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h ¼ perpendicular distance between plates of CPI and PPI (in.)
¼ angle of inclination of plate interceptors with the horizontal
(deg)
9.6.1 Sizing Horizontal Cylindrical Skimmers
The dimensions of the skimmer must be sufficient to allow for separation
of the smallest oil droplet that need to be separated and for the desired
retention time. Let D be the skimmer diameter (in in.) and L s be the
skimmer length (in ft). We shall assume that the skimmer is half full with
water, which moves horizontally from the inlet to the outlet of the
skimmer. Due to the presence of various internal components, the effective
length for separation, L, will be less than L s . To be on the conservative
side, we shall also assume that the oil droplets will settle vertically upward
from the bottom of the skimmer to the surface of the water; that is, the oil
droplets will travel a distance equal to the radius of the skimmer to reach
the surface of the water. The time it takes the oil droplet to rise to the
surface, t o , must equal the time it takes the water to move from the inlet to
the outlet of the skimmer, t w .
From Stokes’ law, the terminal rise velocity of the oil droplet, u o ,is
given by
ð
w
o Þd m ft
6
u ¼ 1:787 10 ð1Þ
pw s
where d m is the diameter of the oil droplet (in m). Therefore,
ðD=2 12Þ
t o ¼ s
u o
D pw
t ¼ 2:34 10 4 s ð2Þ
o
ð
w
o Þd m
The average velocity of the water, u w , is obtained by dividing the flow rate
by the flow area; that is,
½5:61Q pw =ð24 3600Þ ft
u w ¼ 2 1
½0:5ð =4ÞðD=12Þ s
The water retention time is obtained by dividing the effective length, L,by
the average velocity; therefore,
2
D L
t ¼ 41:9786 s ð3Þ
w
Q pw
Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.