analyser house design

HAZARDOUS AREAS AND ANALYSERS

1. Abstract

Subject: Hazardous Area Handling of Analysers and Analyser Housings.

Analysers are often required to be in Hazardous Areas. There are 3 common approaches to meeting Hazardous Area protection standards for analysers. These are to have the analyser itself completely protected with its own certification, to have the analyser housed in a cabinet where the cabinet supplies the necessary protection, or to have the analyser along with other GP equipment housed in an Analyser House with full protection of everything inside the house to the "current best practice" which is best described in IEC1285. This paper presents arguments for all 3 approaches and discusses the common protection methods of individual components and the common mistakes made in the industry. An assessment of benefits obtained by selecting more expensive protection techniques is essential. Feasibility analyses on the instruments' values being protected are also very useful in making a selection. Being a specialty discipline analysers and their peripheral supporting systems need careful design. They cannot be handled in the same manner as mainstream instrumentation because they are reticulating the Hazardous gas within their own battery limits. AS2380.4 specifically excludes rooms with a release of hazardous gas and references the IEC79-16 report which has since been superceded by the Standard IEC1285.

By : Paul Hackett, Measurementation

2. Table of Contents 3. 1. Abstract

2. Table of Contents

3. Aim

4. Background

4.1 Analysers

5. Method 1. Analyser House Protection

5.1 General

5.2 Personnel Protection

5.3 Area Classification of Analyser Houses

5.4 IEC 1285 - Industrial-Process Control - Safety of Analyser Houses

5.5 Benefits / Disadvantages

6. Method 2. Analyser individual protection


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7. Method 3. Cabinet Protection

7.1 General

7.2 Method

7.3 General Requirements


8. Common Mistakes

8.1 Over design

8.2 Temperature Classification

8.3 Modifications

8.4 Other equipment

9. Summary

10. Conclusion

11. References

3. Aim

The aim of this paper is to discuss the three common approaches for the meeting of the hazardous area requirements for analysers. These are to have the analyser itself completely protected with its own certification, to have the analyser housed in a cabinet where the cabinet supplies the necessary protection, or to have the analyser along with other GP equipment housed in an Analyser House with full protection of everything inside the house to the "current best practice" which is best described in IEC1285. Australian standard AS2380.4 specifically excludes rooms with a release of hazardous gas.

4. Background 1. Analysers

Process analyser are used to automatically and continuously measure the characteristics of a continuously flowing process stream. The sample is usually extracted from the process, passed through a sample conditioning system and fed into the analyser. Analysers in simple cases can be directly mounted onto the process stream and no sample extraction would be required.

Figure 1. Shows the components for a standard sample extraction system. Typical analysers using this system are :- Gas Chromatograph, Mass Spectrometer, Physical property analysers (i.e. cloud, freeze point), NIR & IR etc



Figure 1 - Typical P&ID for an analyser system

To protect analysers there are three common approaches or combinations thereof.

1. Purge an analyser house to make a general purpose area internal. 2. Protect all equipment using individual certifications. i.e. Exd, Exe, Exp 3. House equipment within a Exp cabinet.

The benefits of each approach are discussed in the following sections.

5. Method 1 - Analyser House Protection

Australian standard AS2380.4 - Pressurised rooms or pressurised enclosures, covers the purging requirements for cabinets and rooms. The standard covers the use of cabinets with internal hazardous releases but it does not cover the situation in terms of rooms (e.g. analyser houses). For analyser houses reference is made to report IEC 79-16 which has since been superseded by the standard IEC1285.

1. General

An analyser house as distinct from a laboratory can be defined as an enclosed building or part of a building containing process analysers where the process stream enters the building rather than grab samples being brought in for analysis by an operator.

Analysers can be mounted separately or grouped together. Each analyser should be considered separately to determine its best location within the plant. An analyser house may be considered necessary for the ease of maintenance, protection of analysers and equipment, protection of personnel and safety. In general analysers within an analyser house will be more reliable and have less down time.

The benefits of the additional expense of an analyser house need to be carefully considered. A simple analyser used for trending will probably not warrant an analyser house while an analyser used for process control may have substantial dollar penalties for down time and will warrant the use of an analyser house. In general most cases will not be so clear cut.



The selection of an analyser house should consider :-

1. Area classification of the analyser house location 2. Personnel and equipment protection and accessibility requirements for maintenance. 3. Ambient conditions 4. Environmental conditions required by the analyser manufacturer. i.e. hazardous area, temperature, humidity,

dust, snow etc

Figure 2 - Typical Analyser House under construction

2. Personnel Protection

The main difference between the AS2380.4 Australian standard on pressurized cabinets and the international standard IEC 1285 on analyser houses is that that for an analyser room personnel protection must be considered for analyser houses.

The vapour levels that are dangerous to personnel (toxicity & asphyxiation) can be lower than the levels that are dangerous to create an explosive mixture.

The design of an analyser house must address personnel protection for both toxicity and asphyxiation and at the same time as addressing the Hazardous Area protection.

The IEC 1285 standard requires that an analyser house have additional safeguards to ensure that personnel are protected.

1. Purge level must be selected to provide dilution to the lowest level of either toxicity, asphyxiation or explosive limit.

2. Gas detectors must be installed to measure and warn of toxicity levels. 3. Emergency measures such as panic button or telephone must be provided.

3. Area Classification of Analyser Houses

The reason for declaring an analyser house to be in a Hazardous Area is that either an external source of release in a nearby plant may occur or an internal source inside the analyser house may be possible. This second case of a possible internal release makes the analyser house design unique when compared to the design for most other plant areas. Most other purged rooms such as control rooms do not have a reticulation of the hazardous release source.



4. IEC 1285 - Industrial-Process Control - Safety of Analyser Houses

The International standard IEC 1285 standard provides the best practice for the design and construction of analyser houses.

The essential design criteria for any hazardous installation is to identify the hazardous materials and either eliminate the releases or keep them restricted to a minimum.

IEC 1285 provides the best practice requirements for the artificial ventilation of analyser houses.

The ventilation air should preferably be non hazardous. The ventilation should then provide a minimum of five air changes per hour. The ventilation design should also ensure that all areas within the analyser house are adequately ventilated. If Zone 2 air is used then gas detectors should be installed at the air inlet point and purging should be discontinued when a value of 20% LEL is exceeded.

The flow rates should also ensure that with a hazardous leak the atmosphere within the house is kept below the 20%LEL.

The pressure within the analyser house shall be maintained at a level high enough to keep any hazardous materials out of the house. The standard recommends a level of 25 to 50 Pa at a rate of five air changes per hour.

In case of ventilation failure any non-hazardous rated equipment shall be immediately isolated. The use of gas detectors may allow the non-hazardous equipment to have a delay before being switched off while the levels are below 20% LEL.

Ventilation failure is not defined within the standard. For practical purposes this would be the drop in air flow to below that required to maintain purging of the house combined with low analyser house pressure. The use of the analyser house pressure by itself can be a misleading measure of purge failure. Consider that the HVAC filters are blocking and the amount of air flow is reduced. The weighted barometric louvers on the house may well be able to maintain the pressure within the house even though the amount of purge air has fallen to below that required.

The design of the analyser house must ensure that an explosive atmosphere cannot be created by the reticulated hazardous gases. This can be achieved by restricting the hazardous gases introduced (flow restrictors or high flow shut off valves) and / or the ventilation systems can maintain the flammable gases to below 20% LEL. All areas within the house need to be adequately ventilated to ensure that the levels of hazardous gases are below 20% LEL.

The purging system shall also ensure that all of the possible hazardous gases can exit the house. The most widely used method is to use weighted louvers at both high and low points on the wall of the building. At least half of the upper and lower exit louvers should be operating under any abnormal wind conditions. All the usual protection should be employed to guard against the blockage of the vents from insects, vermin, dust, snow, debris etc.

The Australian standard for pressurized rooms that do not contain an internal source of release requires a velocity out all openings to the room (except air locks) to be 0.3 m/sec. This same requirement is not part of the international standard for analyser houses.

5. Benefits / Disadvantages

The benefits of creating a General Purpose area in which all types of equipment can be located and worked on



easily can not be over stated. A single purge system can be implemented to enable multiple analysers to be operated. Other ancillary equipment to the analyser such as printers and plotters can also be used.

The operation and the maintenance of the analysers is greatly enhanced as not only can the analyser be operated / maintained in climate controlled comfort but this can be done without disabling any hazardous protection requirements. ( e.g. opening Exd box)

Many types of analysers can not be obtained with appropriate certification and therefore could not be used in a Hazardous Area without special protection.

Analysers that have an individual certification such as Exd can not be easily worked on in a Hazardous Area. Hot work permits with gas monitoring are required before any live equipment can be opened and worked on. With a purged analyser house the analysers can be maintained with greater ease.

Environmental protection for both the analysers and the operators is a major benefit of using an analyser house. The ambient conditions on a site generally vary significantly. The analyser electronics, mechanics, optics or other components may not be able to operate within its specification with such a large ambient variation. If an analyser house is fitted with a HVAC system then the variation in humidity and temperature that the analyser is subjected to can be drastically reduced. In many cases the repeatability of the analyser is directly related to the stability of its environment.

To increase the reliability of the analyser house a dual purge system can be installed. In this way the loss of one purge fan will not cause the general purpose equipment within the house to be disconnected. Dual power feeds also increase the reliability of the analyser house.

Note that there are disadvantages of using a fully purged analyser house. These include :-

1. common mode of failure for all analysers. If the purging fails for instance then all General Purpose equipment must be disconnected. Electrical supply is the same to all analysers. These can be minimised by design if required.

2. expense of alarming, shutdown and control of the purging system if this is not required for personnel protection.

3. usually most expensive option. 4. may cause sample line lengths to be longer than necessary with inherently longer system response times.



Figure 3 - Inside typical analyser house.

6. Method 2 - Analyser individual protection

The two most common methods of protecting an individual analyser for operation in Zone 1 or 2 areas is Exd (Explosion protection) or Exp (Purge protection).

In a typical Gas Chromatograph or similar device three forms of protection or may be used. The oven and electronics may be protected by purging, the power supply entering the unit and controlling the purging may be Exd and any pressure transmitter may be Exi or Exe.

Analysers with individual protection are not always available. Many analysers are not available with appropriate certification. In this case the analyser will either have to be cabinet mounted ( that is certified ) or housed within a purged analyser house that meets certification requirements.

Figure 4 - Exd rated analyser house within a climate controlled cabinet.


The benefits of having an analyser that is already certified is one of reliability and cost. If the analyser can perform adequately in the general environment and it is not required to be in an analyser house then this will be an economical solution. The analyser may also be more reliable than one that is in a purged enclosure due to the reliance on the purging system.

A certified analyser can be positioned as close as possible to the sample point. This will reduce the system response time and the need to transport sample through other parts of the plant.

The use of a certified analyser within an analyser house can also be an added advantage in two ways.



The first is that the analyser may have greater availability. On loss of purge to an analyser house all non-hazardous equipment must be shut off. If the analyser is itself Hazardous Area rated for operation then it can continue to operate.

The second is the reduced purging requirements for the analyser house itself. If all equipment in the analyser house is appropriately rated then the analyser house does not need to be of purged design for the Hazardous Area reasons.(i.e. explosive gases) It should also be noted that house will create an enclosed area and purging may still be required for protection of the operators due to toxicity and asphyxiation.

Once again there are disadvantages in using individual analyser certification's. These are :-

1. reduced ease of maintenance and operation. 2. additional expense per analyser. 3. IP ratings may be required as extras on the analyser.

7. Method 3 - Cabinet Protection

1. General

An analyser cabinet can be defined as a cabinet or enclosure that contains one or more analyser where the process stream enters the cabinet but personnel entry is not possible. The cabinet may be housed under a shelter or similar structure. Typically a three sided shelter would be used for environmental protection for the operators or alternatively the analyser and cabinet would be located within or beside an analyser house or other structure.

Analysers are generally housed within a cabinet for either environmental considerations (i.e. keep cool) and / or Hazardous Area considerations. In terms of Hazardous Area considerations the analyser can be used in a Hazardous Area when the unit itself is not rated or certified for this duty if the cabinet is designed for this Hazardous Area.

2. Method

Analyser cabinets are often employed as they can convert a non hazardous area rated analyser into a hazardous area suitable analyser.

Australian standard AS2380.4 - Pressurized rooms or pressurized enclosures, covers the purging requirements for cabinets. This standard covers the use of cabinets and rooms intended for Zone 1 & 2 explosive atmospheres. It also covers the uses of cabinets with internal hazardous releases.

The main difference in the design in cabinets that contain an internal source of release is that the purging system additionally be designed to dilute any gases from releases whether they are normal or abnormal..

The following is a list of the main features of a purged cabinet. The list is not exhaustive and the reader is advised to consult the relevant standards for the complete requirements.

3. General Requirements

TEMPERATURE RATING

The temperature classification of the cabinet is the higher of the



1. external surface temperature of the enclosure 2. or the maximum temperatures of the internal equipment that remain energised when the purging system fails.

PRESSURISATION

1. 25 Pa for rooms and 50 Pa for cabinets pressure to be maintained above the external pressure. This should be ensured at all points within the cabinet.

2. For cabinets with an internal source of release a minimum pressure of 50Pa is required. 3. Distribution of the purge gas is required to ensure no pockets of hazardous material remain. 4. Purging is to be completed before electrical supply is switched on.

CONTINUOS DILUTION

1. Continuous dilution is required for cabinets containing possible source of release. 2. The cabinet design is to ensure mixing in all areas of the cabinet. 3. The dilution system should ensure that the gases within the cabinet are kept below the 25%LEL

a. Alarm / Shutdown

AS2380.4 defines the requirements for either alarming or shutdown of the cabinet and the use of inert purge gases in the cabinet. The requirement for either shutdown or alarming is dependant on the Zone that the cabinet is within, the extent of the possible leak within the cabinet (limited, unlimited) and whether the equipment within the cabinet is Zone 2 rated.

The use of inert protective dilution gas is required when an un-limited release of hazardous material is possible within a cabinet.

As a general rule if the cabinet is in a Zone 2 area then on loss of purge an alarm only is required. For a Zone 1 area the power must be disconnected. For unlimited releases of hazardous gas the use of an inert purge gas is required.

b. Cabinet Outlet

The outlet from the cabinet purging system will need to be designed such that in the worst case with the largest leaks the outlet gases do not alter the area classification of the surrounding area.

The outlet of the cabinet may then need to be tubed to a controlled point.

The outlet of the cabinet also needs to be protected against the flow of external air into the cabinet and ejection of any sparks or incandescent particles into the surrounding environment.


There are many analysers that are not available as a certified piece of equipment without a large price increase. By using a non-hazardous rated analyser in a purged enclosure the user can not only use an analyser in a situation where they may not other wise have been able to but at a lower cost. If more than one analyser is contained within the purged enclosure then greater cost benefit can be realised.

The cabinet can be fitted with environmental conditioning equipment such as a heater or air conditioner. This will give it the flexibility to allow the analysers to operate in greater extremes of weather. This of course will not help the operators or maintenance crew.



Once again there are disadvantages of using purged enclosures for analysers. These are :-

1. reduced ease of maintenance 2. reduced ease of operation. 3. high air usage and a resultant loss of protection upon loss of purge air. 4. lack of comfort for operators and maintenance technicians.

8. Common Mistakes

1. Over design

The over design of analyser houses is prevalent when one chooses the most stringent requirements from a number of different sources and brings them together and used in a single specification. The idea of course is to make the analyser house "safer". Unfortunately it usually only adds to the cost without any benefit in safety.

A typical example of over design is the use of high purge rates. The current best practice is for minimum of 5 house volume air changes per hour minimum. (toxicity, asphyxiation, LEL allowing) Many designs require twelve air changes per hour. As rate of air change increase so does the cost of the purge fan, A/C unit, filters and duct work. This can have a considerable affect on the cost. If the analyser house purge rate can be five changes per hour to keep the toxicity, asphyxiation and LEL levels below acceptable limits then to design a purge rate with more than twice that purge rate is simply wasting money.

The IEC 1285 has brought together all analyser house requirements and put them together into a standard that provides a high level of safety at the same time as being cost effective.

2. Temperature Classification

A piece of Exd equipment has a specified ambient temperature classification. The standard AS 2381.2 Flameproof enclosures d states "Electrical equipment with type of protection d is designed for use in an ambient temperature range between -20OC and +40OC, unless otherwise marked." Many ambient conditions vary from this particularly ambient conditions above +40OC. This can be overcome by using an analyser house or a cabinet with a cooling system.

3. Modifications

An analyser house will be designed for a particular purpose with certain hazardous gases at particular flow rates. If at a latter date another analyser is added to the house with a new gas to be analysed then this may alter the hazardous area rating within the house or change the purge requirements.

An example of this is the addition of a gas chromatograph that uses hydrogen carrier gas altering the area classification within the house on loss of purge from IIB to IIC. The installation of the new analyser may also result in an area that is not well purged and potentially could build up explosive gases faster than the rest of the analyser house.

4. Other equipment

An analyser house requires many other pieces of equipment to be installed for personnel safety and to be able to operate the analyser house. Typical mistakes are

a. not enough lighting



b. external alarms can not be heard inside house. c. air conditioning cannot handle load. d. not enough visible warnings on conditions of house before entry. e. non hazardous equipment not disconnected on loss of purge. i.e. telephones

9. Summary

Items to consider when making choice between methods :-

1. Area classification of sample point and possible analyser location 2. System response time required. 3. Affect on other plant areas of any sample transport through them. 4. Environmental requirements of the analyser - temperature, humidity, dust 5. Access requirements for the analyser for operation 6. Access requirements for the analyser for maintenance 7. Certification of analyser 8. Cost of different certifications

A summary of the main points raised in the above discussion is tabled below. :- Note that this is only for the Hazardous Area protection and other factors will play a part in the decision and should be weighted accordingly.

Advantage DisadvantageAnalyser House 1. Climate control

2. Suitable for housing multiple analysers

3. Brings analysers together

4. Operation and maintenance ease

5. Comfort for operators and maintenance technicians

6. Allows use of other non-hazardous ancillary equipment.

1. Cost 2. Single location

may cause sample transport problems

3. Common Mode of failure to be considered.

Analyser Certification

1. Suitable for a single analyser.

2. Easily positioned close to sample point.

1. Maintenance difficulties

2. No climate protection for operations or maintenance



10. Conclusion

The three common methods discussed are not mutually exclusive. An example of the combination is the use of certified Gas Chromatographs Ex d e p used in a fully purged house. This allows the use of other non-critical analysers such as a moisture analyser and ancillary equipment to operate while the house is purged. On loss of purge the other non-critical analysers are shutdown but the GC can continue to operate.

The environment that the analyser is to be installed in plays a great part in the decision as to what type of hazardous protection is the most appropriate. The position of the sample point system, response time and other analysers already in use should be considered in determining the best solution.

11. References

1. IEC 1285, Industrial-process control - Safety of analyser houses, first edition 1994 2. AS2380.4, Electrical equipment for explosive atmospheres - Explosion-protection techniques, Part 4:

Pressurized rooms or pressurized enclosures, 1994 3. AS2380.2, Electrical equipment for explosive atmospheres - Explosion-protection techniques, Part 2:

Flameproof enclosures d, 1994

personnel.

3. Restricted to certified analysers.

Cabinet certification

1. Suitable for multiple analysers

2. Can be climate controlled

3. Easily positioned close to sample point

4. Suitable for non-certified analysers.

1. Reliance on compressed air.

2. Maintenance and operation difficulties

3. No climate protection for operations or maintenance personnel.

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