Carbon dioxide fire protection system evaluation

Carbon Dioxide is a fire extinguishing agent which has the advantage that no ‘clean-up’ is required. As a non-conductor, CO2 is suitable for the protection of electrical equipment, and is commonly used in environments such as maritime platforms and data server rooms.

Carbon Dioxide is a fire extinguishing agent which has the advantage that no ‘clean-up’ is required. As a non-conductor, CO₂ is suitable for the protection of electrical equipment, and is commonly used in environments such as maritime platforms and data server rooms.

Our client installed a ‘total flooding’ CO₂ fire suppression system in a ship compartment, but under physical testing O₂ sensors indicated that the oxygen reduction rate was below that required by regulatory requirements. In fact, the issue persisted despite modifications being made to the system to improve its CO₂ release rate and capacity.

Frazer-Nash Consultancy was tasked to identify the cause of non-compliance, and a number of possible root causes were postulated:

• Insufficient rate of discharge of CO₂
• Inadequate mixing of air with CO₂
• Direct leakage of CO₂ to outside the space requiring protection.

We carried out multiple investigations including:

• A site inspection of the system and compartment
• An interrogation of data recorded during the physical testing, which included O₂ and CO₂ concentrations, pressure and temperature, and a video of the release

Plus theoretical CO₂ discharge rate calculations via:

• Methodology given in the NFPA 12 Standard, and
• A 1-D two-phase flow simulation derived from first principles, accounting for the CO₂ being physically ‘boiling’ all the way down the release pipework.

Site inspection identified that there was little CO₂ leakage potential, and blockages that could significantly impede gas mixing were also unlikely. These were supported by sensor readings and video footage that showed uniform mixing was achieved shortly after the initiation of gas discharge. The room pressure built-up was also in agreement with that calculated for a low CO₂ leakage.

While NFPA 12 is typically used at the design stage to ensure a system meets the industrial requirements, an iterative approach was developed in MATLAB to enable the evaluation of CO₂ discharge from an existing system. In parallel, our 1-D flow simulation provided not only an independent validation of the NFPA methodology, but also an extension to conditions not accounted for by the international standard, namely the variation in CO₂ storage temperature, and the cooling of the bottles during discharge; both have notable effects on the CO₂ pressure and flow. From these calculations, we confirmed the discharge rates were highly consistent with measurement data, providing crucial evidence in supporting the root cause of non-compliance identified. 

The investigation has provided our client with confidence in understanding the issues, and how to redesign the system to rectify them, which is currently ongoing.