Forensics

Plastic Pipe Failures

Forensic Services Newsletter

Fires get all the publicity. So also do pipe ruptures when they occur on an oil or gas line and result in massive conflagration. There is however a continuing incidence of small, unspectacular pipe failures in industry that result in large insurance claims. In our experience most often these pipes contain nothing more harmful than water. It is their strategic location that makes these claims significant, with the archetype example of the failures described in this newsletter a rupture of a US$3 elbow or valve located above a US$1 million machine in an electronics factory.

In days past such pipe and fittings would have been made of steel or cast iron. Nowadays they are usually of plastic, being the subject of this newsletter. When steel pipes fail there is often warning, such as a leak from a thread or a sagging line poking through a ceiling. When plastic fails it is usually more dramatic, with total rupture and the venting of a lot of liquid.

Judging by the accidents we are consulted on, the causes of pipe failures are as follows, in approximate order of importance.

1. Manufacturing faults




Broken fitting on left. Photograph on right shows close-up of thread, with manufacturing defect (weld lines) arrowed.
2. Poor fitting
Separated fittings at top left and right, in both cases a result of insufficient penetration by one part into another, and presence of dirt. Photograph at bottom left shows polished section through a joint, with voids and dirt visible.
Top left : Failure of thermally fused HDPE pipe joint. Top right : Porosity visible in failed portion. Bottom : Porosity (arrowed) in unfailed portion.
3. Insufficient Support
This can cause failure in two ways. The pipe can fracture because the lack of supports can result in overloading, particularly when the pipe is full. Also lack of sufficient supports can allow the pipe to vibrate, with fatigue occurring as described below in 5.
4. Corrosion
Failure of ABS pipe in four days from contact with sulphuric acid. ABS is not suitable for containing or conveying sulphuric acid.
5. Fatigue
Left : Failed fitting. Centre : Fractured thread. Right : Progression marks indicating fatigue failure. 

In regard to poor fitting and insufficient support, there is a feature of these failures that stands out. Most of the people reading this newsletter will have been on a behind-the-scene inspection of a hotel and will have been struck by the contrast between what the public sees and what the employees' experience. The same contrast exists in many high tech factories. Rows of electronic machines in pristine surroundings, serviced by skilled technicians flown in from around the world.  Above the ceiling can be a maze of pipes, erected by local contractors who submitted the lowest bids. The differences in engineering standards can be startling and is responsible for many of the claims.

Regards,





Barry Dillon