Page 245 - International safety guide for oil tankers and terminals

SHIPBOARD OPERATIONS
---ใช้เพื่อการศึกษาเท่านั้น---
งานห้องสมุด ศูนย์ฝกพาณิชย
11.8.2.3
A material of intermediate conductivity, such as wood or natural
fibre, generally has sufficient conductivity as a result of water
absorption to avoid the accumulation of electrostatic charge. At
the same time, the conductivity of these materials is low enough
to ensure that instantaneous release of a charge is not possible.
It is prudent to assume that the surface of a non-conducting liquid์นาวี
There should be a leakage path to earth from such materials, so
that they are not totally insulated, but this need not have the very
low resistance normally provided for the bonding and earthing of
metals. In practice, such a path usually occurs naturally on ships,
either by direct contact with the ship or by indirect contact through
the operator of the equipment.
Static Accumulator Oils
(static accumulator) may be charged and at a high potential
during and immediately after loading. Metallic dipping, ullaging
and sampling equipment should be bonded and earthed to avoid
sparks. There remains, however, the possibility of a brush
discharge between the equipment and the charged liquid surface
as the two approach each other. Since such discharges can be
incendive, no dipping, ullaging or sampling with metallic
equipment should take place while a static accumulator is being
loaded, due to the possibility of the presence of a flammable gas
ึ
mixture.
There should be a delay of 30 minutes (settling time) after
the completion of loading of each tank before commencing
these operations. This is to allow the settling of gas
bubbles, water or particulate matter in the liquid and the
dissipation of any electrical potential.
The situations in which these restrictions on the use of metallic
equipment should be applied are summarised in Figure 11.5.
Non-Metallic Equipment
Discharges between the surface of a static accumulator oil and
non-metallic objects have not in practice been found to be
incendive. Dipping, ullaging or sampling with non-metallic
equipment lowered on clean natural fibre line is therefore
permissible at any time.
Section 3.2.1 should be referred to regarding the use of
non-metallic sampling containers.
Sounding Pipes
Operations carried out through sounding pipes are permissible at
any time, because it is not possible for any significant charge to
accumulate on the surface of the liquid within a correctly designed
and installed sounding pipe. A sounding pipe is defined as a
conducting pipe which extends the full depth of the tank and
which is effectively bonded and earthed to the tank structure at its
extremities. The pipe should be slotted in order to prevent any
pressure differential between the inside of the pipe and the tank
and to ensure that true level indications are obtained.
The electrostatic field strength within a metal sounding pipe is
always low due to the small volume and to shielding from the rest
© ICS/OCIMF/IAPH 2006 211