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Foundation fieldbus in hazardous areas

Nov. 17, 2015
Why and how Saudi Aramco plans to migrate from non-incendive to intrinsically safe.
Authors

Mokhtarudin Abdul Razak, instrumentation and control engineer, and Hamad Balhareth, PE, instrumentation engineer, are in the Process & Control Systems department, Engineering
Services at Saudi Aramco.

At Saudi Aramco, the protection scheme non-incendive energy limited (Ex nL) is used for Foundation fieldbus (FF) spurs to allow for "live maintenance." The Ex nL was withdrawn from the IEC 60079-151, and replaced with a new intrinsically safe protection method (Ex ic) per the IEC 60079-112. Considering that Ex nL-certified FF devices are gradually diminishing, appropriate studies, solutions and mitigation plans were explored to ensure safety and reliability is maintained with the existing installed base. For grassroots projects, the situation is a bit more complicated because end users have to correctly interpret the new international standards addressing the installation requirements. This issue is purely related to the installation of electrical equipment in classified areas, and is not related to FF technology itself. This article describes Saudi Aramco's experiences for a migration plan from Ex nL to Ex ic, and highlights their major differences from an end-user perspective.

Understand the standards

Selection and design of suitable protection schemes are essential for electrical equipment in classified areas. Improper selection and designing the inappropriate schemes will result catastrophic incidents, especially in the oil and gas industry. Various solutions are specified, such as explosion-proof (Ex d), intrinsic safety (Ex i) and increased safety (Ex e), just to name a few.

The International Electrotechnical Commission and National Fire Protection Association are two well-known organizations that govern the electrical equipment protection schemes in classified areas internationally and in North America, respectively. IEC sets the requirement via IEC 60079 standards, and NFPA 703 provides rulings via the National Electrical Code (NEC), which are NEC 500 and NEC 505. The electrical equipment protection schemes in Europe are governed by the European Union (EU) via the ATEX directive.

Saudi Aramco hasn't heavily depended on the intrinsically safe (IS) method. Instead, the protection scheme of Ex d has been used in classified areas.

Saudi Aramco has the largest FF installed base worldwide, and has been enjoying its benefits, such as a smaller control hardware footprint and valuable diagnostics. Its first use of nonincendive, energy-limited (Ex nL) concept started with using the FF protocol, where segment wiring block manufacturers developed their control drawing. As end-users, we're required to verify whether energy storage from the instrument and the cable exceeds the values published in the control drawing for each spur. This exercise is fairly easy.

IEC 60079-11 (2011) and NEC 505 (2011) have now added Ex ic with more requirements. For instance, in addition to what was mentioned above for Ex nL, the designer shall ensure a voltage limitation mechanism for each spur, and 50 mm (2 inches) separation distance between IS and non-IS connections. Field devices must be certified as ic-type IS (Ex ic), and interface to the H1 cards must be evaluated carefully, so either separation distance requirement is complied with or H1 cards have only pure passive circuits.

Though the new additions may sound less confusing, the implementations aren't so straightforward. Each FF supplier designs its segments based on its own interpretation of the new standard. Also, the SIRA certification body, CSA Group UK, has published a paper addressing suppliers' concerns about whether Ex ic is adequately replacing Ex nL or not in Zone 2 applications. This creates differences among suppliers, and when the international standards responsible party is called to explain or take a ruling, limited information or none has been acquired.

Compare the schemes

Though the intent of both techniques is similar, the two schemes have dissimilarities including cable marking, creepage distance, control drawing and descriptive system drawing requirements, and power supply voltage. After the Ex nL withdrawal, FF manufacturers indicated no new devices would be certified to Ex nL. Existing devices, those already with Ex nL certification, will continue to be offered for a limited time.

At Saudi Aramco, the electrical area classification of flammable gaseous/vapors is performed using the Class I/Zone/Group method per our own engineering standard SAES-B-0684. Accordingly, appropriate protection schemes of any electrical equipment can be selected per NFPA NEC 505 and IEC 60079 standards. The protection schemes can be summarized (Table 1).

Existing installations

At Saudi Aramco, FF segments are used in Class I, Zone 2 areas, where the protection schemes are Ex nA and Ex nL for trunk and spur, respectively. The FF segment typically consists of redundant interface cards (H1) and bulk power supplies (BPS), supplying power to redundant FF power supply conditioners (FFPS). All power supplies reside in a safe area inside the process interface building (PIB5).  The FF trunk cable is powered from a FFPS, and extends to the field junction box, where it terminates on a non-arcing wiring block connection(s).

Table 1: Protection schemes for different zones
FF spurs branch out of the wiring block, and consist of spur cables and field devices (Figure 1). The wiring block and field devices shall be certified as Ex nL, so installation is safe and live maintenance is allowed only on the spur. The wiring block shall also be certified as Ex nA for a trunk terminal connection. Live maintenance isn't allowed on the trunk, unless the segment is powered off.

Future installations

In compliance with the latest edition of the IEC 60079-11 standard, protection schemes include Ex ic as a direct replacement for Ex nL for Zone 2 applications. Since each FF supplier has its own interpretation of the new standard, Saudi Aramco has come out with a typical arrangement of the Ex ic installations based on its understanding of the new IEC requirements (Figure 2). The typical FF installation shall have the following

New Foundation fieldbus installation

Figure 2: A typical arrangement of Ex ic installations should have the trunk maintained as Ex nA,  Foundation fieldbus (FF) H1 input/output cards that meet PELV/SEL4 requirements, FF power supply conditioners with voltage limitation, wiring block with Ex nA certifications, current limitation at the wiring block spur, and field devices certified as Ex ic.

  • The trunk is still maintained as Ex nA
  • FFH1 input/output cards shall meet PELV/SEL6 requirements
  • FFPS shall have voltage limitation mechanism
  • Wiring block shall have certifications of Ex nA [ic]. The current limitation is done at the wiring block spur.
  • Field devices shall be certified as Ex ic
  • The spur cable shall be colored blue

Resolution and conclusive plan

As mentioned, the certification change from Ex nL to Ex ic is purely related to the installation of electrical equipment in classified areas. The protection schemes discussed can also be implemented in a conventional system. Saudi Aramco has brought these issues to the Fieldbus Foundation to collaborate with the IEC to find appropriate mitigations for its FF installations. Meanwhile, Saudi Aramco has explored possible methods to ensure the continuous safety and technical integrity of FF installations.

Saudi Aramco will continue with the protection type Ex nL for spurs in existing FF installations. This is based on the clause in IEC60079-147, version 2007 (paragraph 14), which says, "Equipment to type of protection nL may be used in an intrinsically safe ic circuit." In the event of any fault of Ex nL devices, direct (like-for-like) replacement of Ex nL-certified devices is recommended as long as they're available. Should like-for-like replacement not be possible, faulty Ex nL devices should be replaced with Ex ic-certified devices provided that the entity safety parameters calculations are done, matched, and properly documented per the control drawing requirement. All these calculations shall be documented for future traceability and reference during maintenance. In this case, the loop is still considered Ex nL unless voltage limitation is applied. For marking requirement, the particular spur cable connecting the Ex ic device is recommended to be marked with a blue sleeve slipped over the orange jacket at both termination points.

In grassroots projects, the protection scheme Ex ic is to be used, and is only applicable for FF spurs, such as between the FF wiring block and field devices. The FFPS and H1 card shall have a voltage limitation mechanism and meet PELV/SELV requirements, respectively. The protection scheme at the trunk between the FFPS and the wiring block shall remain as non-arcing Ex nA.

Notes and references

  1. IEC 60079-15 (2010); Explosive Atmospheres – Part 15: Equipment protection by type of protection "n", 2010 edition.
  2. IEC 60079-11 (2011); Explosive Atmospheres – Part 11: Equipment protection by intrinsic safety "i", 2011 edition.
  3. NFPA 70 (2014) – NEC 500 and 505; National Fire Protection Association, National Electrical Code, 2014 edition.
  4. SAES-B-068; Saudi Aramco Engineering Standard, Electrical Area Classification, Rev. 14 August 2014.
  5. PIB is an unmanned building near plant process areas that's used to termination and distribution instrument wiring from the field. It typically houses marshalling cabinets, system cabinets and relevant diagnostics workstations.
  6. PELV/SELV definition per IEC60079-11(2011): i. SELV is a safety extra-low-voltage system (i.e., normally not exceeding 50 Vac and 120 V ripple-free dc) electrically separated from earth and other systems in such a way that a single fault cannot give a rise to an electrical shock. ii. PELV is a protective extra-low-voltage system which is not electrically separated from earth but which otherwise satisfies the requirements for SELV.
  7. IEC60079-14 (2013); Explosive Atmospheres – Part 14: Electrical installations design, selection and erection, 2013 edition.

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