CO2 gas suppression is still a popular choice for industrial applications, the question is which design method do we use?

Unlike Inerting or Halocarbon agents which have one means of calculation CO2 has four types of calculation to choose between.

Total flooding design differs for deep seated fires and surface fires. Local application design differs from surface type applications i.e. flat surfaces of tanks or the volume method which is used for three dimensional irregular objects like transformers or printing presses. A further complication is added for high pressure systems or low pressure systems used for CO2 storage.

CO2 Local ApplicationWhen CO2 is the selected agent, the designer will use SANS 306:2019 for reference and calculation, the designer must select the correct application method before starting the design. His second action is to calculate the gas quantity by hand as the hydraulic pipe design, unlike the other clean agents, does not calculate the gas quantity.

When the gas quantity is calculated this is input along with the pipe design layout parameters to the computer programme that will then provide the pipe sizes.

This all straightforward until we are given a transformer room that has large vents in the room where leakage will occur. How do we allow for the leakage?

Some traditional designers will calculate the size of the leakage area and add another 5 or 10% CO2 agent to cater for the leakage. This is total guess work and not in compliance with any standard.

With this example we choose a total flooding application for a deep seated fire and use table 3 to obtain the gas quantity. The complication is how do we cater for the leakage of gas we know is going to occur? This is achieved by utilising paragraph 10.3.2 in SANS306:2019

For total flooding systems a well-enclosed space is assumed in order to minimize the loss of CO2. Any un-closable openings shall be compensated for by additional CO2 as specified in 10.3.2.2. A simplified calculation is available in Figure 1

If the quantity of CO2 required for compensation exceeds the basic quantity of CO2 required for flooding without leakage, the system shall be designed for local application in accordance with clause 11, Local application.

Should we find leakage is excessive we then have to use the Volume method of calculation as found in SANS 306:2019 11.8. As the transformer will be an irregular shape we build an assumed box around the transformer to achieve an assumed volume. The following formula can be used to calculate the required quantity of CO2.

ASSUMED VOLUME x DISCHARGE RATE x DISCHARGE TIME x VAPOUR PHASE x MATERIAL CONVERSION FACTOR

CO2 NozzleIt must be remembered if one uses this local application method of calculation, the installation must be done with local application nozzles installed to distribute gas directly onto the transformer.