FIRE, GAS AND SMOKE DETECTOR SELECTION

 

FGSs and their related detection systems shall comply with the requirements of instrument protective functions. This includes determining the required SIL: the required degree of fault tolerance and the maximum probability of failure on demand for the defined set of detectors that are to detect the identified fire or gas release.

 

The exact number and location of FGSs and their associated equipment (horns, beacons, etc.) shall be determined during detailed engineering, and such detectors and their location shall be indicated on the master plan of fire safety systems. The make and specific type of FGSs shall be stipulated in the project specification. Determining the correct location to site fire and gas detectors is not straightforward, particularly in congested areas. Special software has been developed for this purpose, and such software has the potential to optimise the effectiveness of the FGSs. It does this by enabling an engineer to consider different detector locations and evaluate different voting strategies. The result is a compliance (performance) map that shows if the chosen detector locations reduce the identified risks to ALARP. Such software is commonly referred to as “Fire & Gas Detection Mapping” and is available from a number of sources.

Fire & Gas mapping software needs a 3D model of the facility, and this is typically supplied by the Contractor. This enables detection compliance maps to be produced that show the safety contribution for each detector; thus enabling the safety versus economic value of each detector to be judged. The software also determines the safety coverage under detector fault conditions.

 

Fire & Gas Mapping software may not be required for all designs. For example, where designs are small or for limited detection refurbishment, where the additional cost may not be justified. However, Fire & Gas Mapping shall be considered.

The hardwired interface between the detector or control unit and the FGS IPS shall be either 0 - 20 mA or potential-free contact, and detectors shall be configured individually for alarm purposes.

 

Individual (point) monitoring Each detector is considered on an individual basis in order to provide a one-to-one alarm / message presentation to the operator, i.e. each alarm / message represents one detector only. This can be achieved by applying: - a serial pulse-code type data transmission (bus) between the detector and the rest of the system, in which case a number of detectors are interconnected in series and terminated to one system input. Serial pulsed coded detectors are not allowed for SIL 1 and higher unless TÜV Bayern or Rheinland approved according to the requirement class (AK class) that is related to the SIL or equivalent AK class; - an analogue data transmission (typically 0 – 20 mA) between the detector and the rest of the system, in which case each detector is connected to an individual system input (star configuration).

 

Zone monitoring A number of detectors are connected to one system input only, to provide an 'n' to one alarm / message presentation to the operator, i.e., one common zone alarm / message represents any one of these detectors. The design of the detector(s) should allow them to be calibrated and verified by a single person.