Safety Programmable Logic Controllers

To protect against random hardware failures, a safety PLC wiU emphasize internal automatic diagnostics, a combination of hardware and software 

Manufacturer Model Description Assessment Type Assessment Agency 

Det-tronios Eagle Quantum Fire and Gas Controller/PLC FMEDA exida 

Emerson DeltaV SIS Safety PLC 61508 Certification exida/RWTUV 

Yokogawa Electric Corporation Prosafe PLC Safety PLC 61508 Certification TUV Sud 

---

LASS is a distributed control system whose architecture is based on safety programmable logic controllers (PLC) of the Siemens 417FH series [SIE 09]. 

Part 5 (Draft) Programmable logic controllers for safety applications (August 1998) VDIA DE 3542, Safety terms for automation systems. 

Qualification tests shall be carried out in order to check the transmission of one-bit binary information sent by sensors and safeties to the computerized control system or programmable logic controller. 

As a rule, this Ex i-isolator is installed in a safe area and connected to additional apparatus not ex-protected, e.g. to a single loop controller or to a PLC (Programmable Logic Controller). Therefore, in the field of intrinsic safety two types of apparatus are distinguished ... 

The assessment should be performed at the requirements capture stage and developed more fully at later stages in the life eyele. This assessment should cover in its broadest sense GxP, safety, and enviromnental issues. It should also eover an evaluation and eategorization of the software. SCADA systems need eritieality assessment whether they stand alone or network to Industrial Personal Computers or Programmable Logic Controllers. 

Requirements for the use of programmable logic controllers (PLCs) are defined in NFPA 86 (sect. 5-3). All critical protection interlocks (e.g., combustion safety interlocks, flame supervision, high and low oxygen and fuel flow interlocks, excessive temperature limits, etc.) shall be wired to de-energize the safety shutoff valves directly, and their operations shall result in a safe system condition. 

Process safety interlocks can be used to prevent ttnd/or mitigate tin upset condition. Salety interlocks consist of outputs from process control systems, such as programmable logic controllers or distributed control systems, which trigger tut action designed to compensate I ot an upset condition and thus avoid an accident even in the e em of human or computer error. An example of a salety interlock would be the introduction of full cooling if the temperature of a reaction exceeded a safe level. 

Programmable logic controllers (PLC) often seem a logical choice for safety instrumented system logic solvers. After all, the PLC was designed... 

During the 1990s the concept of Safety Integrity Levels (SIL) was developed [1]. It serves to assess safety-related systems and concerns aU components and subsystems required to realize safety functions from the sensor to the final element. Apart from that it applies to application software, which was developed for systems with limited variability language (no branching) or programmable logic controllers (PLC). 

Certain critical safety functions require systems more reliable than a DCS, and they must be handled separately. These functions are associated, for example, with rectifiers, hydrogen compressors, and brine purging of membrane cells. In the past, redundant programmable logic controllers (PLCs) often provided the necessary reliability, but newer systems with the robustness of a standard PLC also have internal redundant logic and input checking and may also have redundant outputs. 

Use of FTA for physical failure in hardware is well proven. In this section, discussions wfll be mainly on use of FTA in PEs to be specific on FTA for software. In today s industrial automation, almost all control systems deploy PE systems. Therefore, software is very critical for control systems. They are increasingly used to handle safety-critical system functions also. As a natural consequence, a high number of hazards in such systems are known to be caused by software which controls the systems. In this section, the focus will be on PEs, for example, programmable logic controller (PLC) faults. PE system faults can be categorized in three classes depending on whether they are caused by ... 

H. Bezecnyl, D. Inverso, V. Maggioli, G. Rabe, A. Weinert, Guidelines for the use of programmable logic controllers in safety-related systems, in European Workshop on Industrial Computer Systems Technical Committee 7 (Safety, Reliability and Security) Working Paper 6009 Version 13, October 1997. 

Conformity of component safety integrity level (SIL) and safety loop (SIF). PLC, programmable logic controller PFD, probability of feilure on demand. 

Hardwired alarm for safety. DCS, distributed control system ESD, emergency shutdown LVM, limit value monitor PLC, programmable logic controller PS, power supply. 

Design steps for a safe system. ALARP, as low as reasonably practicable BPCS, basic plant control system DCS, distributed control system IPL, independent protection layer PLC, programmable logic controller SIS, safety instrumentation system. 

Burner management system safety integrity level (BMS SIL) calculation. BMS, burner mam agement system IIP, input O/P, output PFD, probability of failure on demand PLC, programmable logic controller SIS, safety instrumentation system SV, solenoid valve / shutdown... 

---