Field sensors Final control elements

It may also be able to justify less fault tolerance than required by Table 6, when the dangerous failure modes of the SIF devices and associated process interfaces are well understood. Clause 11.4.4 states that if the selection of a device is based on prior use, then, under specific conditions, the fault tolerance for sensors, final control elements, and non-PE logic solvers can be reduced by 1. The reduction of fault tolerance is acceptable, since prior use establishes the field application data, which includes the random hardware failures for the device itself and the random failures due to the process and field device interfaces. 

The sensor, transmitter, and control valve are physically located on the process equipment ( in the field ). The controller is usually located on a panel or in a computer in a control room that is some distance from the process equipment. Wires connect the two locations, carrying current signals from transmitters to the controller and from the controller to the final control element. 

Requirements of SIS for field bus, sensor, logic solvers, and final control elements failure rates, specification, selection procedure. Alarm management layers of protections. 

Detailed discussions at component level starting from sensors, safety field bus, safe logic solvers, and final control elements are a unique addition to the book. The book also covers the security aspects of various networks, viz. firewalls and zone-conduit to name a few, which is also another unique feature of the book. The book also covers application of SIS in various plants covering fossil fuel power stations, nuclear power stations, oil and gas sector such as upstream, midstream, refinery, and petrochemicals. There have been supplementary data and information on statistical approaches, embedded controls, and cost impact and life cycle cost analysis to take care of advancement in technology and systematic approach toward the problem. 

In the above example, there was no distinction between process field equipment (sensors and final elements) and the logic solver. In an actual installation, it may be possible to provide independence between the field hardware used for control and that used for risk reduction. Consider a common logic solver with one control loop having its own sensor and control valve and one SIF with its own sensor and final element. For this example, the control loop can be an initiating event for the hazard scenario that the SIF is designed to prevent. 

Even the tight controls in siUcon integrated circuit manufacturing are not yet sufficient to produce absolutely identical sensors on a single wafer. Cahbration of the final product is usually necessary, often by adjusting the value of a circuit element on the IC such as a resistor. The caUbration process can be automated, but it stiU adds to the cost of batch-fabricated sensors. Clever means of self-caUbration, particularly in field use, are constantiy being sought. 

Thus, additional risk reduction can be achieved using existing BPCS field elements. However, this approach should only be used when fault tolerance has been provided in accordance with ANSI/ISA-84.00.01-2004, Clause 11.4. For SIL 1 and SIL 2, this requires at least one (1) SIS sensor or valve in addition to the control sensor or valve using fault tolerant architectures, such as 1oo2, 2oo3. This approach should not be used for architectures that do not have fault tolerance, such as lool, 2oo2, etc. Further, for SIL 3, two sensors and final elements independent of the initiating cause should be provided to meet the fault tolerance requirements. 

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