Page 257 - Petroleum and Gas Field Processing

Using Eq. (25) and the dimensions of standard CPI packs, the
number of CPI packs needed, N, is determined using

Q pw pw
N ¼ 0:11 ð27Þ
ð
w
o Þd m

9.6.6 Sizing of Skimmers with Cross-Flow Devices

As described earlier, cross-flow devices require the installation of internal
spreaders and collectors for uniform distribution of the water among the
plates. Because spreaders do not guarantee a 100% uniform distribution of
the flow, a spreader efficiency must be introduced in sizing skimmers with
cross-flow devices. It has been suggested to use a spreader efficiency of
75% for this purpose. Equation (25) could be used for sizing after dividing
the right-hand side by 0.75; therefore,

Q pw h pw
HWL ¼ 6:432 ð28Þ
ð
w
o Þd m cos

9.6.7 Determining the Reqired Number of SP Packs
As previously described, the SP packs are used to coalesce the oil droplets
and greatly increase their size to speed the separation process. Depending
on the treatment requirements, more than one stage may be needed. In
such a case, the stages will be arranged in series. The number of SP pack
stages, n, can be determined as follows. Let C i be the oil concentration in
the influent water and C o be the maximum allowed oil concentration in the
effluent (treated) water. Therefore, the overall separation or treatment
efficiency, E, is determined from
C o
E ¼ 1 ð29Þ
C i
Alternatively, because the droplet size distribution is approximately linear,
E could be determined from

d o
E ¼ 1 ð30Þ
d i
where d o is the largest oil droplet diameter in the effluent and d i is the
largest oil droplet diameter in the influent water.
Now, let d m be the oil droplet diameter that can be separated by a
skimmer of a certain size. The function of the SP pack is to coalesce such
oil droplets to increase the diameter to d max (typically, d max for standard

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