safety instrumented systems: can they be integrated but separate

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