Jimo A. Arasi
Phillips Petroleum Co.
Tananger, Norway
The geometric method is generally preferred over the probabilistic method in classifying the hazardous areas for electrical equipment on offshore production facilities.
Determining the hazardous area classification is part of safety engineering, and it is governed by the company, national, and other industrially accepted practices. Because in many cases the standards and practices overlap, area classification is an inexact science that relies on risk evaluation and human judgment.
AREA CLASSIFICATION
Area classification involves safety in the application and installation of the electrical apparatus and systems in hazardous atmospheres. The objectives are to:
- Identify and classify the sources of release of flammable vapors and gases
- Reduce the magnitude of hazards
- Specify the electrical apparatus applicable to a specific hazard zone.
The type of zone has an impact on the capital, operating, and maintenance costs of the electrical apparatus and in some cases the process shutdown costs.
Area classification often appears simple and straightforward. Onshore classification fits neatly into the classical procedures established by relevant national standards. The conditions offshore are much different.
As a result of the constraint imposed by space, the layout of the various modules and equipment is of prime importance. The layout can be a major factor in classifying a platform after a significant change to the installations.
Offshore area classification can be a challenging assignment. Different national standards compound the challenge.
For example, one standard may supercede or replace another. A British standard, BS 5345, supercedes another British standard, CP1003. These standards are widely used for area classification studies.
In addition, a radical change in design philosophy may involve a change of standards. On some offshore installations the European standards have been used after extensive application of North American standards.
Methods for classifying areas vary among companies. The basic principles can easily be traced to two major groups of standards:
- The North American standards produced by the API and NFPA (see abbreviations).
- The International Electrotechnical Commission (IEC) standards.
API 500B is a widely used standard in the United States. IEC 79 is a very popular standard used in Europe.
NORTH AMERICAN METHOD
In North America, principally the U.S., the area classification is usually aided by NFPA and API standards.
This approach identifies three classes of hazards. These are hazards due to the presence of flammable gas or vapor, combustible dusts, and ignitable fibers or flyings.
In the petrochemical industry, the main concern is the locations in which flammable gases or vapors are, or may be, present in the air in quantities sufficient to produce explosive or ignitable mixture.
These locations are further subdivided into two divisions. (See box for definition of terms used to classify hazardous areas.)
A popular criterion for Division 1 is that the probability of flammable atmospheres occurring is estimated to be more than 1 hr/10,000 hr (more than 1 hr/year),
A corresponding criterion for Division 2 is that the probability of flammable atmospheres occurring is estimated to be less than 1 hr/10,000 hr and more than 1 hr/l million hr.
The probability criteria are debatable. Some area classification investigators have suggested associating the probabilities to the existence of the lower explosion level (LEL) of the gaseous atmospheres.
Allocating the probabilities to the existence of the releases has also been advanced. These probability concepts will be very difficult or expensive to verify.
The definitions in the box and criteria stated above serve as guidelines for classifying the specific areas. To further simplify classification,
the process gases are grouped into four classes (A, B, C, and D) in accordance with their explosive properties. Most hydrocarbons belong to Group D.
For each group, the temperature classes are also created. These are depicted in Fig. 1. The electrical apparatus in the temperature class, T3, can be used in the areas in which the gases and vapors have ignition temperatures higher than the maximum surface temperature of 200 C.
These standards contain diagrams which facilitate the geometrical method of area classification. The extents of the Divisions 1 and 2 areas from a source of release are defined.
Implicit in this method are frequency, duration and quantity of release, ventilation, construction, plant layout, and industrial experience.
These generic geometrical representations are usually conservatively extended by the users.
The standards also recommend the types of electrical equipment for each division. The list of the electrical equipment includes motors, cables, conduits, circuit breakers, light fixtures, and heaters. The wiring methods are also described.
The area classification exercise becomes an apparently simple one. The specialist considers the characteristics of:
- Hazardous material
- Probability of simultaneous occurrence of sufficient energy and air at the specified location
- Site topography
- Construction of the plant or building (i.e., outdoor, exposed, open sided)
- Past history of fire and explosion for the particular plant
- Affiliated industry.
From this evaluation, a geometrical representation is produced of the extents of the divisions.
The specialist specifies, in addition, the electrical equipment that must be approved for the classified location. As a matter of common practice, the maximum surface temperature of any exposed electrical equipment should not exceed 80% of the ignition temperature of the specific gas or vapor in 'C. The temperature classes are depicted in Fig. 1.
Various forms of protection exist for the electrical apparatus. These forms include explosion-proof enclosures, intrinsically safe circuits, and pressurized enclosures.
It is very simple to use the geometrical approach. But despite its shortcomings, it is still widely used.
EUROPEAN METHOD
The European approach is very similar to the North American method. The classification of hazardous areas is adequately described in IEC 79, BS 5345, and other publications.
Generally, there are three categories of explosive atmospheres. In Category 1, the risk is due to the presence of flammable gas or suspended droplets. In Category 2, flammable dust constitutes the risk, while in Category 3, the substance itself has to be explosive.
Category 1 is the primary concern in the petrochemical industry. In this category there are three zones of hazardous atmospheres: Zones 0, 1, and 2. Any area which does not belong in these zones is nonhazardous.
The classification depends on several characteristics of the released gas or vapor and its environment. These characteristics include lower and upper explosive limits, relative density, equipment layout, ventilation, and the grade of the source of release.
The grades are closely related to the zones.
A concept which is similar to the probability criterion also exists. This is expressed in terms of frequency and duration of the releases of gases and vapors. The concept is as follows:
- Low frequency/short duration-less than 2 hr/time or 10 hr/year in total.
- High frequency/long duration-more than 10% of total process of time or more than 1,000 hr/year in total.
This concept varies among companies.
Accordingly, in Zone 0, the explosive gas atmosphere is expected to exist for more than 1,000 hr/year (6 weeks/year). This is an acceptable meaning of continuity in many studies.
This probability statement has the same shortcoming as described in the North American approach.
Associated with the zones is the concept of the grades of the sources of release. The source of release is the location from which the gas, vapor, or liquid droplets emanate into the atmosphere to form an explosive gas atmosphere.
The sources are graded in accordance with the frequency and duration of their releases. Grading is not a function of the rate, chemical, or thermophysical properties of the released material.
The three types of vapor/gas release are:
- Continuous sources, release continuously or for long periods.
- Primary sources are likely to release in normal operation.
- Secondary sources are not likely to release during normal operations, but if they do, the durations are limited.
There is a correlation between the sources of release and the zones. The simplified relationship is schematically shown in Fig. 2. Examples of Zones 0, 1, and 2 locations are documented in the standards and other industrial publications.
The explosive atmosphere is split into two groups: coal mine and noncoal mine atmospheres. Group 1 is for gases produced in coal mines. This is mainly methane. Group 2 gases are hydrocarbon, hydrogen, acetylene, methane, ethylene, and others produced elsewhere besides coal mines.
Group 2 is further divided into groups 2A, 2B, and 2C. These correspond to the aforementioned Groups A, B, C, and D.
As might be expected, there are temperature classes also for the electrical equipment used in the Group 2 atmospheres. These classes are identical to those described earlier.
The procedure for area classification involves identifying and grading the source of release. Several factors, including ventilation and density, are considered to determine the zones and their extents.
Unlike the American system, the popular standards BS 5345 and IEC 79 do not provide the desirable diagrams to facilitate a geometrical approach to area classification.
Several company standards, industrial publications, and guidelines provide very useful information and diagrams with respect to the geometrical representations of the area classification. The geometric representation, because of its dependence on different sources, is not unique.
As in North America, the diagrams are conservatively applied.
Electrical equipment for use in Zones 0 and 1 must have a test certificate from a recognized test institution. The test must be carried out in accordance with IEC or Cenelec standards.
Electrical equipment for use in Zone 2 requires a declaration, at least, from the manufacturers, to confirm the standard to which the equipment is constructed.
Only the manufacturer's guarantee is required for the nonsparking equipment. Such a guarantee must state the maximum surface temperature and the type of enclosure.
The equipment protection categories include flameproofed enclosure (Ex d), increased safety (Ex e), pressurized (Ex p), intrinsic safety (Ex i), etc.
Recommendations for cables, conductors, flexible cables, and conduit systems used in the hazardous area are clearly stated in the various standards and guidelines.
APPLICATION
What happens if two methods of area classification have been applied at different times over the years on a platform? Different types of equipment would have been installed in similar situations because of the application of the different guidelines and standards. Lack of uniformity would be evident.
There are some platforms that are characterized by the application of two international standards. The North American standard might have been used before the platform operators decided to use the European standards.
In this situation, does the area classification specialist recommend changing all affected electrical equipment to conform to the new standard? Or is the recommendation to upgrade, recertify, and request manufacturers' guarantees on the electrical equipment?
Does the specialist conduct a safety audit to determine how the flammable gas accumulation can be mitigated by effective mechanical ventilation and safety instrumentation?
The decision has to be based partly on other factors. Such factors include the plant maintenance history, equipment, breakdown statistics, plant fire and explosion history, and cost of the replacement or upgrade of the electrical equipment.
For example, based on these factors, it may be sufficient to request a manufacturer's guarantee to use a Class 1, Division 2 (ClD2) motor in a Zone 2 environment.
The ClD2 motor is usually a squirrel cage induction motor, nonexplosion proof, totally enclosed and fan cooled, and nonarc-producing. The temperature rating is compatible with the gas or vapor atmosphere.
Similarly, a Zone 2 motor is usually an Ex(n) type. It has no arc-producing parts. Its temperature rating is compatible with the surrounding gaseous atmosphere. The enclosure is dust and splash proof.
How much work and cost will be involved in guaranteeing that the ClD2 motor is functionally equivalent to an Ex(n) motor? The result could be quite illuminating. It could also be cost-effective without compromising safety.
VENTILATION
What happens if there is a change in the effectiveness of ventilation? Lack of adequate ventilation has a tremendous effect on vapor dispersion and extent of zones.
On a platform which has been modified significantly, it will be necessarily to review the effectiveness of the ventilation. Ventilation is a basic component of the platform safety.
In this type of review, the area classification specialist has to refer to the fundamental principles to determine or define what constitutes an adequate ventilation in observable and reproducible terms.
Guidelines and standards provide different and useful information.
API 500B describes three types of ventilation. Adequate ventilation is the ventilation outdoors.
The second type, called limited ventilation, is required to prevent the accumulation (or at least extended accumulation) of flammable concentration of gases or vapors in most cases of abnormal operation.
The third type, inadequate ventilation, provides minimum or even a complete lack of ventilation.
Furthermore, NFPA 30 provides that adequate ventilation should be sufficient to prevent the accumulation of significant volumes of flammable gases or vapors in concentrations exceeding 25% of the LEL of the gases or vapors at steady-state conditions.
This statement facilitates determining the number of air changes per hour that provide the degree of adequate ventilation described in API 500B.
This statement is responsible for the application of the commonly quoted "steady state, twelve fresh air changes per hour for adequate ventilation."
Some guidelines use the wind speed as a criterion for adequate ventilation. Areas with natural ventilation (adequate ventilation, unlimited open air movement) are characterized by wind speed which normally exceeds 7.2 km/hr, and only exceptionally less than 1.8 km/hr.
Areas with limited ventilation (limited air movement) do not satisfy the wind speed requirement.
Accordingly, the objective is to evaluate air-flow patterns on decks using any of the criteria.
To assess the general ventilation on a platform, the platform should be surveyed. The wind velocities should be measured and recorded at specified locations on the platform. Also, the global wind velocities should be recorded.
Measurements should be taken before and after any major obstruction that can reduce ventilation. These measurements and the corresponding meteorological data should give a good indication of the ventilation condition.
Several other studies could be carried out. These include the explosion risk analysis on the decks where the ventilation is restricted. Simply described, the gas leakage and dispersion would be modeled.
The explosion probability is estimated during the cloud development. The analysis can confirm if the platform ventilation is restricted.
The specialist could also conduct a complex flow analysis, if the layout allows. The wind flow at and around the platform can be described by the Navier-Stokes equations.
The flow simulation would specify the physical properties of the flow along with the appropriate boundary conditions. This analysis would provide only a good guide for assessing the ventilation.
Sometimes wind tunnel analysis is carried out. These results provide further information.
The information derived from the various studies, a modified wind speed criteria (if necessary), and further off shore surveys can be used to categorize platform areas either adequately ventilated, inadequately ventilated, or not ventilated.
With the problem of ventilation "solved," it becomes a simple exercise to apply the standard procedures to classify the areas, assuming there are no other problems.
GENERIC DIAGRAMS
Another concern is the use of the generic diagrams. The generic representations of the area classification may not yield safe classification if applied to nonstandard situations. the layout of an offshore platform presents a challenge to the geometrical approach to area classification.
For a platform that has seen several significant modifications which affect the ventilation over the years, the drawings should be made more conservative by increasing the extents or the hazard radiuses.
Increasing the extents could affect the compatibility of the equipment with the new zones or divisions. It will also impact the cost of platform modification.
To produce an approvable set of area classification drawings may require a combination of the knowledge of the various international and national standards and guidelines. It could be a formidable task.
INTERPRETING THE STANDARDS
Interpreting standards and guidelines is a common problem. One will be amazed at how different specialists analyze and apply the same standards in complex-area classification investigations.
Should the intent or the letter of the standards and guidelines be adhered to? These questions usually arise on borderline cases.
A common example is the question of what constitutes a nonhazardous atmosphere. Is it a gas-free environment? Is it an environment whose ventilation is adequate to instantaneously disperse any release?
Should we base the hazard on the LEL of the gases or vapors in the atmosphere? Any of the questions could deserve a "yes" depending on the situation.
There is another example.
What is the boundary line between "no ventilation" and "inadequate (limited) ventilation" according to the standards? The interpretation or definition of these types of ventilation can affect the type and/or extent of the zones.
API 500B recognizes a reduction from Division 1 to Division 2 in the case of limited ventilation.
IEC 79-10 will degrade a zone. Zone 1 becomes Zone 0, or Zone 2 becomes Zone 1 in the case of no ventilation.
BS 5345 Part 2 applies ventilation in the determination of the extents of the zones.
The specialist has to base decisions on the company design premises, guidelines, any applicable regulations, and personal judgment. In all cases, safety must be preserved.
When two alternatives meet the safety standards, economics is applied to make a decision. Some national standards actually stipulate regulations that specify the minimum safety standards. These regulations provide an extra, usually restrictive, dimension in the interpretation.
Interpretation, simple as it may sound, can consume substantial man-hours in meetings and correspondence. Interpretation also has a big impact on the verification (conformity with regulations) of the area classification and other safety documents.
The impact of the interpretations on installation and operating costs are enormous for offshore installations.
PROBABILISTIC APPROACH
Should a new approach be established? Several area classification investigators have considered risk assessment as opposed to the geometrical approach.
In the risk-assessment principle, area classification, a safety subject, starts with the definition of the acceptable risk level, such as the limit on the probability of explosion. Areas and equipment are assigned probability coefficients with respect to their propensities to become explosive or ignition sources.
The probability coefficients weighted by such factors as ventilation, topography, maintainability, and plant and industrial explosion history, are combined statistically.
Nevertheless, a probabilistic approach will be extremely difficult to verify because no standards exist to use as a basis for evaluation. It is practically impossible to apply the probabilistic method because it relies heavily on human judgment.
Many companies are interested in applying the proven, or a variation of the proven, safe methods. They will not accept less than a near-zero risk level in area classification.
Safety is the highest priority. Such being the case, esoteric ideas are not usually advanced to classify the areas on the offshore platforms. The geometrical approach is usually agreed upon. The standard guidelines, diagrams, and their logically derived and accepted variations, are utilized.
CHANGING CLASSIFICATIONS
What should be done if several areas that were formerly safe should become hazardous?
The extension of the zones of divisions can affect the mechanical ventilation of some enclosed areas. The ventilation air intake should normally be in a safe area. With the new zones or divisions and their extents, some air intakes could be in Zone 2 or Division 2 areas. The same could happen to the combustion air intakes.
The ventilation systems would have to be evaluated and modified. New systems might be installed. The associated fans may demand more power from the normal or emergency electrical generators. Accordingly, the generator capacity and demand would have to be evaluated.
Bigger generators could be required. The associated electrical power transmission and distribution systems would accordingly be reviewed. Next in line would be the control and instrumentation systems, and so forth.
Electrical equipment, especially motors, could become unsuitable for their newly classified areas. On some platforms, several totally enclosed fan cooled (TEFC) motors may have to be replaced by Ex(d), or Ex(e), or Ex(n) motors, depending on the classification. It should be pointed out that many of these TEFC motors could have been running for several years without any problems.
Emergency equipment and installations such as the fire water pump, control room, and emergency power-supply system, should be installed in areas which are safe by the virtue of their location, i.e., not mechanically ventilated for the purpose of area classification.
The change of classification could affect the status of these emergency systems.
This problem has to be addressed on a case-by-case basis. The general principle is to further enhance the level of safety.
The enhancement could be in the form of system relocation, increased mechanical ventilation, and safety instrumentation and procedures.
Should the intent or content of the intrinsic safety concept be observed? An intrinsically safe system is incapable of releasing electrical energy under normal and abnormal conditions to cause ignition of a specific atmospheric mixture.
Thus, the basic requirements are: the hazardous atmosphere, ignition energy, and normal and abnormal conditions.
Shouldn't these requirements be evaluated in the light of a reduced ventilation or change of zones or divisions? A formerly intrinsically safe circuit, in a naturally ventilated environment (i.e., no risk of hazardous atmosphere), is now in an area classified as Zone 2 or Division 2 (i.e., possible risk of hazardous atmosphere under abnormal conditions) because of the platform changes.
The ignition energy level may also change because of a possible accumulation of a different type of gas in the new Zone 2 or Division 2 environment through which the originally intrinsically safe circuit passes.
The intent of the intrinsic safety system is to prevent the nonintrinsically safe portion of the system from transmitting sufficient energy to the intrinsically safe portion to cause ignition of the atmosphere, even under failure.
This prevention usually is ensured by installing barriers at the critical interfaces of the intrinsically safe and nonintrinsically safe sections. The intrinsic safety of the instrumentation on the platform will have to be evaluated.
MANAGEMENT
There are management problems too.
First, which disciplines should be represented in a typical area classification task force? A basic task force may consist of representatives from process, HVAC, safety, electrical, instrumentation sections, plant operation, plant maintenance, and plant/project management.
However, a realistic task force should select representatives from disciplines of most concern.
How should we show the modifications of an area classification on the drawings for presentation to the management? Should the drawings show what exist now (issue for information or comments or review), the situation when all the upgrades and changes are complete (issue for construction or issue for design), or both?
Presenting the wrong set of drawings to management could invite further problems as a result of any misinformation.
A decision of this nature has to be taken in light of the safety and continuity of the platform operation.
Other current and proposed projects that rely on the area classification drawings have to be considered before any set of drawings is issued. A set of drawings should be agreed upon and issued.
The drawings should be issued for both comments and construction to facilitate better communication. However, IEC 79-10 suggests that the area classification be established assuming that the mechanical ventilation is completed.
BIBLIOGRAPHY
- American Petroleum Institute, "API Recommended Practice for Classification of Areas for Electrical Installation at Drilling Rigs and Production Facilities on Land and on Marine Fixed and Mobile Platforms," API RP 500B.
- International Electrotechnical Com-mission, "Publication 79-10, Electrical Apparatus for Explosive Gas Atmospheres, Part 10: Classification of Hazardous Areas."
- Det norske Veritas, "Recommended Practice, Volume C: Facilities on Offshore installations, Group Cl 00: General Safety, Area Classification and Ventilation."
- Royal Ministry of Industry and Handicrafts and the Water Resources and Electrical Board (NVE), "Regulation for Electric Installation of December 1963, including later revisions."
- Norwegian Petroleum Directorate, "Acts, Regulations and Guidelines for the Petroleum Activity."
- National Electrical Code, NFPA 70-1978.
- British Standard 5345. Part 2: 1983 Code of Practice for Selection, Installation and Maintenance of Electrical Apparatus for use in Potentially Explosive Processing and Manufacture). Part 2: Classification of Hazardous Areas.
- Magison, E.C., Electrical Instruments in Hazardous Locations, Third Edition, Instrument Society of America, Pittsburgh.
Copyright 1991 Oil & Gas Journal. All Rights Reserved.
