Smoke Detection in Hazardous Areas


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Interesting editorial from Peter Mundy explaining how to protect an Hazardous Area (ATEX) with ASD systems.

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Smoke Detection in Hazardous Areas

  1. 1. Protection of hazardous areas using High Sensitivity, Early Warning Aspirating Smoke DetectionA petrochemical site. Pic courtesy of Vision Systems (Europe) Ltd a 20 hole system with equal quantitiesSUMMARY By Peter Massingberd-Mundy, of air entering each sampling point. To ost Fire Engineers, ConsultantsM and Fire Prevention Officers know what an aspirating smokedetection (ASD) system is and can M.Eng, C.Eng, M.IMechE Technical Product Manager – Europe have an effective sampling point sensi- tivity of 4%/m – which is widely con- sidered to be normal for a standard point detector – the central detectordescribe a centralised smoke detector Vision Systems – VESDA must have a Fire Alarm threshold ofwith a fan or aspirator drawing air Vision House, Mark Road 0.2%/m.samples from the protected area through Hemel Hempstead, This illustration shows how an aspi-holes in a 25mm pipe running above the HP2 7BW England rating system with multiple holes mustprotected area or across the air intake have a high sensitivity detector togrille of an Air Handling Unit (AHU). match the performance of the generic What many may not appreciate is the benefits and features that match the point detectors. However, one of theincreasing diversity of applications for price you pay. Naturally the fan or aspi- unique benefits of an aspirating systemwhich these detection systems are rator used to transport the samples into is its ability to detect lower concentra-being used and the major benefits they and along the sampling pipe is sized to tions of smoke than a normal pointcan offer. reflect the maximum pipe runs (which detector when smoke enters more than This article highlights the major can be up to 200m) but more impor- one sampling point. In this case thebenefits of high sensitivity ASD systems tantly, the sensitivity of the central smoke concentration at the centraland then concentrates on its applica- detector is key to its performance detector is less diluted. As a result,tion in high risk hazardous environ- capability. Aspirating Systems have a naturalments particularly with regard to ASD systems can have many holes in ability to detect diffused smoke – themeeting the imminent requirements of the sampling network. In order to have more diffused the smoke is the morethe ATEX Directives. confidence that each hole has at least sampling holes it enters. This is known the equivalent performance to a point as the cumulative effect.HIGH SENSITIVITY ASD SYSTEMS detector the scenario of smoke only To illustrate with figures: ConsiderIn their simplest form ASD systems are entering one hole must be considered. the 20 hole system. If 2%/m smokeeasy to understand; they continually In this case all other holes will be draw- enters 2 holes then the detector withdraw samples of air from the equip- ing clean samples and the sensitivity of its sensitivity set at 0.2%/m will declarement or area requiring protection and the central detector must be sufficient an alarm. If 1%/m smoke enters 4 holesassess these samples for the presence of to account for this dilution. the 0.2%/m detector will similarlysmoke. Many variations exist with Illustrating this with figures: consider declare an alarm. And so on. INTERNATIONAL FIRE PROTECTION 3
  2. 2. control. Depending on the particular application they may be required to prepare for the full alarm condition. For example, procedures in an office might direct them to close windows, shut down PCs, and warn other person- nel in the area that a fire Alarm is imminent. The underlying message is “take action”. In an IT or industrial application the Action alarm may start to initiate back up procedures or shut down machinery. After these vital early warnings (that are only available on top range ASD detectors) there is the Fire condition. This is generally connected as an Alarm input to the central panel and is often considered to be equivalent to the standard point detector response. On some ASD systems that have a wide Fire Growth Curve. sensitivity range a fourth alarm level is While this cumulative effect is an “thermal event” first time but after a also available that can be interfaced important feature of all ASD systems it number of incidences the warning into automatic suppressions systems. does not in itself provide the early might be traced, say, to an electric Such detectors can therefore provide detection capabilities on which the heater. Depending on the particular multiple alarm outputs that can initiate good reputation of high sensitivity ASD site the source of the alarm can be the appropriate response. systems is founded. This reputation is removed or the alert threshold can built on the utilisation of very sensitive be permanently adjusted to compen- APPLICATION OF ASD IN HIGH RISK AREAS central detectors to give alarm warn- sate for this regular occurrence. The reputation of High sensitivity ASD ings that are significantly earlier than If the smoke condition continues to systems originates in the protection of conventional detectors – even when escalate then a second stage warning is the high risk, high value environments smoke is only entering one hole. It generated – this is the Action Alarm. of Telecom switches and Electronic should be noted that the discussion Generally this is configured to raise a Data processing areas. These areas typi- and illustrations thus far relate to the pre-alarm warning on the central panel, cally have large Air Handling Units for ALARM condition. For true early warn- the CIE (Control and Indicating Equip- cooling the electronic equipment and, ing what matters is not that the ASD ment). Local personnel might be due to the dilution of any smoke by the system can match and in many cases expected to inform the relevant author- large airflows; effective smoke detection exceed the performance of a point ity if they have identified the source is only practical using high sensitivity detector – what really matters is that it and ask for the unit to be isolated systems. Over the years, the technology can provide a warning as soon as any temporarily until the cause is under has been applied successfully in many abnormal conditions are discernable. In this context, Early Warning should not be confused with Pre-alarm. Pre- alarm provides an indication that an ALARM condition is approaching. Early Warning provides an indication that normal conditions are no longer present – something unusual is happening. This is best illustrated by considering the multiple alarm outputs available on the top range ASD detectors. MULTIPLE ALARM THRESHOLDS First level Early Warning alarm Alerts key personnel to a potential problem – perhaps the on site security staff are informed of the condition and/or the local room occupants are alerted to the unusual conditions. The response to this Alert warning might be that local personnel stop an unapproved activity (e.g. soldering) thereby avoiding an unnecessary evacuation of the premises. They might not find the source of the Hazardous Area Detectors. Pic courtesy of TEPG4 INTERNATIONAL FIRE PROTECTION
  3. 3. ATEX 95, the “equipment” Directive (94/9/EC) specifies the Essential Health and Safety Requirements for equipment that may be used in an explosive atmosphere. It places requirement on manufacturers of such equipment both in terms of performance and product quality, which was not included in the majority of the national requirements that preceded it. The essential safety requirements for equipment covered by the directive include requirements for the selection of materials, marking, user instructions and design and construction. Typically manufacturers will use the appropriate European and/or international standards to demonstrate their compliance with many of these essential requirements. The directive defines a number of categories, which correspond to the classification of the hazardous areas in which the equipment is to be deployed. However, it is important to realise that there is no direct relationship defined in the directive between the area zoning and the category of equipment, which must be used. The full list of ATEX categories are as follows (there is a sub-division which separates equipment for use in mines from all other equipment) where the VESDA Exd product image. Pic courtesy final column provides a link to the zoning classification. of Vision Systems (Europe) Ltd Group Category Locations Comments Zone Classification common other areas where business continuity is in Europe critical and/or where risks and potential (Based on IEC 60079-10) losses are high. Examples include ware- I M1 Mines capable of functioning housing, cold stores, industrial processes, safely in the presence n/a cable tunnels, production machinery, of an explosive atmosphere wet benches and prison cells. One specific example of the growing I M2 Mines must be de-energised when an explosive n/a adoption of ASD systems is in haz- atmosphere is present ardous areas. Generally these areas by II 1 Other protection assured in Explosive atmosphere their very nature fall into the category the event of two Zone 0 will be present of high risk and they are often critical independent failures continuously to the business continuity. PetroChem II 2 Other protection assured Explosive atmosphere plants have many hazardous areas; in the event of will be present some foreseeable failures Zone 1 of the time (e.g. due to warehouses storing solvents and alco- hol also have classified areas, which operational reasons) require specialist fire detection prod- II 3 Other protection assured Explosive atmosphere during normal Zone 2 may be present (e.g. ucts. Essentially these are areas where operation in the event of a fault) explosive mixtures of gases or vapors can accumulate which if ignited would cause an explosion. Equipment intended for use in the higher risk categories is required to under- go independent certification by a Notified Body (an independent and suitable ASD IN HAZARDOUS AREAS authorized certification body). Equipment for lower risk categories may be self- Within Europe there have historically certified by the manufacturer in order to affix a CE mark to the product. been different approaches to Ex envi- The actual relationship between the category and the certification require- ronments within different member ments is as follows: states. However, with the imminent mandatory CE marking of Electrical Category Requirement equipment installed in hazardous areas 1 Product certification and review of quality control system by under the ATEX directive there is some 2 Notified Body required harmonization and renewed emphasis (electrical) on the suitability and application of 2 (non electrical) Self-certification by manufacturer permitted supported by electrical equipment in such areas. Declaration of Conformity and Technical File There are many web pages devoted to 3 the full explanation of the ATEX Directives, of which there are two, as summarized (see tables, right and over). l Intrinsically safe (Exi) equipment l Pressurized apparatus (Exp) prevents In summary equipment installed in limits the electrical energy in the explosive mixtures reaching the hazardous areas must be CE marked devices installed in the area so that potential source of ignition by and have appropriate Ex rating. There no significant spark or overheating means of a positive pressure within are a number of alternative approaches can occur, which may ignite an the electrical enclosure. to Ex ratings (ref. EN50014). explosive atmosphere.6 INTERNATIONAL FIRE PROTECTION
  4. 4. common are intrinsically safe smoke hazardous area (as depicted in Scheme and heat detectors. Flame detectors A) with a flame arrestor (see illustra- and linear systems such as pressurized tion) to protect the hazardous area pipe or temperature cable/fibre are also from possible ignition through the available for such environments. How- pipe. There is clearly a problem with ever, none of these technologies can this arrangement in that the ASD sys- provide the Early Warning smoke tem is exhausting the hazard into the detection capability of an ASD system. safe area. To overcome this, the exhaust When deploying ASD systems for the can be piped back to the hazardous protection of hazardous areas some area through another flame arrestor installers have previously tried to (Scheme B). However, serious consider- exploit the “remote sensing” capability ation must be given to the failureFlame arrester. Pic courtesy of AMAL of such systems by installing the detec- mode of this arrangement in the event tor in a safe environment and installing of an explosion. In accordance withl Flameproof enclosures (Exd) contain the electrically passive pipework in the ATEX 137, the behavior of the ASD ignition sources in such as way that any ignition of the hazard inside the enclosure will not be transmitted ATEX 137, the “user” Directive (1999/92/EC), is specifically to the atmosphere outside the concerned with worker safety and places requirements on employers whose enclosure. staff may work in an explosive atmospherel Increased Safety (Exe) use mechani- It places a number of requirements on employers. The main ones are to: cal construction safeguards to l assess explosion risks and draw up an explosion protection document ensure that the apparatus does not (Articles 4 and 8); contain normally arching or sparking l prevent and provide protection against explosions (Article 3); devices, or hot surfaces that might l implement measures to ensure work in explosive atmospheres can be cause ignition.l Oil immersion (Exo), Powder filling carried out safely (Article 5); (Exq) and encapsulation (Exm) are l coordinate the implementation of health and safety measures with other other approaches that may be used. employers (Article 6); l classify, zone and mark areas where explosive atmospheres may occur Fire detection products generally use (Article 7).either Exi or Exd protection. Most
  5. 5. does not become a hazard. In Scheme D the ASD is mounted in the hazardous area and a remote display is provided to provide information and interfaces within the safe area. Clearly Scheme D is the most appropriate but this does not preclude the use of other schemes where appropriate risk assessments are undertaken and recorded. When designing the fire protection arrangements for hazardous areas the benefits of installing high sensi- tivity ASD systems are clear. The ability for these systems to provide early warning ensure the best possible protection for these high risk, high value areas. However, careful consideration of the explo- sion risks is essential and simply installing the ASD system in a safe area with flame arrestors in theenclosure must be considered in rela- ASD detector in a flameproof enclosure pipework to the hazard may betion to the safety of personnel (and and provide flame arrestors directly into flawed. The preferred approach isequipment) in the vicinity. Furthermore, the enclosure (as shown in Schemes to install the ASD system in an Exdthe integrity of the pipework must be C&D). In scheme C the detector is enclosure, which can be mountedgiven due consideration in the event of mounted in the safe area, which may in the safe area or the hazardousflame front travelling from the detector be possible as long as the leak rate of area depending on the particulartowards the flame arrestors. the Exd enclosure is sufficiently low to site arrangements. The best solution is to enclose the ensure that the surrounding atmosphere