C factor of 90 probably is a little bit higher for old pipes if your system contains old Asbestos concrete pipe. Based on my knowledge, most asbestos concrete pipes were laid between &#;s and &#;s. Most of them are more than 60 yrs old. The age of ductile pipe may between 20 and 50 yrs old.
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You can find some references on the book of &#;Effective Modelling of Urban Water System, Monograph 13&#; (published by CHI, ISBN ). On page 125, Figure 7.4 gives C factors varying with time (from new to 75 yrs old). This data was obtained from a survey of 7 cities in North
America. From the figure, the range of C factors,
for 10 yrs old pipe, is from 113 to131;
for 20 yrs old pipe, is from 98 to 124;
for 30 yrs old pipe, is from 85 to 120;
for 40 yrs old pipe, is from 71 to 117;
for 50 yrs old pipe, is from 62 to 118;
for 60 yrs old pipe, is from 56 to 113;
for 70 yrs old pipe, is from 54 to 111.
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