Deluge Nozzle Pressure

[AlisonM]

NFPA 15 section 8.1.2 states "Minimum operating pressure of any nozzle protecting outdoor hazards shall be 20 PSI (1.4 bar)." Whilst NFPA 15:A8.1.2 states "The minimum operating pressure is required for proper pattern development and to overcome the effects of wind. For nozzles with orifices of 3/8" (9.5mm) or less, a minimum pressure of 30PSI (1.4kPa) is recommended." My question is this: Is the minimum operating pressure of outdoor nozzles 1.4 bar and that will overcome the effects of the wind? How is "wind" defined i.e., is this a coastal high wind or a general daily wind? Is the higher pressure of 30PSI only required for nozzles with orifices of 9.5mm or less i.e., if a nozzle is in a windy location but is 10mm is 1.4 bar OK? Also anyone else notice that 30PSI is not 1.4 kPA but just below 2.1 bar? so which is correct? I have an installation and the nozzles are reaching 1.4bar but when tested on a windy day the spray just blew back. Any ideas?

 

[UFT12]

The explanatory comment in the annex is meant as follows:

A) Nozzles are to be calculated with a minimum design (often also called 'start') pressure of 1.4 bar. But that does not mean you just jump into it like that doing the calcs. This is considered the absolute minimum. Your application may require higher pressure. This is determined in conjunction with the nozzle manufacturer recommendation for the minimum pressure and depending on the K factor you choose for the given flow as well as what discharge you want to achieve.

B) The wind is not quantified in the standard. It is just a recommendation for ordinary winds but this is by its nature quite vague. The best thing to do is to check with the nozzle manufacturer describing them the type and geometry of the hazard as well as the intensity of the prevailing winds in the area of your hazard.

C) The smaller the K factor the smaller the droplets. Smaller droplets are carried away easier and therefore the committee put a demarcation line of 3/8" which under that your droplets will need more momentum which in turn can only be achieved by increasing the pressure.

As for the units, just forget about it. I 've personally found tons and tons of errors in conversions in almost any handbook or standard that uses the IP system as the primary unit system. Just follow my advice, ignore all units in parentheses and convert yourself.