Power stations around the world historically use steam pipework throughout the plant. These pipes are typically made from 1/2CrMoV steel and while this type of steel is best suited to this application, it has a tendency to age, just like all other metals, and is also susceptible to fatigue failure from the high operational pressures and temperatures. Experience at these power stations indicates that catastrophic failures of these pipes is more likely on pipe bends rather than welded sections, and pipes bends most susceptible are those closest to the boilers which have to withstand temperatures of around 600 oC (~1,110 oF).
These steam pipe failures obviously have huge cost, and, of course, enormous safety implications. A power plant shutdown due to any failure can result in millions of dollars in losses to the operating company, not to mention the intangible losses to industry and homes that are dependent on this power.
The most common tests used in power plants to monitor pipe robustness and safety are strain measurement, metallurgical replication, dimensional measurements, hardness tests and chemical testing. Unfortunately these testing techniques cannot help predict remaining useful life of the pipes and subsequently indicate when these pipes need to be replaced.
Small scoop sampling and impression creep testing is fast becoming a viable alternative to these traditional testing techniques and is gaining global recognition simply because these new techniques support finite analysis and have been shown to have better repeatability. The test results are also able to classify which pipes are in good condition and which ones are in critical condition.
The sampling for the test is done by a unique machine specifically developed for this application. The machine samples a scoop from existing pipework without damaging the pipe and while the plant is operational. This is obviously a huge benefit since the testing can be done without shutting down the plant.
The impression creep test or indentation test is used to determine the level of material fatigue and the more worn the metal, the greater the indentation under a constant applied load; results are expressed as a compressive extension in mm. This impression creep test uses a Tinius Olsen model H25kS tester, furnace, dual LVDT averaging extensometers, an alignment fixture and machine control and data analysis software. The test has been refined and implemented by NPower in the UK (part of the EU RWE power generation group). Click here to learn more about RWE and NPower, by Bristol University and Tinius Olsen.