15Chapter 4 Siting Considerations
Searchline Excel™ Plus / Searchline Excel™ Edge – Technical Manual
4 Siting Considerations
When designing an installation for a Searchline Excel detector it is important to give due consideration as to the specific
application requirements, location and potential sources of problems that may be encountered.
Users are strongly recommended to consult Honeywell experts experienced in site mapping and siting of gas detectors.
4.1Safety Case Framework
In compliance with IEC 61508, IEC 61511 and EEMUA recommendations, the Safety Case Framework is used as a method
of reducing risks at hazardous installations to acceptable levels. The framework is based on the concept of Layers of
Protection which is widely recognized by the process industry and clearly defined in industry safety standards.
Considering the application of all types of the protection layers − some layers are preventative (e.g., emergency shut
down), while some are there to mitigate the impact of an incident should it occur (e.g., fire and gas protective systems or
plant emergency response systems). Other layers of protection can counter occurrence of incidents in the first place (e.g.,
plant and physical asset protection, constraint and boundary management, operator training, and asset management);
while others can provide detection and alerting, and associated guidance (e.g., operator alarms, early event detection,
and integrated operator procedures). Layers can either be automated, such as emergency shut down (ESD) equipment,
or require human interaction such as operator responses to process alarms. Some layers offer easily quantifiable
risk-reduction benefits but require that the risks all be identified before. And still others are less tangible and offer subtler
benefits.
An Open Path Gas Detector will generally be used as part of the Layered Safety Model. (for more information, refer to
https://www.honeywellprocess.com/library/marketing/brochures/A%20layered%20approach%20to%20plant%20
safety.pdf)
Figure 3.The Layered Safety Model
4.2Location for Best Coverage
Guidance on the positioning of gas detectors to provide the best detection coverage is contained in BS EN 6007929
2 & IEC 60079292 and other national Codes of Practice. It is recommended that the installation designer consults
these Codes of Practice when determining where detectors are to be located. Appropriate positioning should be
determined by the customer based on the Safety Case. In general, the following positions can provide the best results for
Searchline Excel Plus:
• Below potential leak sources for gases that are heavier than air
• Above potential leak sources for gases that are lighter than air
• Near to leak sources, along the expected leak trajectory, taking account of the prevailing wind direction and any other
factors which will influence propagation of the leak
• Between possible leak sources and any potential sources of ignition
The above is also valid for Searchline Excel Edge including:
• Siting along edges/boundaries of areas to be monitored
• Siting at height above vegetation and avoiding vehicles or other line of sight blockages
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