【Mechanical Equipment Technology Exchange Edition】Mechanical Equipment 【Daily Question】20191210

The losses incurred by a turbine during operation due to air leakage.

Losses in a steam turbine during operation due to air leakage

In my opinion: it is proportional to the gap and pressure difference!

Losses in a steam turbine during operation due to air leakage

The losses incurred by a turbine during operation due to air leakage. Since the turbine power cannot reach 100%, there will inevitably be some losses. The reasons for the losses are both mechanical and related to processes or operations

The losses incurred by a turbine during operation due to air leakage.

Due to the different structures of the impulse stage and the reaction stage, the amount of steam leakage within each stage as well as the impact of this leakage on the efficiency of each stage vary, hence it is necessary to discuss the steam leakage issues for these two types of stages separately. In the impulse stage, there is a significant pressure difference before and behind the partition, and there is also a gap between the partition and the shaft; as a result, some steam must leak from in front of the partition through this gap into the vapor chamber located between the partition and the impeller of this stage. Since this portion of steam does not pass through the nozzle, it does not contribute to work, thereby resulting in leakage losses across the partition. Furthermore, the steam that leaks into this steam chamber may also flow into the moving blades through the gap between the nozzle and the root of the moving blades. Since these leaks do not enter the moving blades in the correct direction, they not only fail to generate power but also disrupt the main steam flow within the moving blades, resulting in losses. To prevent steam leaking from the diaphragm from entering the moving blades and disturbing the main steam flow, on the one hand, balance holes are provided in the impeller so that the steam leaking from the diaphragm can flow through these holes to the area behind the stage; on the other hand, steam seals are installed at the roots of the moving blades to prevent such leakage. Additionally, a suitable degree of reaction is chosen during design to ensure that no suction or leakage of steam occurs at the roots of the moving blades. At the top of the moving blade, in order to avoid relative expansion between the rotor and the cylinder as well as collisions that may occur when the rotor vibrates, there should be certain axial and radial gaps between the top of the moving blade, the partition, and the retaining ring. Even at the impulse stage, there is a significant reaction degree at the top of the moving blade, meaning that there is a large pressure difference across the top of the blade. This inevitably causes some of the steam exiting the nozzle to leak out through the gap at the top of the moving blade rather than passing through the blade’s vapor passage, and thus to end up behind the stage. Since this portion of steam does not participate in doing work, it constitutes tip leakage loss. Since the steam leakage rate is proportional to the gap area and the pressure difference across the gap, reducing steam leakage losses should be achieved by decreasing both the gap area and the pressure difference on either side. Practice has shown that the use of high and low tooth steam seals can meet these two requirements simultaneously. Since the gap between the upper and lower tooth steam seals can be made very small, and the steam flow undergoes throttling each time it passes through a tooth, resulting in a decrease in pressure, each tooth bears only a small portion of the total pressure difference, as shown in Figure 1.5.5(b). Since the steam flow in each steam seal tooth is roughly similar to that in a converging nozzle, the steam leakage rate can be calculated by referring to the nozzle flow formula. For the reaction stage, it is not difficult to analyze based on its basic structure and working principle that its steam leakage loss is greater than that of the impulse stage. This is because: 1) The steam leakage amount through the inner-diameter steam seal is higher than that through the partition steam seal in the impulse stage, mainly due to the larger diameter of the inner-diameter steam seal compared to that of the partition steam seal, along with a smaller number of seal teeth. 2) The pressure difference before and after the moving blade is large, so the steam leakage at the blade tip is considerable. To reduce steam leakage losses, the radial clearance should be minimized as much as possible. However, during startup and other operations, the stationary and rotating parts of the turbine are heated unevenly, resulting in large temperature differences; to avoid friction between them, the clearance cannot be too small. Therefore, a radial and axial steam seal structure is adopted to reduce steam leakage. For longer twist vane stages, in the absence of a shroud, the top of the moving vanes is often thinned to reduce the gap between the moving vanes and the cylinder (or partition sleeve), thereby achieving steam sealing. Furthermore, every effort should be made to reduce the tip reaction degree, so as to prevent an excessive pressure difference across the leading and trailing edges of the moving blade.

The losses incurred by a turbine during operation due to air leakage. ①Vapor leakage through the partition ② Vapor leakage at the top of the moving blades

The losses incurred by a turbine during operation due to air leakage. Since the turbine power cannot reach 100%, there will inevitably be some losses.

The loss incurred by a turbine during operation due to air leakage is called air leakage loss. Air leakage loss is related to the following factors: inadequate sealing of the seals ； Poor sealing of the wheel cover, etc