Safety Instrumented Systems

Minimizing Risk, Delivery Times and Costs in the Implementation of Safety Instrumented Systems Facility and worker safety is critical in process industries, but building and implementing safety instrumented systems can be complex and challenging. Sergio Diaz, Product Marketing Manager responsible for process safety systems at Emerson, explains how pre-engineered hardware solutions can simplify project execution and help save valuable time and resources. Safety Instrumented Systems (SIS) are commonly used in process facilities to bring equipment or a process to a safe state. Consisting of a designed set of sensing elements, logic solvers and final elements, SIS is designed to respond to facility requirements and produce the correct output before a dangerous event occurs. Today's SIS is based on intelligent safety sensors and final control elements with logic solvers built for digital communication, and includes advanced diagnostics and predictive intelligence to increase the reliability of all Safety Instrumented Functions (SIF). The IEC 61511 standard provides guidance on SIS implementation in process industry applications. To prevent safety issues stemming from excess pressure or tank overflows, a SIS requires a range of equipment to operate correctly and precisely. Sensors must be able to detect abnormal operating conditions such as high flow, low level or incorrect valve positioning. The Logic Solver must make appropriate decisions based on information provided by the sensor and accordingly change output according to user-defined logic. Output results in a final element that performs an action to bring equipment or process to a safe state (for example, closing a valve).

SIS Design

SIS is designed to meet the characteristics of the facility and specific safety applications. The system may require one or more SIF implementations, and these may be based on single or multiple signals. Therefore, each SIS installation will differ significantly from one application to another. Development and implementation of SIS always requires serious work since it demands tremendous effort to develop processes and procedures to address each phase of the project cycle. Working with competent SIS design engineers is critical to reduce or eliminate systematic failures resulting from inadequate specification or engineering. Once all SIS requirements are determined and Safety Integrity Levels (SIL) of the SIF are defined, appropriate technology is selected and installation is performed, after which the installed system is tested for functionality and operability.

SIS Projects

Although safety is the primary criterion and correct implementation is critical, SIS projects are subject to the same scrutiny as other automation projects in terms of cost and time scale. There is pressure to manage project risk, capital costs and planning. In projects where new systems or system expansions are undertaken, the planned downtime understood cannot be exceeded. Projects must be completed on time to ensure production restart. Failure to do so can result in significant financial losses in terms of production loss. For this reason, strict planning is mandatory and every effort should be made to minimize overall project risk. Project execution generally proceeds sequentially because in many cases completion of one matter is required before starting the process. For example, the number and type of I/O must be known before design of a SIS cabinet can be initiated or before proceeding to SIF configuration. When each team is rushed with tasks, time can be wasted simply waiting for another team to continue their work.

Conventional Approach

Generally, SIS has been installed the same way for many years. Essentially, different types of I/O cards and conditioning devices must be cross-connected to conventional marshalling terminals for each signal type. The company providing the SIS must know the I/O layout before sending cabinet designs for customer approval. I/O cannot be finalized until Hazard and Operability (HAZOP) analysis is completed and required SIFs are designed to meet the target SIL rating. Time pressures typically push projects forward with the assumption that at some point there will be I/O changes requiring redesign and rework. Software and hardware are ultimately tested by the customer at the supplier's factory. The entire process requires various interactions between the two parties that can consume considerable time and resources.

Disadvantages of Custom-Built Cabinets

Although the requirements and implementation of each SIS differ, certain elements offer opportunities to simplify the design and implementation process. The SIS cabinet is one such element and a good place to start the process. Normally, a "custom-built" SIS cabinet would be designed by extending the cabinet's features and customization as desired. On the surface, this sounds logical; however, when analyzing the work involved, this often outweighs the benefits. Engineering and design time is required for planning, configuration and design of any custom panels or connection boxes. This costs time and money. The project schedule must typically account for the time required to design control panels and field connection boxes, including the required Factory Acceptance Test (FAT). The project will also include time for detailed design, project management and administration. As the size of a project grows, there will be a significant increase in the time required to design, manufacture and test custom cabinets. Timing and costs can be affected by errors made during manufacture and wiring of custom cabinets. Many wiring-related errors are typically found during FAT, and correcting these errors takes time, which can extend the entire project schedule.

Impact of Late Changes on I/O

Any late changes to I/O will affect nearly every aspect of the cabinet. While they may not require complete replacement, they may require extensive modification and even redesign. I/O changes will certainly affect engineering drawings and likely impact wiring. Logic Solvers and power supplies may also need to be relocated and/or added. These changes and additions tend to be expensive. Changes can occur even after cabinets have been shipped to the field. When this happens, it may be cheaper and faster to send the cabinet back to the panel manufacturer rather than rely on inexperienced local labor to make changes in the field. In any case, cost and scheduling are seriously affected (for large projects this can take weeks or even months), which can impact startup. If IEC 61511 is being followed, rework also affects the verification process required by the standard.

Alternative Approach

For several years, end users have been able to simplify distributed control system design and implementation processes by taking advantage of the availability of cabinets configured to order. The same form of flexibility can be provided in engineered or pre-engineered SIS. These offer an easy-to-implement option that can provide significant savings in time and resources. Pre-engineered solutions are useful for most safety applications in small and medium-sized projects. Standard solutions exist for applications such as emergency shutdown, burner management, fire and gas detection, High Integrity Pressure Protection Systems (HIPPS) and Automatic Overfill Protection Systems (AOPS). These solutions are also available for applications replacing aging safety systems that could cause reliability issues, or for concerns regarding IEC 61511 compliance. Pre-engineered SIS provides end users with simple options to specify pre-configured standard cabinets or field enclosures as needed. The aspect requiring attention is cabinet size, depending on total I/O capacity.

Electronic Marshalling

A key component of these pre-engineered solutions is electronic marshalling. This technology eliminates the need for traditional marshalling cabinets. Signals can be terminated directly at electronic marshalling terminals from the field and connected in any order. Intrinsically safe and non-intrinsically safe signals can reside in the same cabinet. For each signal, a characterization module appropriate to electrical requirements is selected. Characterization modules are available for nearly all commonly encountered I/O signal types and need not be specified until the final stages of projects. These can support different voltages, analog and digital inputs or outputs. The cabinet itself can be shipped without knowing the I/O signal type. Characterization modules can be added when the signal type becomes known and can even be left until I/O signal cables are terminated in electronic marshalling terminal blocks. This flexibility benefits the project by allowing field wiring termination much earlier in the project. Alternatively, final design decisions in the project can be accepted later in the process. End users must select a cabinet size based on the maximum I/O capacity they anticipate. If intrinsically safe signals are required, the IS and non-IS I/O distribution must be determined to ensure signal separation in the cabinet. End users should determine with the SIS how it will integrate with existing systems and facility operators. Is it an expansion project with full integration with existing DCS? Or an independent SIS connected to an existing DCS through open communication protocols such as Modbus TCP? Other considerations include whether a local HMI panel is required.

Late Binding of I/O

Two important requirements exist to separate hardware and software design. First, I/O references are defined based on tag name rather than hardware location; in this way hardware and software design are completely separated. Second, the ability to automatically bind hardware to I/O reference. This method is necessary to easily bring together independently executed hardware and software designs.

Standards Compliance

From a hardware standpoint, all standard requirements are met by the factory and documents are sent with standard boxes facilitating IEC 61511 compliance.

Simple Decisions

In general, configuring system hardware is reduced to three simple choices: cabinet style, system size and required system interfaces. Pre-engineered hardware solutions can be shipped ready for field installation. End users only need to make power, ground and communication connections. Gökhan Karatoprak Senior Sales Engineer Emerson Process Management