Safety of Analyser Houses
13 January 2016
The petrochemical and chemical industries need to control continuous or batch processes to ensure safety and environmental compliance and to optimise operations for maximum profitability. On-line process analysers are a valuable tool to help achieve these objectives and in this article, Peter Pergande, Chairman of the EEMUA On-Line Analyser Committee, looks at safety considerations when using these facilities.
Internal view of walk-in analyser house - Image: Thyson Technology
On-line process analysers can provide frequent continuous or cyclic analytical information on key process properties which allows on-line optimisation of the plant and rapid identification and correction of off-spec or undesirable operating conditions. Analyser precision is also usually better than laboratory reference methods so closer control and targeting to product specification can be achieved using an on-line analyser.
To obtain the best performance (both precision and reliability are important) from on-line analysers, it is normally necessary to install them in an environmentally controlled environment. This usually means they should be installed in an enclosed walk-in, ventilated, temperature and humidity controlled analyser house. Several analysers are commonly grouped together in a single walk-in analyser house to reduce installation costs and to provide a single common location for connecting utilities and signal cables.
There is sometimes a misconception that walk-in analyser houses are for the convenience of the analyser maintenance technicians but except for extreme climates, they are primarily used to ensure best analyser performance to maximise the benefits from the analyser. The fact that they also provide a good environment for the analyser technician is a secondary benefit. Many process analysers are similar to or are automated versions of the equivalent laboratory analyser which would not be used or be expected to function properly with good precision in an uncontrolled environment.
However the use of enclosed walk-in analyser houses introduces some additional safety hazards to the plant which must be mitigated in the design and operation of the analyser installation. Toxic, flammable and asphyxiant process samples and utilities are brought into an enclosed space where plant personal may be present and required to work for extended periods when checking and performing maintenance on the analysers. Figure 1 (below) shows the configuration for an on-line analyser system using an analyser house.
For walk-in analyser houses, the primary risk reduction mitigation is the use of continuous forced draft ventilation to dilute any potential release of flammable, toxic or asphyxiant materials inside the analyser house where practical to below hazardous levels. To facilitate this and reduce ventilation requirements, the flows of these materials (process samples and utilities) into the house are restricted to the minimum needed for correct operation of the analysers. All necessary sample conditioning and disposal components are mounted outside the analyser house and only the small final sample flow needed by the analyser is brought inside the house.
Safety Considerations for the Design and Operation of Analyser Houses
This article focuses on hazards arising from release of toxic, asphyxiant or flammable materials inside an analyser house, but it is also important that personnel coming into contact with analyser systems do not suffer injury from additional hazards such as burns, electric shock and cuts from exposed sharp edges.
A number of statutory requirements pertaining to safe design and practices are in force in many countries e.g. the ATEX and PED Directives in Europe. There are also specific standards relating to the safety of analyser systems e.g. IEC 61825 Safety of Analyser Houses. See the applicable standards section at the end of the article for a fuller list of relevant standards and publications.
1. Toxic and Asphyxiant Hazards
When an analyser house system is designed, the toxicity of the materials being handled inside the house must be considered so that under the worst fault conditions i.e. sudden rupture or failure of one of the sample or utility lines entering the analyser house, the legal short-term exposure limits of the substances in the atmosphere are not exceeded. The design must also ensure that under the worst fault conditions that the atmosphere inside the house cannot reach asphyxiating levels.
Analyser houses into which toxic or asphyxiant materials are introduced should therefore be provided with ventilation systems designed to ensure that the relevant occupational exposure limit or dangerous asphyxiant levels for these materials is not exceeded in the general atmosphere inside the house under normal or any likely fault condition.
A warning sign should also be provided on entry doors indicating the possible presence of a toxic or asphyxiating hazard within the house and that personnel should not enter the house without suitable protection if any gas detection alarms or the a ventilation air flow alarm are active based on alarm indications provided externally by the analyser house entry door(s).
It should be noted many flammable materials are also toxic and asphyxiant and will reach the short-term toxic exposure limit or asphyxiant levels long before the lower flammable limit value is reached. Therefore the toxic and asphyxiant hazard from flammable materials entering the analyser house must be taken into consideration when designing the analyser house ventilation systems as well as the flammable hazard.
2. Flammable Vapour Hazards
The same regulations apply to the installation of analysers as with installation of any other electrical equipment in electrically classified hazardous areas and the appropriate relevant legal regulations and standards should be followed for the country in which the equipment is being installed. These documents will define the appropriate area classification inside the analyser house. All equipment used in these housings should be suitable for use in accordance with the area classification.
Ventilation System Design
The analyser house ventilation should be provided by forced draft ventilation fans. The design basis is to provide sufficient flow of non-hazardous ventilation air to dilute any source of release (normal or abnormal) within the general atmosphere inside the house to below hazardous levels and to prevent egress of hazardous material in the area outside the analyser house from entering the house by providing a small overpressure inside the house.
Figure 1 - Typical analyser house installation
The required forced ventilation flow can be can be minimised by restricting the likely size of an internal release in the analyser house by:
• Minimising and restricting the quantity of potentially hazardous materials entering the housing to the minimum required for the correct operation of the analysers.
• Installing all other sample conditioning, validation/calibration materials and sample disposal facilities outside the analyser house.
• Reducing the number of joints in sample lines
• Using the lowest possible operating pressures
The flow of material on the sudden rupture or failure of a sample or utility line entering the house is limited by either a fixed restriction in the line before it enters the house e.g. flow restriction orifice or maximum cV of a regulator or final flow control device of the flow into the house or by the use of excess flow cut-off valves.
2. Ventilation Flow Requirements
The ventilation air flowrate must be sufficient to:
• Dilute to below the short term occupational exposure limit any toxic gases/vapours introduced into the house by accidental rupture of any one sample or utility line within the house.
• Dilute to below the asphyxiant level any asphyxiant gases/vapours introduced into the house by accidental rupture of any one sample or utility line within the house.
• Dilute escaping vapours from the rupture or failure of the most hazardous sample or service line to less than 20% LEL around any potential sources means of ignition.
The calculations must also cover any lines containing volatile liquid materials entering the house which will potentially vaporise if the line is ruptured. The equivalent vapour rate is based on the vapour pressure and temperature of the material in the line.
The required ventilation flow must be calculated for every line entering the analyser house which contains toxic, asphyxiant and/or flammable materials (and each hazard type for each individual line) based on the maximum restricted flow of the material into the house. The final minimum ventilation air flow design is then that required for the worst case. The required ventilation air flow is only based on the failure of a single line inside the house as the probability of the simultaneous rupture or failure of two separate lines is very low.
The referenced publications and standards give examples and further details on of how to make these calculations.
In practice, a minimum flow (normally 6-10 air changes pare hour) is specified for internal environmental reasons (temperature control and heat dissipation) unless a larger flow is required based on the internal release calculations.
The ventilation system must also be designed to keep toxic gases, asphyxiant and flammable vapours either lighter or heavier than air outside the house from entering the house. It is normal practice to operate at an internal overpressure relative to the atmosphere of 0.2 to 0.5 mbar.
3. Air Intake System
The air intake should be through a ducting stack provided with a weather protection cowl (rain hood). The air source should be from a non-hazardous area where corrosive or toxic gases do not occur. The design of the intake duct and the diameter of the stack should be sized to limit air velocity to a maximum of 15 m/s. Any ducting to the analyser house which passes through hazardous areas should be leak-tight. Ducting through Zone 1 hazardous areas should be avoided where possible.
In some instances, the distance to a non-classified area may be excessive and in these cases air may be drawn from a Zone 2 area. In this case all analyser house equipment must be certified as suitable for Zone 2 as a minimum.
4. Ventilation Fan Requirements
The ventilation should be provided by centrifugal or axial fans. The ventilation fans should be mounted outside the analyser house integral with the air conditioning system if used.
Dual ventilation fans each sized for the minimum flow with auto-changeover should be used to ensure continued dilution of leaks of flammable, toxic and asphyxiant materials in the event of failure of one fan and to minimise trips of non-certified equipment on ventilation failure. To facilitate maintenance, fans should be fitted in parallel with non-return valves and have suitable means of mechanical isolation. Power supplies to the fan motors should also be independent of each other.
External view - Image:Thyson Technology
5. Ventilation Air Flow Distribution
The direction of flow of air within the analyser house should be such as to ensure a movement throughout the housing and around all equipment installed inside irrespective of wind direction and strength. For larger houses, internal air ducting should be used to evenly distribute the air flow inside the house. The position and number of exit ports is determined by the nature of the hazardous materials handled inside the house (e.g. heavier or lighter than air, toxicity and location of potential sources of release). The air exit ports should be louvers or weight-balanced flaps sized to provide the required internal overpressure.
Figure 3 shows the external view of a typical walk-in analyser house. The air intake ducting can be seen at the back on top of the house. This would be extended to a non-hazardous are when installed in the plant. The stainless steel sample conditioning system cabinets and utility distribution headers and gas bottles are located outside the house to minimise the amount of hazardous material inside the house. Figure 4 shows the combined ventilation fan/air conditioning system at the other end of the house.
Safety Monitors and Alarms
The primary safeguard for the analyser house is the correct ventilation air flow and certification of the electrical equipment to the required hazardous area classification. Alarms should be provided to indicate failure of the analyser ventilation system. Flammable and toxic gas detectors should be installed inside the analyser house as an additional mitigating safeguard. The gas detecting heads should be located at key points within the analyser house.
Analysers should preferably be certified for use as a minimum in a Zone 2 hazardous area. This is to allow continued operation for short periods even when ventilation and combustible gas alarms exist to minimise disruption to process operations though loss of analyser control or information. If any general purpose non-certified electrical equipment is used inside an analyser house into which flammable material is introduced or if the analyser house is located in a Zone 2 hazardous area then this must be shutdown via a suitable shutdown/logic system on ventilation system failure or flammable gas detection inside the house.
The analyser house shutdown/alarm logic systems is usually mounted within the analyser house and incorporates all alarm interfaces, logic indicators, key operated alarm/shutdown override switches and equipment reset switches. Key safety alarms should also be repeated outside the analyser house entrance door and to a manned control room to allow immediate investigation and resolution of any hazardous situation inside the analyser house.
To allow continued operation during ventilation failure and/or gas detection the analyser house lighting, and equipment associated with the shutdown/alarm system including all initiators, logic and alarm indicators should be certified for use in a Zone 1 hazardous area.
Uninterrupted Power Supplies (UPS) should be used for all of the alarm and shutdown system including the gas detectors.
The following alarms/shutdowns should be provided as appropriate:
1. Ventilation Flow Failure
An independent low flow switch positioned to detect loss of air flow through the house should be used. This should be located in the ventilation air inlet ducting. A differential pressure switch across the ventilation fans should not be used to detect low ventilation air flow as it may not detect loss of flow due to a blocked air filter or an incorrect circulation of air around the fans. The flow switch should be set to indicate flow failure when flow falls below the equivalent of approx. 60-80% of required design flow. A time delay of up to 1 minute may be used to prevent spurious operation during short term disturbances and during automatic switchover for dual backup fans.
Where equipment other than that suitable for Zone 1 or Zone 2 operation i.e. general purpose is used the low flow detection shall initiate the following trip functions.
• Immediately isolate any non-certified electrical equipment
• Immediately isolate electrical wall convenience sockets in case any non-certified maintenance equipment is being used
• In the absence of flammable gas detection, isolate Zone 2 certified equipment after an optional time delay up to a maximum of 24 hours.
• For Zone 2 air-purged equipment, purge failure coincident with ventilation failure should initiate isolation of the purged equipment after an optional time delay up to a maximum of 24 hours.
Upon restoration of ventilation, power shall not be permitted to be restored to isolated equipment until at least 10 analyser house volumes of air have been exchanged. This should be automatically controlled via a delay-on timer initiated when ventilation air flow is established. Activation of a local manual reset facility will then be allowed to restore power.
Alarms for ventilation system failure or low house overpressure should be provided outside the analyser house entry of doors as well as on the shutdown logic/alarm system interface inside the house. The ventilation system failure alarm should also be part of a common hazard alarm to a manned control room location.
A visual indication of the pressure differential between the analyser house and the external atmosphere should be provided within the analyser house.
Petrochemical physical property analysers - Image: Icon Scientific
2. Flammable. Toxic and Asphyxiant (Low Oxygen) Gas Detection
Gas detectors should be calibrated and positioned according to the nature of the gases expected to be released within the house either from the analyser systems or via the ventilation system if ventilation air is taken from a Zone 2 area,
Flammable gas detectors should also initiate the following trip functions:
• Immediately isolate non-certified equipment on 20% LEL detection.
• Immediately isolate wall sockets on 20% LEL detection.
• For Zone 2 purged equipment, purge failure coincident with LEL detection should initiate isolation of the purged equipment.
The above trip functions should operate independently of the ventilation failure trip functions. Gas detection is an added safeguard and is not a substitute for ventilation failure trips other than allowing for the optional requirement for the time delay on tripping Zone 2 and purged equipment to be removed.
On removal of the hazardous conditions as indicated by the gas detection equipment (below 20% LEL), power shall not be restored to isolated equipment until at least 10 analyser house volumes of air have been exchanged. This should be automatically controlled via a delay-on timer initiated when the required ventilation air flow is established and the LEL falls below 20%. Activation of a local manual reset facility should then be used to restore power.
If toxic and asphyxiant gases are handled inside the house, toxic gas and oxygen deficiency detection should be provided. Detection of toxic gas above preset alarm or oxygen below preset limits should initiate a visual and audible alarm in the house and at a manned location elsewhere.
Alarms for each type of gas detection and for gas detector faults should be provided outside the analyser house entry of doors as well as on the shutdown logic/alarm system interface inside the house. The gas detection alarms should also be part of a common hazard alarm to a manned control room location.
Gas detection and ventilation failure should also initiate a flashing warning beacon light above each entry door and sound a horn to immediately alert plant personnel in the vicinity of and inside the analyser house to potentially hazardous conditions inside the analyser house.
Applicable Standards and Publications
The above is an overview summary of the potential hazards and mitigations for the design of analyser houses handling toxic, asphyxiant and flammable materials. There are a number of useful International Standards and Publications available that provide full details on these and other aspects of the safe design and operation of analyser systems and installations.
*BS EN 61285:2015: Industrial Process Control – Safety of Analyser Houses (also published as IEC 61285)
*EEMUA Publication 138 Edition 2: Design and Installation of On-Line Analyser Systems (also published as IEC TR 61381)
*EEMUA Publication 187 Edition 2: Analyser Systems - A Guide to Maintenance Management (also published as IEC TR 62010)
*EEMUA Publication 209 Edition 1: Guide to the Development and Implementation of On-Line Analyser Applications
Some companies find it useful to develop checklists based on the above standards and any applicable internal company practices that can be used during HAZOP reviews of new analyser system designs and for existing plant installations.
Further information and advice on the safety of analyser houses can be obtained from the EEMUA On-Line Analyser Committee. The EEMUA On-line Analyser Committee actively participates in the IEC Analysing Equipment Committee developing International Standards for on-line process analysers and systems. EEMUA is an international non-profit membership organisation representing companies that are significant purchasers and users of engineering equipment and materials of construction, helping them improve the safety, environmental and operating performance of industrial facilities in the most cost-effective way. More information on EEMUA can be found at www.eemua.org