Previously, we discussed broadly the dangers posed by the toxic gas Hydrogen Sulphide (H2S), why this is a problem in the industry, specifically with reference to onsite personnel, the physiological effects, exposure limits and detection limits typical for toxic gas detection. This article continues to examine how a performance-based gas detection system can be designed using a methodology that is easily repeatable, auditable and results in an effective detection arrangement, whilst ensuring compliance.
Compliance & Performance Based Design
Figure A aims to outline the basic steps of what should be considered applying a holistic approach for an effective toxic gas detection design. It is simply not enough to note there is H2S present on the site and base the design on this. One must review how much is in the stream, at which positions this becomes a hazard of concern and to which equipment it is associated, therefore analysis of the stream compositions must be carried out as part of the hazard analysis.
As mentioned in part one of this article, the codes, standards and guidance recommended for use can vary from country to country and site to site. Frequently referenced documents include; HSE No.6/2009[1], ISO 10418:2003[3], API RP 14C[4] and HSE 2013 RR973[5]. Consideration must also be given to the on-site philosophies and operator engineering technical practices (ETPs).
While these documents give advice of the acceptable exposure limits to H2S and hence points towards suitable detector alarm set-points, there is no all-encompassing document that details where these devices should be placed, how many units are required, and which detection technology should be used. Hence reliance on a strictly prescriptive based approach is highly impractical, which is why we must tailor each design on a case by case basis founded on the objective performance of the system.
As with any performance-based approach the designer must hone in on what is expected from the system, and define the performance required. Figure A touches on important factors of setting performance targets for a toxic gas detection system.
Mapping, Repeatability & Auditability
Once the performance expectations of the system are defined, an effective method of proving these targets have been met is to carry out a modelling exercise. Micropack’s market leading fire and gas mapping software, HazMap3D, is equipped with all the tools to map an effective design whichever standard, guidelines or ETPs are specified. Utilising 3D mapping tools verified by fire & gas experts allows the user to apply the predefined detection thresholds to equipment/areas of concern and assess, resulting in quantifiable outputs for the end client. This then aids in proving that an area of concern is protected to an acceptable level. The entire process is effortlessly repeatable and traceable and is easily auditable when required.
The following case study outlines the basic steps to consider during the design of a performance based toxic gas (H2S) detection system.
Case Study – HazMap3D Toxic Gas Detection Assessment (TGDA)
This case study relates to a greenfield extension platform of an established oil and gas production platform. The mapping results are shown for one level of this new module.
It is essential to review/analyse the process stream compositions to pass judgement on whether the streams; exceed acceptable limits of concentration for H2S, or, yield a concentration for which it is permissible to infer the presence of H2S by responding to flammable gas detection. Therefore, determining if an appropriate dedicated, fixed toxic gas detection is required or reliance on existing adequate flammable gas detection arrangement is appropriate. The following figure attempts to outline the basic steps to consider during the performance based design.
Analysis & Mapping
The analysis of the stream compositions revealed the H2S concentration was >500ppm, hence a dedicated H2S fixed detection system in compliance with onsite philosophy and ISO 10418 (where applicable) was needed.
During the geographic review of the area the access/egress routes and entrances/exits were noted, with reference to the escape route and safety equipment diagrams; this information was input into HazMap3D for assessment, in the form of a grade map as depicted in Figure B.
Before the detection is mapped, we must consider detector technology/make/model. Some ETPs have guidelines on which to use on their sites. Technologies include, electrochemical, semiconductor and laser based open path. HazMap3D is equipped with a detection library to aid the user with the selection process. Each of the detection types has strengths and limitations, these will not be presented here however the reader is encouraged to get in touch if required. For this case study the detector type was chosen to maintain site continuity.
The detectors are then added, firstly based on the FEED/as built F&G layouts and then, dependent on their coverage achieved, a proposed/recommended arrangement. Due to the nature of toxic gas they should be focused along egress routes along with entrances and exits from these areas; where personnel can be expected to be present during normal working conditions.
The software examines the proximity of the detectors to the egress paths based on the generated grademap and produces coverage results based upon a desired 5m spacing between detectors, which was compliant with the project specific onsite philosophy. Many toxic gas detection philosophies also require further ‘manual’ assessment of egress routes in ensuring that entrances/exits are adequately covered.
The 3D & representative 2D results of the proposed arrangement are shown in Figure C. The 2D assessment is shown for a horizontal ‘slice’ through the module, selected at a height close to the ‘breathing zone’ level, to best show the protection to personnel. Toxic gas detectors have been recommended to be located at an elevation of between 1.2m and 1.5m above local deck in order to best protect personnel.
It is important to note that an appropriately designed fixed toxic gas detection should not be considered a replacement for effective working practices, training and suitable personnel protective equipment (breathing apparatus etc).
In summary, this article previously discussed the dangers posed by Hydrogen Sulphide (H2S), particularly to personnel, the physiological effects, exposure and detection limits. Here, we have examined how a performance-based gas detection system can be designed using a repeatable and auditable methodology to ensure effective detection, whilst ensuring compliance with site requirements. Different hazards, such as heavy gas, flammable/combustible gas or special hazards (e.g.: methanol/condensate vapours) and require separate consideration.
For more information go to www.micropackfireandgas.com
References
- HSE Offshore Information Sheet No. 6/2009 Managing Hydrogen Sulphide Detection Offshore
- HSE EH40/2005 Workplace Exposure Limits
- EN ISO 10418:2003 Annex F, Toxic Gases
- API RP 14C Appendix F, Toxic Gases Section
- HSE 2013 RR973 – Review of alarm setting for toxic gas and oxygen detectors
- HSE EH40/2005 Workplace Exposure Limits
- EN ISO 10418:2003 Annex F, Toxic Gases
- API RP 14C Appendix F, Toxic Gases Section
About the Author
Written by Gemma Finnegan MEng MSc CFSP MIET Senior Safety Consultant Micropack (Engineering) Ltd.
