Gas Detection Systems

On oil tankers, two portable gas detectors are to be provided as a minimum requirement. They should measure the flammable content in % LFL and oxygen levels. Many types are multi-function detectors, capable of measuring flammable gas, oxygen levels, hydrogen sulphide as well as carbon dioxide built into one unit. If the vessel is equipped with an inert-gas system it must be made sure the portable device is capable of detecting flammable gases in a low-oxygen environment. Other vessel types will be equipped with portable gas monitors of a similar type for use in enclosed space entries. They may well also be used for cargo work on bulk carriers or work on the vehicle deck on ferries where harmful gases may well be present during the operation. Suitable means for calibration and testing of the devices must be kept on board. The calibration should be conducted in accordance with the manufacturer's instructions and, on tankers, conducted weekly.

Electro-catalytical sensors for combustible gases work on the principle of catalytic oxidation and are commonly referred to as “pellistors”. They are designed to detect the presence of most combustible gases below 100% Lower Explosive Limit (LEL). The normal lifespan of such sensors is about 5 years but will be considerably less if they are exposed to silicones in the air, salt, dust, or similar particulates on bulk carriers. This makes them perfect for tanker use but can be limited in their utility on bulk carriers when used for the detection of flammable gasses evolving from solid cargoes. They do not give a warning when their operational life is over. Only testing can give notice of their proper function so they must have a regular bump test using the manufacturer's instructions and bottled gas samples.

These types of detectors are simple and cost-effective, with chemically doped resistors reacting to hydrocarbon gases. They are much less sensitive to dust and particulate matter which makes them much more appropriate for detecting hydrocarbon gases on bulk carriers. Advantages: - Stable zero. - Long operational life. Disadvantages: - Widespread measuring spectrum. It will usually detect a large range of flammable gases and is not very precise. - Non-linear must be calibrated and tested within the alarm limits set.

Infrared (IR) gas detectors use a beam of infrared light within the detector to determine gas concentrations. Through the lens, an infrared beam is sent out, and depending on the carbon content of the gas, the signals will be partly absorbed and not returned via the mirror. The difference in outgoing and ingoing IR signals is used to interpret the gas content. The sensor is therefore not sensitive to other gases, dust, or particulate matter. Maintenance mainly consists of cleaning the lens and mirror inside the gas chamber. There are no parts exchangeable on an IR detector or a need to calibrate it regularly but, calibration is a very specialist job that will usually require external contractors to perform. Advantages: - Very precise advanced measuring. - Fast response. - Long life. - Low zero drift. - Poisoning resistant. - Linear response over the full range. - Will measure hydrocarbons also in a fully inert (0% oxygen) environment. Disadvantages: - Expensive. - Requires specialized tools for calibration and maintenance.

Open path gas detection is used to measure gas in open areas such as vehicle decks or engine rooms. IR technology is used, making the open path type an extended infrared detector. These types of detectors are suitable for open deck areas and in engine rooms to detect pipe leakages. This is due to them being easy to site and highly accurate in detecting flammable gases. They are only for the detection of flammable gases as they are not able to detect toxic gases or oxygen levels with any accuracy. Hose lengths up to 200 meters are possible, but the typical ship and offshore installations have hose lengths of 5-10 meters. The length of the hose may have a delayed effect on the detection but will not affect the performance otherwise of the system. Only aspects such as the diameter of the hose may affect the thresholds of detection due to a possible restriction of the volume of gas being detected.

These sensors are highly flexible and can be used for the detection of oxygen, hydrogen sulphide, ammonia, and many other specific gases. They use a chemical compound that reacts with the sample gas and the reaction is linear to the concentration of the sample gas. On electrochemical detectors, the sensor is replaceable and can be replaced to detect different gases as required. The entire detector is only replaced if problems are present in the electronics. To avoid failing due to the short lifespan of the detector chemicals, annual replacement is recommended for these types of sensors. Advantages: - High sensibility. - Good accuracy, linear response. Disadvantages: - Relatively short sensor life, typically 1 year on oxygen sensors for example. - Pressure-sensitive.