D. GEORGE, C. JELLY and C. J. WEI, Cygnus, Dorchester, UK
When operating in a potentially explosive atmosphere, several crucial aspects must be considered. These include understanding the potential hazards and choosing the correct tools for the identified hazardous areas.
The authors’ company’s ultrasonic thickness gaugea is certified intrinsically safe to Zone 0—the most hazardous area—negating the need for hot work permits. But what does this really mean? The rules and regulations surrounding the use of equipment in hazardous areas are deciphered and explained here.
What are explosive atmospheres (Ex)? The term Ex is recognized globally for explosive atmospheres and the schemes used to prevent explosions.
Hazardous areas are locations with a risk of fire or explosion due to the presence of ignitable or flammable substances in the air. These substances include gases, vapors and dusts at concentrations high enough to produce an ignitable mixture.
Ex schemes and standards around the world. The International Electrotechnical Commission (IEC) provides an international standard for the classification of hazardous areas and Ex-equipment as well as a standardized labeling system. The IEC standard is a United Nations (UN)-endorsed standard that provides a common point of reference for understanding the various Ex rating standards used worldwide. The IECEx system is the IEC system for certification to standards for equipment used in explosive atmospheres.
FIG. 1 shows which schemes and standards are adhered to in each country. Ex systems are used worldwide, and most systems are based on IECEx.
Regions like the European Atmosphere Explosible (ATEX) Standard and the North American National Electrical Code/Underwriters Laboratory (NEC/UL) Standards use the IEC 60079 Standard as a source (TABLE 1).
The authors’ company’s gaugea has been approved to IECEx, ATEX and UKEX requirements and is the only gauge that meets the highest classification of ATEX Zone 0, equivalent to NEC (UL) Class I, Division 1, Group “ALL.”
Zones and divisions. These are classification of hazardous areas: “Zones” is the term used within the European ATEX scheme, while “Classes,” “Divisions” and “Groups” are terms used in the North American NEC/UL scheme.
In both the IECEx and ATEX systems, hazardous areas are classified into zones (FIG. 2) based on an assessment of the frequency of the occurrence and duration of an explosive atmosphere:
Zone 0: An area in which an explosive gas atmosphere is continuously present or for long periods. Zone 0 is the most hazardous zone.
Zone 1: An area where an explosive gas atmosphere is likely to occur in normal operation.
Zone 2: An area in which an explosive gas atmosphere is not likely to occur in normal operation and, if it occurs, will only exist for a short time.
In the North American NEC/UL Ex scheme, divisions are used instead of zones (FIG. 3).
Division 1 = Zone 0 and Zone 1.
Division 2 = Zone 2.
Plants and facilities are comprised of many different zones or divisions. Once these hazardous areas have been established, signs must be installed to warn and inform anyone entering the area.It is important to be aware that these area labels can change, especially due to a leak. For example, if a welded joint, flange or valve leaks, the zone around it can potentially be elevated from Zone 1 to Zone 0, as shown in FIG. 4.
Ex standards vs. other testing methods. Other methods for testing equipment in hazardous areas are used, but it is important to be aware of how they differ from Ex standards.
The MIL-STD-810G only tests a piece of equipment to see if it can cause an explosion on the day of the test. It does not consider use in the field, being dropped, becoming faulty or short-circuiting batteries. This means it provides no guarantee that the equipment cannot cause an explosion.
Conversely, the Ex standard provides assurance that the equipment, even if faulty, cannot cause an explosion. Ex equipment must be designed to be safe. Redundant safety circuits are incorporated into the design so that if one fails, the next will still provide protection. Equipment is drop tested, IP tested and batteries are short-circuited to check they do not explode or get hot. Materials used must be anti-static to avoid static charge buildup that can cause sparks.
TABLE 2 illustrates the differences between Ex and MIL-STD-810G standards.
Intrinsically safe vs. explosion-proof. Intrinsically safe is one of the protection concepts used to define Ex equipment designed for safe operation in hazardous locations. The authors’ company’s gaugea uses intrinsic safety protection to limit the energy present in its system, meaning it is insufficient to ignite a hazardous atmosphere under any conditions. This includes low power levels and low stored energy. Intrinsically safe equipment can be Ex-certified to Zones 0, 1 or 2.
Another concept often used is explosion-proof, or flame-proof. This type of equipment is sealed and rugged, such that it will not ignite a hazardous atmosphere. However, it is possible for a spark or explosion to be generated within the equipment itself. For this reason, explosion-proof equipment can only be Ex certified to Zones 1 or 2.
This means that only intrinsically safe equipment can be used in Zone 0.
Equipment protection levels (EPLs). All certified Ex equipment (portable or fixed) designed for operation in hazardous areas will have an Ex Certificate of Conformity (C-of-C) that states its EPL, represented by a “G” code.
TABLE 3 shows the correlation among zones, EPLs and different protection concepts. The authors’ company uses the intrinsically safe protection type for its Ex products.
Hot work permit not required. Hot work permits are used to mitigate the risks of hot work tools or processes capable of creating a spark or generating enough heat to ignite a gas. Thanks to its intrinsic safety design, the company’s gaugea cannot generate sufficient energy to cause an explosion and, therefore, can be used safely within hazardous areas at any time without needing a hot work permit.
This prevents obstacles, as obtaining a hot work permit is normally costly and time-consuming. A formal application with a risk assessment and methods to remove or reduce the risks is required.
Typically, for a permit in a hazardous area:
The permit must be authorized by a senior person.
A competent person is required to check gas levels.
Oxygen levels and flammable substances must be frequently monitored.
Personnel should be competent in emergency response and first aid.
Specialist rescue equipment on standby is required.
Pipes and tanks may need to be purged with nitrogen.
Plant operations should be shut down.
Hot work permits can be very inconvenient and must be planned. It is far easier to use Ex equipment.
The authors’ company’s gaugea is Zone-0 UTM gauge certified to ATEX, IECEx and UKEX for use in hazardous Zones 0, 1 and 2. Featuring three measuring modes and the Deep Coat function, this versatile UTM gauge can measure the wall thickness of various materials through thick coatings or extreme corrosion. The A-Scan and B-Scan functions provide additional measurement verification and cross-sectional display, while the data logging capability records measurements with A-Scans for hassle-free reporting and analysis. Furthermore, the Manual Measurement Mode allows manual gates and gain to be configured to suit various applications.
When coupled with the company’s high-temperature Ex probe, the gaugea transforms into a high-temperature thickness gauging kit for efficient corrosion inspection on hot assets in service without needing a plant shutdown or hot work permit.
Despite various advanced functionalities, the gaugea is surprisingly easy to operate, thanks to four measurement screens, four function keys for easy controls and dynamic views, and an intuitive menu. HP
NOTES
Cygnus 1 Ex Ultrasonic Thickness Gauge
Within the Cygnus team, David George works as a Technical Director, Claire Jelly is a Creative Marketing Executive, and Chih Ju Wei works as Marketing Director.