FTA vs FID: The Basics

Why have so many fires and explosions occurred in industrial processes that were believed to have adequate protection against such hazards? In many cases, the wrong analyzer was used, or the sampling method was incorrect. Selecting the correct analyzer - and obtaining an accurate sample - is essential to maintaining a safe process. 

Ideally, the analyzer will have an accurate response to a broad range of flammables, will be able to operate in high temperature process environments, and will have a fail-safe design:

1. The Need for Continuous LFL Monitoring - Fire and safety laws require that process ovens and dryers be designed not to exceed 25% of the lower flammable limit (% LFL). When continuous monitoring is employed, however, the process may be designed to operate at flammable concentrations up to 50% LFL (and provide system shutdown above 50% LFL). The use of continuous analyzers not only greatly improves safety, but also reduces operating costs.
2. Measuring Flammability - Any analyzer used to prevent explosions in a process application must accurately measure the total flammability of the process atmosphere, which consists of: Volatile Organic Compounds (VOCs) including alcohols, halogens, aromatic and aliphatic hydrocarbons; fuels used to heat the process (such as natural gas, propane and hydrogen); and carbon monoxide, a by-product of inefficient combustion. 

Over the next few weeks, let's specifically compare two analyzers, Flammability (FTA) and Flame Ionization (FID):

• A flammability analyzer measures the total flammability of the sample in the 0 to 100% LFL range. A carefully metered pilot flame incinerates the sample; the resulting change in flame characteristics is proportional to the total concentration of flammable vapors present. 
• A flame ionization detector measures ionized carbon; it does not measure total flammability. A carefully metered pilot flame incinerates hydrocarbons in the sample – the resulting ionized carbon passes through an electrical field, creating a current flow proportional to the amount of ionized carbon present. An electrometer measures this current flow, and the resulting electrometer output is amplified and displayed, typically as a parts-per-million (PPM) meter reading. The FID signal is sometimes translated into a 0 to 100% LFL meter reading, but this translation is complicated and prone to error because parts-per-million (PPM) is only a fraction of the LFL concentration.