Page 252 - Petroleum and Gas Field Processing

9.6.3 Sizing Vertical Cylindrical Skimmer
In this type of skimmer, the oil droplets move vertically upward against
the vertially downward flowing water. For settling to occur, the velocity of
the oil droplet, u o , must be at least equal to the average velocity of the
water, u pw , determined by dividing the water flow rate by the circular
cross-sectional area of the skimmer; that is,
5:61Q pw =ð24 3600Þ ft
u w ¼
2
ð =4ÞðD=12Þ sec
Equating u w to u o expressed by Eq. (1) and solving for the diameter of the
skimmer, D, we obtain

2
D ¼ 6691 Q pw w in: 2 ð16Þ
ð

Þd m
o
w
Equation (16) is good for diameters up to 48 in. For larger diameters,
where turbulence and short-circuiting occur, Eq. (16) should be multiplied
by a factor F that is greater than 1. The value of F depends largely on the
design of the inlet and outlet flow spreaders, oil collector, and baffles.
The retention time is obtained by dividing the height of the water in
the skimmer, H, by the average flow velocity of the water:
1
5:61Q pw =ð24 3600Þ
t rw ¼ H s ð17Þ
2
ð =4ÞðD=12Þ
Convert t rw into minutes in Eq. (17) and solve for the height H:
t rw Q pw
H ¼ 0:7 ft ð18Þ
D 2
Equation (16) determines the minimum diameter of the skimmer that
satisfies the settling constraint. Any diameter that is equal to or larger
than that determined from Eq. (16) is acceptable. Equation (18) determines
the height of the water for the selected diameter to satisfy the retention
time constraint. The total height (seam-to-seam length) of the skimmer, L s ,
is obtained by adding 3 ft to H; that is,
L s ¼ H þ 3ft ð19Þ

Example 1

A field separation and treatment plant produces 8000 BPD of water
containing 2000 mg of oil per liter of water with a maximum oil droplet size
of 500 mm. The water viscosity and specific gravity are 1.1 cP and 1.07,

Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.