During the investigation it appeared that only 2 (out of a bank of 6) Inergen bottles discharged into the enclosure. Initially it was thought the firing and/or release mechanisms may have been defective. The system has never been either fully function tested or used in anger previously. Understanding of the functionality of this system was limited. This unit was retro-fitted as a bespoke modification in c.1997 to replace the Halon systems. The design of the system is poor with no apparent evidence of suitability as a safety-critical system. It is complex and descriptions in the supporting documentation were lacking in detail and were ambiguous. Subsequently there was a general lack of knowledge and varied understanding of the system functionality by those expected to operate and maintain the Inergen system. The design relies on a signal from the Fire &Gas system triggering release of nitrogen from a small (70barg) cylinder (Figure 1). The pressure from this cylinder fires off the first cylinder of Inergen (Figure 2) and also holds open a spring loaded valve which diverts all the released Inergen from a main manifold to the relevant turbine enclosure (see Figure 3). The initial release of Inergen from the first cylinder backpressures the release mechanisms on the remaining 5 bottles (Figure 5). The conclusion is that the nitrogen pressure has bled away, causing the spring-return valve to close (the valve has no latch mechanism to keep it open) thereby preventing the full discharge of Inergen. Upon further review of the design of the package the investigation team and platform staff identified several potential single point failure mechanisms. System causes relate to original design inadequacies and lack of a suitable assurance routine on this safety-critical system. Documentation to support the design does not appear to have been subject to HAZOP review or FMEA analysis was not evident and it was unclear whether this had not been carried out or was just missing from the handover of information. Maintenance regime had focussed only on the Nitrogen trigger mechanism and a visual check of the pressure gauges for the Inergen Bottles and did not include a function test.
No details available.
1. Ensure all Fire Team members are made aware of this incident and given basic familiarisation with the Inergen package functionality.
2. Review maintenance and assurance routines for sufficiency to ensure continued suitability of this safety critical system. Review verification scheme performance standard for appropriateness.
Inergen Fixed Fire Suppression System in a Gas Turbine Enclosure