PVC PIPE TUTORIAL
SDR or DR or Schedule XX:
PVC pipe may be classified as SDR or DR or Schedule XX.
SDR and DR are essentially the same thing (Standard Dimension Ratio). Both are the average outside diameter divided by the minimum wall thickness. A French engineer came up with a series of preferred numbers and these are the "Standards" (thus, SDR). Many others have since been developed and are frequently referred to as DR. Some references use the "S" on all. It really doesn't make the least bit of difference. I'll use SDR from here out to refer to both.
Since SDR is a diameter to thickness ratio, all pipes with the same SDR have the same pressure rating.
Schedule 40 pipes all have the same wall thickness. Schedule 80 pipes all have the same. Thus, all pipes that are Schedule 40 do not have the same pressure rating. Same for Schedule 80. There's also Schedule 120.
The four standard specifications for underground PVC are as follows:
D2241, C900 and C905 follow the SDR system.
D1785 follows the Schedule XX system.
You can find pressure ratings at Uni-Bell. If you want to know how to calculate the pressure rating, here are the formulas:
For D1785, D2241 and C905, use the following:
For C900, use the following:
Now notice a difference in the above! If you calculate the same pipe through both formulas you'll get different results. So, DR18 run through the D2241 formula will come out as 235 psi. Calculate the same pipe for C900 and you'll get 150 psi. C900 has a higher safety factor and a surge allowance.
C900 (which is smaller pipe) is generally used on lines with more valves and loops. C905 is larger pipe where there are fewer valves. Thus, one uses the surge pressure in it's rating calculation. So, if you're using C905 and know that you're going to have surges, you'd best take them into consideration since the formula doesn't.
C900 usually comes in SDRs of 14, 18 and 25. The pressures, respectfully, are 200, 150 and 100 psi. If you compare these between C900 and D2241 you'll see how the formulas come up different. 100 psi pipe meeting C900 will be DR25. But 100 psi for D2241 will have a DR of 41.
You'll also run across the "Class" designation. Class 100 is 100 psi. Etc...
Gravity Sewer Pipe:
Gravity sewer generally uses SDR35 pipe. This pipe is ASTM D3034 for 4" through 15" and it is ASTM F679 for 18" through 27". The pipe uses gasketed joints and can take some low pressure but is not designed to. Larger sewer pipe is ribbed and is called Profile Pipe.
Gravity sewer pipe is usually tested with air pressure.
PVC pipe does well under vacuums but you usually don't want one in a water system.
Deflection and Burial Depth:
Under a soil load, a pipe will deflect (become egg-shaped). The diameter will reduce in the vertical direction while widening in the horizontal. ASTM allows a deflection of 7.5% and AWWA allows 5%. The Uni-Bell website can help you with the calculations. Look for their FAQ.
Any flexible pipe will deflect, not just PVC. Pressure pipe should not deflect over 5% long term. Gravity sewer pipe should not exceed 7.5%.
The Uni-Bell website also has some tables for determining the maximum depth of burial for PVC pipe. Be careful since their tables have no safety factors built in. Note that their C900 table allows for highway loads (H20) while their gravity sewer pipe (SDR35) table does not. Even better, look in their Download section and get a free program!
PVC Pipe Tables:
You can find a great set of PVC pipe tables at Uni-Bell.
This entire web site is Copyright 1999 to 2010 by Mark Turner.