Page 66 - International safety guide for oil tankers and terminals

INTERNATIONAL SAFETY GUIDE FOR OIL TANKERS AND TERMINALS

The gases from crude oils and many petroleum products are composed
essentially of paraffinic hydrocarbons and there are a number of suitable
2.4.13.2 Sampling Proceduresษาเท่านั้น---
materials available for flexible sample tubing. The problem of material
selection is more difficult for those gases containing substantial
proportions of aromatic hydrocarbons, in particular xylene. It is
In order to prevent liquid from being drawn up the gas sampling line andย์นาวี
recommended that in such cases suppliers of sample tubing should be
asked to provide test data showing the suitability of their product for the
purposes for which it will be employed.

Sample tubing should be resistant to hot wash water.

Sample tubing which is cracked or blocked, or which has become
contaminated with cargo residues, greatly affects instrument readings.
งานห้องสมุด ศูนย์ฝกพาณิช
Users should check the condition of the tubing regularly and replace any
found to be defective.

causing contamination of the line, manufacturers provide a float
termination or a probe termination to prevent the ingress of liquid.
Operators should consider using these fittings, but should be aware of
---ใช้เพื่อการศึก
any limitations on their use to avoid static hazards.

ึ
Every tank has ‘dead spots’ where the rate of change of gas
concentration during ventilation or purging is less than the average in the
bulk of the tank. The location of these dead spots depends on the
positions of the inlet and outlet through which ventilating air or inert gas is
admitted and expelled and also on the disposition of the structural
members in the tank. Generally, but not invariably, the dead spots are to
be found within the tank bottom structure. The sample line should be long
enough to permit sampling in the bottom structure.

Differences in gas concentration between the bulk volume of the tank and
the dead spots vary depending on the operating procedures in use. For
example, the powerful water jets produced by fixed washing machines are
excellent mixing devices which tend to eliminate major differences in gas
concentration between one location in the tank and another. Similarly, the
introduction of ventilating air or inert gas as powerful jets directed
downwards from the deckhead produces good mixing and minimises
variations in concentration.

Because of the hazards associated with these dead spots, it is important
to refer to Chapter 10 before entering any cargo tank or other enclosed
space.

2.4.13.3 Filters in Sample Lines

Cotton filters are used to remove water vapour in some hydrocarbon gas
meters, of either the catalytic or non-catalytic filament types, and
additional filters are not normally needed. In extremely wet conditions,
e.g. during tank washing, excessive water can be removed from the gas
sample using materials that retain water but do not affect the
hydrocarbons. Suitable materials are granular anhydrous calcium chloride
or sulphate. If required, soda asbestos selectively retains hydrogen
sulphide without affecting the hydrocarbons. However, it also retains
carbon dioxide and sulphur dioxide and should not be used in tanks
inerted with scrubbed flue gas.

32 © ICS/OCIMF/IAPH 2006