FIXED AND VARIABLE AREA DISCHARGES (FAVAD)
Fixed and Variable Area Discharges (FAVAD) recognizes that flow rates of some types of leaks vary more with pressure than others, some with pressure to power 0.5 (less sensitive), others with pressure to power 1.5 (more sensitive).
This has been known since 1994, but the 1999 UARL equation assumed, for simplicity, a linear relationship half-way between 0.5 and 1.5. UARL with FAVAD calculation requires pipe materials to be broadly classified as rigid (power 0.5) or flexible (power 1.5).
Fixed Area Leaks
For Fixed Area Leaks, occurring in rigid pipes, leak flow velocity is not very sensitive to changes in pressure; leak flow rate (volume per hour) varies with pressure to the power 0.5 (N1=0.5).
A 10% increase in pressure results in a 5% increase in leak flow rate.
Variable Area Leaks
For Variable Area Leaks, occurring in flexible pipes and background leakage at joint and fittings, leakage area also varies linearly with pressure. Leak flow rate (volume per hour) is therefore much more sensitive to changes in pressure and varies with pressure to the power 1.5 (N1=1.5).
A 10% increase in pressure results in a 15% increase in leak flow rate.
