For values of A falling to the left of the material/temperature line see Step 7 1

[taba94]

hi
what is the technical and theoretical meaning of this statement in ug-28 of asme bpvc?
in other words whta happen when value of a falling to the left of the mat./temp line?

For values of A falling to the left of the material/temperature line, see Step 7.
Step 7.
For values of A falling to the left of the applicable
material/temperature line, the value of Pa can be calculated
using the following equation:

 

[metengr]

I am not sure of your question. ASME Section VIII Div 1 is a design by rule code book which means you follow the rules, as applicable. It seems clear that the rule for A (A = factor determined from Section II, Part D, Subpart 3, Figure G and used to enter the applicable material chart in Section II, Part D, Subpart 3. For the case of cylinders having Do/t values less than 10, see (c)(2).) you go to step 7 for values of A above.

 

[taba94]

thanks for your reply
i know that i should go to step 7 but im asking that what does it mean that we dont have such value of factor A in the chart ?

 

[AWDMIKE]

As you may know, the "B" factor is defined as follows:
B = AE / 2 = sig.e / 2 = P.cr * Do / (4*t)

where
A is the critical value of strain
E is the modulus of elasticity
sig.e is sigma sub e, or the theoretical elastic buckling circumferential stress
P.cr is the critical external pressure
Do is the cylinder outer diameter
t is the shell thickness

You may recognize the last part of the equation as sig.e = "P*r / t" or "P*D / (2*t)"

Using a value of FS = 3, the allowable external pressure, Pa, is defined as follows:
P.cr = FS*Pa = 3*Pa

Value of A is not to the left of material/temperature line:
Substituting P.cr = 3*Pa into the equation above, and solving for Pa in terms of B:
Pa = 4*B / ((3*(Do/t))

Value of A is to the left of material/temperature line:
Substituting P.cr = 3*Pa into the equation above, and solving for Pa in terms of AE/2
Pa = 2*A*E / (3*(Do/t))

By inspection, if you see the left portion of each of the material/temperature lines is a straight line which can be calculated by A*E/2. If you can imagine, the straight portion of the line would continue downward and to the left, and the equation within Step 7 simply represents that extension of the line.

The "B" factor charts look like allowable stress to strain plots which are a function of temperature.