The Canute FHC hydraulic calculation software can be used to calculate water mist systems for low, intermediate and high-pressure systems as defined by NFPA 750. Careful consideration needs to be given on which option you use in FHC for your calculation. This will depend on the system pressure, water velocity and if the system has any additives.
NFPA 750 allows the Hazen Williams formula to be used for water mist systems, including high-pressure systems (pressure exceeds 34.5 bar) if certain conditions are met, for intermediate and high-pressure systems, the water velocity must be below 7.6 m/s, and the smallest pipe diameter must not be less than 20 mm. The infographic below summarises the differences between the three pressure classifications and when the Hazen Williams pressure loss formula can be used.
The Hazen Williams pressure loss formula is the standard method Canute FHC when calculating pressure losses in the pipework. Therefore, you do not need to make any changes, but you must be careful that you selected the appropriate minimum pipe size and your water velocity does not exceed 7.6 m/s.
Our experience has shown that you will benefit from lower system pressure by using the Darcy Weisbach pressure loss formula for all water mist calculations. If you are designing a high-pressure system, then for practical purposes, you should select the Darcy Weisbach pressure loss formula by limiting yourself to pipe velocities of less than 6.7 m/s, and a minimum pipe size of 20 mm is probably impractical.
When making water mist calculations and your system meets the criteria which allow the Hazen Williams pressure loss formula to be used, you can specify a maximum water velocity in the calculation options as shown to the left. This dialogue is displayed after you select calculate, you can now enter 7.6 m/s as your maximum velocity and if this is exceeded FHC will display a dialogue box at the end of the calculation and will display the pipes exceeding this velocity.
You can also find the pipes with the highest velocities by going to the menu -> reports -> pipe summary and you will find that any pipes exceeding the set velocity will be displayed in red.
