Description

Technical Data Sheet covering water mist system calculations in FHC, including NFPA 750 pressure classifications, pressure loss formula selection, project settings and automatic pipe sizing.

Technical Data Sheet TDS-01 explains how to set up and run water mist hydraulic calculations in FHC. It is written for fire protection engineers who are already familiar with calculating a project in FHC and with water mist design principles, and who need to know which pressure loss formula applies and which project settings to change.

What it covers

• The low, intermediate and high pressure classifications defined by NFPA 750, with nozzle pressure ranges and typical applications.
• The conditions under which NFPA 750 permits the Hazen-Williams formula, including the 7.6 m/s velocity limit and the 20 mm minimum pipe diameter for intermediate- and high-pressure systems.
• Switching a project to Darcy-Weisbach using the DWF button on the Project data tab, and setting water temperature, density and viscosity.
• The DWF command statement and its DENS and VISC parameters, and when to edit them.
• Selecting the correct design authority, and the risk of using a formula the standard does not permit.
• Project settings that do and do not apply to water mist work: design area, water supply pump curves, head area and head density.
• Automatic pipe sizing on tree systems, including zeroing pipe sizes, setting the sizing criteria, and capturing the final sizes.

Standards referenced

The guidance follows the NFPA 750 Standard on Water Mist Fire Protection Systems for pressure classifications, permitted pressure-loss formulae, and water density and viscosity values. It does not replace NFPA 750 or any other standard you are designing to, and it does not cover system design, nozzle selection or acceptance testing.

Related resources

For background on the underlying friction-loss theory, see our Darcy-Weisbach article, which includes an interactive Moody diagram. The pipe velocity calculator is useful for checking designs against the 7.6 m/s limit, and full details of the program are available on the FHC product page.

TDS-01 is one of a series of Technical Data Sheets supplementing the FHC User Manual. Issue 4 applies to FHC version 1.8.6 and later. Use it alongside the User Manual to set up water-mist hydraulic calculations in FHC.

Issue 4: 31-Aug-26, Reviewed and corrected; procedures renumbered; screenshots recaptured.