Page 248 - Petroleum and Gas Field Processing

the engineer may have to assume reasonable values for missing or
unattainable data. The following data and information are required for the
design of the treatment system:
1. Flow rate of the produced water, Q pw (BPD)
2. Specific gravity,
pw , and viscosity, pw , of the produced water
3. Concentration of oil in the produced water (in mg/L or ppm)
4. Oil droplets size distribution in the produced water
5. Specific gravity of oil at flowing conditions,
o
6. Required effluent (treated water) quality (in mg/L or ppm)
7. Concentration of soluble oil in the effluent
8. Rainfall rate, Q rw , and flow rate of washdown water, Q wd
It should be noted that the equipment discussed in this chapter could not
separate the soluble oil. The treating equipment are designed to separate
dispersed oil only. Therefore, the soluble oil concentration should be
subtracted from the allowable oil concentration of the effluent to obtain
the allowable dispersed oil concentration used for design purposes. This is
used to determine the size of oil droplets that must be removed from
the water, which greatly affects the size and selection of the treating
equipment.
In the following subsections the equations used to determine the size
of the various treating equipment are first presented, then a guideline of
equipment selection procedure is highlighted. Throughout the derivation
of the equations, the following notations and units are used:

Q pw ¼ produced water flow rate (BPD)
Q rw ¼ deck’s rainwater flow rate (BPD)
Q wd ¼ deck’s washdown water flow rate (BPD)
pw ¼ produced water viscosity (cP)
2
¼ produced water viscosity (lb-s/ft )
¼ smallest diameter of oil droplets to be removed (mm)
d m
t rw ¼ retention time for water (min)
¼ retention time for water (s)
t w
¼ settling time for oil droplets (s)
t o
V w ¼ average water velocity through the skimmer (ft/s)
¼ settling velocity of oil droplets (ft/s)
V o
D ¼ diameter of cylindrical skimmer (in.)
L ¼ effective length of skimmer (ft)
¼ total (seam-to-seam) length of skimmer (ft)
L s
H ¼ height of water in the skimmer (ft)
W ¼ width of rectangular cross-section skimmer (ft)
2
A ¼ flow area for water (ft )

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