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Hazardous area classification: points to consider

15 November 2017

A hazardous location is designated as any location in which an explosive gas atmosphere is or may be expected to be present, in quantities such as to require special precautions for the construction, installation and use of equipment. This paper, by Brazilian consultant Estellito Rangel Junior, discusses some precautions to be taken when classifying hazardous locations, when using US sources such as API RP-505, as well as IEC documents.

The primary objective of an area classification plan is to define and show the volumes where the concentration of flammable gases can exceed their Lower Flammable Limits (LFL), which will allow the selection of the adequate explosion protection types for all electrical and electronic equipment to be installed without compromising the plant’s safety.

Methodology

 The area classification assessment can be described in steps, as follows:

1) Define the standard that will be used as basis;

2) Obtain pertinent information, e.g., process flow diagrams, material safety data sheets, processes data sheets and updated plot plan;

3) Identify and assess all continuous, primary and secondary sources and their interactions with the local ventilation;

4) Define the boundaries of each Zone and plot them on appropriate drawings (that must include plot plans and elevation views).

The definitions of classified area extents for gas explosive atmospheres can be developed using the following methods:

1)  Direct example - applicable to standardized installations such as vehicle refueling stations.  Care should be taken to ensure that the same products and conditions of the plant under consideration are the same of the example;

2)  Gas dispersion theory - considering the plant process data, what is emphasized in IEC 60079-10-1 ed. 2.0.

Documentation

The area classification documents must be reliable, as they will be used as reference not only for equipment selection, but also for developing operational, maintenance and safety procedures.

A survey was carried out in the American Oil, Gas and Petrochemical industries in the nineties, and revealed that regarding area classification documents:

1) 20% of designers used just a text description;

2) 20% failed to mention the gas group;

3) 25% included the recommended temperature class for electrical and electronic equipment to be installed in the area;

4) The reliability of these documents was assigned a rating of 6, 10 being the highest rating. 

Fig. 1 - API RP-500: 1957 - figure 3 - “Process area with restricted ventilation”
Fig. 1 - API RP-500: 1957 - figure 3 - “Process area with restricted ventilation”

It is a point of concern that in many companies the area classification drawings are not updated.  The same documents issued in the early design stage were kept unchanged for 20 years or even more, and this situation can compromise the plant safety. 

Reference documents

If the area classification drawings do not reflect the current installation details, and the extent of the classified areas was defined by direct reproduction of the figures found in American documents such as API RP-500/505 without any justification, they cannot be considered as reliable.  Such documents are not “standards”, but “Recommended practices”, is to say, they are considered only as guides that require the application of sound engineering judgment. 

So, in a real plant the classified areas’ extents may vary from the figures examples in RP-500/505, depending on the location, conditions, equipment, and substances involved in any given situation.  It is important to clarify that just reproducing such examples can compromise the plant safety.

API RP-500 was firstly issued in 1951, when computational resources were not easily available, with the objective to show some area classification extents adopted by some petroleum companies.  The second edition, in 1957, received more “reference figures”, as shown in Fig. 1. (Above)

Note 1 under that figure was: “Distances given are for average refinery installations; they must be used with judgment, with consideration given to all factors discussed in RP-500”.

The current API RP-500 edition kept as figure 22 the same illustration, shown below.

The API RP 500: 2012 Note 2 under figure 22 (Below) is: “Distances given are for typical refinery installations; they must be used with judgment, with consideration given to all factors discussed in the text. In some instances, greater or lesser distances may be justified”. 

But what does “typical refinery installations” mean?  As the figures are the same, can one say that a 1957 "typical refinery" had the same characteristics and products of a 2017 installation, despite many improvements and process changes since then?  Unfortunately, no.

Additionally, it is important to note that the classified area extents have not been imposed in this document to be used for whichever gas is processed in the plant, as we know that different gases have different LFLs, and consequently the classified area extents are not equal.

Therefore, just copying and pasting such example figures into an area classification plan cannot be correct.

Today’s process plants are larger and more complex, using new chemical products and equipment.  It is well known that changes in the product mix or introduction of new flammable materials into a plant can drastically change the classified area extents, as can be foreseen when using area classification software.

Fig. 2 - API RP 500: 2012 - figure 22 – “Inadequately ventilated process location with heavier than air gas or vapor source”.
Fig. 2 - API RP 500: 2012 - figure 22 – “Inadequately ventilated process location with heavier than air gas or vapor source”.

Mathematical models

Area classification software uses mathematical models based on the estimated media flow through specially designed openings to ensure the maximum flow rate with minimal energy.  These models are developed under the discipline of Fluid Dynamics.

The extent of hazardous areas also depends on the wind speed in the vicinity of the sources of release and the ventilation availability.  For these estimates, some equations are valid for outdoors with wind speed between 0.5 and 2 m/s.

IEC 60079-10-1 ed. 2.0 gives some equations in Annex B that can be used in particular conditions, and as highlighted on B.7.2.1 “the equations and assessment methodologies presented in this clause are not intended to be applicable to all installations and only apply to the limited conditions noted in each section”.  In Annex C, a ventilation guidance is presented, also giving some equations applied for natural ventilation.

There is a concern regarding the promotion of mathematical equations in the text of standards because by concept, the standard’s objective is to define the minimum requirements for a given product or service.  Experienced professionals might prefer to use more adequate models and this can lead to discussions with clients, as the latter usually require to “follow the standard”.

Another concern is that persons without the due knowledge may use the suggested equations in inappropriate conditions.

The current  IEC 60079-10-1 ed. 2 also states on its sub clause 4.3 that "a risk assessment may be carried out to assess whether the consequences of ignition of an explosive atmosphere requires the use of equipment of a higher equipment protection level (EPL) or may justify the use of equipment with a lower equipment protection level than normally required"

This statement can be interpreted that equipment only approved for use in Zone 2 can be used in Zone 1 locations, but, considering that the prime objective of using Ex equipment in hazardous locations is to avoid explosions, it is not reasonable to use such EPL “underrating” to allow foreseen “small explosions”.  As it is well known, “small explosions” can lead to tragedies due to domino effect, and considering that there is no proven way to predict the consequences of an explosion, this “underrating” cannot be considered as a safe alternative.

It is interesting to show that gas dispersion is affected by the process temperature.  A simulation was made with isopentane at the same pressure, but at three temperatures: 30 oC, 90 oC and 120oC.

This simulation alerts that the area classification can’t be made by simple reproduction of “example figures”, because the influences of temperature are not addressed on them.

For atmospheric releases, the volume of the explosive atmosphere is a balance between the rate of vaporization (dependent on volatility, i.e. flash point and boiling point of liquids), and the rate of dilution (ventilation).  Where the gas/vapour is heavier than air, it is established that the explosive atmosphere may collect in hollows and enclosures, taking on the size and shape of the containing structure.

On rare, windless days, explosive atmospheres can form “trails” and migrate over considerable distances, resulting in a shape considerably different from that “example figures”.

Warning signs

An item that contributes to the plant’s safety and also helps the inspection activity is the signalisation of hazardous locations.  The location of each warning sign can be indicated on the area classification drawings, in order to alert workers that perform services in industrial plants.  

With a clear and simple design that follows ISO recommendations, it has the “Ex” inside a yellow triangle over a red colour background and shows the zone, the gas group and the temperature class of permitted electrical and electronic equipment to be used in a particular classified area.  It provides important information not only for maintenance workers, but also for safety officers and installation inspectors. 

The referenced area classification drawing number is also indicated on the sign, making easier for the user to get additional information.

Conclusions

It is known that the most common characteristic found in area classification drawings is the reproduction of generic figures presented in some reference documents as “examples”. 

Although these documents express in many parts of their texts that it is necessary to consider the real conditions of the plant under study and not only the “example figures”, there is a misconception that such figures can be used as a “cookbook”, which could allow persons with no experience to “quickly” create area classification drawings, just by reproducing them.

In fact, the IEC 60079-10-1 ed. 2.0 presents on Annex K: “where examples from industry codes or national standards are used, then they shall be quoted as the basis for classification and not IEC 60079-10-1”.  This highlights that each industry code or company specification is based upon a particular approach to area classification and these approaches may vary. 

So, if a particular plant was classified just mixing information from different codes without considering the real process data, such area classification plan can’t be considered as reliable.

Considering that IEC 60079-10-1 ed. 2.0 shows many equations taken from Fluid Dynamics discipline, it is important to say that area classification can’t be put under the duty of a sole electrical professional.  To be reliable, the area classification assessment is to be made by a group consisting of process engineers, safety professionals and plant operators, considering the real processes data. 

References

[1]  IEC 60079-0, Explosive atmospheres – Part 0: General requirements, ed. 2013.

[2]  API RP-500, Recommended practice for classification of locations for electrical installations at petroleum facilities classified as Class I, Division 1 and Division 2, Edition 3, 2012.

[3]  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, 2012.

[4]  Estellito Rangel Jr., “Brazil moves from Divisions to Zones”, in IEEE PCIC Conference Record 2002, New Orleans, USA.

[5]  R. J. Buschart, “Electrical area classification drawings - a comparison”, in IEEE PCIC Conference Record 1995, Denver, USA.

[6]  API RP-500, Recommended practice for classification of areas for electrical installations in petroleum refineries, Edition 2, API: 1957.

[7]  NI-2706, Presentation of the area classification plan, Petrobras: 2007, Brasil.

[8]  Estellito Rangel Jr., Aurélio Luiz and Hilton Madureira, “Area classification is not a copy and paste process”, in: IEEE PCIC Conference Record 2014, San Francisco, USA.

[9]  A. Bozek and V. Rowe, “Flammable mixture analysis for hazardous area classification”, in IEEE PCIC Conference Record 2008, Cincinnatti, USA.

[10]  R. Tommasini, E. Pons, “Classification of hazardous areas produced by maintenance interventions on NG distribution networks and in presence of open surface of flammable liquid”, in IEEE PCIC Conference Record 2011, Toronto, Canada.

[11]  IEC 60079-10-1, Explosive atmospheres – Part 10-1: Classification of areas – Explosive gas atmospheres. Edition 2.0. IEC: 2015.

[12]  Estellito Rangel Jr. and C. Sanguedo, “International standards on explosive atmospheres: harmonization is a hard but necessary task”, in VIII PCIC Europe Conference Record 2011, Rome, Italy.

[13]  A. H. Otsuka, “Quantitative analysis of classified areas extent”, UFS, Brasil, 2011.

[14]  Webber, D. M., et al., Ventilation theory and dispersion modelling applied to hazardous area classification, Journal of Loss Prevention in the Process Industries, 2011.

[15]  T. Rains and B. Satavalekar, “Harmonizing ventilation requirements for indoor lighter than air hazardous atmospheres”, in IEEE PCIC Conference Record 2010, San Antonio, USA.

[16]  Estellito Rangel Jr., “Assessment and definition of classified locations using international standards”, in VIII ENASSMA National Conference on Health, Safety and Environment 2001, Florianópolis, Brasil.

About the author

Estellito Rangel Junior is an electrical engineer and an area classification senior consultant at ABPA (Brazilian Association for Accidents Prevention), with more than 30 years' experience on risk assessment and design of electrical systems for the oil and gas industry. He was also a member of the IEC MT-60079-10 working group.

 


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