You are entering an interesting field. I see that after more than a month you have not really gotten answer to the specific question you asked (prompting you to ask it again). I am aware that in some of the earlier days of piping, unlined or various thin bituminous lined iron and steel pipes were in various fashion flow tested to see what head loss occurred in that (of course then most often relatively “new”) piping. I suspect some designers probably went out and designed more piping, assuming that was the flow performance they would have in the field. It was eventually found that over a period of time there was more pressure drop in (or less water delivery from) some of that piping than what there was initially. In the most prevalent cases where substantial changes were seen (when there were not other more obvious causes such as closed valves, obstructions, sediments, large leakages etc.), it was eventually found that the reductions in flow were either the result of scale build-up due to very hard waters, or instead internal tuberculation/corrosion build-up in pipes carrying certain soft and aggressive water conditions (see prior thread on this forum, before I joined, that discusses some aspects of this at
http://www.bossintl.com/forums/showt...ight/hdpe.html). Both these conditions in their most severe forms in effect substantially reduce the internal flow diameter of pipe, of course with an understandable, substantial reduction in time in the “C-factor”.
With that industry knowledge and the subsequent development of cement mortar linings (first used e.g. in some USA iron pipes in 1922), I believe there was then greater sensitivity to flow properties and the subject of maintenance of flow properties over time. For this reason I know many utilities worked with first the Cast Iron Pipe Research Association (CIPRA) and then later the Ductile Iron Pipe Research Association (DIPRA) to run flow tests on sections of their piping, to get a better feel for what flow was actually being achieved by that piping in the field (with normal quality of installation and other real world effects etc.), and in some cases they repeated these tests over time to see what was happening in these areas of their systems. Voluminous results of this field research are of course now available as a result of this experience e.g. in Tables 1 and 2 of the publication at
http://www.dipra.org/pdf/cementMortarLinings.pdf (I think more than sixty flow tests of working cement lined iron pipelines over a near 80 year time span, and at least in recent decades I believe virtually all accomplished by registered professional engineers and with no little interest of the utilities involved, are represented in these tables), as well as many other published studies by other third-parties. I would also recommend reading the passage, “Flow Coefficient…” on page 6 of this DIPRA document, as I believe it refers/has relevance to other types of pipes. I believe it is not unusual to see just a few points drop in any installed pipeline over many years, including even cement lined pipes that don’t significantly tuberculate (as illustrated in Table 2). This can occur in some cases for many reasons, including many that may have absolutely nothing to do with the condition of the contact surface of the pipe. However, you do not normally see drastic drop as in unlined pipes (unless there is a reason other than tuberculation). E.g. I believe any types of material pipelines can develop air locks and/or sedimentation, or formation of “scales” etc., if the flows and/or water chemistry etc. is such so as to result in such effects.
It seems quite odd however (considering the fact that they claim to have been around at least to some extent for many decades) that similar published databases are apparently not available for the plastics. I believe flow performance of installed pipelines is thus far more complicated than laboratory testing of a few pipes, and for that matter should be far more than perception bourne by plastic marketing hype (or even the “feel” of a pipe wall). In this regard (I think as also taught by the aforementioned earlier post on this forum) the flow performance of most at least contemporary types of pipe with effectively clean water laboratory flow performance only a few percent off the “smooth pipe curve”, including perhaps hdpe piping at least that is not too “wavy” and with the internal “beads” removed, is most likely best predicted by the actual internal flow diameter of the pipe (of course the outside diameter minus the sum of the wall thicknesses). I would be very careful otherwise about plugging in more favorable flow coefficients for even treated water piping that are not reasonably substantiated by field testing/experience.
I believe gravity sewer pipes and force mains (some subject to low or intermittent flows) can additionally be a special case, due to more likelihood of significant slimes, sedimentations, obstructions etc., that I believe incidentally can occur in any piping material.
ASCE MOP No. 60 (WPCF MOP No. FD-5), “Gravity Sanitary Sewer Design and Construction” suggests the same range of Mannings “n” values for “Cement-lined and seal coated cast iron” and “plastic pipe (smooth)” (both namely 0.011-0.015). The 1997 version of “Recommended Standards for Wastewater Facilities” (Ten States Standards) stated under Section 48.6 Design Friction Losses (sub-section 48.61 Friction Coefficient), “…for other smooth pipe materials such as pvc, polyethylene, lined ductile iron, etc. a higher “C” value not to exceed 120 may be allowed for design.”
While I realize the plastic pipe industry advertises/claims extremely favorable flow coefficients, you may be interested in the threads
http://www.eng-tips.com/viewthread.c...=124331&page=9 , the site
http://www.canclay.com/htdocs/Slimin...es%20Large.pdf , the thread at
http://www.eng-tips.com/viewthread.c...73633&page=19, and the table “C-factor Values…” in the “C. Tables” section of the relatively new reference at
http://www.haestad.com/library/books...tml/wwhelp.htm , that may give other glimpses of the real world. Sorry for the long post, but I noticed you were a student (who might have some interest in this history) and perhaps also this will prompt better response for you. Good luck with your project.
Randy Conner