3.1 In arrangements where the total air pipe sectional
area is 10% or more of the cross-flooding sectional area, the restrictive
effect of any air back pressure may be neglected in the cross-flooding
calculations. The air pipe sectional area should be taken as the minimum
or the net sectional area of any automatic closing devices, if that
is less.
3.2 In arrangements where the total air pipe sectional
area is less than 10% of the cross-flooding sectional area, the restrictive
effect of air back pressure should be considered in the cross-flooding
calculations. The following method may be used for this purpose:
The k coefficient used in the calculation of cross-flooding
time should take into account the drop of head in the air pipe. This
can be done using an equivalent coefficient ke
,
which is calculated according to the following formula:
ke
|
= |
kw +
ka · (ρ
a/ρ
w) · (Sw/Sa)2
|
where:
kw
|
= |
k
coefficient for the cross-flooding arrangement (water) |
ka
|
= |
k
coefficient for the air pipe |
ρ
a
|
= |
air density |
ρ
w
|
= |
water density |
Sw
|
= |
cross-section
area of the cross-flooding device (water) |
Sa
|
= |
cross-section
of air pipe |