Refinery: an environment where the margin for error is minimal.
A refinery operates with flammable hydrocarbons, toxic fumes, and pressures that, if they fail, release enough energy to compromise entire facilities. The combination of heat, fuel, and ignition potential is not a hypothetical scenario: it is the permanent condition of distillation columns, pressure vessels, heat exchangers, and transfer lines.
The history of accidents in the sector is sufficiently documented to dispense with speculation. From Flixborough in the 1970s to more recent incidents at refineries in Brazil and around the world, most serious occurrences involved failure to detect leaks or out-of-range temperatures early. Therefore, emergency detection systems exist not as a bureaucratic requirement, but as a technical barrier between normal operation and irreversible consequences.
Gas leak: the risk that doesn't warn you.
Light hydrocarbons such as propane, butane, and naphtha evaporate upon contact with the atmosphere and, in concentrations between 2% and 10% of the air volume, form explosive mixtures. The practical problem is that the operator often does not notice this accumulation in time: gases such as methane and propane are odorless in their pure form, and H₂S, which has a characteristic odor, rapidly paralyzes the olfactory nerve at lethal concentrations.
Fixed gas detectors This is addressed through two technological routes. Catalytic detection operates by oxidizing the gas in a heated sensor element, generating a resistance variation proportional to the concentration. Electrochemical detection, in turn, is the standard choice for toxic gases such as H₂S and CO. For environments with silicon or silicates, infrared (IR) technology eliminates the problem of sensor poisoning, meaning it is technically preferable, although more expensive.
The effectiveness of a detection system, however, does not depend solely on the type of sensor. It depends on mapping where to position the detection points. A gas dispersion study, for example, considers molecular weight, ambient temperature, and ventilation pattern of the area to define which regions accumulate concentration first. Without this mapping, installing detectors is an exercise in trial and error with serious consequences.
Temperatures outside the range as a sign of impending failure.
Temperature is the most information-dense process parameter in refineries. A 15°C increase above the normal profile in a hydrotreating reactor, for example, may indicate an uncontrolled exothermic reaction. A sudden drop in a heat exchanger, in turn, signals fouling or tube rupture. In distillation units, the temperature profile along the column is the operational signature of the process—therefore, any persistent deviation requires immediate investigation.
Therefore, emergency detection systems incorporate continuous temperature measurement at critical points, with alarms configured for process deviations and automatic triggering of emergency procedures. The response needs to be quick: in catalytic reactors, the interval between abnormal temperature and thermal runaway can be minutes.
Thermocouples and RTDs The devices installed at these points feed into SCADA and SDC systems that correlate multiple variables simultaneously. Isolated temperature measurement is not sufficient; rather, it is the correlation with pressure, flow rate, and composition that qualifies a signal as a real alert and not as an expected operational variation.
What is a Fire & Gas system?
Fire & Gas (F&G) is the technical term for the safety subsystem dedicated to detecting fire, flame, and gas in industrial plants. It integrates flame detectors (UV, IR, or combined UV/IR), fixed gas detectors, temperature sensors, and smoke detectors, all connected to a logic control panel certified as a Safety Instrumented System (SIS), according to IEC 61511.
The Safety Integrity Level (SIL) of the system determines the probability of dangerous failure on demand. Consequently, it directly influences sensor redundancy, test frequency, and controller architecture. The higher the SIL, the more stringent the technical requirements for each component, including the temperature sensors that make up the system.
In a refinery, the F&G system does not operate in isolation. It communicates with the Emergency Shutdown System (ESD) and also with the fire suppression system, whether deluge, foam, or CO₂, depending on the area. Flame detection in a processing unit automatically triggers the closing of shut-off valves, fuel cutoff, and deluge activation, that is, without human intervention.
Classified areas and the requirement for ATEX/IECEx certification.
The concept of a classified area defines zones where the presence of an explosive atmosphere due to gas, vapor, or dust is probable, possible, or eventual. The classification follows standards such as ABNT NBR IEC 60079-10-1 for gases and IEC 60079-10-2 for combustible dusts.
In refineries, most operational areas are classified as Zone 1 (presence of an explosive atmosphere under normal conditions) or Zone 2 (possible under abnormal conditions). Therefore, all electrical equipment installed in these zones must be certified by a recognized body, with tests proving that it does not constitute a source of ignition under the specified conditions.
ATEX certification, aimed at the European market, and IECEx certification, with international acceptance including in Brazil, meet this requirement. A sensor without ATEX/IECEx certification installed in Zone 1 constitutes an uncontrolled potential ignition source and represents an irregularity under NR-10, NR-13 and NR-20.
However, certification is not a generic stamp. Each certificate specifies the gas group (IIA, IIB, IIC), the maximum surface temperature class (T1 to T6), and the protection mode (Ex d, Ex e, Ex ia, among others). A sensor certified for group IIA, suitable for propane, does not offer equivalent protection in environments with hydrogen (group IIC). Therefore, specifying a sensor only as "ATEX" without verifying these parameters is insufficient.
The complete protection chain: how the sensors fit together.
Thermocouples and RTDs certified for classified areas are components of the F&G system just as much as flame and gas detectors. For measurement in classified regions, ATEX/IECEx certification is not a product differentiator: it is a requirement determined by the area classification.
The design of a safe installation considers the complete protection chain: sensor element, sensor head, temperature transmitter, zener barriers or galvanic isolators in the field panel, and the associated wiring. A failure in any link invalidates the integrity of the Ex system as a whole; therefore, each component needs to be specified individually.
The choice of thermocouple takes into account the operating range (type K covers up to 1.260°C; type N, up to 1.300°C), the chemical resistance of the sheath to the process environment, and the response time required by the system's SIL (Surface Inspection License). These parameters are defined in the SIS (System Inspection and Control) design requirements and are audited during commissioning and periodic revalidations.
Alutal manufactures ATEX and IECEx certified thermocouples For installation in Zones 1 and 2, with transducers and transmitters suitable for each process configuration. The specification includes gas group, temperature class, and protection mode, not just the generic certificate.
Best practices for deployment and maintenance
Emergency detection systems degrade over time if there is no structured maintenance program. Catalytic detectors lose sensitivity due to poisoning with silicone, chlorine, or sulfur compounds. Thermocouples in process environments accumulate calibration drift due to thermal cycling and chemical attack on the sheath. Flame detectors, in turn, have their field of view reduced by dirt on the lenses.
IEC 61511 requires periodic proof tests to confirm that the system's SIL (Safety Indicator) is maintained throughout its lifecycle. For F&G (Fire and Gas) systems, this includes: functional testing of detectors with calibrated reference gas, verification of the field of view of flame detectors, complete circuit testing up to ESD (End-of-Sight Test), and documented recording of each result.
One frequently overlooked point is the revision of the area classification study after process modifications. Any change in layout, processed products, or ventilation can alter the classified zones and, consequently, the correct specification of the installed equipment. Refineries that have been operating for decades also frequently have area studies that are outdated in relation to the actual plant.
For refineries seeking to adapt or expand their emergency detection systems, Alutal offers technical support in specifying certified temperature sensors for classified areas.
Get in touch with our experts Describe your application — we'll identify the correct specification for each zone and each process condition.
