Include Velocity Pressure

The inclusion of velocity pressure normally has very little effect on the hydraulic calculations, however certain approving authorities require velocity pressure to be included. Checking this check box will cause the program to include velocity pressures.

According to NFPA:

There are two methods of calculation of pressures throughout sprinkler/hydrant systems;- the Total Pressure method or the Velocity   Pressure method. The term Velocity Pressure method is something of a misnomer in that this method utilizes the “Normal” pressure to determine the flow through a sprinkler. In order to find the normal pressure, the velocity pressure needs to be calculated and subtracted from the Total pressure.

Hydraulic calculations of sprinkler systems using the Total pressure method are the most common. This method simplifies the calculations and in most cases, builds a safety factor into the calculations because the assumption is made that the total pressure (which is always higher than the normal pressure)  is responsible for pushing the water through the sprinkler.

When using the velocity pressure method of calculation, the normal pressure is used to determine the flow through the sprinkler except at the last sprinkler on the branch. For these sprinklers the total pressure is used because in these situations the velocity pressure will be acting in the same direction as the normal pressure.

In the program, Sprinklers connected directly along a pipe (but not at the end) with opposing flow into the sprinkler, it is assumed  that  the flow through the sprinkler is based on total pressure.

NFPA further states that:

NFPA 15, Standard for Water Spray Fixed Systems for Fire Protection requires the use of the Normal pressure method of calculation in many situations because of the possibility of long runs of small diameter pipe to the end nozzle and because it is common for large flow demands to discharge from the side of a tee.

Whether this applies to Mist systems is not stated.