THE USE OF COMBUSTIBLE GAS DETECTION IN HAZARDOUS LOCATIONS

Transcription

1 THE USE OF COMBUSTIBLE GAS DETECTION IN HAZARDOUS LOCATIONS Copyright Material IEEE Paper No. PCIC-PH-56 Allan Bozek, IEEE Senior Member EngWorks Inc th Ave. SW Calgary, AB, T2T2T7 Canada Tim Driscoll, IEEE Fellow OBIEC Consulting Ltd. 604 Bearspaw Village Road Calgary, AB, T3L 2P1 Canada Jon D. Miller IEEE Member Detector Electronics Corp West 110 th St. Bloomington, MN USA Vince Rowe IEEE Life Senior Member Marex Canada Ltd Butcher Road Nanaimo, BC V9T5Z2 Canada William G Lawrence, IEEE Senior Member FM Approvals 1151 Bos-Prov Tpke Norwood MA USA william.lawrence@fmapp rovals.com Abstract This document explores the use of combustible gas detection as a means of supplementary protection in hazardous locations. The use of combustible gas detection in the context of a hazardous area classification design is reviewed and its use as a means to interlock ignition capable equipment is discussed. IEC and North American codes, standards and recommended practices are referenced. A case example is provided to illustrate the use of concepts presented. Index Terms Flammable gas detection, Combustible gas detection, Hazardous Area Classification, Hazardous Locations, Explosion protection. I. INTRODUCTION Combustible gas detection is often used to detect flammable gas/vapor releases in enclosed areas in order to identify the need for corrective measures. The technology is used in one of two ways: 1) in conjunction with adequate ventilation to define the hazardous area classification design; or 2).to de-energize electrical equipment that is not rated for a specific hazardous location when higher than normal concentrations of flammable gas or vapor in the air are detected. The difference in how combustible gas detection is applied in each case has led to industry confusion on how the technology should be employed. This is further complicated by conflicts between the area classification recommended practices and the National Electrical Code (NEC) [1], Canadian Electrical Code (CEC) [2] and IEC installation codes and practices. This often leads to misapplication of the technology and non-code compliant installations. The intent of this document is to clarify the requirements and provide guidance on how combustible gas detection should be employed in hazardous locations. In this paper, flammable limits are used rather than explosive limits. The acronyms used for the lower and upper flammable limits and lower and upper explosive limits are LFL, UFL, LEL and UEL. Flammable and explosive limits are often used interchangeably for gas/air mixtures; however they are not the same. Flammable limits are the range of a mixture where self-sustaining flame propagation can occur. The difference for explosive limits is that flame propagation throughout the mixture will occur, and this range will be narrower than the flammable limits. Combustible gas detectors measure flammable and combustible concentrations, i.e. LFL. II. THE FIRE TRIANGLE The fire triangle can be used to describe how combustible gas detection can be used in hazardous locations. Fig. 1 Fire Triangle Explosion safety in a hazardous location where flammable gas/vapors may be present depends on ensuring that all three sides of the explosion triangle do not occur simultaneously. Since oxygen (in the form of air) is always present under normal circumstances, the fuel and ignition sides of the triangle must be properly managed to ensure safety. The fuel side of the triangle is addressed by the hazardous area classification design. This is considered an engineering design task that is performed using guidance provided by industry recognized design standards and recommended practices. The frequency and duration of a flammable atmosphere being present is assessed and the appropriate Division or Zone classification assigned. Combustible gas detection is used to monitor the levels of flammable gases in air, and can be used as a tool to help manage the fuel side of the triangle. Once an area is classified, the ignition side of the triangle can be addressed by selecting equipment certified for the location and installing the equipment in accordance with appropriate installation codes as defined by the authority having jurisdiction (AHJ). In the US, Articles 500 and 505 of the NEC apply. In Canada, 1

2 Section 18 and Appendix J of the CEC apply. In countries using IEC standards, the installation practices are defined by IEC [3] III. COMBUSTIBLE GAS DETECTION AS A PREMISE FOR CLASSIFYING LOCATIONS Hazardous areas, where flammable gas/vapors may be present, are divided into Zones or Divisions based on the frequency of occurrence and the expected duration the flammable or explosive gas atmosphere (i.e. 100% LFL) will exist. American Petroleum Institute, API RP 505 [4] and Energy Institute, EI 15 [5] (formerly IP 15) suggest the following relationship for classifying zones based on a potential exposure to a flammable or explosive gas atmosphere. Zone 0 1,000 or more hours/year Zone 1 < 1,000 hours/year and > 10 hours/year Zone 2 < 10 hours per year While API RP 500 [6] does not have a similar table, it is generally accepted that Zone 2 and Class I, Division 2 are equivalent; therefore it is reasonable to conclude that Class I, Division 1 is equivalent to Zone 0 and Zone 1 combined. Combustible gas detection is often used in the development of a hazardous area classification design. The primary purpose of combustible gas detection is to ensure the classification remains within the definition of a Zone/Division classification as defined by the NEC and the CEC. The standards described in A, B, C and D below are not mandatory requirements in Canada or the United States. However the standards described in E, the NEC and CEC, are mandatory in each. A. API RP 500/505 These standards are recommended practices, and as such are not mandatory requirements. They use words such as shall, should and may. Where the word shall is used, it is not a requirement; rather it is a strong recommendation, being that these are recommended practices. Another term that is used throughout these standards is engineering judgment, sometimes preceded by sound, good or individual. Wherever used, it is meant to indicate that there are assessments that should be made based on various factors such as risk, pressures, maintenance, etc. that can affect the area classification decisions. For example, recommended boundary distances may be increased due to the nature or volume of material that could be released in a specific area. One major criterion in classifying for both API RP 500 and 505 is ventilation. As part of that discussion, where the ventilation rate in enclosed areas is less than 3 air changes per hour (ACPH), it is recommended that permanent continuous combustible gas detection be applied. API RP 500 for Division classified facilities allows for the use of combustible gas detection to classify an inadequately ventilated enclosed area containing equipment that may release gas or vapor, as Class I, Division 2. However, it cautions that if the equipment may release gas or vapor during normal operations, combustible gas detection should not be used for this purpose (in which case the area would be Class I, Division 1). API RP 500 also allows for designating an enclosed area not containing equipment that can be a source of release that is adjacent to or in a Class I, Division 2 area, as unclassified. The room would need to be of vapor tight construction where it meets the classified area. There are some requirements listed for the gas detection system in order to achieve this classification; Permanently mounted, point detectors. Note - open path type would not be acceptable. Nationally recognized testing laboratory (NRTL) listed Detection equipment for hazardous locations shall also be performance tested, and ANSI/ISA ( ) [7] is recommended, and additionally for installation, operation and maintenance An adequate number of sensors be installed to ensure detection of releases Triggering an alarm and possibly some remedial actions on 20% LFL Automatically de-energizing equipment not suitable for the area on 40% LFL and gas detection malfunction, and take remedial action Calibration of the detectors in accordance with manufacturer s recommendations, but at least once every 3 months API RP 505 for Zones, contains essentially the same requirements as in API RP 500 for the gas detection systems, however there are some distinct differences in the recent 2nd Edition. ANSI/ISA [8] is referenced for gas detector installation, operation and maintenance. An inadequately ventilated Zone 1 enclosed area containing equipment that can be a source of release may have electrical equipment suitable for Zone 2 installed. The area classification shall document the basis for selecting the equipment suitable for Zone 2. This is a significant difference from the previous edition and from API RP 500, where the classification could be designated as Zone 2 (and Class I, Division 2 in API RP 500) by the use of combustible gas detection. An enclosed area not containing equipment that can be a source of release, that is adjacent to or in a Zone 2 area, may have electrical equipment suitable for unclassified locations installed. The room would need to be of vapor tight construction where it meets the classified area. The area classification shall document the basis for selecting equipment suitable for unclassified locations. This is a significant difference from the previous edition and from API RP 500, where the area could be unclassified by the use of combustible gas detection. A new clause has been added for enclosed areas that are unattended or unmonitored, where a gas/vapor release may go undetected for an extended period of time. This would normally be considered Zone 1. However combustible gas detection can be used to 2

3 monitor the area, triggering alarms and responses, so as to limit a release to a short time period, where the enclosed area can then be classified as Zone 2. Another key change has been made to API RP 505 in the definitions and throughout the body, and that is to use flammable limits rather than explosive limits to define an ignitable mixture. The upper and lower flammable limits (UFL/LFL) are based on material properties whereas the explosive limits (UEL/LEL) are also dependent on other factors such as enclosure geometry or igniter location. The flammable limits are generally wider than the explosive limits. Wherever flammable or explosive limits are mentioned in the standard, the term flammable or explosive is now used, along with the acronyms, which has the effect of making flammable limits the minimum requirement. As described above, there are some major differences between API RP 500 and the recent edition of API RP 505. It is expected that the next edition of API RP 500 will incorporate these changes. B. NFPA 497 NFPA 497 [9] is a recommended practice for the classification of chemical process areas subject to the installation requirements of the NEC. The document does not discuss the use of combustible gas detection in performing a hazardous area classification design, however, it does state that existing facility history is an important factor in classifying a facility. Gas detection can be a useful tool in monitoring the operating characteristics of a facility under normal and abnormal conditions and determining the appropriate classification. Other NFPA documents may specify the use of gas detection as part of an overall fire protection scheme; however, the use of gas detection does not influence the hazardous area classification of such facilities. C. IEC IEC [10] does not discuss the use of gas detection as a basis for a hazardous area classification design. The use of gas detection is mentioned in IEC for the design of type pz pressurized buildings sourcing pressurized air from a Zone 2 location. Gas detection is used to monitor the air intake to a pressurized room and upon detection of a 40% LFL, the air intake to the building must be shutdown. The detection and electrical equipment used for alarming and emergency actions/interlocks must have a minimum equipment protection level of Gb (suitable for Zone 1 areas). D. EI 15 EI 15 does not recognize the use of gas detection as a basis for a hazardous area classification design. The document does however recommend the use of gas detection for unmanned enclosed facilities designated Zone 2. The rationale used is that in certain situations, the ability to detect and respond to a flammable gas/vapor release may be compromised in facilities that are not monitored on a frequent basis. If there is a possibility of releases extending beyond the 10 hour per year short time criteria often defined for a Zone 2 location, the installation of gas detection is an appropriate precaution. Practically this would be a 2-3 hour limit for any single release. The application of gas detection does not allow an enclosed location to be defined Zone 2 by itself. It is viewed as additional protection in the event of a flammable gas/vapor release. E. NFPA 70 (NATIONAL ELECTRICAL CODE) AND CSA C22.1 (CANADIAN ELECTRICAL CODE) The NEC and the CEC do not mandate the use of combustible gas detection as a part of a hazardous area classification design. They only require that an area classification is performed according to the definitions provided. However it would be difficult to achieve the Zone 2/Division 2 area classification criteria in many enclosed spaces without the use of combustible gas detection. This is best understood by examining the NEC and CEC Zone 2 definitions: Zone 2 a location in which ignitable concentrations of flammable gases or vapors are not likely to occur in normal operation and, if they do occur, they will exist only for a short period. Note: The above wording is the NEC definition. The CEC wording varies slightly but has the same meaning. The CEC and the NEC recognize Zone 2 and Class I, Division 2 as being equivalent. They both allow the same electrical equipment in Zone 2 and Class I, Division 2. The CEC has therefore changed the Class I, Division 2 definition to match the Zone 2 definition. When the Zone classification was adopted in Article 505 of the NEC, the Zone 2 definition used the existing Class I, Division 2 definition and added the slightly modified version of the IEC definition. This Class I, Division 2 definition in the NEC includes several additional criteria elaborating on specific situations which are intended to provide clarification, whereas the IEC and CEC definitions are much simpler. As such, the importance of gas detection in meeting a Zone 2 or Class I, Division 2 classification for enclosed spaces may be less apparent in the NEC. As outlined above, industrial reference documents such as API RP 505 and EI 15 suggest as a Rule of Thumb, the total length of time an explosive gas atmosphere should be present in an enclosed Zone 2 location be no more than a total of 10 hours per year. During normal operation the fugitive emissions in Zone 2/Division 2 buildings are diluted to safe levels by their ventilation systems. In Canada and the US, the ventilation requirements for new facilities are most often determined using the procedures in section 6 of API RP 500 or RP 505. However, if an abnormal situation occurs which results in elevated concentrations of flammable gas or vapor within a Zone 2/Division 2 building, it is required that action be taken to eliminate it within a short time. Before action can be taken, operating staff must first be aware of the abnormal situation. In buildings that are not checked a number of times daily, combustible gas detection is the only practical means of alerting operating staff of the need to take corrective action. Therefore, for many Zone 2/Division 2 buildings, it would be difficult to meet the Zone 2 or Class I, Division 2 definition without the use of combustible gas detection. 3

4 Application of API RP 500 and 505 recommendations for achieving adequate ventilation, particularly the recommendation for 6 air changes per hour (ACPH), frequently results in unnecessarily high costs for heating buildings during cold weather, particularly in Canada and the northern United States. Very often this can be traced back to inexperience on the part of the designer. Where excessive ventilation rates cause problems (such as freezing process liquids or excessive heating costs) in cold weather, combustible gas detection systems can be a valuable tool for justifying a reduction in building ventilation. A combination of permanently installed combustible gas detection systems and portable combustible gas detectors can be used to monitor the gas concentration in buildings as the ventilation rate is progressively reduced. Essentially existing process buildings are full scale models. Measuring gas concentration in the air within process buildings is a considerably more accurate approach for determining if a building is adequately ventilated than are estimates done using fugitive emissions calculations. This process should only be used under proper engineering supervision. Clause (2) of NFPA 30 [11] outlines a method for sampling actual vapor concentrations in enclosed processing areas. The application of gas detection by itself does not designate a hazardous area classification. Within enclosed areas, the area classification design will be determined by the expected release rates, volumes and duration as well as the ventilation provided. Gas detection provides additional protection in situations where enclosed areas are not visited on a frequent basis to ensure that flammable emissions resulting from a process containment failure do not persist undetected for long periods of time. As ignitable concentrations of flammable gases or vapors can only be present in Zone 2 areas for a total of 10 hours per year, it is reasonable to interpret a short period as stated in the Zone 2 definition as a fraction of 10 hours for any single occurrence. If a flammable gas/vapor release can exist for a long period of time, then the application of combustible gas detection is recommended for monitoring an enclosed location designated Zone 2/Division 2. Operations can then take action to ensure the short period criterion is maintained. IV. THE USE OF COMBUSTIBLE GAS DETECTION AS SUPPLEMENTAL PROTECTION The use of gas detection is not recognized as a method of protection in the traditional sense. There are no North American or IEC product standards that specify the construction, testing and marking of systems employing gas detection as a method of protection. North American installation codes and practices recognize the use of combustible gas detection equipment to provide supplemental protection from explosion or fires by minimizing the possibility of an accumulation of combustible gases reaching ignitable levels. In effect, the hazard is mitigated by early detection of increased concentrations of flammable gas/vapors in air and subsequent control actions to alarm, ventilate and if necessary, shutdown potential ignition sources. The application of an explosion protection system employing gas detection is required to conform to the appropriate electrical installation codes. ANSI/NFPA 70 (NEC) The National Electrical Code (NEC) sections 500.7(K) and 505.8(I)) include provisions for fixed combustible gas detection equipment to permit the installation of equipment that would otherwise be unsuitable for that hazardous location. The NEC references ANSI/ISA- TR [12], for application guidance. The NEC restricts the use of combustible gas detection as a method of protection to three specific situations (see description in section B. below) in industrial establishments with restricted public access and where the conditions of maintenance and supervision ensure that only qualified persons service the installation. A. ANSI/ISA Given the complications involved with establishing advisory text on the use of combustible gas detection equipment for equipment protection purposes, the International Society of Automation (ISA) Standards Practice (SP) Combustible Gas Detection Committee was approached for development of a recommended practice. ANSI/ISA-TR , Guide for Combustible Gas Detection as a Method of Protection, was subsequently released and referenced as further guidance material within the NEC. The three applications permitted by the NEC are covered in detail within the recommended practice: 1) Inadequate Ventilation; 2) Interior of a Building; 3) Interior of a Control Panel. Gas detection may be used to install Zone2/Division 2 certified equipment within an inadequately ventilated Zone 1/Division 1 enclosed location. The system should initiate a gas alarm at the 20% LFL level and provide some means of mitigating the rate of increase resulting from the gas release. This usually is accomplished through increased ventilation of the enclosed area. Should the gas concentration reach the 40% LFL level, the protected equipment should be de-energized. A malfunction of the gas detector equipment should also initiate a shutdown of the protected equipment. Refer to Figure 2 for an example of the system configuration. Gas detection may also be used to permit nonhazardous location rated electrical equipment to be installed in an unclassified room that is surrounded or incorporates an opening adjacent to a Zone 2/Division 2 location. The unclassified room must not contain a flammable source of release. Detection of a 20% LFL should initiate a low level gas alarm. Detection of a 40% LFL high alarm or a malfunction of the gas detection equipment should initiate a shutdown of the protected equipment located within the unclassified room. Refer to Figure 3 for an example of the system configuration. Gas detectors may also be used to protect instrumentation utilizing or measuring combustible liquids or gases within a control panel. Instrumentation equipment must be certified for a Zone 2/Division 2 4

5 Fig. 2 Class I, Division 2/Zone 2 Equipment in an Inadequately Ventilated Division 1/Zone 1 Location Fig. 3 Non-hazardous Rated Equipment in an Unclassified Room within a Class I, Division 2/Zone 2 Location Fig. 4 Control Panel Application location and the gas detection equipment should initiate a low level alarm at 20% LFL and de-energize the protected equipment at a 40% LFL alarm level. A malfunction of the gas detection equipment should also initiate the shutdown of the protected equipment. Refer to Figure 4 for an example of the system configuration. Gas detection as a method of protection should be used only when necessary with otherwise unsuitable equipment in a hazardous location (as stated in ANSI/ISA-TR ). Use of suitably certified/listed electrical equipment is recommended wherever possible. B. CSA C22.1 (CEC) The Canadian Electrical Code Rule permits the use of electrical equipment suitable for non-hazardous locations in Zone 2 locations and equipment suitable for Zone 2 in Zone 1 locations, based on the following criteria being met: 1) no suitable equipment is available; 2) the equipment is non-ignition capable during its normal operation; and: 3) the location is continuously monitored by a combustible gas detection system that will activate alarms and additional ventilation at 20% LFL where supplemental ventilation is provided automatically de-energize the electrical equipment being protected when the gas concentration reaches 40% LFL or on failure of the gas detection equipment where supplemental ventilation is not provided, automatically de-energize the electrical equipment being protected when the gas concentration reaches 20% LFL One criterion that has often been misapplied, is the first item listed above no suitable equipment is available. The intent of this part of the rule is that it can only be used if no certified product is available for that location on the market. The rule is also specific to equipment and does not apply to wiring methods. Additional clarification on the intent and on the application and maintenance of combustible gas detection for this purpose is contained in informative Appendixes B and H. C. IEC Standards The International Electrotechnical Commission (IEC) installation practice does not presently align to other installation practices around the world (e.g. NEC, API RP14F [13], CEC, EN [14], AS2865 [15]) for use of gas detection as a means of accepting non-hazardous certified electrical equipment in hazardous locations. IEC is in general agreement that IEC should include details related to gas detection as a method of protection (discussions within IEC SC31J, Fall 2014); however, there has not been any implementation of requirements to date. Although there is no IEC installation practice governing gas detection as a method of protection, the IEC Series Combustible Gas Detection standards provide requirements for performance, selection, installation, use and maintenance for gas detectors and systems. The scope of these standards is centered around combustible gas detection equipment intended to provide an 5

6 indication, alarm or other output function for the purpose of which is to indicate a potential explosion hazard and in some cases, to initiate automatic or manual protective action(s). There are a variety of gas detectors on the market today that have Safety Instrumented System SIL 2 certification per IEC [16] Series standards, which among other aspects demonstrates a level of instrument reliability as a safety function. The SIL certification additionally supports a high level of electrical and software design rigor and product validation. Although SIL certification of gas detectors presently is not mandatory for their use as a method of protection, the SIL certification demonstrates a high level of product reliability. Further details on incorporation of gas detection within a Safety Instrumented System can be found in IEC [17] and ISA TR [18]. D. Resolution of conflicts between standards Work continues through standard development committees to include and enhance the requirements for gas detectors. The collaborative efforts between UL, CEN, ISO and IEC (to name a few) have had a major focus in recent years to consolidate requirements and application of gas detectors. There is also the intent to harmonize the CEC and the NEC with respect to the use of gas detection. It is expected that the next edition of ANSI/ISA-TR will address both NEC and CEC requirements. V. APPLICATION CONSIDERATIONS The following are some important considerations when incorporating combustible gas detection into a design. A. Selection of appropriate combustible gas detection technology Where hydrocarbons are handled, processed or stored, combustible gas detectors should be used. There are a variety of combustible gas detector technologies available on the market today; however, there are advantages and disadvantages that should be considered for the various technologies. Table 1 provides a summary of the various combustible gas detector technologies. In summary, catalytic sensors and infrared sensors have been most prominently employed as a method of protection based upon proven reliability and performance for purpose. Current gas detection technology, with the use of infrared sensor technology, is much improved from technology decades ago. Infrared gas detection technology provides both improved accuracy and reliability from past gas detection technologies. However, infrared sensor technology does not offer detection of hydrogen (H 2 ) gas, so applications such as these typically incorporate the historically proven catalytic sensors. Although catalytic sensors are prone to poisoning, they are capable of detecting all flammable gases that may be present in a given location. Table 2 provides advantages and disadvantages of these technologies for consideration. Infrared sensor technology offers both Point gas detection and Open Path gas detection. The Point gas detector provides a gas concentration measurement of % LFL at the detector; whereas, the Open Path gas detection system does not provide an actual concentration measurement but rather an LFL-meter output. Given that the Open Path gas detection system is not capable of providing an actual %LFL gas concentration measurement, application as a means of protection within a precise location is NOT recommended for the protection of equipment. Only gas detectors performance certified (conforming with) ANSI/ISA ( ) per the NEC or CSA C22.2 No. 152 [19] per the CEC are permitted to be used as a means of safety protection. The gas detector must comply with the response time requirements within the performance standard. A gas detectors response time characteristics are independent of the time duration of a release for defining a zone/division. TABLE I GAS DETECTION TECHNOLOGY OVERVIEW Operation Technology Advantages Disadvantages Principle Catalytic Sensor Thermal Conductivity Sensor Infrared Sensor Semiconductor Sensor Electrochemical Sensor Flame Ionization Detector Flame Temperature Analyzer Photo Ionization Detector Paramagnetic Oxygen Detector Oxidation of flammable gas Heat loss by conduction Absorption of light energy Electrical conductance by chemisorption Electrolyte chemical reaction Electrical charging of burnt organic compound Flame temperature rise on constant flow of combustion Ionization by ultraviolet radiation Proportion of oxygen paramagnetic force Detects all flammable gases Detects individual gas with high or low relative thermal conductivity up to 100% v/v Extended life expectancy with selfdiagnostic measures Sensitivity at very low concentration Detects H2 & CO at LFL levels, & O2 at 25% v/v High sensitivity, wide measuring range, small measuring uncertainty, poison resistance, fast response time Fast response to total flammable gas at LFL levels High sensitivity, poison resistance, fast response time Measurement selectivity, long term stability, poison resistance Requires sufficient O2 content, prone to poisoning Requires low variance of background gases No H2 response, sensitive to pressure variance Vulnerable to humidity change & interfering gases Electrolyte lasts for short period Signal critically dependent upon flow rate, no environmental sensitivity Requires fuel gas & O2 for flame No detection of compounds with higher ionization potential than lamp energy, e.g. H2, CH4, CO Shock/vibration sensitive 6

7 TABLE 2 GAS DETECTION TECHNOLOGY SUMMARY Technology Advantages Disadvantages Catalytic, Point Infrared, Open Path Infrared, Point - Low initial cost - H2 detection - Versatile, broad range - Large area coverage - Fail safe - Infrequent calibration - Low maintenance - Environmental superiority - Fail safe - Low cost of ownership - High reliability - Frequent calibration cycle - Susceptible to poisoning - Not fail safe - Monthly degradation - Alignment challenges - LFL x meter read out / output - Expensive - No H2 detection B. Placement of gas detectors A fixed gas detection system should be capable of giving an early warning of both the presence and the general location of an accumulation of flammable gas or vapor. Mitigation action, either automatic or under manual control, needs to be initiated in order to retain a safe state. The placement of the combustible gas detector is critical for proper operation of the mitigation actions. In order for the gas detection system to signal an alarm, the gas sensor must be in the gas or vapor cloud. The gas sensor, whether catalytic or Point infrared, is designed to be positioned within the potential source of gas or vapor release without causing an explosion hazard itself. The gas sensor should be placed adjacent to the equipment being protected. It is imperative that flow considerations from ventilation, fans, wind, convection, etc. be taken into account for proper placement and the number of gas sensors. Proper orientation of the gas sensor in accordance with the manufacturer s instruction manual must be followed for correct operation. As a general rule, the gas detector should be positioned below the exhaust ventilation openings and close to the floor for the detection of gases heavier than air, and the gas detector should be positioned above the level of exhaust ventilation openings and close to the ceiling for the detection of gases lighter than air. C. Calibration requirements To ensure correct operation, it is essential to carry out both inspection and recalibration periodically. All types of gas sensors will require periodic inspection and recalibration using appropriate calibration gases. The required frequency may be specified by regulations of the responsible authorities. In most cases, advice or recommendations can be obtained from the manufacturer. Ultimately, it will depend on the severity of the application, and is best determined by starting a process of regular frequent checks and logging the results (amount of adjustment required etc.) in the maintenance records. After installation on site, each sensor should be calibrated according to the manufacturer's instructions, unless it carries currently valid factory calibration certification for the gas of interest. Calibration should only be carried out by suitably trained and competent persons. Gas detection system calibration is normally carried out by application of a zeroing gas (e.g. bottled air or nitrogen), or by verifying that the area is gas free, and the signal of the equipment is then zeroed. Then a span gas is applied and the sensitivity of the equipment adjusted to an appropriate value. Proper calibration is of vital importance to the ongoing reliability of the system. An accurate calibration requires the use of calibration gas as recommended by the manufacturer along with any required adjustments. It is acceptable to use a calibration gas different from the gas or vapor being monitored provided an appropriate correction is made so that the gas detector when calibrated gives the correct response to the monitored gas or vapor. Whenever a calibration gas other than the gas or vapor of interest is used, it is imperative that additional safety margins are employed by lower alarm settings in order to compensate for the added uncertainty of relative response. D. Maintenance requirements Gas detectors require on-going maintenance for proper operation and to provide a higher level of safety and longevity. Sensors are subject to degradation due to factors such as dust, solvents and other contaminants in the air, and the maintenance program should take this into consideration. It is extremely important that the manufacturer's instruction/safety manual(s) be followed completely. Routine maintenance of any combustible gas detection system is an extremely important factor affecting the reliability of the units. Optimum system performance and reliable operation will only be achieved if there is informed management, producing a responsible and practical program which yields complete, dedicated maintenance on a high priority level. Management of such a program depends on setting responsibilities for the various aspects, for example; who is supposed to perform functional checks, who is supposed to perform inspections and recalibrations, and who is responsible for maintenance, ensuring that the personnel concerned are adequately trained and periodically retrained. Part of this management responsibility is setting operational limits, for example; determining acceptable tolerances on functional checks that are to be permitted before re-calibration becomes mandatory, determining the frequency of regular re-calibrations, frequency of maintenance. Further details regarding placement, calibration and/or maintenance of a gas detection system can be found in ANSI/ISA ( ). VI. CASE EXAMPLES Three examples describing the application of gas detectors in an appropriate manner are described below. A. Zone 2 classified building in a remote located facility 7

8 The following case example describes the use of combustible gas detection as a premise for a hazardous area classification design. A natural gas compressor is installed in a building which is not physically monitored on a regular basis, and is remotely controlled from a central location. The building is naturally ventilated under normal operating conditions with supplemental ventilation fans provided for building temperature control. Figure 5 is an example of this application. To designate the building as Zone 2, the installation must meet the definition of a Zone 2 location which is, a location where ignitable concentrations of flammable gases or vapors are not likely to occur in normal operation and, if they do occur, they will exist for a short period only. To meet the criterion of not likely to occur in normal operation the enclosure must have sufficient ventilation (natural or mechanical) to dilute the fugitive emissions that occur during normal operation. Fig. 5 Compressor Building Application The second criterion, if ignitable concentrations of gas or vapor do occur, they will exist only for a short period, requires the building be monitored to ensure that any abnormal releases of flammable gases or vapors that do occur will be quickly detected. This is function of permanently installed combustible gas detection systems. It will then be necessary to take action to ensure the ignitable concentrations of flammable gas or vapor exist only for a short period. As outlined earlier in the paper, a short period will typically be a fraction of the rule of thumb of a total of 10 hours per year. This will allow for the possibility of multiple abnormal occurrences over the course of a year. This is typically accomplished by automatic shutdown and depressurization or by dispatching staff to the site to take some other manual actions. When the gas concentration in the air reaches the alarm point, some companies may choose to start additional ventilation in an effort to increase the ventilation to a point where the gas concentration in the air is controlled to a level below the high alarm point (typically 40% LFL). If that does not lower the gas concentration, shutdown or dispatch of personnel takes place. When using this approach it is important to understand that pockets of gas exceeding the LFL may still exist in certain areas of the building. Where ventilation is based on the fugitive emissions calculation method outlined in API RP500 and 505, experience has shown the concentration of flammable gas or vapor in the air in most process buildings is in the order of 1% LFL or lower. Combined with that knowledge, some operators are setting their gas detector early alarm point to 10% LFL to ensure developing abnormal situations are detected as early as possible. In past years, this may have resulted in nuisance alarms, however improvements in the accuracy and stability of combustible gas detection systems now makes this practical. Note that the application of gas detection does not designate the location as Zone 2 by itself. Adequate ventilation is required to ensure the building can continuously dilute the expected rate of fugitive emissions under normal operating conditions to safe levels. Without adequate ventilation, the fugitive emissions would eventually accumulate to ignitable concentrations within the building. Calculations or measurements are required to determine or confirm the appropriate ventilation rate to control the buildup of expected fugitive emissions to a level well below the lower flammable limit for the process gas. B. Non-hazardous location certified analyzer in a Division 2 classified building using CEC standards An analyzer is required to be installed in a Division 2 classified building using CEC installation standards. The analyzer is not available with a hazardous location certification due to the specialized nature of the equipment itself. The equipment does not contain any electrically heated parts or components that normally operate at a temperature equal to or above the auto-ignition temperature defined for the classified location. There are no electrical components that produce incendive arcing or sparking during their normal operation. The building incorporates a high rate ventilation system for stabilizing flammable gas releases. The analyzer may be suitable for installation in the location using gas detection under Rule of the CEC. The gas detector(s) will be configured to activate an alarm signal should the gas concentration exceed a 20% LFL within the building. The high rate ventilation system will also be activated to assist in stabilizing the release. Should the LFL reach the 40% level, the gas detector(s) would activate a high level alarm and initiate automatic disconnection of power to the analyzer unit. In the event of a gas detector malfunction, power to the analyzer would be disconnected. A program to calibrate and adjust the combustible gas detection system would be implemented in accordance with the manufacturer s recommendations. The calibration gas used would consist of a known mixture (nominal 50% LFL is recommended) of diluents and methane or other gas anticipated to ensure accurate readings. Operation of the interlocks would be checked regularly to verify the integrity of the system. C. Division 2 certified analyzer in a Division 1 classified building using NEC standards A Division 2 certified analyzer is required to be installed in a Division 1 classified building using NEC installation standards. The building is inadequately ventilated and is within an industrial establishment with restricted public 8

9 access and uses qualified maintenance personnel. Section (K)(1) of the NEC may be used as a basis for the installation. To implement this within the rules of the NEC, the guidelines provided in ANSI/ISA-TR should be met. The first is that the combustible gas detection equipment used must be of the stationary type and suitable (per Section 500.8) for a Class I, Division 1 location. More than one gas detector may be necessary to sense gas in all areas of the building. The interlocking of gas detectors using a voting scheme may also help to eliminate nuisance alarms. The gas detector(s) will be configured to activate an alarm signal should the gas concentration exceed a 20% LFL within the building. If available, supplemental ventilation in the building would be activated to dilute the release. Should the LFL reach the 40% level, the gas detector(s) would activate a high level alarm and initiate automatic disconnection of power to the analyzer unit. If the electrical equipment required to disconnect the power to the analyzer is located within the building and has arcing components, it would be required to be explosionproof or flameproof. The power circuit for the combustible gas detection equipment should be independent from any power circuit for the equipment within the space in order to permit continued gas detection monitoring. A program to calibrate and adjust the combustible gas detection system would normally be implemented in accordance with the manufacturer s recommendations. Operation of the interlocks would normally be checked regularly to verify the integrity of the system. VII. CONCLUSIONS Combustible gas detection is a useful means of providing additional safety in hazardous areas where flammable gas or vapors may be present. In situations where an enclosed area is designated Zone/Division 2, gas detection provides an effective means of assuring the short time period criterion of a Zone/Division 2 is maintained. Gas detection may also be used as additional means of protection where unsuitably certified/listed electrical equipment is installed in hazardous locations. The installation must be done in accordance with hazardous location installation standards adopted by the AHJ as the requirements differ between Canada and the US. It is recommended that this approach only be used where suitably certified/listed equipment is unavailable. This approach should never be used as a means to avoid obtaining the appropriate certification/listings for electrical equipment. The flammability limits (LFL, UFL) should be used in consideration of hazardous locations for standards and gas detection measurements. IEC and the latest edition of API RP 505 have already adopted this change. Explosive limits (LEL, UEL) are not synonymous with flammable limits. The use of combustible gas detection in all cases requires the appropriate selection of the detection technology and proper installation and maintenance. Used in this manner, combustible gas detection provides additional measures of safety in facilities handling flammable fluids (gases, vapors, and liquids). VIII. REFERENCES Note The reference to these standards in the body of this paper is to the most recent edition. [1] ANSI/NFPA 70 National Electrical Code, National Fire Protection Association, Quincy, MA [2] CSA 22.1 Canadian Electrical Code, Part l, Canadian Standards Association, Toronto, ON [3] IEC Explosive atmospheres - Part 14: Electrical installations design, selection and erection, International Electrotechnical Commission, Geneva, Switzerland [4] ANSI/API RP 505, Recommended Practice for Classification of Locations for Electrical Installations at Petroleum Facilities Classified as Class I, Zone 0, Zone 1, and Zone 2, American Petroleum Institute, Washington, DC [5] EI 15 (formerly referred to as IP 15) Model code of safe practice Part 15: Area classification code for installations handling flammable fluids, Energy Institute, London, UK [6] ANSI/API RP 500, Recommended Practice for Classification of Locations for Electrical Installations at Petroleum Facilities Classified as Class I, Division 1 and Division 2, American Petroleum Institute, Washington, DC [7] ANSI/ISA ( ) Explosive Atmospheres Part 29-1: Gas detectors Performance requirements of detectors for flammable gases, International Society for Automation, Research Triangle Park, NC [8] ANSI/ISA ( ), Explosive atmospheres Part 29-2: Gas detectors Selection, installation, use and maintenance of detectors for flammable gases and oxygen, International Society for Automation, Research Triangle Park, NC [9] ANSI/NFPA 497, Recommended Practice for the Classification of Flammable Liquids, Gases, or Vapors and of Hazardous (Classified) Locations for Electrical Installations in Chemical Process Areas, National Fire Protection Association, Quincy, MA [10] IEC , Electrical Apparatus for Explosive Gas Atmospheres Part 10 Classification of Hazardous Areas, International Electrotechnical Commission, Geneva, Switzerland [11] NFPA 30, Flammable and Combustible Liquids Code, National Fire Protection Association, Quincy, MA [12] ANSI/ISA-TR Guide for Combustible Gas Detection as a Method of Protection, International Society for Automation, Research Triangle Park, NC [13] API RP14F Design, Installation, and Maintenance of Electrical Systems for Fixed and Floating Offshore Petroleum Facilities for Unclassified and Class 1, Division 1 and Division 2 Locations, American Petroleum Institute, Washington, DC 9

10 [14] EN Explosive atmospheres Explosion prevention and protection Part 1: Basic concepts and methodology, CENELEC, Brussels, Belgium [15] AS 2865 Safe Working in a Confined Space, Safe Work Australia [16] IEC (Parts 1 to 7) Functional Safety of Electrical/Electronic/Programmable Electronic Safety-Related Systems, International Electrotechnical Commission, Geneva, Switzerland [17] IEC , Explosive atmospheres Part 29-3: Gas detectors Guidance on functional safety of fixed gas detection systems, International Electrotechnical Commission, Geneva, Switzerland [18] ISA TR , Guidance on the Evaluation of Fire and Gas System Effectiveness, International Society for Automation, Research Triangle Park, NC [19] CSA C22.2 No.152 Combustible Gas Detection Instruments, Canadian Standards Association, Toronto, ON IX. VITAE Allan Bozek, P.Eng., MBA, graduated from the University of Waterloo in 1986 with BSc in Systems Design Engineering and a MBA from the University of Calgary in He is a Principal with EngWorks Inc. providing hazardous location consulting services to industry. He is a registered professional engineer in the provinces of Alberta, Ontario and British Columbia, Canada and has been a member of the IEEE since Allan s areas of expertise include hazardous area classification design, application of hazardous location codes and standards to facilities and the design and certification of equipment in hazardous locations. Tim Driscoll, P.Eng. (BSc.EE 76) received his Bachelor of Science, Electrical Engineering degree in 1976 from the University of Calgary, Calgary, Alberta, Canada. Since graduation he has been employed at Shell Canada in various positions including control engineering, project management and electrical engineering. Responsibilities included electrical engineering support for all Shell Canada s facilities in the areas of operations, maintenance, safety, energy and capital projects. Currently retired from Shell, he runs a small engineering firm in Calgary, OBIEC Consulting. He has co-authored several papers and presentations at the IEEE PCIC Conference, the IEEE PCIC Europe Conference, the IEEE Electrical Safety, Technical & Mega Projects Workshop and the IEEE Electrical Safety Workshop. He is a member of the Association of Professional Engineers and Geoscientists of Alberta. He is also chair of the Canadian Electrical Code section 62 and the Technical Content Subcommittee on the CSA Objective Based Industrial Electrical Code, and participates on several other Alberta Codes, and CSA, API, IEEE and IEC standards. Vince Rowe (BSc. EE 60) received his Electrical Engineering Degree in 1960 from the University of Manitoba. Since graduation he has worked for an Electrical Utility, an ammonia production facility, two potash mines and a nickel mine before joining Shell Canada in Calgary in He retired as Head Electrical Engineer in 1994 and has since worked part time as a consultant and is currently a semi-active Partner in Marex Canada Limited. He is past chairman of section 18 of the Canadian Electrical Code and is currently a member of sections 18, 20, 56, 62 and 86. He is an IEEE Life Senior Member. Jon D. Miller received his BSEE degree from the University of Minnesota in 1989 and MBA degree from the University of St. Thomas in Since BSEE graduation he has been employed by Rosemount, ITS, and Det- Tronics working in the field of hazardous locations. His current employment at Det-Tronics since 1996 has included working in the field of gas detection. He is Chairman for the US Gas Detection Standards Development Committees for UL STP60079 TG79-29 (Combustible) and UL STP9200 (Toxic), and he is Convener for the International Gas Detection Standards Development Committees for IEC TC31 MT (Combustible) and IEC TC31 JWG45 (Toxic). He is also a member of several ISA, UL, and IEC committees responsible for hazardous location electrical equipment. William G. Lawrence, P.E. received his BSEE degree from the Pennsylvania State University in 1978, working initially in the high-voltage and product safety fields. Since 1983, he has been employed by FM Approvals (and its predecessor Factory Mutual Research Corporation) and working in the field of hazardous locations. He is the Technical Advisor for the US Technical Advisory Group for IEC TC31 (responsible for the IEC standards for Explosive Atmosphere s) and is the International Convener for TC31/WG22 (responsible for IEC ) He is also a member of several NFPA, ISA, and UL committees responsible for hazardous location electrical equipment. He is a registered Professional Engineer in the Commonwealth of Massachusetts and is a senior member of IEEE. 10