Not just a pipe dream

26.01.2018

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Be it in nuclear power plants (NPPs), industrial plants, drinking water piping networks, biogas plants or in the home: piping sort of represents the life veins of many technical installations. Leakages from such pipes may turn into a safety risk, e.g. if they transport poisonous, radioactive or flammable substances into the environment and the human organism. This is added by the fact that such damage will normally go in hand with economic losses when the substance to be transported gets partially lost on the way to its destination or the leakage causes a downtime of the installation.

Why the researchers of GRS deal with leakages

Due to the potential effects of leakages on the safe operation of an NPP, experts at GRS specialising in structural mechanics have for decades been carrying out intensive research into this issue. For the generator to be able to generate electricity, the driving turbine has to be supplied with steam. In an NPP with pressurised water reactor, this supply comes from the steam generators via the so-called main-steam line.  The steam generators are in turn connected with the reactor by piping. Through this piping, the heat generated by the fuel is transported to the steam generators. The turbines also require oil for their operation, which is also guided via piping to where it is needed.

In order to be able to predict even better the behaviour of leaks and their effects, special calculation codes are used to analyse e.g. the leak and break probabilities as well as the leak rates in piping and vessels. Here, “leak rate” is defined as the speed at which a volumetric unit flows from a leak, or in short: how much flows out in what time. It depends above all on the pressure, temperature and aggregate phase of the medium contained in the piping. In addition, conditions on the outside of the piping also play a role. If one assumes, for example, piping in an NPP that contains water at high pressure and high temperature with the ambient temperature and the pressure on the outside of the piping being lower, then a water-steam mixture will escape from the leak.

Among the different types of leak that can occur in the area of nuclear engineering, crack-like leaks in pipes are of particular relevance. As a rule, these are initially narrow, which means that only a slight amount will escape from the piping into the environment. However, if pressure inside the pipe increases, leaks may widen, and in the worst case, such crack growths can even lead to a pipe break.

As it is only possible to detect leaks from a particular size onwards with any certainty, the researchers are particularly interested in the leak sizes between the „extremes“: starting from the point where a leak can already be detected but is still stable up to the point where the pipe breaks. The delimitation and definition of this range is done within the framework of so-called leak-before-break demonstrations (see Figure 1). The nuclear regulations in Germany require that such demonstrations must be provided for certain piping in German NPPs. 

Figure 1: Schematic diagram of the leak rate as a function of the leak length: the light blue area in the middle shows the leak lengths at which a leak is detectable but still sufficiently stable. In this area it is possible to find the leak so that repairs can take place in time. (Illustration: GRS)

Analyses regarding the leak-before-break demonstration

A leak-before-break demonstration shows that a through-wall crack (leak) in a pressurised piping section of an NPP remains sufficiently stable and that the leak can be detected in time before it can grow and may perhaps lead to a pipe break. Hence it is of advantage if piping shows such leak-before-break behaviour. This would result in a time window during which it would be possible to initiate counteractions, like the shutdown of the plant. Whether or not piping does show such leak-before-break behaviour depends on different characteristics of the component. For example, piping of large diameter tends rather more to show leak-before-break behaviour than small-diameter piping. Wall thickness, on the other hand, has a rather more negligible influence on the leak-before-break behaviour.    

Benefits for drinking water supply

The scientists at GRS are currently examining the extent to which the findings from the analyses regarding the leak-before-break demonstration in nuclear engineering may be applied to the maintenance of the drinking water network.

The consequences of climate change in the form of droughts, floods, etc. are already having an impact on the availability of drinking water in many regions of the world. Where drinking water is already scarce, additional water losses caused by damaged piping represent a major problem. Industrialised nations in regions with sufficient water reserves, on the other hand, face economic challenges in terms of drinking water supply. Here, it is important to avoid pipe bursts in an ageing network, in particular through appropriate maintenance measures.

The knowledge about the behaviour of leaks in piping can increase the reliability of the drinking water supply. Specifically, in the field of structural mechanics, hydraulic descriptions of leaks, such as those found in the water industry, were compared with leakage flow models from nuclear engineering. Here, it could be shown that the leak blowdown models of GRS provide a good justification for the properties of leaks observed in water distribution systems that could not previously be explained with simpler models. The first calculations in this respect were presented at the 15th International Computing & Control for the Water Industry Conference in Sheffield. The article "Leakage Modeling: High Leakage Exponents Due to Friction Effects"submitted by our GRS colleagues was awarded Best Paper.

In addition to water management, the GRS experts are currently examining other possible fields of application for the calculation of leaks, e.g. process engineering (e.g. in connection with oil and gas pipelines) or the infrastructure for hydrogen-based mobility.

Related links

GRS simulation code WinLeck
GRS simulation code PROST (Program for the determination of the reliability