Static Electricity 2000 Edition

7.4.5 Filtration.
7.4.5.1 Microfilters.
Microfilters typically have pore sizes less
than 150 
µm. These filters generate very large streaming cur-
rents with nonconductive liquids, due to their large contact
area. (Conductive liquids typically dissipate their charge to
ground through the body of the liquid.) Streaming currents
will frequently be greater than those for the pipe flow entering
the microfilter by two orders of magnitude and the charge
density added to the liquid can exceed 2000 
µC/m
3
.
To prevent these charges from entering the receiving ves-
sel, the filter should be placed far enough upstream so that the
charge can decay to the same magnitude as it would be in the
pipe flow. Common industry practice is to provide 30 seconds
Note: One straight line through the scales simultaneously solves
the relations.
10
5
100
50
1
0.5
10
5
0.1
S
υ
(
µC/m
3
) (m/sec)
(
µA)
I
s
100
10
1
0.1
0.01
10
–3
10
–4
gal/min (m
3
/hr)
F
d
100
10
1
1
10
100
10
3
10
4
10
3
10
5
10
4
10
5
1 m
500 mm
100 mm
50 mm
10 mm
1 in.
2 in.
3 in.
4 in.
6 in.
8 in.
10 in.
12 in.
14 in.
16 in.
20 in.
24 in.
I
s
= 2.5 
× 10
–5
υ
2
d
2
S = 3.18 
× 10
–5
υ
F = (
π/4) υd
2
(To convert from ft/sec to m/sec multiply by 3.28)
(To convert from bbl/hr to m
3
/hr multiply by 0.159)
F = (
π/4) υd
2
(To convert from bbl/hr to m
3
/hr multiply by 0.159)
Note: One straight line through the scales solves the flow relation.
1
0.8
gal/min (m
3
/hr)
F
d
100
10
1
1
10
100
10
3
10
4
10
3
10
5
10
4
10
5
1 m
500 mm
100 mm
50 mm
10 mm
1 in.
2 in.
3 in.
4 in.
6 in.
8 in.
12 in.
14 in.
16 in.
20 in.
24 in.
0.5
0.1
υd
(m
2
/sec)
υ
ft/sec (m/sec)
10
5
100
10
1
0.5
0.1
1
50
20
7
10 in.

---

77–
20
STATIC ELECTRICITY
2000 Edition
of residence time in the pipe or conductive hose downstream
of the microfilter, especially if the conductivity of the liquid is
not known. For nonconductive liquids that have both very low
conductivity (i.e., less than 2 pS/m) and high viscosity (i.e.,
greater than 30 centistokes) at the lowest intended operating
temperature, longer residence times might be appropriate. In
these cases, a residence time of up to three times the relax-
ation time constant of the liquid should be considered.