PVC - Solvent Cemented Joint Strength / Failure Modes
PVC - Solvent Cemented Joint Strength / Failure Modes
(OP)
I'm investigating some joint failures in a 10" diameter, Sch 80, PVC pressure piping system. The pipe sections and fittings are joined by solvent cementing. This is a new installation and many leaks showed up during a pressure test. All the leaks are from cemented joints - from between the pipe and the fitting bell.
I'd like to rule out the possibility that this is caused by stress, so I'm wondering what the strength of a properly made cemented joint is relative to the pipe. What would the failure mode be assuming that the joints are properly made but the piping is overstressed from thermal expansion/contraction due to insufficient flexibility in the system.
The piping is ASTM D1785 and it is an above ground installation.
Thanks
I'd like to rule out the possibility that this is caused by stress, so I'm wondering what the strength of a properly made cemented joint is relative to the pipe. What would the failure mode be assuming that the joints are properly made but the piping is overstressed from thermal expansion/contraction due to insufficient flexibility in the system.
The piping is ASTM D1785 and it is an above ground installation.
Thanks





RE: PVC - Solvent Cemented Joint Strength / Failure Modes
Insufficient flexibility in a piping system usually does not result in immediate failure, only excessive lateral deformation and possibly buckling of pipes in compression, possibly exacerbated by eccentricities introduced by thermal deflection. It's most severe effect can be on attached equipment, as it tends to bend pump flanges and misalign pump and motors. In other words it creates a lot of maintenance headaches, rather than direct failures.
However coincident lateral deflection of pipes from thermal deformation would introduce bending stresses, which if happened across a joint, would tend to separate it on the tension bending side of the joint.
Another mode of failure could be from expansion due to end cap pressure load on a closed pipe system, which introduces axial tension across the entire pipe cross-section. Joints failing from that might exhibit a uniform pullout as opposed to pullout of one side only, as was the case in bending failure. Normally a lot of that axial tension due to pressure can be resisted by thermal expansion compressive forces in restrained systems, but if no or little axial restraint was provided, the pipe would be free to expand and pull out any badly cemented joint.
RE: PVC - Solvent Cemented Joint Strength / Failure Modes
So, in practice, would you expect to see the joint failure modes you describe to occur in a properly cemented joint at a much lower load than pipe failure?
For example, in a welded steel or fused HDPE pipe, I would expect the joints to handle as much, or close to the load that the pipe itself can. If I saw a lot of joints failing in bending without evidence buckling or large deformations in the pipe, I would suspect the quality of the joints. I'm not sure if the same logic applies to PVC cemented pipe.
RE: PVC - Solvent Cemented Joint Strength / Failure Modes
Were the joints prepared properly before cementing? Were the contact surfaces sanded?
Can you post a picture of the failure.
RE: PVC - Solvent Cemented Joint Strength / Failure Modes
Just degrease the surfaces and then there is no problem.
RE: PVC - Solvent Cemented Joint Strength / Failure Modes
RE: PVC - Solvent Cemented Joint Strength / Failure Modes
RE: PVC - Solvent Cemented Joint Strength / Failure Modes
The reality is these types of joints are very susceptible to poor installation or movement / stress before the joint is cured.
If undertaken properly the joint will not fail under other stresses, due to the thicker pipe at that location.
Movement or stress before curing, not enough solvent or poor surface preparation are all much more likely.
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RE: PVC - Solvent Cemented Joint Strength / Failure Modes
10" PVC solvent-welded joints pretty much require equipment or jigs to install correctly. The push-back force of the components when they are properly primed and glued is so large that it's tough for even a couple people to resist them. Even at 6" it starts to get pretty challenging to keep the joint from pushing back apart to some degree before the glue finally "grabs" and holds the parts fast.
RE: PVC - Solvent Cemented Joint Strength / Failure Modes
From my experience, I don't think it is practical to solvent weld pipe as large as 10" diameter, Sch 80, PVC. This may be practical for a small pipe segment, but not for any significant length of piping.
I would add that due to the difficulty of making the solvent weld joints on large pipe, the joints may not be properly made up. If the joints were made up properly, BigInch is correct and the joints are twice the thickness of the pipe and are stronger than the pipe.
If the joints are leaking during the initial pressure test, the joints were not made up properly.
I have seen installations of large PVC installations such as this fail due to water hammer from transient flow conditions and poorly executed pipe restraints.
If nothing else, consider installing mechanical pipe connections.
RE: PVC - Solvent Cemented Joint Strength / Failure Modes
RE: PVC - Solvent Cemented Joint Strength / Failure Modes
What is support span of pipe? If not continuous supported, support span shall be calculated and in my experience, it is way shorted than steel pipe.
RE: PVC - Solvent Cemented Joint Strength / Failure Modes
RE: PVC - Solvent Cemented Joint Strength / Failure Modes