Replies

  • We also have to assume pipe will not be subject to improper water chemistry(which caused damage you showed in this picture).

    Steve Wiencek said:

    3423775658?profile=original

     

     

             Richard-

                           The coefficient of friction is a misleading value.  As in the case of the photo above showing new flex versus 15 year old flex. The coefficient of friction is an empirical measurement – it has to be measured experimentally, and cannot be found through calculations. While your values may differ somewhat from my values, the overall point is that they are both based on new pipe and, in theory, should remain a near constant value throughout the life of the pipe. But how can they when one compares the interior roughness of the two pipes above? How does one even begin to quantify the cof of flex pipe when it is variable (read gets worse) over time?

     

     

    • how does the comparisson stack up on the basis you use less fittings with flex than you do with rigid?

      100ft say with two short radius bends at best or two 90 deg fittings vs no fittings on the flex?

  • 3423775659?profile=original

     

     

             Richard-

                           The coefficient of friction is a misleading value.  As in the case of the photo above showing new flex versus 15 year old flex. The coefficient of friction is an empirical measurement – it has to be measured experimentally, and cannot be found through calculations. While your values may differ somewhat from my values, the overall point is that they are both based on new pipe and, in theory, should remain a near constant value throughout the life of the pipe. But how can they when one compares the interior roughness of the two pipes above? How does one even begin to quantify the cof of flex pipe when it is variable (read gets worse) over time?

     

     

  •  

     The friction loss of 100' of 2" sch 40 pvc @ 40 gpm = 2.663 ft of head.

     The friction loss of 100' of 2" flex pvc @ 40 gpm = 2.75 ft of head. 

     This is obviously for new pipe

     

    Steve, where did you get this info?  I had a hard time finding data on flex PVC until I found the roughness here, but with the much greater absolute roughness of flex PVC (0.2 mm vs. 0.005 mm) I show 2" @ 40 GPM ft/100ft as going from 2.56 to 3.88.  The 2.56 I show vs. 2.663 you show for rigid PVC are essentially the same since there is a range of roughness assumptions as well as temperature, but the flex number you show would imply a flex PVC pipe that was nearly as smooth as rigid PVC.

     

    WAIT!  I called FlexPVC and they said just to add 5% to standard PVC losses since it is very close to rigid PVC.  He said the roughness was not 0.2 mm but far less in the hundreths of a millimeter.

  • There's a summary of issues with flexible PVC including the significantly lower working/burst PSI levels and temperature derating (the latter is the same as regular PVC).  There is also a FAQ.

     

    The absolute roughness for rigid PVC is from 0.0015 to 0.007 mm with an average of 0.004 mm which is 0.0001" so is significantly smoother than flexible PVC, but flex PVC is not rough everywhere -- it only has spaced ridges that come up around 0.01" to 0.02".  This link gives effective inner roughness of rigid PVC as 0.005 mm while flexible PVC is 0.2 mm so I was wrong in my initial post to assume they were similar.  I have a spreadsheet where you can enter the pipe size as well as the absolute roughness to get tables of head loss.  I do this below comparing rigid PVC (using 0.005 mm) vs. flexible PVC (using 0.2 mm).

     

    1.5" Pipe head loss (ft per 100 ft) .......... 2" Pipe head loss

    GPM .... Rigid PVC .... Flex PVC ........ Rigid PVC .... Flex PVC

    10 ............ 0.71 .............. 0.97 ................ 0.22 ............. 0.27

    20 ............ 2.45 .............. 3.68 ................ 0.74 ............. 1.01

    30 ............ 5.09 .............. 8.13 ................ 1.53 ............. 2.21

    40 ............ 8.57 ............ 14.31 ................ 2.56 ............. 3.88

    50 .......... 12.87 ............ 22.22 ................ 3.83 ............. 6.01

    60 .......... 17.97 ............ 31.87 ................ 5.33 ............. 8.60

    70 .......... 23.85  ........... 43.25  ............... 7.06 ........... 11.65

    80 .......... 30.51 ............ 56.37 ................ 9.01 ........... 15.16

    90 .......... 37.93  ........... 71.22  ............. 11.18 ........... 19.13

    100 ........ 46.11  ........... 87.80  ............. 13.57 ........... 23.57

  • Go back and replumb one pool due to termite damaged pipe and you will never go use it again.  Most of the builders in my area simply tell the customer it is no the fault of the installer and do not back thier job.  The flex pipe is sold with a disclaimer from the manufacturer.  Use it at your own risk.   The last time I talked to a pipe rep, he said there is nothing they can due to correct the problem.
  • "...is there any significant difference in calculating friction head loss?"

     


     The friction loss of 100' of 2" sch 40 pvc @ 40 gpm = 2.663 ft of head.

     The friction loss of 100' of 2" flex pvc @ 40 gpm = 2.75 ft of head. 

     This is obviously for new pipe

    John Warner said:

    Termites are also found in the "frigid north" common wherever there is "Yellow" sand. especially where builders have created "stump dumps". They are equally as destructive to vinyl liners. A coating of DE will deter them (it actually cuts them, and they ooze to death. Back to the pipe - you can also find chemical damage in flexible PVC downstream of In-line tablet chlorinators. Again the acidity of the trichlor tablets leaches out the plasticizers. Also fillers used in foreign manufacture make for an inferior quality. Given that I am aware of all of this, and that any flex I've used appears to me to be "bumpier" on the inside than sch 40 rigid (which is not perfectly smooth on the interior either); is there any significant difference in calculating friction head loss?
  • Termites are also found in the "frigid north" common wherever there is "Yellow" sand. especially where builders have created "stump dumps". They are equally as destructive to vinyl liners. A coating of DE will deter them (it actually cuts them, and they ooze to death. Back to the pipe - you can also find chemical damage in flexible PVC downstream of In-line tablet chlorinators. Again the acidity of the trichlor tablets leaches out the plasticizers. Also fillers used in foreign manufacture make for an inferior quality. Given that I am aware of all of this, and that any flex I've used appears to me to be "bumpier" on the inside than sch 40 rigid (which is not perfectly smooth on the interior either); is there any significant difference in calculating friction head loss?
  • That is an issue.  We don't have any insect damage like that in Ohio, so it is not an issue up here.  Definitely if that is an issue than it should never be used where you have termites. 

    (although i would trade warmer weather and termites for cold and snow)

  • 3423775273?profile=original"....it is not a failure of the pipe, it is a failure of the homeowner/service company for not being educated."

    If we could only just educate the termites not to eat it. Herein lies the problem.

    Matt Cellura said:

    I think the issue lies in the different types of flexible pipe that have been used over the years.  I too am a fan of rigid pvc, especially above ground on the system.  it is a neater installation and looks like a professional did it.  However, the new schedule 40 flexible PVC is perfectly acceptable for underground buried installation and in many cases will actually work better than rigid PVC.  there are no joints to ever fail, and since you can make large sweeps instead of elbows and 45s you are reducing the friction loss.  When done properly and backfilled properly it will last as long as rigid PVC. 

    The skimmer in the above pictures is a result of chemicals being added directly to the skimmer, most likely trichlor tabs.  The heavy concentration of chlorine causes the damage to the pipe, it is not a failure of the pipe, it is a failure of the homeowner/service company for not being educated. 

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