Pinprick predicts lifetime of gas pipes

IMPACT researcher develops new measuring technique

26 January 2010 

The first plastic gas pipes in the Netherlands were made of hard PVC and fitted from the sixties up to the mid-seventies. Their lifetime was originally estimated to be 50 years. Now, 50 years on, their lifetime is proving to be longer. However, there is no method for determining the residual lifetime of these pipes simply and cheaply. To this end, Roy Visser of the University of Twente has developed a measuring technique which gas grid operators can use to determine their replacement policy. Visser received his PhD from the Faculty of Engineering Technology on 22 January 2010.

Plastic pipes were first used to lay the gas grid in the Netherlands in the sixties. Until the mid-seventies, hard PVC was used for this. The Netherlands has about 22,500 km of hard PVC gas pipes. The expectation was that these pipes would last for about fifty years. Now, fifty years on, the pipes are still functioning well. The question is whether this will continue to be the case for the next few years. As it is hugely expensive to replace the pipes, research into the residual lifetime of hard PVC gas pipes was initiated on behalf of four major gas grid operators: Cogas Infra en Beheer, Enexis, Liander and Stedin.
Roy Visser of the University of Twente has developed a measuring technique which can be used to determine the residual lifetime of these plastic pipes. 

Embrittlement
There are two types of breakages in gas pipes, a clean break and a brittle break. In the case of the first type, the consequences are limited. The second type of break, however, can occur as a result of ageing; the material becomes gradually more brittle.
The risk of a brittle break is that this occurs over a longer length, which means that a large amount of flammable gas is released and it is tricky to repair.
Hard PVC is known to display embrittlement symptoms. That is why it is important to know which ageing processes can lead to embrittlement in the material. Visser has established that physical ageing is the main cause of gas pipe embrittlement. Severe embrittlement restricts the lifetime of the pipe material.

Ripe fruit
To monitor the embrittlement process closely, Visser applies the method of micro-indentation, which means that the gas pipes do not need to be dug up. In this method, a small needle is used to make a pinprick in the pipe. How far this needle can be inserted into the pipe and what force is required to do so tells us something about the mechanical properties and ultimately the residual lifetime. It's just like when you use your fingers to feel whether a piece of fruit is unripe, ripe or overripe. The practical application of this method is currently being investigated further. Other gas grid operators, the water pipe sector and the plastic pipe industry are also involved in this follow-up project.

Note for the press
Roy Visser received his PhD from the Faculty of Engineering Technology on 22 January. He conducted his research within the Design, Production & Management research group and the IMPACT research institute. His supervisors were Prof. Remko Akkerman (UT), Prof. Mannes Wolters (UT) and Dr Leon E. Govaert (TU/e). His thesis entitled 'Residual lifetime assessment of uPVC gas pipes' can be obtained in digital format on request.