Why is the ASTM standard for SDR 'pressure rated' wheras the ASTM standard for Sch 40 is not designated 'pressure rated'? ie - what is the significance of 'pressure rated'?
My understanding of the significance of "pressure rated" is this. "Pressure rated", in regards to SDR21 pipe, means just that. Since the ratio of wall thickness to pipe diameter is constant, all SDR21 pipe regardless of diameter will have the same pressure rating. That is all "pressure rated" means. SDR21 pipe has a pressure rating of 200 PSI.
SDR26 pipe has a pressure rating of 160 PSI. Every pipe diameter in a particular SDR is rated at the same pressure. This can be very useful if you are designing a system with a range of pipe sizes operating at the same pressure throughout.
Schedule 40 pipe on the other hand has a varying ratio of wall thickness to pipe diameter. Therefore, there is no way one can make the general statement that Schedule 40 pipe is rated at XX psi. Because the ratio varies, every pipe diameter is capable of withstanding a different pressure. However, due to the physical characteristics of the pipe, the "pressure rating" of Sch. 40 pipe tends to follow the trend of varying inversely proportional to the diameter. That is, the smaller the diameter of the pipe, the larger the pressure it is capable of withstanding.http://www.harvel.com/tech-specs-pvc-pipe-40.asp
The systems that I design generally do not exceed 40,000 GPD. These systems have never required any pipe greater than 4" in diameter. I have no use in specifying a whole range of pipe diameters that are "pressure rated" at a constant pressure. Especially when I can specify a material that is far more robust than any readily available SDR product.
Additionally, schedule pipe allows for up to 20% non PVC content, and SDR pipe doesn't.
This might have some bearing on the argument they are making. Where does that statement come from? If I specify that the pipe be made of PVC 1120, Type I, Grade I Cell Classification 12454 per ASTM D1784 (as is typical of both types of pipe) are they not then made of the same material?For anyone else following this thread that might find this information useful:
Here is what Harvel had to say on the matter:
Thank you for contacting Harvel with your inquiry. Please note that Harvel manufactures both Sch 40 and SDR series PVC pipe from our own blend of NSF Listed PVC material. There is no difference in the PVC material that we utilize to manufacture these products (PVC 1120, Type I, Grade I Cell Classification 12454 per ASTM D1784) nor any differences manufacturing processes. As you are aware, the difference is in the physical dimensions of the products (wall thickness/pressure bearing capability). Perhaps they may have Sch 40 PVC solid wall pipe confused with Sch 40 foam core PVC pipe?.
Harvel solid wall Schedule 40 PVC pipe has been used for direct burial applications since its inception over 45 years ago (as have the other schedules and SDR's that we produce). I have attached a copy of our product specification sheet applicable to our Sch 40 piping for your review and submission to MDEQ. It should be noted that there are basically two separate ASTM standards pertaining to Sch 40 PVC; ASTM D2665 for DWV (includes deflection load requirements for buried lines) and ASTM D1785 for pressure (includes hydrostatic pressure requirements for long-term pressure bearing). Harvel Sch 40 PVC pipe is Listed by NSF as meeting both of these requirements, and is dual-marked as such. This information is referenced in the attached product specification. Please also refer to the information below from our on-line Technical Support Center pertaining to Critical Collapse Pressure of the various piping products we produce. As can be seen, small diameter Sch 40 piping (4" & smaller) can handle a higher load than the same size SDR 21 piping due to the difference in wall thickness of the products. I have also attached information below pertaining to the installation of buried pipe which may prove beneficial for future reference.
Of particular interest was the attached engineering data pertaining to burial pressures. Here is a link to that information:http://www.harvel.com/tech-support-eng-neg.asp
It shows a comparison of PVC duct, SDR 41, SDR 26, SDR 21, SCH 40, SCH 80, and SCH 120 pipe.
Thanks to everyone for all of the information and comments.