Page 80 - International safety guide for oil tankers and terminals

INTERNATIONAL SAFETY GUIDE FOR OIL TANKERS AND TERMINALS

2.6.2 Formation of Pyrophors
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2.6.2.1 General

As described above, the formation of pyrophors is dependent on three
the rate of sulphidation. Experiments have supported the view that thereย์นาวี
factors:
Presence of iron oxide (rust).

Presence of hydrogen sulphide gas.

Lack of oxygen.

However, it also depends on the comparative influence of these factors.
The presence of oxygen will inhibit the conversion of iron oxide to iron
งานห้องสมุด ศูนย์ฝกพาณิช
sulphide. Also, while the concentration of hydrogen sulphide gas has a
direct influence on the formation of pyrophors, the degree of porosity of
the iron oxide and the rate of flow of the gas over its surface will influence

is no safe level of hydrogen sulphide below which a pyrophor cannot be
generated.

2.6.2.2 In Terminal Operations
In terminal operations, pyrophoric iron sulphide is well recognised as a
potential source of ignition. Pyrophoric deposits are apt to accumulate in
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storage tanks in sour crude service and in process equipment handling
sour streams. When such tanks or equipment are taken out of service, it
is normal practice to keep all internal surfaces thoroughly wet during
ventilation so that there can be no pyrophoric reaction before the
equipment is made hydrocarbon gas free.
Deposits and sludge should be kept wet until removed to a safe area
where subsequent ignition will cause no damage. Numerous fires have
occurred when deposits have dried out prematurely.

2.6.2.3 In Marine Operations

While pyrophoric iron sulphide is a widely recognised ignition source in
shore based operations, it has rarely been cited as the cause of a marine
ignition and in those few cases the hydrogen sulphide levels were very
high. It is believed that marine operations have been free of this hazard
because the cargo tanks of non-inerted ships normally contain some
oxygen in the vapour space as a result of tank breathing.

However, the use of inert gas on crude carriers may, by decreasing the
initial oxygen level as well as that of subsequent replenishments, increase
the possibility of forming pyrophoric deposits. Although tanker flue gas
normally contains 1-5% oxygen, this level can be further reduced by
absorption into the crude cargo. Furthermore, as the cargo tanks are kept
pressurised with inert gas with a low oxygen content, no air will enter the
ullage space. If the pressure needs to be increased, it will again be done
with inert gas having a low oxygen content.

2.6.3 Prevention of Pyrophoric Ignition in Inerted Cargo Tanks

As long as the cargo tanks remain inerted, there is no danger of ignition from
a pyrophoric exothermic reaction. Therefore, it is imperative that the
atmosphere in the tank is not allowed to become flammable. Flammable

46 © ICS/OCIMF/IAPH 2006