Measurement and control of oxygen and hydrocarbon vapour levels in Inert Gas (IG) systems on board ships


In the previous article we learnt about the various alarms and controls which monitor conditions in the IG system based on different parameters such as pressure and temperature at various points. Another important parameters which needs to be measured are the oxygen levels which should be below 8% and the hydrocarbon vapours which should not be within the lower and upper flammable limits.

Oxygen Analyzer & Hydrocarbon Meter

Oxygen analyzers come in various types depending on usage, sensitivity, manufacturing company and so forth. You can see the shape of a typical oxygen analyzer commonly used in the industry in the picture in the next section. The basic principle of operation of an oxygen analyzer is that it draws a sample of air through an oxygen sensor known as the lambda probe. The probe works on different principles based on type of sensor such as voltage change, resistance change etc in proportion to the amount of oxygen content in the sample and the signal is further sent to the oxygen analyzer which converts this signal into the corresponding value of the oxygen level which is displayed at the output of the analyzer.

The measurement of hydrocarbon vapours is similarly done using instruments which have somewhat similar working principle and have a sensor and an analyzer for measurement and display of hydrocarbon vapours. A portable hydrocarbon vapour meter is shown along with the oxygen analyzer below.

Oxygen Analyzer & Hydrocarbon Meter

Oxygen Analyzer
Hydrocarbon Meter

Use in IG Systems

Obviously the analyzers have to play an important role in the inert gas systems since the whole arrangement exists for maintaining oxygen levels in the inert region in the system when required, while ensuring that hydrocarbon vapours are not within the flammable range. The oxygen samples should be analyzed at the position after the IG blowers but before the regulating valve (see diagram in this article for layout). Also there should be sampling points in cargo tanks for measurement as and when required using portable analyzers for oxygen as well as hydrocarbon vapour levels. Normally a long tube is employed to analyze the vapour sample from random locations which give overall picture of tank atmosphere. This should be intrinsically safe to prevent sparks which could be dangerous. There should also be a sampling point before the deck seal where portable instruments can be used for measurements.

There are certain things which should be taken care of while incorporating the sampling probe such as the presence of a dust filter in the probe, installation of the probe at place where temperature is optimum so as to prevent condensation and minimum number of joints in the pipe to prevent mixing of outside air with the sample which would result in a false reading.

In the next article we will learn a few things about the drainage arrangements and sea water supply to IG systems.


Oxygen Analyzer Image – Tech Diving Ltd.

Hydrocarbon Vapour Meter Image – Thermo Environmental Instruments Inc.