Are your Ex loops legitimate?
AA: Towards the end of 2008 SA Instrumentation and Control ran a two-part article on ARP 0108 in which we described some of the changes to legislation relating to hazardous area instrumentation. We do have unique requirements in South Africa. Could you walk our readers through how an end-user should design a loop, select instrumentation and obtain certification to ensure that he remains on the right side of the law?
GF: Well, I cannot describe the whole process. Extech supplies zener barriers and galvanic isolators for intrinsically safe applications, but we do not design the instrument loop: that would typically be done by a site engineer or a company like Explolabs.
JA: Explolabs sometimes provides a service to prepare instrument loops. I prefer the term ‘prepare’ as the primary intention is to certify the loop in our capacity as an approved/accredited Ex test laboratory (ATL).
Because many users of intrinsically safe loops do not have in-house capacity to design a safe loop and prepare a loop (installation drawing), the lab has provided this service when requested.
Design methodology
JA: Firstly we determine if the area in which any part of the loop is to be installed is considered as hazardous. The questions we need to answer are:
* Is there flammable gas (including vapour) or dust present that could cause an explosion hazard?
* What is the gas or dust?
* Under what circumstances can this gas or dust be exposed to atmosphere (air) and form an explosive mixture?
* How often might this occur and for how long per annum?
* Is the application for underground mining where methane may be present?
* What is the applicable zone or zones? For example, a probe may be installed in a Zone 0 tank vapour space and its controller in a Zone 1 location.
* Which level of Equipment Protection Level (EPL) is appropriate?
The answers to these questions influence the selection of apparatus used in the loop.
AA: OK, with input from the two of you, could we look at these steps in some more detail, highlighting any legislative requirement that needs to be taken in designing a loop for a hazardous area? For our example I have chosen an RTD temperature measurement loop connected to a DCS. The RTD will be connected to a temperature transmitter (TT) and the signal wiring from the TT will connect back to the safe area and ultimately to the DCS/power supply rack.
Who can do this?
AA: I think the first question is: Can anybody follow these guidelines to design a loop as we are describing, or do they need to hold a specific formal qualification?
GF: In many foreign countries the petrochem, oil and gas companies would have their own in-house professional responsible for Ex loop design, documentation and approval. Locally the Ex i probably has more applicable legislation that other protection techniques.
JA: The two South African safety laws (MHS Act and OHS Act) do not directly specify loop design qualifications. Since the loop will anyway need to be certified by an ATL, anybody with suitable background knowledge can do the initial design.
But things are changing and Ex standards are starting to incorporate qualification requirements for Ex designers and other competencies.
AA: I recently read that the IECEx has initiated a Certified Persons Scheme covering personnel who install, inspect and maintain equipment in hazardous areas, as well as their supervisors and those responsible for plant design and area classification.
What are zones?
AA: Could you explain what Zones are and how they differ from Categories and Divisions.
JA: Zones are the IEC terminology with which most European-centric instrumentation practitioners are familiar. There are three zones defined for flammable gases and three for flammable dust-laden atmospheres. Zone 0 describes areas in which there is a flammable gas atmosphere present for more than 1000 hours per annum (‘continuously’); Zone 1 covers areas where a flammable gas atmosphere is present intermittently for more than 10 hours pa but less than 1000 hours pa.; Zone 2 covers areas where such an atmosphere is present for less than 10 hours pa. (‘abnormally’).
For many years dusts were treated the same as gases, but there are some fundamental differences which are recognised by categorising the equivalents of the above zones for dust laden flammable atmospheres as Zones 20, 21 and 22 respectively.
In the North American market the near equivalents are Zone 0 and Zone 1 falls under Division 1 and Zone 2 under Division 2.
AA: In South Africa we speak about zones, but ATEX introduced the concept of ‘categories’ when it introduced the European Directive for apparatus for use in hazardous areas (94/9/EC). Are the two numerically equivalent?
JA: No, under ATEX, Zone 0 is known as Category 1, Zone 1 as Category 2 and Zone 2 as Category 3.
Staying with the IEC terminology it would be the process engineer who would determine the zone for a particular instrument since that will be the person who knows the gas and its conditions, and also the frequency or duration for which the hazard is present. For the sake of this example let’s assume the gas is butane at 10 bar and a nominal gas temperature 60°C and the condition is continuous. Clearly this is a Zone 0 application.
AA: Recently a new term has been introduced, ‘Equipment Protection Level’ (EPL). How does that tie in?
JA: In 2007 the IEC introduced the EPL terminology into the IEC 60079 series of standards. There is a close relationship between Zones 0, 1 and 2 and EPL Ga, Gb and Gc. But the idea is that the actual selection of an EPL can take into account additional factors such as consequence of explosion. For instance a zone may be categorised as Zone 1, but because of the potential consequences of an explosion the designer may specify the EPL as Ga.
AA: Right, so now we know what we are dealing with and so far we have avoided any legislative red tape. What is next?
What techniques are available?
JA: Knowing the Zone in which the instrument will be placed and the process information we now need to decide on a technique.
There are several techniques that can be applied to prevent ignition of a flammable atmosphere. We need to ensure that no spark is introduced to the vapour that can cause ignition and we need to ensure that no surface becomes hot enough to raise the vapour above its flash point.
For the former, there are four broad techniques: physically isolate any possible source of a spark from the flammable atmosphere (protection by gas exclusion – Ex o, p, q); physically prevent the propagation of an explosion (Ex d); or limit the available energy in the hazardous area to a value that precludes the possibility of ignition (low energy methods – Ex ia, ib, ic). Lastly, sparks can be completely eliminated (Ex e, nA).
GF: Looking at generic instrumentation, our experience is that low energy methods, referred to as intrinsically safe, inevitably result in lower cost installations that are easily maintainable.
Maintainability
AA: How does the selection of technique impact on maintainability?
GF: When you think about it, it is quite simple. If you use techniques other than low energy techniques then it can be very challenging to fault-find in the hazardous area. Gas exclusion techniques and propagation prevention techniques mean that junction boxes cannot be opened while there is the possibility of a hazardous atmosphere arising. But if low energy techniques are employed, maintenance personnel can still measure signals of live loops in hazardous areas as long as their test equipment is suitably rated, without purging the area and needing a ‘gas clearance certificate’.
JA: That does not mean that Ex techniques other than Ex i are impractical, but Ex i is an important technique with which instrument personnel should be familiar.
Three levels of intrinsic safety
AA: Earlier on you mentioned that intrinsic safety comes in three flavours: ia, ib and ic. Why do we need more than one level?
JA: Well, it all boils down to balancing the probability of the presence of an explosive atmosphere versus the probability of a situation occurring that could ignite that atmosphere, which brings us back to the Zones that we spoke about earlier.
There is a close correlation between Zones 0, 1 and 2 and the use of ia, ib and ic levels.
‘ia’ offers the highest level of protection and is generally considered as being adequately safe for use in Zone 0 because it considers the possibility of two ‘faults’ and uses a factor of safety of 1,5 in the safety assessment.
‘ib’ apparatus, which takes into consideration one fault and a factor of safety of 1,5, is considered adequately safe for use in Zone 1.
‘ic’ apparatus which is assessed in ‘normal operation’ with a unity factor of safety is generally acceptable in Zone 2. The ‘ic’ concept was introduced in 2005 and is replacing the ‘energy-limited’ (nL) technique of the type ‘n’ standard IEC 60079-15 and possibly the ‘non-incendive’ concept of North American standards.
AA: What do you actually mean when you say that one or two faults are taken into consideration in assessing safety?
GF: It gets a little complicated here and unless readers are designing an instrument they don’t need to be too concerned about this. But basically a countable fault is one which adversely affects the safety of the equipment, and within the standards there are certain specially designed components that are considered as infallible.
Gas group
AA: How is the specific gas taken into account?
JA: Gases are grouped according to the amount of energy that is required to ignite a gas/air combination of them. This is not directly related to their ignition temperature. For instance a gas like propane has ignition energy of 180 μJ and falls into group IIA, whereas hydrogen has much lower ignition energy of 20 μJ and falls under gas group IIC. The firedamp (predominantly methane) hazard in mining applications (fiery mines) falls under group I and has ignition energy of 250 μJ.
Another consideration is the ignition temperature. With the exception of carbon disulphide, most gases have ignition temperatures above 135°C. What this means is that we are generally safe in specifying equipment that is compatible with gas group IIC and temperature classification T4.
This will certainly meet the requirements of our example since butane falls into gas group IIA and has ignition temperature of 372°C.
Simple apparatus
GF: It is important to remind readers that our example instrument is an RTD – something that does not store energy and is limited in its ability to generate energy. Devices like simple switches, thermocouples, RTDs and junction boxes fall under the definition of ‘simple apparatus’.
Equipment used in intrinsically safe loops must be either ‘certified’ or fall under the ‘simple apparatus’ definition.
JA: In South Africa, as in many foreign countries, legislation does not require any certification or legislative process for simple apparatus. Having said that, issues such as insulation properties and temperature rating must be determined and may become the responsibility of the end user if not specified on the certified loop drawing. This aspect makes certification of simple apparatus desirable, and I have personally recommended certification to end users.
GF: Of course, even though no IA certification is required for simple apparatus, the loop as whole must still be certified and the apparatus needs to be assessed as part of the loop certification process.
Look out for Part 2 of this article in the April 2010 issue of SA Instrumentation and Control.
Intrinsic safety
IEC apparatus standard IEC 60079-11 defines intrinsic safety as ‘a type of protection based on the restriction of electrical energy within apparatus and of interconnecting wiring exposed to the potentially explosive atmosphere to a level below that which can cause ignition by either sparking or heating effects’.
About Gary Friend
Gary completed his B.Sc Electrical Engineering at the University of Witwatersrand in 1990 and worked for Temperature Controls at various stages between 1986 and 1992. In July 1992, Gary moved to the UK, where he worked as a design engineer for LTH Electronics in Luton, designing process control instrumentation for water quality monitoring.
In November 1995 Gary joined MTL, starting as a design engineer on zener barriers. He then moved to the MTL8000 Process IO team, where he was responsible for product validation, integration testing and customer support worldwide. From 1998 Gary led the Technical Support Group supporting all MTL products worldwide.
In 2005 Gary returned to the development arena as programme manager of the MTL4500 galvanic isolator development.
In September 2006, Gary returned to South Africa and he joined Extech Safety Systems. Gary sits on the Ex-steering committee and IS sub-committee of SA Flameproof Association and is vice-chairman of the Fieldbus Foundation Southern African Marketing Committee.
About the author
Andrew Ashton has electrical, mechanical and business qualifications and has been active in automation and process control since the early 1980s. Since 1991 he has headed up a company that has developed formulation management systems for the food, pharmaceutical and chemical manufacturing industries and manufacturing solutions involving the integration of various communication technologies and databases. Developed systems address issues around traceability, systems integration, manufacturing efficiency and effectiveness. Andrew is features editor for S A Instrumentation and Control and editor of Motion Control in Southern Africa.
For more information contact Gary Friend, Extech Safety Systems, +27 (0)11 791 6000, [email protected], www.extech.co.za or Dr Johannes Auret, Explolabs, +27 (0)11 316 4601, [email protected], www.explolabs.co.za
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