Static Electricity 2000 Edition

FIGURE 7.4.1(a)
Nomograph for estimating the charge on a 
nonconductive liquid flowing through a smooth pipe.
(Pratt, 1997, p. 112.)
7.4.2* Nonconductive Pipe and Lined Pipe.
Nonconductive
surfaces affect the rates of charge generation and charge dissi-
pation during flow through a pipe. The rate of charge gener-
ation is similar in conductive and nonconductive pipes, while
the rate of charge loss can be significantly slower in noncon-
ductive pipes. For charged, nonconductive liquids, insulation
by the pipe wall can result in charge accumulation of the
opposite polarity on the outer surface of the insulating liner or
pipe. Charge accumulation can eventually lead to electrical
breakdown and pinhole punctures of either the liner or, in
the case of nonconductive pipe, the entire wall thickness.
7.4.3* Flexible Hoses and Tubing.
Flexible hoses and flexi-
ble tubing are available in metal, lined metal, nonconductive
plastic, reinforced rubber and plastic, and composite-ply
types. Where nonconductive hose or tubing must be used
because of process conditions, the hazards of static electric
charge generation should be thoroughly investigated. As a
minimum, all conductive couplings (e.g., end fittings) and
components should be bonded and grounded. If hoses are
used immediately downstream of filters in nonconductive liq-
uid service, they should be of metal or other conductive mate-
rial. Semiconductive liners might be necessary to prevent
charge accumulation and pinhole damage to the hose. Con-
ductive hoses should be electrically continuous and the conti-
nuity should be periodically checked.
FIGURE 7.4.1(b)
Nomograph for estimating fluid flow
parameters. (Pratt, 1997, p. 114.)
7.4.4 Fill Pipes.
Fill pipes should be conductive and should
be bonded to the filling system. Fill pipes should extend to the
bottom of the vessel and can be equipped with either a 45
degree cut tip or a tee to divert flow horizontally near the bot-
tom of the vessel being filled. The design should prevent
upward spraying during the initial stage of filling. A “slow
start” might be necessary so that the inlet velocity is held to less
than 1 m/sec until the outlet of the dip pipe is covered by at
least two pipe diameters of liquid.