To follow up on Artisi's point about actual pipe diameter (you may or may not know the following, but I am providing this discussion to help others as well as yourself):
Your 2.5" and 4" diameters are almost certainly nominal sizes, not actual diameters. Many pipe standards show significant differences between nominal sizes and actual diameters and these differences are not the same for each size or for each pipe class. Also, different pipe types (based on their particular standards) have different differences between nominal sizes and actual diameters.
Because these differences can be significant, your final hydraulic calculations should be based on actual diameters, not nominal sizes. Depending on circumstances, I sometimes use nominal sizes for preliminary calculations to get close to the desired pipe size, but I always based final calculations on actual diameters.
Here is an example of this "chaos":, for 4" nominal pipe size:
4" AWWA C900 PVC (typically used as water pipe)
- 305 psi (DR 14): nominal ID = 4.07" (for 6" and larger, nominal ID is less than nominal size; because of manufacturing tolerances with plastic pipe, "nominal ID" is used instead of "actual ID")
- 235 psi (DR 18): nominal ID = 4.23" (for 8" and larger, nominal ID is less than nominal size; for a 30" pipe, the nominal ID is 28.23", which is almost 2" smaller than the nominal size)
- 165 psi (DR 25): nominal ID = 4.39" (for 16" and larger, nominal ID is less than nominal size)
C900 pipe is sized to match the ODs of cast and ductile iron pipe so they can use the same fittings (this makes it easier to sell PVC). The IDs are smaller than the same nominal size iron pipes because iron is stronger than PVC.
4" ASTM D3034 PVC (typically used as sewer pipe)
- DR 26 (PS 115): nominal ID = 3.891"
- DR 35 (PS 46): nominal ID = 3.975"
4" HDPE DIPS Pressure Pipe, PE 4710
- DR 7 (333 psi): average ID = 3.346" (the average ID for a 20" pipe is only 15.058")
- DR 9 (250 psi): average ID = 3.670"
- DR 11 (200 psi): average ID = 3.876"
- DR 13.5 (160 psi): average ID = 4.045"
- DR 17 (125 psi): average ID = 4.202"
- DR 19 (111 psi): average ID = 4.264
- DR 21 (100 psi): average ID = 4.315"
- DR 26 (80 psi): average ID = 4.408"
- DR 32.5 (63 psi): 4.486" (the average ID for a 20" pipe is 20.190 and the average ID becomes less than the nominal size only for 30" through 48")
4" HDPE IPS pressure pipe has different dimensions than 4" HDPE DIPS pressure pipe. These two pipe standards are sized to match two different iron pipe standards. As above, the IDs are smaller because iron is stronger than PVC.
Ductile iron pipe has its own set of IDs for different pressure classes. If you specify double lining thickness, subtract a little more.
Steel pipe has it's own set of IDs for different classes and can be had with different lining thicknesses. Larger diameter spiral weld pipes with cement mortar lining can be made so that the actual diameter matches the nominal diameter.
I have probably gone on too long, and there are more pipe types out there that have other dimensional standards. So, long story short, use actual ID's for hydraulic calcs.
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