Off Shore

Gas Detection is the First Line of Defense

When dealing with flammable or explosive liquids or gasses, gas detection systems are the early warning systems that can buy you time before a potential disaster.  Refineries and gas processing plants are a complex system of heaters, coolers, separaters and pumps converting liquids or gasses from one state to another.  Under design conditions there is no gas or liquid leakage; however the state of the process is in constant change.  It is not unknown for processes to drift from their parameters, environmental conditions to change extremely or worse still for a component to fail.  These are the circumstances that lead to leaks in the system.

Safety is employed in many ways to prevent a disaster, such as process design, process monitoring, procedures, entrinsic safety, and early detection.

Gas detection systems are often misunderstood as to what they are indicating as a safety system as opposed to a process measurement.  Gas detection systems operate to give alert or alarm in a safe concentration envelope where the concentration of the gas in air is too weak or lean or explode in the presence of an ignition source, (although they may still deflagrate).  The threshold at which a gas will explode is different for each type of gas, but is termed the "Lower Explosive Limit" and is expressed as a percentage concentration (%LEL) in air.

In safety systems, this is the safety envelope in which alerts and alarms are given at thresholds to give enough time to respond to the problem.  The alarm thresholds are not prescribed and may vary between companies, sites or countries, but the alarm threshold is almost universally 50% LEL, with the first alert threshold being around 15-20%LEL.

Intrinsic safety (IS) is a protection technique for the safe operation of electronic equipment in explosive atmospheres.  The intrinsic safety is designed into the system by ensuring that the available electrical and thermal energy in the system is too low for ingnition of the hazardous atmosphere to occur.  The voltages and currents that enter the hazardous area are limited by protection devices such as Zener or galvanic isolators. Thus ensuring that in the event of a gas leak there are no sources of ignition within the hazardous area.

These two safety aspects need to be applied in balance, as now situations are arising where gas leaks go undetected on plants with good intrinsic safety and little or no gas detection, allowing large gas clouds to grow and pass beyond the perimeter of the plant which can be exposed to uncontrolled sources of ignition.  Perimeter gas detection systems are an essential component to plant protection.