Static Electricity 2000 Edition

s
, can be expressed by the following:
where:
I
s
= charging current
N = constant (characterizing flow conditions, see text)
x = approximately 2
y = approximately 2
v = flow velocity (m/sec)
d = diameter of conduit (m)
Various values for the constants can be found in the litera-
ture. For I
s
, in amperes, the constant N has been reported to
range from 3.75 
× 10
−6
C-sec/m
4
to 25 
× 10
−6
C-sec/m
4
. The
low value corresponds to turbulent flow through a long,
smooth pipe, while the high value corresponds to turbulent
flow through spiral-wound composite hose. An order of mag-
nitude value for N is 1 
× 10
−5
C-sec/m
4
. While more recent
studies suggest that y is equal to 1, it has been most commonly
reported that both x and y are approximately equal to 2, so
that the charging current is roughly proportional to the
square of (v 
× d). An important outcome of the studies is that
(v 
× d) can be used as a means of characterizing the charging
current in pipe flow and as a basis for setting flow limits when
filling tanks. (See Sections 7.5 and 7.6.)
A.7.4.2
All-plastic nonconductive pipe is not recommended
for handling nonconductive or semiconductive liquids, except
where it can be shown that the advantages outweigh any risks
associated with external static electric discharge or leaks from
pinholes or where tests have demonstrated that the phenom-
ena will not occur. Grounded, plastic-lined metal pipe does
not pose either of these risks directly, but tolerance for liner
pinholes should be considered. For example, if the liquid is
corrosive to metal piping, gradual loss of metal because of pin-
holes could lead to unacceptable product contamination and
eventual loss of containment. Conversely, minor pinhole dam-
age might be acceptable, if the liner is intended only to mini-
mize product discoloration caused by rust and scale.
Where nonconductive and partly conductive liquids need
to be transferred through plastic piping systems, mitigating
strategies include the following:
(1) Reducing the rate of charging by decreasing flow velocity
(2) Eliminating or relocating microfilters further upstream
(3) Reducing wall resistivity, possibly to less than 10
8
ohm-m
(4) Increasing the breakdown strength of the pipe wall by
increasing the thickness or changing the material of con-
struction
(5) Incorporating an external grounded conductive layer on
the piping
I
s
N v
x
( ) d
y
( )
=

---

77–
40
STATIC ELECTRICITY
2000 Edition
Combinations of these strategies can be considered. For
example, in many cases, the presence of an external conduc-
tive layer on a plastic pipe will not by itself eliminate punctur-
ing of the internal plastic wall, and, if the layer does not
provide containment, it will not prevent external leakage.
A.7.4.3
For all-metal conductive hoses, the resistance to
ground from any point should normally be 10 ohms or less,
except where insulating flanges are used to avoid sparks from
stray current. For conductive hoses that contain a continuous
bonding element, such as wire or braid, the resistance to
ground from any metal connector should normally be 1000
ohms per meter or less, with the same exception being appli-
cable. Resistance to ground through semiconductive hoses
whose current-limiting design eliminates a low-resistance
bonding element and resistance to ground through insulating
flanges should be between 10
3
ohms/m and 10
5
ohms/m. In
either case, the total resistance to ground from a metal hose
connector should not exceed 10
6
ohms.
While a resistance to ground of less than 10
6
ohms will pre-
vent accumulation of static electric charge in most cases, if
periodic testing reveals a significant increase in the as-installed
resistance, this could be the result of corrosion or other dam-
age that could lead to sudden loss of continuity. The hose or
insulating flange, or both, should be inspected to determine
the need for replacement. Where conductive hoses have dou-
ble spirals, one for bonding and the other for mechanical
strength, continuity between the end connectors only con-
firms the continuity of one spiral. A fire was reported during
draining of toluene from a tank vehicle through such a hose.
It was found that the inner spiral was not only broken but was
not designed to be bonded to the end connectors. For han-
dling nonconductive liquids, one option is to use a hose with
a semiconductive or conductive liner, so that a broken inner
spiral cannot become isolated from ground and form a spark
gap. Ideally, the inner spiral should be separately bonded to
the end connectors.
It is especially important to ensure continuity with end con-
nectors (or nozzles) where a hose is used in an ignitible atmo-
sphere. In general, it is safer to use a properly designed fixed
fill system, such as a dip pipe arrangement, for filling tank
vehicles, rather than to use a hose.
Utility Hoses. Where used in flammable atmospheres, such
as inside tanks, utility hoses should be conductive or semicon-
ductive. In particular, all metal connectors and nozzles should
be grounded. Ungrounded hose connectors on nonconduc-
tive hose can become charged by a variety of means, such as by
inserting a nitrogen hose into a tank containing charged liq-
uid or mist, by rubbing, or by steam impact. While clean and
dry gases do not generate charge, a nonconductive hose will
become highly charged by the flow of steam.