Safety Standards of Level Control Devices

Level Control Devices and SILMalfunctioning level controls allegedly contributed to the 1986 Chernobyl meltdown and the 2005 Buncefield depot explosion north of London, to name just two of the more notorious incidents. For decades, the industrial firewall against safety incidents as they relate to level controls has been governmental and professional association standards that require manufacturers to make their products according to safety guidelines. The International Standards Association, however, lists some 180,000 varieties of international standards. The key health and safety standards that can affect level control devices and applications fall into three categories: (1) Instrument and Component Standards, (2) Safety Integrity Levels, and (3) Hygienic Standards.

Instrument Standards. The largest group of standards relates to equipment, component and enclosure performance. These standards are authorized by NEMA (National Electrical Manufacturers Association—USA); Underwriters Laboratories (UL—USA); American National Standards Institute (ANSI); American Society of Mechanical Engineers (ASME); The Instrumentation, Systems, and Automation Society (ISA—originally named the Instrument Society of America); Canadian Standards Association (CSA); the European EN 60529 = DIN VDE 0470; IEC (International Electrotechnical Commission); VDE (Institute of German Electronics Engineers); the International Electromechanical Commission (IEC); and TÜV (Germany). Principal organizations with market-relevant technical standards for materials, products, and systems also include the European Union’s ATEX directives (ATmosphere EXplosion); America’s OSHA (Occupational Safety and Health Administration); the American Society for Testing and Materials (ASTM), and the United Kingdom’s Health and Safety Executive (HSE).

Safety Integrity Level (SIL). Another group of directives that relate specifically to level control safety performance are those of the IEC concerning risk reduction. These directives refer to the classification of Safety Instrumented Systems (SISs) according to their Safety Integrity Level (SIL)—that is, according to their potential risk for people, manufacturing processes, and the environment in case of a malfunction. Four SIL levels are defined in these directives, with SIL 4 being the most stringent and SIL 1 being the least. (No standard process controls have yet been defined and tested for SIL 4).

There are two ways an instrument manufacturer can determine and declare their devices suitable for a SIL level. For pre-existing devices, the supplier takes over the “proven in use” procedure, in which the instruments are tested and described according to IEC 61508 and 61511. For new devices, the supplier makes a direct declaration to IEC. This declaration comprises an evaluation of the device based partly on a Failure Modes, Effects and Diagnostics Analysis (FMEDA) and partly on an assessment of the proven-in-use documentation.

Today, many instrument and plant engineers use an instrument’s SIL suitability level as “shorthand” for an instrument’s overall reliability.

Hygienic Standards. A third group of standards relates to hygienic issues concerning the food, beverage, dairy and pharmaceutical industries. These include the Federal Drug Administration (USA authority), the 3-A Sanitary Standards (a group of three USA milk and dairy authorities), the European Hygienic Equipment Design Group (EHEDG—a consortium of European equipment manufacturers, food industries, research institutes, and public health authorities), and the Federal Health Department (Germany).

Understanding Safety Integrity Level SIL

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