The numerous flame sources in CPI facilities necessitate the installation of advanced flame-detection technologies
Fire is a primary and very real threat to people, equipment and facilities in the chemical process industries (CPI), especially in the refining and storage of petrochemicals. The consequences of failing to detect flames, combustible gas leaks or flammable chemical spills can have dire consequences, including loss of life and catastrophic plant damage.
The monitoring of flame hazards is mandated by the U.S. Occupational Safety and Health Administration (OSHA; Washington, D.C.; www.osha.gov) through its comprehensive Process Safety Management (PSM) federal regulation. Internationally, the European Union (E.U.) splits gas and flame safety responsibilities between E.U. directives and European standards organizations, including the European Committee for Electrotechnical Standardization (Cenelec; Brussels, Belgium; www.cenelec.eu), the International Electrotechnical Commission (IEC; Geneva, Switzerland; www.iec.ch) and several other bodies.
Many accidents are the result of either failing to implement these standards properly with suitable flame-detection equipment or the failure to train employees to follow related safety procedures consistently. In either case, it is important to understand the many different sources of flame hazards, the detection sensor technologies that can warn of imminent danger and the proper location of flame detectors in today’s complex chemical plants.
In the petrochemical plant environment, the range of potential flammable hazards is expansive and growing as materials and processes become more complex. These hazards have led to the development of more sophisticated combustible-gas and flame-sensing technologies with embedded intelligence that can better detect the most common industrial fire sources, some of which are listed in Table 1.
Principles of flame detection
Industrial process flame detectors detect flames by optical methods, including ultraviolet (UV) and infrared (IR) spectroscopy and visual flame imaging. The source of flames in CPI plants is typically fueled by hydrocarbons, which when supplied with oxygen and an ignition source, produce heat, carbon dioxide and other products of combustion. Intense flames emit visible, UV, and IR radiation (Figure 1). Flame detectors are designed to detect the emission of light at specific wavelengths, allowing them to discriminate between flames and false alarm sources.
The flame safety industry has developed four primary optical flame-sensing technologies: UV, UV/IR, multi-spectrum infrared (MSIR), and visual flame imaging (Figure 2). These sensing technologies are all based on line-of-sight detection of radiation emitted by flames in the UV, visible and IR spectral bands. Process, safety and plant engineers must choose from among these technologies to find the device that is best suited to their individual plant’s requirements for flame monitoring by deciding upon the importance of the detection range, field of view, response time and immunity against certain false alarm sources.
Ultraviolet/infrared (UV/IR). By integrating a UV optical sensor with an IR sensor, a dual-band flame detector is created that is sensitive to the UV and IR radiation emitted by a flame. The resulting UV/IR flame detector offers increased immunity over a UV-only detector, operates at moderate speeds of response, and is suited for both indoor and outdoor use.
Multispectral infrared (MSIR).Advanced MSIR flame detectors combine multiple IR detector arrays with neural network intelligence (NNT). They provide pattern-recognition capabilities that are based on training to differentiate between real threats and normal events, thus reducing false alarms. MSIR technology allows area coverage up to six times greater than that of more conventional UV/IR flame detectors.
NNT is based on the concept of artificial neural networks (ANNs), which are mathematical models based on the study of biological neural networks. A group of artificial neurons in an ANN process information and actually change structure during a learning phase. This learning phase allows ANNs to model complex relationships in the data delivered by sensors in a quick search for patterns that results in pattern recognition (Figure 3).
Flame detectors with NNT operate similarly to the human brain; they have thousands of pieces of data stored in their memories from hundreds of flame and non-flame events observed in the past. These detectors have been trained through NNT intelligence to recognize flames based upon those data, and determine if they are real events or potential false alarm sources.
Visual flame-imaging flame detectors.The design of visual flame detectors relies on standard charge-couple-device (CCD) image sensors, commonly used in closed-circuit television cameras, and flame-detection algorithms to establish the presence of fires. The imaging algorithms process the live video image from the CCD array and analyze the shape and progression of would-be fires to discriminate between flame and non-flame sources.
Visual flame detectors with CCD arrays do not depend on emissions from carbon dioxide, water and other products of combustion to detect fires, nor are they influenced by fire’s radiant intensity. As a result, they are commonly found in installations where flame detectors are required to discriminate between process fires and fires resulting from an accidental release of combustible material.
Visual flame detectors, despite their many advantages, cannot detect flames that are invisible to the naked eye, such as hydrogen flames. Heavy smoke also impairs the detector’s capacity to detect fire, since visible radiation from the fire is one of the technology’s fundamental parameters.
Flame detection requirements
When configuring a flame-detection system and evaluating the available technology alternatives, there are many performance criteria that must be considered. The following sections outline some of these important detector criteria.
False alarm immunity.False alarm rejection is one of the most important considerations for the selection of flame detectors. False alarms are more than a nuisance — they are both productivity and cost issues. It is therefore essential that flame detectors discriminate between actual flames and benign radiation sources, such as sunlight, lighting fixtures, arc welding, hot objects and other non-flame sources.
Detection range and response time.A flame detector’s most basic performance criteria are detection range and response time. Depending on a specific plant-application environment, each of the alternative flame-detection technologies recognizes a flame within a certain distance and a distribution of response times. Typically, the greater the distance and the shorter the time that a given flame-sensing technology requires to detect a flame, the more effective it is at supplying early warning against fires and detonations.
Field of view (FOV).Detection range and FOV define area coverage per device. Like a wide-angle lens, a flame detector with a large field of view can take in a broader scene, which may help reduce the number of flame detectors required for certain installations. Most of today’s flame detector models offer fields of view of about 90 to 120 deg (Figure 4).
Self diagnostics. To meet the highest reliability standards, continuous optical-path monitoring (COPM) diagnostics are often built into optical flame detectors. The self-check procedure is designed to ensure that the optical path is clear, the detectors are functioning, and additionally, that the electronic circuitry is operational.
Self-check routines are programmed into the flame detector’s control circuitry to activate about once every minute. If the same fault occurs twice in a row, then a fault is indicated via a 0–20-mA output or a digital communications protocol, such as HART or Modbus.
SIL/SIS standards.When plant safety engineers choose detectors certified to safety integrity levels (SIL) and properly integrate them into safety-instrumented systems (SIS), they have again added another layer of safety. Certification to these standards plays a valuable role in effective industrial gas and flame detection.
Normative standards establish minimum requirements for the design, fabrication and performance of flame detectors and other safety devices as necessary to maintain protection of personnel and property. The ANSI/ISA S84.00.01 standard was enacted to drive the classification of SIS for the process industries within the U.S., as well as the norms introduced by the IEC (IEC 61508 and IEC 61511).
Together, these standards have introduced several specifications that address safety and reliability based on optimizing processes for risk. The IEC 61508 standard is a risk-based approach for determining the SIL of safety-instrumented functions. Unlike other international standards, IEC 61508 takes a holistic approach when quantifying the safety performance of electrical control systems — the design concept, the management of the design process and the operations and maintenance of the system throughout its lifecycle are within the scope.
Location and installation
A variety of processes and sources within the plant environment can lead to flame and fire incidents, including leaking tanks, pipes, valves, pumps and so on. Accurate detection while avoiding false alarms is also important because false alarms result in unnecessary process or plant shutdowns, slowing production and requiring time-consuming reviews, paperwork and reporting.
False alarms can, over time, provide a false sense of security, because employees can become complacent if alarms go off frequently for no apparent reason and are continually ignored. The problem is that personnel alone cannot really determine the difference between a false alarm and a serious accident that is about to happen.
Fixed flame- and gas-detector systems are designed and installed to protect large and complex areas filled with process equipment from the risks of flames, explosions and toxic gases. For these systems to be effective, their location and installation are important so that they offer a high likelihood of detecting flame and gas hazards within monitored process areas.
Three-dimensional mapping.Determining the optimal quantity and location of flame and gas detectors is therefore critical to ensure the detection system’s effectiveness. Flame and gas three-dimensional mapping is a solution that assists in the evaluation of flame and gas risks within a process facility and also reduces these risks toward an acceptable risk profile. Flame and gas mapping includes the placement of detectors in appropriate locations to achieve the best possible detection coverage (Figure 5).
The use of three-dimensional flame and gas mapping helps plant, process and safety engineers in a number of ways. First, mapping helps to increase plant safety by improving the likelihood of detecting flame and gas hazards. Also, it allows facilities to quantify their risk of a fire or a gas leak, and then assess the overall effectiveness of their flame- and gas-detection coverage. For new installations, mapping can help improve the design of new fire and gas systems to mitigate risks from accidental gas releases or fires. For existing installations, mapping provides a method for assessing the risk-reduction performance of existing fire- and gas-detector systems and recommends ways to improve coverage.
Mapping assists facilities in understanding their risk of a fire or a gas leak, and then allows them to optimize their flame- and gas-detection protection layout by recommending the appropriate detector technologies, detector locations and quantities. Mapping also equips the engineer with the means to measure detection improvements when small incremental design changes are made. Mapping can therefore help to minimize overall system costs.
With mapping, determining detector layouts becomes much simpler, because mapping provides a methodical and systematic approach for determining the areas with the highest likelihood of flame and gas risks. Understanding the locations and likelihood of risks will help remove guesswork and uncertainties from engineering.
Once the optimal locations are determined for the placement of the flame detectors, then installation depends on the type of flame detector chosen. Most optical-type flame detectors are placed high and are pointed downward either inside or outside buildings or structures to monitor tanks and pipelines running throughout the plant.
In order to protect chemical processes and plants from flame hazards, it is important to understand the basic detection sensor technologies and their limitations. Defining the type of potential hazard fuels, the minimum fire size to be detected and the configuration of the space to be monitored through three-dimensional hazard mapping can influence the choice of instrument.
When reviewing a plant’s flame-safety protection level, be sure to ask for assistance from any of the flame detection equipment manufacturers. They have seen hundreds, if not thousands, of plants and their unique layouts, which makes them experts in helping to identify potential hazards and the best way to prevent accidents.
Remember, too, that no single flame-detection sensing technology is right for every potential plant layout and hazard. For this reason, adding multiple layers of flame- and gas-detection technology provides a multi-sensory approach that increases detection reliability and also can prevent false alarms. ■
Edited by M. Bailey and D. Lozowski
Ardem Antabian is currently the OGP (Oil & Gas Products) segment manager at MSA — The Safety Company (26776 Simpatica Circle, Lake Forest, CA 92630; Email: [email protected]; Phone: 949-268-9523. Website: www.msasafety.com). Antabian joined the company in 1999, and has held various positions, including global assignments in Dubai, U.A.E. and Berlin, Germany. He also helped develop the company’s advanced-point and open-path infrared gas detectors, as well as its multi-spectral infrared flame detector. Antabian holds dual B.S. degrees in chemical engineering and chemistry from California State University, Long Beach.