The Theory and Practice of Variable Orifice in Automatic Sprinklers VOS Fire Protection Engineering Ltd Ralph R Mehr D.Sc. PE

Q=K*P ½ Q = The Flow [gpm] P = The Pressure [psi] K = A constant Coefficient [gpm/psi½]

Q=V*P ½ 5.6 gpm/psi½ ≤ V ≤ 28gpm/psi½ 7 psi ≤ P ≤ 175 psi

V=a*P b V=2.12*P 1/2 Q=2.12*P

The size of V as a function of the Pressure P

V performance compare to K

Add-on Device stand-by position maximum pressure

Special Sprinkler stand-by position maximum pressure

Insert to a Large Sprinkler The Insert stand-by maximum

Droplet Size Description Sprinkler Type Low PressureHigh Pressure The ESFR barely had drops at this pressure. It was closer to a continuous drip of water at the center of the spray pattern with a small canopy at which the droplets were large, but with no thrust. The ESFR sprinkler had large drops but there was also a fair amount of mist that was created by the sprinkler as well. ESFR The VOS had large drops that were also mixed with a little mist. There was quite obviously a better developed spray pattern, but the water in the center of the spray pattern was noticeably reduced. The VOS had smaller drops than the ESFR sprinkler, but the same amount of mist was created. VOS

Spray Pattern Radius [m] (ft)Sprinkler Type Low PressureHigh Pressure 1.30 (4.25)4.04 (13.3)ESFR 2.19 (7.17)4.36 (14.3)VOS

Spray PatternSprinkler Type Low PressureHigh Pressure ESFR VOS

Final Conclusions The Variable Orifice Sprinkler can operate within a wide range of pressures. The Variable Orifice Sprinkler demonstrates a high degree of flexibility, and can operate at all kinds of hazard occupancies from light hazard to extra hazard and special hazard, including storage etc. As the first sprinklers deliver much more water, it is expected that fewer sprinklers will be needed to control or even to suppress the fire, as a result, less water might be needed and less water damage is expected.

Recommendations for additional research Determination of the optimal function of V in the basic equation (Q=V*P ½ ) so that the first sprinklers will deliver the maximum possible water, while all sprinklers calculated will deliver a total amount of water, as required by the standard. As the insert type device seems to be the most cost effective, the insert should be designed to best comply with the optimal function of V. A full scale research to re-determine the minimum number of sprinklers needed for each occupancy hazard should be performed. Investigation to determine if the VOS fulfils the criteria of Quick Response Early Suppression sprinklers (QRES) as defined by the standard (Budnick & Fleming) which may improve the Fire Protection Level with much less water.

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