Flow measurements – why, how and when (part 2)

This is the highly anticipated sequel to the acclaimed “Flow measurements – why, how and when”-series.

The measuring of flow

As you are now familiar with most things relevant when it comes to measuring the flow of a turbine, let’s look into a real flow measurement case: A power plant group is unsure of the performance for one of their plants. They know a lot of stuff about the plant, but not the flow. The tear and wear on the runner (and other mechanical components in the waterway/turbine) makes it close to impossible to estimate the flow. Therefore, the power plant group hires a consultant to do a flow measurement of the plant in question. It is decided that the Gibson method in cooperation with index measurements shall be used to avoid an unseemly amount of shutdowns of the turbine (as the plant has a rather complicated and lively waterway, and shutdowns put a lot of strain on the mechanical components). Some installation and set-up time later, the tests are ready to go. The Gibson test is run at 6 different outputs, to get reference points for the index tests. The index test is performed at 12 different outputs, and is then calibrated towards the Gibson test. This way of doing our flow measurements is easier on the turbine runner and is also more efficient as we don’t have to shut-down the turbine as often as if we only ran the Gibson tests.

The graphs illustrates how the primary measurements are used to calibrate the index measurements. The primary measurement gives us the absolute flow, which we can plot directly into our chart, while for the index measurements the y-axis is an unknown. As per the example earlier, we know the relative flow (compared to the other index flow measurements) at the various test settings, but we don’t know where we should put the points on the y-axis. This is where the primary measurements come into play. We align the points such that they overlap (as best as we can), and in this way we will get a complete picture of the flow through the turbine at all test settings. Clever, practical and easy on the turbines.

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Unfortunately, it is not always that easy to know which methods of flow measurement one should use. It depends on many factors: waterway stability, head, pipe geometry, access and so forth. This makes it paramount to have good knowledge of the plant before one plans the measurements, as down-time is costly as all hell. Measuring the flow is an important tool for operations and maintenance and is a valuable input for the market division when calculating our capacity, pricing and down-time schedule. This enables us to manage Statkraft’s portfolio of power plants optimally and to be both effective and efficient in maintenance and operation of our plants.

Hopefully in the future, integrated systems for continuous flow measurement will be a natural part of our power plants, giving us even more insight into the turbine condition in real time, such that the full potential of our power plants can be made use of optimally. There are a lot of people working on this, and personally I see no reason why this shouldn’t become reality, and common practice, in a few years time.

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