Equivalent Length

The use of equivalent lengths is supported by AFT Fathom. This option is hidden by default, but can be enabled in Analysis Setup on the Junction Loss Models panel. After enabling this feature the properties windows for the bend and valve junctions will have another type of loss model called "Equivalent Length". In the pipe properties window the table in the fittings & losses tab will change as well. The options for bend and valve types come from table 23.4.3.1.1 in NFPA 13NFPA 13, Installation of Sprinkler Systems, 1999 Edition, page 13-139.. If you are not using the NFPA 13 standard then using a user specified equivalent length is recommended. To enable NFPA reporting for use with equivalent length losses see the NFPA panel topic.

The loss factors in table 23.4.3.1.1 assume that a Hazen-Williams C-factor of 120 and schedule 40 steel piping are being used. If your reference pipe is defined using a different Hazen-Williams factor or different pipe schedule, then the equivalent length will be adjusted by equation 6.2 (Equation 1 below) on page 103 of Brock, Pat D. 2012Brock, Pat D., Fire Protection Hydraulics and Water Supply Analysis, Third Edition, Fire Protection Publications Oklahoma State University, Copyright 2012 by the Board of Regents, Oklahoma State University..

(1)

Where:

L_eq = new equivalent length
L₁ = original length
D_eq = diameter of new equivalent length
D₁ = original diameter
C_eq = C-Factor for new equivalent length
C₁ = original C-factor

This equation is intended for taking an equivalent length and adjusting for a new equivalent length based off changes to diameter and C-Factor. This is a general equation, but NFPA table data from table 23.4.3.1.1 is only for schedule 40 steel and a C-Factor of 120. Therefore the adjusted equivalent length can be written as shown in Equation 2.

(2)

Equation 2 (adjusted from Brock et. al. Equation 6.2) will predict an adjusted equivalent length that coincides with the NFPA 13 equivalent length modifier 23.4.3.1.3.1 and the C-Value Multiplier from table 23.4.3.2.1. Note that if the reference piping is using a friction model other than Hazen-WilliamsC_eq is assumed to be 120 and the equivalent length will not be adjusted due to friction.