Static (Normal) or Stagnation (Total) Pressure for Fire Sprinkler Flowrate Calculation? 2

[az5333]

Hi NFPA fellow members,

I posted this question in Mechanical piping forum as well but posting it here as I got to know about this forum a little later. The link to that thread is as mentioned below:

Link

I am working on designing a wet pipe fire sprinkler system for a small room and I am using "Fluidflow software" to perform the hydraulic calculations. I am stuck at a very simple concept but I need your help in clarifying that. Sprinkler manufacturers recommend that the sprinkler operate at or above a minimum residual (flowing) pressure of 7 psi. Now my question is, when we look at the pressure available at the inlet of the sprinkler head, do I look at the stagnation pressure (normal pressure plus velocity pressure) or the static pressure (normal pressure)? Also what pressure is used to determine flow through a sprinkler based on it's nominal K-factor, static or stagnation? I found out very recently that NFPA 13 allows us to use the total pressure (stagnation pressure) to calculate flow through the sprinkler (please see NFPA 13 27.2.4.10.1. Considering this clause, it looks like I have to use stagnation pressure (total pressure) to evaluate the sprinkler flow and this is exactly what the software is doing as well. Appreciate the ingiht from NFPA experts. Thanks.

 

[UFT12]

Your terminology about pressures is not appropriate for water based fire fighting systems. The normal + velocity pressure is called total. The normal pressure is just normal but it is not called static when the water is flowing rather residual because it is the remaining pressure as water has "stolen" energy to move (velocity pressure). That said, NFPA 13 allows to perform calculations disregarding the velocity pressure working only with total pressure which is the pressure to use when working with sprinklers and hydraulic calculations when using this option.

 

[LittleInch]

Please look at the original thread (in the LINK) which has many responses and this really is a signpost to it. There is much more data there also.

It does need someone with NFPA sprinkler design to look at it and see if what has been said is correct or not.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[az5333]

Hi UFT12,

Thanks for your response. Apologies for using the wrong terminology. So from what I have deduced so far, I can use the "Total Pressure" for calculating flow through a sprinkler head using the Q=K*√P formula, where P refers to the "Total Pressure" = "Normal Pressure" + "Velocity Pressure". I am also disregarding the 1-2/" by 3/4" reducer attached between the sprinkler and the drop piping, as NFPA states that the loss for that is included in the K-factor of the sprinkler itself.