Installation of Safety-Related Systems

When locating safety instruments on racks or in cabinets, care must be given to assure that no two redundant instruments are mounted on the same rack or in the same cabinet.

Safety-related wiring, sensing lines, and mechanical signal lines shall not be routed where vibration, abnormal heat, or stress could affect performance.

Installations shall conform to instrument location, installation and isometric (if provided) drawings. These documents shall establish the installation design requirements for safety Class and Safety Significant instruments and their sensing lines, with regard to their safety function, postulated health hazard and their protection against failure.

Safety Class redundant instruments, instrument tubing, and piping (sensing lines) shall be routed and/or protected to withstand the credible effects both during and following design bases accidents for which the instruments/systems are required to perform.

Separation of redundant Safety Class or redundant (as determined by safety analysis) Safety Significant instrument shall be achieved by the use of structures, distance, barriers, or any combination thereof. Any deviation from these methods of separation must be submitted.

Redundant (as determined by safety analysis) Safety Significant instrument sensing lines shall be routed and protected so that the failure of one redundant system will not disable equipment essential to the operation of the other redundant system(s). Sensing lines of one channel shall not crossover or come in contact with equipment of another redundant channel, whether it is in the same or another functional loop of another channel.

The minimum separation between instrument sensing lines of redundant channels shall be at least 46 cm (18 inches) in air in both horizontal and vertical directions in non-missile or jet impingement areas. The 46 cm (18 inches) minimum spacing required between the redundant channels shall be maintained from its starting point at the root valve to the vicinity of the instrument. If this separation is not possible, Engineering shall be consulted to determine if a suitable barrier should be used. A barrier may be equipment, structural steel shapes, building structures such as walls, ceilings, floors and shield walls.

When a barrier is used, it shall extend at least 2.5 cm (1 inch) beyond the line of sight between the two redundant channel sensing lines. Where potential missiles can be identified, additional separation, barriers and/or missile shields may be necessary. Missile shields may be structural steel shapes such as plate, channel and angle, covered tray or pipe guards.

Supports, brackets, clips or hangers shall not be fastened to the sensing lines or their supports for the purpose of supporting other equipment, cables, etc., without specific approval.

Where instrument sensing lines of more than one channel of a redundant set penetrate a wall or floor, the redundant sensing lines shall be routed through separate penetrations and separated by a minimum distance of 46 cm (18 inches). If the use of separate penetrations is not feasible, approval is required to use a common penetration. The use of a common penetration may require the design of:

  • A suitable barrier, such as a guard pipe, to protect instrument sensing lines in one channel or division from postulated effects of a failure of the other channels or divisions.

  • A missile shield, to be installed around the lines until a minimum separation distance of 46 cm (18 inches) is achieved between the different redundant sensing lines.

Instrumentation and sensing lines shall be easily identified and distinctly labeled as Safety Significant. Each instrument sensing line, as a minimum, shall be tagged at its process line root valve connection, at the instrument, and at any point in between where the sensing line passes through a wall or a floor (on both sides of such penetrations).

Barriers used to protect instrumentation (as determined by safety analysis) shall be identified in the field, to prevent inadvertent degradation of this protection.

To prevent the loss of both parts of a redundant set of instruments, separate process pipe connections with sufficient separation shall be used wherever possible.

  • When a single process connection must be used, the system shall be designed for a “safe” trip action of the channel upon tap or sensing line breakage.

  • The single process connection shall be protected from credible sources of damage and separation of the redundant sensing lines shall be achieved as close as possible to the process connection.

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