Calculate intermediate factor related to the geometry of the corroded zone.

`b31gafr(d, wth, l)`

- d
nominal outside diameter of the pipe, [

*inch*]. Type:`assert_double`

.- wth
nominal wall thickness of the pipe, [

*inch*]. Type:`assert_double`

.- l
measured maximum longitudinal length of the corroded area, [

*inch*]. Type:`assert_double`

.

Intermediate factor related to the geometry of the corroded area, [].
Type: `assert_double`

.

ASME B31G-1991.
Manual for determining the remaining strength of corroded pipelines. A
supplement to *ASTME B31* code for pressure piping.

```
b31gafr(30, .438, 7.5)
#> [1] 1.847
# [1] 1.847 # A-factor is less than 5, so the corrosion is not critical
```