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Production • Processing • Handling


Increased demands for HIPPS final elements


Rens Wolters describes the impact of the new IEC 61508 Edition 2010 on final elements and HIPPS.


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n the oil and gas industry protection against high pressure is increasingly performed by means of instrumented systems rather than mechanical safety relief valves. When the risk is high and the response time is short this application is often referred to as HIPPS (High Integrity Pressure Protection System). Te applicable standards - IEC 61508 and IEC 61511 - use the generic term SIF or SIS (Safety Instrumented Function or System), whereas the industry uses HIPPS for this specific application. In the standards the element that shuts-off the incoming flow and isolates the high pressure source (on-off valve) is called the final element. A new revision of the IEC 61508 was published in 2010 and seriously impacts the HIPPS final element.


Prescriptive standards Before the IEC 61508 existed over-pressure protection was governed by prescriptive standards like the DIN and API. Tese standards describe in detail how to design an installation and how to design the safety system required to protect its low pressure part. According to the API the safety system should consist of an emergency shut- down (ESD) valve - which would close upon high pressure down-stream of the control valve - and a full flow flare relief system. Nowadays, when instrumented systems are preferred over flaring, the same redundancy will be applied: the ESD valve remains and the full flow relief system is replaced by the HIPPS which often consists of redundant sensors and final elements. Tis results in an over- pressure protection system with a total of three final elements. A small relief capacity remains to evacuate leakage of the on-off valves.


The IEC 61508 and 61511 standards In the 1990s questions arose regarding the reliability of instrumented systems and the IEC 61508 was born. Tis standard was mainly written for electronic and programmable electronic systems. In these systems it is quite easy to continuously measure, monitor and shut-down on detection of failures. For the final element, being a valve, this detection is not possible.


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Te Safe Failure Fraction - based on the diagnostics capability of electronic systems - it is the fraction of the safe and dangerous detected failures to all failures. Tis means in case there are many safe failures and many detected failures the number of final elements can be reduced. Over the past years this was usually done following a quite loose definition of a safe failure and even the addition of electronic devices to detect failures. Let us focus on Hardware Fault Tolerance (HFT) first. A final element usually is considered type A equipment, according to the IEC 61508-2 (table 2) a single element could be allowed for SIL3 in case a Safe Failure Fraction (SFF) over 90 per cent can be justified. Tis would mean that where the prescriptive standards require 3 final element this table allows only 1 final element if the SFF is over 90 per cent. In 2003 the IEC 61511 standard was published. Tis standard is mainly for end-users and integrators rather than for manufacturers and has a more conservative approach on redundancy. Tere is actually a specific table for final elements and sensors (IEC 61511-1 table 6). Tis table requires


Fig. 1. Full mechanical HIPPS solution suitable for SIL3 application.


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