PVC - Solvent Cemented Joint Strength / Failure Modes 1

[tomecki]

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

 

[BigInch]

"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."

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.

 

[tomecki]

Thanks BigInch.

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.

 

[BigInch]

A properly cemented joint is stronger than the adjacent pipe. Essentially you have twice the wall thickness in that area.

Were the joints prepared properly before cementing? Were the contact surfaces sanded?
Can you post a picture of the failure.

 

[europipe]

I saw PVC joints leaking after sand down.
Just degrease the surfaces and then there is no problem.

 

[BigInch]

Sanded too much.

 

[europipe]

No, it's more a suture problem.

 

[LittleInch]

tomecki,

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.

see attached page 11 for further details

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[moltenmetal]

In services other than Category D, a certified joining/bonding procedure is, or should be, required, and installers should be certified against that procedure. If such a procedure is developed and tested and the installers are certified against it, and then monitored, it is little different than any other operation for which meaningful NDE other than leakage testing is the only option, i.e. very similar to socket welding of steel fittings and pipe.

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.

 

[bimr]

Agree with moltenmetal.

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.

 

[rconner]

I would also recommend a reading of any still available links, references, history/revelations and debate on prior threads on subject e.g.

http://www.eng-tips.com/viewthread.cfm?qid=227506

and

http://www.eng-tips.com/viewthread.cfm?qid=348698

etc.

 

[simplemath2]

Agree with moltenmetal on size of PVC pipe. I would not use PVC over 4" because solvent cemented is less reliable exceeding 4" and HDPE and fusion connection should be used.

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.

 

[BigInch]

I tend to think that all plastic pipes should be more or less continuously supported, as per my suggestion above. O]

 

[moltenmetal]

I'm not against using large solvent-welded piping- I'm merely advising what you need to make the joints correctly. Just like you need tooling/guiding to fusion weld PP or HDPE connections, you need the same sort of guiding or jigs to join large solvent-welded joints. 6" can be done by hand, with care and difficulty. 4" is absolutely no problem for a competent crew. In a lot of cases, Victaulic is another option worthy of consideration once the sizes go up, assuming wall thickness goes up with it.