- MCA amends MGN 657 (M+F) for aerosol fire systems
- Design density clarified using net protected volume
- Approval process moves to UK Approved Bodies
- Test pass/fail criteria and mock-up requirements detailed
The UK Maritime and Coastguard Agency (MCA) has issued an amendment to MGN 657 (M+F) Amendment 1, updating the requirements for fixed aerosol fire extinguishing systems used in small vessel machinery spaces. The amendment provides clarification on design density calculations and formalises testing and approval procedures.
Clarification on Design Density and Testing
The amendment specifies that design density must be calculated using the net volume of the protected space, rather than gross volume. It also clarifies the applicable test criteria, including detailed pass/fail thresholds and the required arrangements for test mock-ups used during system approval.
Background to the Approval Process
Historically, approval of fixed aerosol fire extinguishing systems for use in small vessel machinery spaces was carried out directly by the MCA. Under this previous process, once a system was approved, the manufacturer or supplier was issued an MCA Certificate of Inspection and Test (MSF 1814 form). This certificate confirmed the system’s safe and effective operation following a series of fire-extinguishing tests conducted in an enclosure representative of a small vessel machinery space.
With the publication of the amended MGN, future approvals will now be undertaken by a UK Approved Body. The amendment formalises this transition by publishing the test requirements and mandating that tests are witnessed and confirmed as successful by a UK Approved Body.
Small vessels are defined as those referred to in Section 6 of the appendix to the MGN.
Key Changes to Design Density Calculations
The machinery space containing the fixed aerosol fire extinguishing system is referred to in the amended document as the protected enclosure or protected space. The volume of this protected enclosure must be calculated by the system supplier or installer and agreed with the vessel owner and equipment supplier. This information must then be provided to the surveyor for acceptance, as outlined in Section 5.2 of the MGN.
The required aerosol concentration varies between manufacturers and system types. Agreement on the design concentration for a given space must be obtained from the manufacturer or their agent and be based on fire-extinguishing tests defined within the MGN. The methods used to calculate the design concentration are required to be included in the vessel’s application for certification.
The quantity of extinguishing agent for the protected space is to be calculated at the minimum expected ambient temperature, using the design density based on the net volume of the protected space, including the casing. The net volume is defined as the portion of the gross volume that is accessible to the fire-extinguishing agent.
Updated Success Criteria for Testing
The amendment also sets out detailed success criteria for system testing:
- Class B fires must be extinguished within 30 seconds of the end of discharge, with no re-ignition when the enclosure is opened at the end of the hold period.
- Fuel spray must be shut off 15 seconds after extinguishment. At the end of the hold time, the fuel spray is to be restarted for 15 seconds before reopening the enclosure, with no re-ignition permitted.
- Test fuel trays must contain sufficient fuel at the ends to fully cover the tray bottom.
- Wood crib weight loss must not exceed 30% during the two-minute pre-burn test.
- A re-ignition test must be carried out within 30 seconds after discharge following successful extinguishment of the tell-tale fires in Test 1 (Fire A). Two tell-tale fire containers are to be subjected to attempted ignition, one at floor level and one at ceiling level in diagonally opposite corners. Electrical ignition sources are to be applied at intervals up to 14 minutes after extinguishment. Sustained burning of 30 seconds or more during any attempt constitutes test failure.
The amended MGN is intended to provide clearer, more consistent requirements for system design, testing, and approval, supporting the safe application of fixed aerosol fire extinguishing systems on small vessels.
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Source: IIMS
