ANSI/ISA standard

Valve bodies are also standardized to mate with common piping connections flanged, butt-welded end, socket-welded end, and screwed end. Dimensional information for some of these joints and class pressure-temperature ratings are included in Sec. 10, Process Plant Piping. Control valves have their own standardized face-to-face dimensions that are governed by ANSI/ISA Standards S75.08 and S75.22. Butterfly valves are also governed by API 609 and Manufacturers Standardization Society (MSS) SP-67 and SP-68. 

A similar approach to the IP code is used in the USA and is described in the ANSI/UL and ANSI/ISA standards but a Type Number is used instead of the two or four digit code (n, m, a, s). The basic principles are very similar. Reference 2 Article 500-4 summarises the subject and quotes the appropriate codes and standards. 

Safe Automation and ANSI/ISA 84.01-1996 served as significant technical references for the first international standard, IEC 61511, issued by the International Electrotechnical Commission (IEC). In the United States, IEC 61511 was accepted by ISA as ISA 84.00.01-2004, replacing the 1996 standard. In 2004, the European Committee for Electrotechnical Standardization (CENELEC) and the American National Standards Institute (ANSI) recognized IEC 61511 as a consensus standard for the process industry. IEC 61511 covers the complete process safety management life cycle. With its adoption, this standard serves as the primary driving force behind the work processes followed to achieve and maintain safe operation using safety instrumented systems. 

Where loss of control could lead to severe consequences, the integrity of the basic process control system and the protective safeguards must be designed, operated and maintained to a high standard. Industry standards such as ANSI/ISA-S84.01 (1996) and IEC 61508 (2000) address the issues of how to design, operate and maintain safety instrumented systems such as high temperature interlocks to achieve the necessary level of functional safety. The scope of these standards includes hardware, software, human factors and management (HSE 2000). 

ANSI/ISA-50. Fieldbus Standard for Use in Industrial Control Systems. Multiple part standard. ANSI/ISA. 

This International Standard sets out an approach for safety lifecycle activities to achieve these minimum standards. This approach has been adopted in order that a rational and consistent technical policy is used. The objective of this standard is to provide guidance on how to comply with lEC 61511-1 ANSI/ISA-84.00.01-2004 Parti (lEC 61511-1 Modi. 

To facilitate use of this standard, the clause and subclause numbers provided are identical to the corresponding normative text in lEC 61511-1 ANSI/ISA-84.00.01-2004 Part 1 flEC 61511-1 Mod)... 

For existing SIS designed and constructed in accordance with codes, standards, or practices prior to the issue of this standard fe.a.. ANSI/ISA-84.01-1996). the owner/ooerator shall determine that the equipment is designed, maintained, inspected, tested, and operating in a safe manner. 

The standard recognizes that the specified activities might be structured in different ways, provided that all the requirements are complied with. This restructuring can be beneficial if it allows safety activities to be better integrated into normal project procedures. The purpose of Clause 6 of lEC 61511-1 ANSI/ISA-84.00.01-2004 Part 1 (lEC 61511-1 Mod) is to ensure that if a different safety lifecycle is used, the inputs and output of each phase of the lifecycle are defined and all essential requirements are incorporated. 

Mod) limits the dangerous failure rate, in relation to a particular hazard, that can be claimed to 10 per hour unless the system is implemented according to the requirements of this standard. The reason for the limit is that if a lower dangerous failure rate is claimed, it would be in the range of failure rates within Table 4 of lEC 61511-1 ANSI/ISA-84.00.01-2004 Part 1 flEC 61511-1 Modi. The limit ensures that high levels of confidence are not placed on systems that do not meet the requirements of lEC 61511-1 ANSI/ISA-84.00.01-2004 Part 1 (lEC 61511-1 Mod). 

The international working group that prepared lEC 61508 considered the above factors and specified the extent of fault tolerance required in lEC 61508-2. In preparing this sector-specific standard for the process sector it was considered that the requirements for fault tolerance of field devices and non PE logic solver could be simplified and the requirements in lEC 61511-1 ANSI/ISA-84.00.01-2004 Part 1 (lEC 61511-1 Mod) could be applied as an alternative. It should be noted that subsystem designs may require more component redundancy than what is stated in Tables 5 and 6 in order to satisfy availability requirements. 

IEC 61078 and Annex B of IEC 61508-6 illustrate the reliability block diagram technique for calculating the probabilities of failure for safety instrumented functions designed in accordance with IEC 61511-4-ANSI/ISA-84.00.01-2004 Parti (IEC 61511-1 Modi and this standard. 

One of the more influential documents on SIS was called "Programmable Electronic Systems in Safety Related Applications," which was published by the Health and Safety Executive (HSE) in the United Kingdom (Ref. 2 and 3). Early national standards for SIS include "Grundsatze fur Rechner in Systemen mit Sicherheitsaufgaben," published in Germany (Ref. 4 and 5) in 1990 and ANSI/ISA-84.01-1996, "Application of Safety Instrumented... 

The ANSI/ISA-84.00.01-2004 (lEC 61511) standard (Ref. 1) defines a safety instrumented system (SIS) as an "instrumented system used to implement one or more safety instrumented functions. A SIS is composed of any combination of sensor(s), logic solver(s), and final element(s)." lEC 61508 (Ref. 2) does not use the term SIS but instead uses the term "safety-related system." That term defines the same concept but uses language that can be broadly applied to many industries. 

In ANSI/ISA-84.00.01-2004 (lEC 61511 Mod), 3.2.71, a safety instrumented function is defined as a "safety function with a specified safety integrity level which is necessary to achieve functional safety." This standard, 3.2.68, defines a safety function as a "function to be implemented by a SIS, other technology safety-related system or external risk reduction facilities, which is intended to achieve or maintain a safe state for the process, with respect to a specific hazardous event."... 

ANSI/ISA-84.00.01-2004 (lEC 61511 Mod) requires that equipment used in safety instrumented systems be chosen based on either lEC 61508 certification to the appropriate SIL level or justification based on "prior use" criteria (ANSI/ISA-84.00.01-2004 (lEC 61511Mod), Part 1, Section 11.5.3). However the ANSI/ISA-84.00.01-2004 (lEC 61511 Mod) standard does not give specific details as to what the criteria for "prior use" means. Most agree however that if a user company has many years of documented successful experience (no dangerous failures) with a... 

---