Page 250 - Petroleum and Gas Field Processing

Equating Eqs. (2) and (3), we get
Q pw pw
DL ¼ 557:43 in: ft ð4Þ
ð
w
o Þd m
For turbulence and short-circuiting, an efficiency factor of 1.8 is
recommended; therefore, Eq. (4) becomes

Q pw pw
DL ¼ 1000 in: ft ð5Þ
ð
w
o Þd m
Equation (5) provides a relationship between the diameter D and effective
length L of the skimmer which allows all oil droplets of diameter d m or
larger to rise to the surface of the water. Therefore, any combination of D
and L that satisfies Eq. (5) is theoretically acceptable.
Another constraint on the dimensions of the skimmer is that the size
should be large enough to provide the necessary retention time, t rw . The
retention time is obtained by dividing the volume of the skimmer occupied
by water, V pw , by the water flow rate, that is,

V pw
t rw ¼ ð6Þ
Q pw
where

D 2
V pw ¼ 0:5 L ft 3
4 12

3 !
ft 5:61
Q pw ¼ Q pw ðBPDÞ
min 24 60
Substituting in Eq. (6) and rearranging, we obtain
2 2
D L ¼ 1:43t rw Q pw in: ft ð7Þ
Therefore, the combination of diameter and effective length that satisfies
both settling and retention time constraints should be selected. This is
obtained by plotting D versus L for both Eqs. (5) and (7) and determining
the values of D and L that satisfy both equations. The actual length of the
skimmer, L s , is then determined from

L s ¼ 1:33 L ð8Þ

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