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PVC PIPE ALLOWABLE BENDING STRESS 1
- Thread starter jeg1234
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You will find that the information is not available in the Codes and Standards. Thisis not only AMSE but ISO, AS , JIS etc.
The approach with thermopastic pipe materials has been to focus on hoop stress. The assumption being made is that the bending stresss is a 25% proportion of the hoop stress as a maximum.
The subject is made more complicated by the creep and strain rate properties of the thermoplastic material. The Codes and Standards ignore these properties and refer you to the manufacturer. The manufacturer knows how complex it is and doesnt want to spend time providing you with an education . Mainly because they dont want to scare you off using the material.
The other parameter to take into account is that of fracture toughness. This criteria may limit the use of PVC-U . This material is temperature dependent. Below 5C the fracture toughness is reduced.
You may get more success from the resin rather than the pipe manufacturer. Even then the pipe manufacture may doctor the resin to aid extrusion. Although the "U" stands for "unplasticised", you may find that some of the additives are actually "plasticisers" but described as "fillers"!
The other property that could be used is the ultimate tensile stress.
Refer to Process Piping The Complete Guide to ASME B31.3 Charles Brecht IV. He gives an explanation of the ASME B31.3 approach.
The approach with thermopastic pipe materials has been to focus on hoop stress. The assumption being made is that the bending stresss is a 25% proportion of the hoop stress as a maximum.
The subject is made more complicated by the creep and strain rate properties of the thermoplastic material. The Codes and Standards ignore these properties and refer you to the manufacturer. The manufacturer knows how complex it is and doesnt want to spend time providing you with an education . Mainly because they dont want to scare you off using the material.
The other parameter to take into account is that of fracture toughness. This criteria may limit the use of PVC-U . This material is temperature dependent. Below 5C the fracture toughness is reduced.
You may get more success from the resin rather than the pipe manufacturer. Even then the pipe manufacture may doctor the resin to aid extrusion. Although the "U" stands for "unplasticised", you may find that some of the additives are actually "plasticisers" but described as "fillers"!
The other property that could be used is the ultimate tensile stress.
Refer to Process Piping The Complete Guide to ASME B31.3 Charles Brecht IV. He gives an explanation of the ASME B31.3 approach.
Interesting thread. I think I once read in a 10-15 page chapter of an old "Handbook of PVC Pipe" from Uni-Bell (it appeared devoted to this subject) that "Longitudinal Bending" of pvc pipes "may be done deliberately" or may occur in response to "unplanned conditions" or "unforeseen changes" including, "Differential settlement...", "Uneven settlement...", "Ground movement...", "Erosion of bedding...due to pipeline leakage", "Seasonal variation in soil conditions due to changes in moisture content...", and "Improper installation procedures..." etc.
This reference went on to say that the "ALLOWABLE BENDING STRESSES" for pvc pipe at 73.4 degrees F. varied from 800 psi (5.52 Mpa) to 1,000 psi (6.89 MPa), depending it appears on whether the particular pipe involved was "Pressure Class" or "Pressure Rated" (I guess the difference being a little higher CLAIMED "factor of safety" for the latter). While perhaps not too well known in some areas, I also once read in a perhaps even older paper written more than 20 years ago by the then president of Uni-Bell, that "The allowable bending stress in the PVC pressure pipe where a direct tap is placed should not exceed 800 psi (5.52 MPa)." For some reason or omission specific guidance or warning about tapping longitudinally bent pipes did not appear mentioned at all in the Handbook chapter I referred to above that specifically talked about longitudinal bending of pipes. I suspect a concern about tapping bent pipes is perhaps among other things due to a stress concentration factor, that I believe means that the magnitude of actual localized stress in the pipe wall immediately around the hole may be/is several times what it would be in the absence of the hole with an untapped pipe exposed to the same internal and external loadings (the wall material removed is of course no longer there to help carry the loads!), and then what might be the ramifications of this high localized stress on the present or future steady-state and fatigue lifetimes of the pipe? Frankly, I'm not exactly sure how a pipeline designer (or a pipe material chooser/buyer for that matter) controls all this in the real world (I happened to see about a ten feet long length of 8" C900 DR 18 pvc pipe sitting on a smaller table several days ago, and the pipe appeared "Bent" under its own weight!)
Complicated stress issues in the walls of pipe and subsequent initial tapping at known (and unpredicted/unknown or unrealized in the future?) locations may well become even more of an issue in increasingly congested construction environments, with increasing use of restrained joints and also installation methods like horizontal directional drilling (HDD), that inevitably leave some perhaps additional residual pulling/bending stresses in particularly fused or even rather rigidly joined, longer pipe segments that are pulled into place under the ground.
This reference went on to say that the "ALLOWABLE BENDING STRESSES" for pvc pipe at 73.4 degrees F. varied from 800 psi (5.52 Mpa) to 1,000 psi (6.89 MPa), depending it appears on whether the particular pipe involved was "Pressure Class" or "Pressure Rated" (I guess the difference being a little higher CLAIMED "factor of safety" for the latter). While perhaps not too well known in some areas, I also once read in a perhaps even older paper written more than 20 years ago by the then president of Uni-Bell, that "The allowable bending stress in the PVC pressure pipe where a direct tap is placed should not exceed 800 psi (5.52 MPa)." For some reason or omission specific guidance or warning about tapping longitudinally bent pipes did not appear mentioned at all in the Handbook chapter I referred to above that specifically talked about longitudinal bending of pipes. I suspect a concern about tapping bent pipes is perhaps among other things due to a stress concentration factor, that I believe means that the magnitude of actual localized stress in the pipe wall immediately around the hole may be/is several times what it would be in the absence of the hole with an untapped pipe exposed to the same internal and external loadings (the wall material removed is of course no longer there to help carry the loads!), and then what might be the ramifications of this high localized stress on the present or future steady-state and fatigue lifetimes of the pipe? Frankly, I'm not exactly sure how a pipeline designer (or a pipe material chooser/buyer for that matter) controls all this in the real world (I happened to see about a ten feet long length of 8" C900 DR 18 pvc pipe sitting on a smaller table several days ago, and the pipe appeared "Bent" under its own weight!)
Complicated stress issues in the walls of pipe and subsequent initial tapping at known (and unpredicted/unknown or unrealized in the future?) locations may well become even more of an issue in increasingly congested construction environments, with increasing use of restrained joints and also installation methods like horizontal directional drilling (HDD), that inevitably leave some perhaps additional residual pulling/bending stresses in particularly fused or even rather rigidly joined, longer pipe segments that are pulled into place under the ground.
PUMPDESIGNER
Mechanical
All that the guys above have said is true. Basically, there is no official statement because that cannot be done easily, too many variables.
I do have a scan of a page from Certainteed pipe regarding this issue. You can download the JPG at this link:
PUMPDESIGNER
I do have a scan of a page from Certainteed pipe regarding this issue. You can download the JPG at this link:
PUMPDESIGNER
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