SIS-TECH knows how to do it right, because we have worked directly for companies like Amoco, Arco, Lyondell, Ethyl, Union Carbide, ChevronPhillips, and others. We understand what it feels like to get called out at 3:00am, because a faulty instrumented system is not working. We also know how good it feels to execute a project that meets the needs of plant operations and fulfills the requirements of ANSI/ISA 84.00.01-2004 (IEC 61511 modified).

Our capabilities in SIS design and software development are recognized worldwide. We are frequently invited to speak at industrial conferences, to participate in new standard's development and to teach workshops and courses. If we have a "been there, done that" attitude, it is because we have. Our clients have often stated that they initially chose SIS-TECH because they knew that we would provide an ANSI/ISA 84.00.01-2004 (IEC 61511 modified) compliant safety instrumented system. They formed an alliance with us because our designs yielded superior performance and our professionalism made project execution easy.

The following are just some of the design and engineering services that SIS-TECH provides:

• Safety Requirements Specification
• Safety Function Sheets
• Safety Function Design Documents
• Software Requirements Specification - Logic Flow Charts
• Application Software Development
• Factory Acceptance Test (FAT)
  

At SIS-TECH, the safety requirements specification (SRS) is developed in two parts: the safety function sheets and the SIS design documents. A proven-in-use work process is used that integrates the ANSI/ISA 84.01-2004 lifecycle with a project management process. The work process provides new facilities with means to incorporate the standard's requirements into the project process with minimum disruption of the normal process. It also provides existing facilities with effective methods for documenting that the safety functions achieve the required risk reduction, thus fulfilling the "Grandfather Clause" requirements.

For new projects - The safety function sheets provide a clear description of each safety instrumented function (SIF) and are initially developed during front-end loading. The safety function sheets also provide the preliminary SIL verification for more accurate estimation of the detailed design and installation cost. As detailed design progresses, the safety function sheets and average probability of failure on demand (PFD avg) calculations are updated to incorporate design and logic modifications. Also, during detailed design, safety function design documents are generated, which serve as programming, commissioning, training, and procedural tools. These design documents include overview drawings, cause and effect matrix, instrument index, power distribution overview, and approved communication list.

For existing installations - the SIS standards require that you document how the existing systems achieve the allocated risk reduction. Although design documents were created during the original detailed design, there is typically little documentation defining the functional and integrity requirements for each individual safety function. The safety function sheets provide the functional, diagnostics, manual shutdown, testing, and bypassing requirements necessary to achieve the required risk reduction. The safety function sheet fulfills the "Grandfather Clause."

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The safety function sheet includes the following information:

• Input
  • Type
  • Redundancy
  • Voting Architecture
  • Testing Interval
• Logic Solver Type
• Actuator
  • Type
  • Redundancy
  • Voting Architecture
  • Test Interval
• Final Element
  • Type
  • Redundancy
  • Voting Architecture
  • Testing Interval
  • Diagnostic Requirements For All Devices
• Alarms
• Maintenance Provisions
• Bypass Requirements
• Manual ESD Requirements
• SIL Verification
• Predicted Spurious Trip Rate

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SIS Overview
SIS Overview drawings are developed to depict the SIS field instrumentation and auxiliary components. These drawings are depicted in P&ID format, but should not be confused with Unit P&IDs. The SIS Overview drawings do not contain all field instrumentation. The drawings are restricted to the instrumentation specifically related to each safety function. Process equipment and piping are shown as necessary to provide the general location of the safety instrumentation.

Cause and Effect Matrix
The cause and effect matrix shows the process conditions (cause) that are monitored and the mitigating actions (effect) that are taken as the result of the process conditions. The cause and effect matrix also provides the voting, trip points, and any required explanatory comments.

Power Distribution Drawing
Power distribution is critical to the safe operation of energize-to-trip safety systems. A dependable power distribution improves the reliability of de-energize to trip SIS. A simplified power distribution drawing is provided for the overall SIS.

SIS Instrument Index
The SIS instrument index identifies the input/output points, including field devices, local panel instruments, control room panel instruments, and DCS. The instrument index contains the following information:

• Tag Number.
• Service Description.
• Device Type.
• Signal Type.
• Testing Interval.
• SIS Overview Drawing Number.
• Comments.
• Revision Number.
• DCS Message and Alarm List.

Any communication between the DCS and SIS must be performed in a manner that does not impact the integrity or functional independence of the SIS. Consequently, any communication must be reviewed carefully to verify that the SIS application program is secure during start-up, normal operation, and shutdown. While it is common to communicate start-up permissives, shutdown resets, diagnostic alarms, and valve position indications between the DCS and SIS, any communication must be approved. The DCS Message and Alarm List provides the approved communications between the DCS and the SIS.

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ANSI/ISA 84.00.01-2004 requires the development of a software requirements specification that defines the following:

1. the SIF functional logic to be performed by the logic solver.
2. the non-SIF functional logic to be performed by the logic solver.
3. any diagnostics that are to be performed by the logic, how the logic should be modified when faults are identified, and when and where alarms should be displayed.
4. any communication between the SIS and other devices, such as the operator HMI.
5. the sequence of data processing with respect to the input/outputs (I/O).

At SIS-TECH, logic flow charts are used to document the software requirements specification. The logic flow charts are used for the application software validation at the Factory Acceptance Test (FAT) or Site Acceptance Test (SAT). Their use has been proven to reduce time spent writing the program and testing the logic during the FAT. Logic flow charts are excellent tools for change management when used in conjunction with the provided status and flag listing. You can quickly determine how program changes can impact the functioning of various safety instrumented functions. An examination of the status and flag listing reveals how your changes to internal variables affect the application software operation. Typically, the logic flow charts include the following:

• Automatic SIS functions
• Process permissives
• Manual functions
• Operation actions
• Operator interface display/indications

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The application software implemented in the SIS logic solver must meet the intent of the safety requirements specification. It should be developed in a modular format, capable of diagnostic testing for inputs and outputs, written in a structured order, capable of safe modification, and provide traceability. Adequate notation should be provided so that the program can be understood by those responsible for making changes in the future.

SIS-TECH uses "proven-in-use" function blocks that have been field-proven to provide safe operation. Guidelines are used to establish a consistent programming style from project to project. SIS-TECH follows the intent of the ANSI/ISA 84.01-2004 Part 1 Clause 12 in the development of your application software. A formal design review is performed at integral stages of the programming lifecycle demonstrating specific requirements are being met.

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Effective FAT procedures demonstrate the functionality of the SIS hardware and logic. SIS-TECH generates step-by-step procedures incorporating sign-off spaces for critical steps. These procedures have sufficient detail to address testing of the critical hardware and logic functionality. SIS-TECH procedures contain the following information:

• SIS logic description
• Trip values
• Instrument ranges
• Testing tolerances
• Communications to occur between the SIS and the BPCS
• Bypass initiation, if required
• Alarms
• Indications

SIS-TECH provides FAT support and conducts the FAT in accordance with the procedure.

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