How do I determine overpressure during a clean agent discharge?
The leakage rate IS important because there is a direct correlation between leakage area and peak pressure. If the leakage is small enough, every room will face destruction upon discharge. The reality is that most rooms are so leaky that the opposite is a problem. The agent typically leaks out so quickly that the enclosure doesn't pass the hold time requirements which means there is very little peak pressure which is why many systems do not require venting. For most FM200 systems there is a leakage range where the enclosure is tight enough to maintain sufficient hold time but not leaky enough to require vents. You'll only know if you're in that zone by performing a door fan test according to Annex C of NFPA 2001. ISO has a near identical Annex E.
Both Standards require a peak pressure evaluation to determine whether or not the enclosure will be damaged upon discharge. Unfortunately neither standard provides the peak pressure formula. Traditionally, manufacturers of inert systems have provided the necessary formula but none supplied them for halocarbons (chemical agents). A four year study of peak pressure at Fike which I was a lead participant with the involvement of Kidde Fenwal, 3M, DuPont and every other major player, determined that the inert formula in current use under-predicted peak pressure for inerts by 50%. We also uncovered the peak pressure characteristics for all the halocarbons which were reviewed by Joe Senecal, hear researcher at Kidde Fenwal and have since been published under FSSA and FIA's name. Other equations in common use are VdS and the FIA inert formula for which I have never seen any data. In the case of VdS halocarbon formula, they don't even predict negative peak pressures at all so are most likely based on theory only. I believe the FSSA formulae are correct since I have personally taken the data which has all been made public. No other data has been made public. Still, some insist on using VdS data because it requires smaller vents.
In every case the installer or testing agency must measure the leakage of the enclosure. From that value the hold time is determined using the ISO or NFPA standard. The peak pressure must also be calculated from this leakage value. In Europe, they typically do not rely on room leaks for peak pressure relief and install a vent large enough to vent the enclosure even though it may not be needed. Their theory is that accidental enclosure leaks might get sealed up but in reality they never do.
The second reason for not taking enclosure leaks into account is that in the design phase, the accidental leakage cannot be known. While this is true, the other is that enclosures must be extensively sealed at great cost of money and time to get them tight enough to pass the hold time evaluation so the air sealing can simply be stopped when the optimum hold time is reached. What actually happens in Europe, is that expensive air sealing is undertaken then a large pressure relief vent it added to put leakage back in. Even though this might appear to make little sense, there is some value in that the enclosure will be tighter with respect to smoke migration into the enclosure, temperature and humidity will be easier to control as will dust infiltration.
Just using an equation to determine peak pressure and putting in a vent to satisfy free venting is not enough.
Simply relying on vent manufacturers free vent area rating is insufficient because some are overstated since they do not open at a low enough pressure and for others the venting path is blocked, meaning there might be a vent there but there are other restrictions that reduce the vents effectiveness. The only sure way is to measure the free vent area in the enclosure directly using a door fan.
Pressure Relief Vents (PRVs) must also be checked upon installation and regularly thereafter that they actually open at the prescribed pressure and open in the correct direction. Many are installed backwards and have little value.
The reason why more enclosures have not been damaged is that they will take well over 1000 Pa in most cases without a problem and the enclosures themselves are usually too leaky to contain pressure in any case.
A properly tested system must have the enclosure tested with a door fan to determine hold time and peak pressure. Well over 90% of all the enclosures out there use Retrotec's FanTestic Integrity to perform this function and for use as a design tool. We invented this type of testing and have spent 1000s of hours of our own time in research and understand every aspect of enclosure design from a peak pressure and hold time perspective. We support over 4000 enclosure testers worldwide and have responsibly maintained both standards' enclosure testing annexes since 1984 for NFPA and a few years later for ISO. Most of the major manufacturers of agents and systems rely on our knowledge for normal and abnormal situations and we welcome your support questions whether you are a customer of ours or not.