Industrial processes such as oil and gas production facilities, refining, chemical production and power generation often involve toxic and combustible gases, which can create serious hazards if they escape into the air. Toxic hydrogen sulfide (H2S) and combustible methane (CH4) are among the most widely produced and most dangerous industrial gases.
To minimize risk to personnel and plant safety, plants often deploy early warning devices such as gas detection systems, which require expensive infrastructure and seldom cover all of the potential leak points. This concern is driving a trend toward augmenting existing wired gas detection coverage with wireless sensors, which can significantly increase monitoring coverage with minimal new financial outlay.
In this blog series on wireless gas monitoring, we explore this emerging solution for improving plant safety and kick off by taking a look at some gas sensor basics. In the subsequent posts, we will describe gas detection system architecture, applications of wireless gas monitoring and explore some operating issues that wireless gas monitoring may raise.
Gas sensor basics
A typical gas detector identifies and monitors gas leaks, outputting an electronic signal such as a 4-20 mA, HART, or Modbus wired signal, to a dedicated fire & gas (F&G) system, controller, PLC or control room which sets alarms or guides corrective action. For toxic gases, those signals represent the presence of gas in parts per million (PPMs). For combustible gases they represent the percent of the the lower explosive limits (LELs).
A number of technologies are used to detect the presence of gases. Infrared (IR) technology, which is among the most commonly deployed, monitors gas concentration based on the principle of infrared absorption. Gases like methane absorb specific infrared wavelengths. The electronics module computes the gas concentration based on the amount of absorption.
Electrochemical sensing, another commonly applied technology, measures the concentration of a target gas by reacting with the gas and producing an electrical signal proportional to the gas concentration. A typical electrochemical sensor consists of a sensing electrode and a reference electrode separated by a layer of electrolyte.
Sensing technologies vary in reliability, depending on the properties they are measuring and on the types of gases involved. Electrochemical detectors, for example, tend to be more reliable for specific gases, like hydrogen sulfide, but may lose effectiveness after continuous exposure. Infrared technologies on the other hand tend to be more reliable for detecting LELs and may perform well for up to five years or more. In addition to the sensors deployed in gas detectors, a detection system typically includes a controller or a plantwide fire & gas control system with HMIs, alarm systems, and relays connecting to valves, pumps, or whatever final element might be needed to suppress a gas leak and/or HVAC and fire suppression systems to suppress a fire.
In our next post, we will zoom a level out and examine how a gas sensor is fitted to a typical detector and how that detector plays a part in a gas detection system architecture.
by: Toyin Ajileye, Channel Marketing Specialist at United Electric Controls