Several Recognized and Generally Accepted Good Engineering Practices (RAGAGEPs) exist to help someone make their selection and placement of gas detectors (e.g. ISA-TR84.00.07, NFPA 72, UL-2075). However, there are no real consistent approaches widely used by companies. Historically, gas detection has been selected based on rules of thumb and largely dependent on experience. Over the last several years there has been a growing interest in determining not only the confidence but also the effectiveness of those gas detection systems. In fact, incorrect detector placement far outweighs the probability of failure on demand (of the individual system components) in limiting the effectiveness of the gas detection system.
An effective gas detection system has three elements:
1. A comprehensive Gas Detection Philosophy
2. Appropriate Detector Technology Selection
3. Correct Detector Placement
The Gas Detection Philosophy clearly specifies the chemicals of concern and the intended purposes, i.e. detection of toxic or combustible levels, voting requirements, alarm rationalization, and control actions.
Appropriate Detector Technology Selection includes consideration of the target gas and the required detection concentration levels.
The primary approaches for Detector Placement are geographic and scenario-based coverage. Geographic coverage places detectors on a uniform grid, and sometimes areas risk ranked to reduce the number of detectors required. Scenario-based coverage has a range of leak models and places gas detectors based on the dispersion modeling results.
All three elements for effective gas detection (philosophy, technology, and placement) are interdependent but understanding their relationships is of paramount importance to design an effective gas detection system.
The intention of this paper is to present the main considerations that design engineers and process safety professionals should address for each gas detection system element in order to obtain the best return on your investment when placing your gas detectors.
Keywords: Instrumentation, Reduction of Risk, Risk Assessment, Protection, Detection System, Alarms and Operator Interventions, Detector, Gas Detection/Dispersion Prediction