The wall rate coefficient can depend on temperature and can also be correlated to pipe age and material.

It is well known that as metal pipes age their roughness tends to increase due to encrustation and tuburculation of corrosion products on the pipe walls. This increase in roughness produces a lower Hazen-Williams C-factor or a higher Darcy-Weisbach roughness coefficient, resulting in greater frictional headloss in flow through the pipe.
There is some evidence to suggest that the same processes that increase a pipe's roughness with age also tend to increase the reactivity of its wall with some chemical species, particularly chlorine and other disinfectants. EPANET can make each pipe's wall reaction coefficient (Kw) be a function of the coefficient used to describe its roughness. A different function applies depending on the formula used to compute headloss through the pipe:

Headloss Formula Wall Reaction Formula
Hazen-Williams Kw = F / C
Darcy-Weisbach Kw = -F / log(e/d)
Chezy-Manning Kw = F* N
where

C = Hazen-Williams C-factor

e = Darcy-Weisbach roughness
d = pipe diameter
N = Manning roughness coefficient
F = wall reaction - pipe roughness coefficient

The coefficient F must be developed from site-specific field measurements and will have a different meaning depending on which headloss equation is used. The advantage of using this approach is that it requires only a single parameter, F, to allow wall reaction coefficients to vary throughout the network in a physically me

Bulk Flow Reaction Rates

Bulk flow reactions are reactions that occur in the main flow stream of a pipe or in a storage tank, unaffected by any processes that might involve the pipe wall. HydrauliCAD accesses the EPANET engine to model these reactions using n-th order kinetics, where the instantaneous rate of reaction (R in mass/volume/time) is assumed to be concentration-dependent according to

where Kb = a bulk rate coefficient, C = reactant concentration (mass/volume), and n = a reaction order. Kb has units of concentration raised to the (1-n) power divided by time. It is positive for growth reactions and negative for decay reactions

EPANET can also consider reactions where a limiting concentration exists on the ultimate growth or loss of the substance. In this case the rate expression for a growth reaction becomes

where CL = the limiting concentration. (For decay reactions (CL - C) is replaced by (C - CL).)

Thus there are three parameters (Kb, CL, and n) that are used to characterize bulk reaction rates. General values can be selected for these parameters that lead to several well-known kinetic models. These include:

Simple First-Order Decay (Kb < 0, CL = 0, n = 1)
First-Order Saturation Growth (Kb > 0, CL > 0, n = 1)
Two-Component, Second-Order Decay (Kb < 0, CL ¹ 0, n = 2)
Michaelis-Menton Kinetics (Kb ¹ 0, CL > 0, n < 0)

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