be exempted from impact testing.

But you need to refer to Fig UCS-66 in ASME Code Section VIII Div 1 and see in which A, B, C or D curves your MOC is listed. All ASME carbon steel and low alloy steel material is distributed in these 4 groups (Curves) of materials.

You need to know that the materials listed in curve D have the best toughness property, better than the materials listed in curve C. Similarly materials listed in curve C have better toughness properties compared to materials listed in curve B and materials listed in Curve B have better toughness than materials listed in Curve A .

See Following Fig UCS-66(a):

MDMT ASME VIII

When you determine your MOC curve, then you have to review UG-20(f) and look for the possibility of exemption from the ASME impact test requirement. There are some other conditions in this clause, which you should consider for exemption.

For instance, the vessel should be hydrostatically tested after completion, and the thermal and mechanical loading can not be a design controlling factor.

For example, if your MOC is a normalized SA 516 Gr.70 with 0.75 inch thickness you will be exempted from ASME impact test requirements.

Your thickness, in this example, is 0.75 of an inch, and is listed in curve D which is up to 1 inch, you are exempted by this clause. Of course, you will be carry out hydro-static test and ensure that the mentioned loadings are not a design controlling factor in your considered pressure vessel.

UCS-66(a)

So assume that in the above example, your MOC thickness is 1.125 inch instead of 1 inch, you will not be exempted by UG-20(f) and you have to refer to UCS-66(a);

But for assessment, based on this clause, you need to know your pressure vessel MDMT (Minimum Design Metal Temperature). Assume that is -20 degree F, so you should now go to Fig. UCS-66 and locate 1.125 inch in the horizontal axis and draw a vertical line.

In a similar way, locate -20 degree F in the vertical axes and draw a horizontal line. These two lines will cross each other.

MDMT ASME VIII

See above Figure. The lines are identified in red.

If the cross point falls above the curve D (because your MOC is listed in curve D) you are exempted. Otherwise you are not, but for the current example, you are above the curve D so you are exempted from impact testing.

To simplify your assessment for the ASME impact test requirement, the Fig-66 has been converted to the table (table UCS-66). For any MOC with specific thickness you can go to this table and see what is the minimum permissible temperature without impact testing.

See following UCS-66(a) Table:

MDMT ASME VIII Table

In the above example (normalized SA 516 Gr.70, Curve D, 1.125 inch thickness), the minimum permissible temperature without impact test is -26 degree F. This means that, if in the above example your MDMT changes from -20 degree F to -27 degree F, then you cannot be exempted from the ASME impact test requirement by UCS-66(a), and you have to proceed to UCS-66(b).

UCS-66(b)

Let us explain this clause with the above example. Your MDMT from above is -27 degree F, nominal thickness is 1.125 inch, normalized SA 516 Gr.70 listed in curve D and you are not exempted by UCS-66(a)

So you are here to continue your assessment to find a chance for exemption. You have to refer to Fig UCS-66.1 and calculate the following formula:

Ratio= tr E / (tn –c)

tr is the required design thickness for all applicable loading. We assume for the above example that is 0.95 inch. E is your joint efficiency, and we assume for this vessel it is 1. This means your vessel is RT2, tn is your nominal thinness, which in the example from above it is 1.125 inches, and C is your corrosion allowance, and we assume it is 0.125 inches; so let us calculate:

Ratio = 0.95×1/(1.125 – 0.125) Ratio= 0.95

See following Fig UCS-66(b):

MDMT ASME VIII Ratio

Then go to Figure UCS-66(b) and in the vertical axes locate Ratio and draw a horizontal line. Then locate the cross point with the graph and draw a vertical line to cross the horizontal axis.

You will be able to reach a value of 8 in the horizontal axes. This 8, is your 8 degree F bonus from table UCS-66, which you can reduce by 8 degrees F minimum permissible temperature in table without impact testing.

In the above example, your MDMT is -27 degree F, and in the UCS 66 table, the minimum permissible temperature without impact testing is -26 degree F. So with this clause you can reduce it to -36 degree F(-26 -8 = -34). Your MDMT is -27 degree F, so you are exempted from impact testing with this clause.

UCS-68(c)

Let us change one variable in the above example. Let’s assume you need to have -45 degree F for your MDMT. Other variables are the same; it means normalized SA 516 Gr.70 listed in curve D, thickness 1.125, so you can see you are not exempted by UCS-66(b);

This is because the minimum permissible temperature is -36 degree F, but your MDMT is -45 degree F, so UCS-68(c) might be helpful.

It says that if post weld heat treatment is not a code requirement and your P-No is 1 and you carry out post weld heat treatment, a 30 degree F bonus will be granted to you to reduce the minimum permissible temperature in table UCS-66.

So what post weld heat treatment is the code requirement?

It is code requirement when your service is lethal and when your thickness for P-No. 1 is greater than 1.5 inches;

So, for example, our service is not lethal and our P-No.1 thickness is 1.125 and it is less than 1.5 inches: therefore, post-weld heat treatment is not a code requirement.

It means if you carry out post-heat treatment, a 30 degree F bonus will be granted by this clause. For example, our minimum permissible temperature would be -36-30=-66 degree F, and your MDMT is -45 degree F, so you are exempted from impact testing.

Now the worst case: in the above, assume you need to have -70 degree F for your MDMT; you can see with this new condition you cannot be exempted even by UCS-68(c) and you have to carry out impact testing.

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