UNDERSTANDING THE FOLIAS FACTOR

FITNESS FOR SERVICE ASSESSMENTS (FFSAs)

UNDERSTANDING THE FOLIAS FACTOR

We like to keep things simple on our FFS training courses at Wilkinson Coutts. In FFS codes, the term shells describes pipes, vessels, and tanks collectively. The FOLIAS factor (M) is a critical component in assessing these shells. So, what is it?

It’s just a stress concentration factor to acknowledge the existence of a locally thinned area in an internally pressurised shell. The thinned area wants to bulge, causing stress concentrations between it and the un-thinned areas nearby, reducing its resistance to failure.

How does M affect things?

Simplistically, the M-factor reduces the safe MAWP by affecting the RSF (Remaining Strength Factor) of the flawed component like this

• MAWP (thinned) = MAWP (undamaged) x RSF (a factor less than 1)

As the M-number increases, it decreases RSF, so reducing the safe MAWP. Here’s the relationship in API 579 (eqn 5.12)

The parameter R_t comes from API 579 Eqn (5.5) …it’s a measure of how deep the local wall thinning is compared to the surrounding undamaged thickness.

The greater the wall loss, the small Rt becomes.

If you increase M (keeping R_t the same), you’ll see RSF decreases (hence so does the safe MAWP).

What affects M?

M is a function of several things:

• The Diameter (D) and uncorroded wall thickness (t) of the shell
• The length of the thinned area (let’s call it s)

Mathematically, it would be shown as

• M = f (D, t, s)

It’s only a short journey to track this through API 579 … calculate a ‘length parameter called Lambda’ (λ) from Eqn 5.6 below and drop that into table 5.2 to give you the Folias factor M

What M -factor numbers can you expect?

A component with no local thinning (so no bulging stress concentrations) has a Folias factor M=1. Hence, RSF =1, and MAWP is not reduced.

As shell diameter D increases (i.e., a thinner-walled shell), M decreases until it gets very close to M=1 (again, good). You can think of this as a very thin-walled shell deflecting quite uniformly, with negligible bulge-induced local stress concentrations.

As a shell gets thicker-walled (smaller D, larger wall thickness t), the equation causes M to increase (bad), decreasing the RSF, and hence the safe MAWP. Stress concentrations are higher in bulged shells, where thicker walls limit uniform deflections.

How does defect length s affect M?

Looking at the above equations shows:

• If s doubles, this also doubles λ
• Doubling λ (say from 1.0 to 2.0) in table 5.2 increases M from 1.199 to 1.618

Let’s check the effect on RSF, assuming, for convenience, R_t =0.4

For M = 1.199    RSF calculates to 0.8

For M = 1.618    RSF calculates to 0.636

For a shell with a design MAWP of (say) 100 bar this gives a Folias-factor -induced reduction in safe MAWP of about 20%, as a result of doubling the longitudinal defect length. It’s a rough example but shows the principle.

Where do the M equations come from?

There are many versions of equations used to find M.API 579 and ASME B31G use different variations and other technical papers contain numerous special cases (for example for shells with D/t >50). All the mathematical options are semi-empirical, which means they’re not perfect. That’s why RISK is a part of all FFS assessments.

Find out more in our API 579 practical course on conducting Level 1 and 2 assessments. More here…. want to study in your own time? We also have an e-learning course!