safety instrumented systems: can they be integrated but separate

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This evolutionary system is really only integrated at the HMI level. Theengineering and operations functionality is isolated from the actual logic solverusing a safety write protection layer of software. This layer protects the logicsolver from receiving unsolicited data from elements in the BPCS system.Although the engineering/configuration environment appears to be identical tothe BPCS and utilizes a common format there are significant differences in theactual operation. The safety system configuration tools require a completelydifferent level of security for access and use. When an individual logs on, he/shemust have Safety security clearance to be allowed into the safety configurationenvironment. Once in the safety configuration mode a completely different andunique set of TUV approved function blocks are available that can be configuredand downloaded to a safety logic solver. Safety blocks cannot be downloaded toa BPCS controller and BPCS function blocks and code cannot be downloaded toa SIS module.The HMI layer combines data in the event summaries, historical data base, etc.and allows the BPCS access to all the information from the SIS but does notallow the BPCS to write to the SIS. This feature eliminates the need for the datamapping and buses previously needed to integrate these elements. Each tag isdefined one time in the system as either an SIS or BPCS tag. It is then availablefor trending, display on graphics, historization, etc.The BPCS and SIS systems share a common Alarm Summary Screen. SISAlarms are shown in a different category from process alarms. SIS events arealso integrated into the BPCS Event Chronicle. This allows for SIS events to besorted separately from the BPCS events or sorted by time to see process andSIS reactions to a certain event. The SIS Network is integrated into theDiagnostics Explorer as well for easy viewing of SIS network and hardwarediagnostics.AMS ADVANTAGESThe architecture discussed above is an integrated SIS system that takes intoaccount the HART diagnostics from the end devices in the SIF. This system wasdesigned to comply with IEC 61508 and to make it easier for end user’s tocomply with IEC 61511.Using intelligent field devices communicating using the HART protocol providesimproved safety and significant cost reductions in operations and maintenance.These savings result from extensive diagnostics allowing longer test intervals.HART data is used within the SIS to determine the data integrity of point beforethe SIS reacts to it. This improves plant availability by lowering the number ofspurious trips caused by malfunctioning devices.By eliminating the need for muxes there are fewer third party componentsneeded within your BPCS and SIS systems. It also allows for a seamless
interface for both systems without the need for serial or OPC hardware andsoftware links.Moving the focus back to the SIF as a whole, it is important to look at the finalelement and risks traditionally associated with it in an SIS system. The finalcontrol element (valve) in an SIF has a very simple job. When an event occurs, itneeds to go to a safe position to mitigate the risk of injury to plant personnel.This seems simple enough, but in reality these valves are rarely used so withouttesting there is no way to ensure that the valve will go to a safe position whencalled upon to do so. Because of these concerns, good engineering practicerequires the testing of the final control element at regular intervals to be able toclaim a certain RRF. This testing, while simple in nature, can introduce risk intothe plant environment. Many plants have installed bypass valves, double blockand bleed valves and expensive pneumatic panels to be able to test the final SIScontrol elements without shutting down the plant. Manual installation ofmechanical valve interlocks (pins, etc) can put operations and maintenancepersonnel in hazardous areas of the plant during installation and removal ofthese devices. If the devices fail to be removed after testing, they can preventthe final element from closing during a safety event.Using HART and automated partial stroke testing allows for the extension of thetime intervals between full-stroke testing on plant final control elements. This canhave a significant positive effect on plant shutdowns and turnarounds. Assumingthe valve used has diagnostic capabilities and it is integrated with an assetmanagement package that supports this functionality, pneumatic supply, actuatorpressure, and valve position are tested to verify that the components will performin an actual event. Since this testing is done automatically by the device itself, itreduces human error, risks to operations and maintenance personnel and leadsto better maintenance practices. Partial stroke testing can also be scheduled torun automatically on a set time interval. The testing and the results will becaptured in the Event Chronicle, which can be used as documentation to provethat the test was performed.SIS ADVANTAGESThe combination of integration and separation will allow this system to achieve arating of SIL 3 with a single simplex logic solver. A redundant logic solverconfiguration is also available to increase availability but is not required for a SIL3 rating. The integration of the SIS and BPCS will also reduce maintenancecosts by providing a common operator interface for SIS and BPCS functions,managing bypasses during start up sequences, synchronizing time and collectingevents between both systems and by performing continuous diagnostic testing ofsensors and final control elements. Since the same vendor supplies bothsystems, it also minimizes the number of people to contact to get the support andanswers needed if there is a problem. Training and maintenance costs can also
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safety instrumented systems: can they be integrated but separate

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