Logic solver

An interlock is a protec tive response initiated on the detection of a process hazard. The interlock system consists of the measurement devices, logic solvers, and final control elements that recognize the hazard and initiate an appropriate response. Most interlocks consist of one or more logic conditions that detect out-of-hmit process conditions and respond by driving the final control elements to the safe states. For example, one must specify that a valve fails open or fails closed. 

House logic-solver components of BPCS, and safety instrumented systems (SIS) in a controlled environment... 

SIL1 (PFD = 10-1 to 10 2) These SIFs are normally implemented with a single sensor, a single logic solver, a single final control element, and requires periodic proof testing. 

Safety instrumented system (SIS) Any combination of separate and independent devices (sensors, logic solvers, final elements, and support systems) designed and managed to achieve a specified safety integrity level. An SIS may implement one or more safety instrumented functions. 

Remote automatic—based on a sensing element, logic solver, and output signal to close the valve. In most cases, these automatic valves are designed to fail safe. Automatic isolation can be associated with the automatic activation of a shutdown system for equipment or a process unit. 

At PPG, Class 1 Prooftesting also covers 250 Safety Instrumented System loops in the PSM Safety Systems. A Safety Instrumented System (SIS) is composed of sensors, logic solvers, and final control elements for the purpose of taking the process to a safe state when predetermined conditions are violated. SISs are normally controlled by a PLC with the sole function of monitoring a process to insure operation is maintained within the safe operating envelope. 

This International Standard addresses the application of safety instrumented systems for the Process Industries. It also deals with the interface between safety instrumented systems and other safety systems in requiring that a process hazard and risk assessment be carried out. The safety instrumented system includes sensors, logic solvers and final elements. 

The safety instrumented system logic solvers addressed include Electrical (E)/Electronic (E)/ and Programmable Electronic (PE) technology. Where other technologies are used for logic solvers, the basic principles of this standard may also be applied. This standard also addresses the safety instrumented system sensors and final elements regardless of the technology used. This International Standard is process industry specific within the framework of the lEC 61508 series. 

The targets for average probability of failure on demand or frequency of dangerous failures per hour apply to the safety instrumented function, not to individual components or subsystems. A component or subsystem (for example, sensor, logic solver, final element) cannot have a SIL assigned to it outside its use in a specific SIF. However, it can have an independent maximum SIL capability claim. 

A 2oo3 transmitter set, a 2oo3 logic solver and a 1oo2 final element set which yields a SIS with a PFDgyg of 3,05 X 10 . Thls SIS achieves a risk reduction of approx. 3,3 x 10. ... 

The objective of this subclause is to provide guidance in the design of the SIS. Each SIF has its own SIL. A component of a SIS, for example, a logic solver, may be used by several SIFs with different SILs. 

Manual means that are independent of both the SIS logic solver and the BPCS control system may be provided to allow the operator to initiate a shutdown in an emergency. The requirements for manual shutdown are normally defined in the SRS. 

The emergency stop may be connected to the SIS PE logic solver (for example, when a sequenced shut down is required) provided that it is necessary and deemed appropriate by the H RA team. 

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. 

Table 6 of lEC 61511-1 ANSI/ISA-84.00.01-2004 Part 1 flEC 61511-1 Modi defines the basic level of fault tolerance for sensors, final elements, and non-PE logic solvers having the required SIL claim limit in the first column. The requirements in Table 6 are based on the requirements in lEC 61508-2 for PE devices with a SFF between 60 and 90 %. The requirements are based on the assumption that the dominant failure mode is to the safe state or that dangerous failures are detected. 

This subclause allows the hardware fault tolerance of all subsystems except PE logic solvers to be reduced by one on certain conditions. These conditions will apply to devices such as valves or smart transmitters and reduce the likelihood of systematic failures such that the requirements are aligned to the requirements of lEC 61508-2 for non PE devices. 

In general, the relevant aspects of the operating profile of field devices are different from those of a logic solver. 

For logic solvers, the following points contribute to the operating profile ... 

Requirements for the selection of LVL programmable components and subsystems (for example, logic solvers) based on prior use... 

This subclause lists additional requirements for LVL PE logic solvers having SIL 1 or SIL 2 capability. LVL PE logic solver with SIL 3 or 4 capability should be in accordance with lEC 61508-2 and lEC 61508-3. 

This subclause lists additional requirements to achieve SIL 1 and SIL 2 capability for a safety configured PE logic solver. For additional considerations, see Annex D. 

Safety-configured or lEC 61508 series compliant PE logic solvers typically include diagnostics which detect various faults. The types and diagnostic coverage will generally be described in the safety manual. 

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