Monday, September 8, 2014

NO hit in Safety Factor in federal regs for LF-ERW pipe!

I'm trying to decypher the federal formula for Class, MAOP, PIR, HCA, etc.
These federal regs are really convoluted.

Here's one thing I've found so far--
...specifically, Lap, ERW, and EFW welds,
there is NO HIT in the Safety Factor
under federal regulations for any of these

To explain:

Barlow's Formula is used to determine MAOP: 
MAOP=2*S*T * 1/(D*SF)

S=Material strength (psi)
D=Diameter of the pipe
T=Wall thickness in inches
SF=Safety Factor (or Design Factor)

Notice the Safety Factor (SF) in Barlow's Formula is using the common definition, where an SF=1 means the system will endure the design stress, and no more. Whereas with a SF=2, the system will endure 2x the design stress. Safety Factor must be >1 by definition.  

So you can easily see, because the SF is in the denominator, that if the SF goes up, the resulting MAOP goes down. This is what we would expect. 

However, the Federal MAOP formula is a variant of Barlow's formula:
MAOP=2*S*T * (1/D) * F * E * T

F=Design Factor (or Safety Factor)
E=Longitudinal Joint Factor
T=Temperature derating Factor

Notice in the federal formula, the it twists Barlow's formula, so that the Safety Factor is in the numerator! This means you must take your Barlow's Formula SF and use instead 1/SF, written as an inverse. So you want a Safety Factor of 10, you have to use 1/10 in the federal MAOP formula.
So if F*E*T = 0.1 in the Federal Formula,
this is equivalent to a Safety Factor of 10 in Barlow's formula.

And if  F*E*T = 1, this is like a SF=1, meaning, no safety factor.

49 CFR 192.111 - Design factor ( F ) for steel pipe

This is solely a function of Class:
  • Class 1=0.72  (SF=1.39)   Rural
  • Class 2=0.6  (SF=1.7)
  • Class 3=0.5  (SF=2)
  • Class 4=0.4   (SF=2.5)   Cities

49 CFR 192.113 - Longitudinal joint factor ( E ) for steel pipe.

Range is from E=1 (SF=1, no safety factor)
to E=0.6 (SF=1.67)

NOTE that ERW, and EFW are specifically mentioned in the regs,
however they have an E=1 (no safety factor!!)

49 CFR 192.115 - Temperature derating factor ( T ) for steel pipe.

Range from T=1.0 for 250'F (no safety factor)
to T=0.867 for 450'F (SF=1.15)

So if we calculate the cumulative
F(min) * E(min) * T(min) = 0.4 * 0.6 * 0.867 = 0.20808, SF=4.8
F(max) * E(max) * T(max) = 0.72 * 1 * 1 = 0.72, SF=1.39

This does not take into account pipelines rated under Alternate MAOP.

More soon...


May you, and all beings
be happy and free from suffering :)
-- ancient Buddhist Prayer (Metta)

Don't forget to sign the
Pledge to Resist
the Constitution Pipeline:

No comments: