【Mechanical Equipment Technology Exchange Edition】Mechanical Equipment 【Daily Question】20191125

I’ve heard of this “expulsion loss” for the first time. Is it that: steam leakage loss refers to the situation in a turbine where, due to pressure differences, a portion of the steam escapes through various gaps between moving and stationary parts, rather than passing through the nozzles and blade channels; this steam does not contribute to power generation and thus represents a loss. Or: Moisture loss refers to the situation in the low-pressure area of a turbine where the steam is in a wet state; the water contained in this wet steam not only expands to generate work more rapidly, but it also consumes the kinetic energy of the steam flow. It further exerts a braking effect on the movement of the blades, thereby wasting useful work, and it also causes erosion of the blades.

What are the main internal losses in a turbine, and what are the causes of these losses? Within a turbine stage, the main losses include nozzle loss, blade kinetic loss, residual velocity loss, blade height loss, fan loss, partial steam injection loss, frictional blowing loss, steam leakage loss, and wet steam loss. 1) Nozzle loss and blade loss are caused by the mutual friction between the steam flow as it passes through the nozzle and blades, as well as the friction between the steam flow and the surface of the blades. 2) The residual velocity loss refers to the fact that steam still possesses a certain velocity when it leaves the moving blades; this kinetic energy is not utilized in this stage, and thus it represents a loss for that stage. However, when the vapor stream flows into the next stage, its kinetic energy can be partially utilized by that stage. 3) Leaf height loss refers to the loss caused by vortices formed by the steam flow at the base and top of the nozzle and rotor blades. 4) Fan-shaped loss refers to the fact that, due to the radial arrangement of the blades around the rim, the flow passage cross-section takes on a fan shape. As a result, the pitch, circumferential velocity, and inlet angle vary along the blade height, which causes the steam flow to strike the blades and result in energy loss. Additionally, the steam flow generates radial movements that further consume its energy. 5) Some of the steam inlet losses are due to \"blowing\" losses that occur when the moving blades pass through the arc sections without nozzles, as well as gas expulsion losses that occur when the moving blades move from the non-operating arc sections into the operating arc sections with nozzles. 6) Frictional blowing loss refers to the fact that the high-speed rotating impeller comes into friction with the steam surrounding it, causing this steam to rotate; as a result, part of the impeller’s useful work is consumed. Additionally, the steam flow between the partition and the nozzles forms vortices due to centrifugal force, which also consumes the impeller’s useful work. 7) Steam leakage loss refers to the loss that occurs in a steam turbine due to pressure differences; as a result, some of the steam escapes through various static and dynamic gaps, rather than passing through the nozzles and blade passages, and thus does not contribute to power generation, leading to losses. 8) Wet steam loss refers to the situation in the low-pressure section of the turbine where the steam is in a wet steam state. The water contained in this wet steam not only expands to generate work, but it also consumes the kinetic energy of the steam flow; it further exerts a braking effect on the movement of the blades, thereby wasting useful work, and it also causes erosion of the blades.

Steam rejection loss occurs in stages with partial steam inlet; when the moving blades pass from the arc section without nozzles to the arc section with nozzles, there is stagnant steam between the blade rows. The steam emerging from the nozzles must first displace this stagnant steam before it can push on the moving blades, which inevitably results in a decrease in the speed of the steam acting on the blades and thus causes loss. Since these two streams of steam appear to exert a repulsive force on each other, this phenomenon is known as steam rejection loss.

This post was last edited by Zhongyuanren on 2019-11-25 at 17:25. Due to the high-speed rotation of the impeller and the pressure difference, steam leakage occurs in the axial gap at the exit end of the nozzle assembly, while vapor suction occurs at the starting end of the nozzle assembly. This allows the low-speed steam present in the gap to enter the flow channels of the moving blades, disrupting the main flow and resulting in losses; these losses are known as vapor rejection losses.

When the moving blade enters the arc section equipped with nozzles again, the working steam flow must first push aside and accelerate the steam that is stagnant in the blade’s steam channel; this consumes some energy, a phenomenon known as steam rejection loss

In some steam-inlet stages, the steam exiting the nozzles passes only through the flow channels of some of the rotor blades, while the flow channels of the other rotor blades are filled with stagnant steam. When this section of the moving blades rotates back into alignment with the nozzle, the main steam stream emerging from the nozzle must first push out this stagnant steam, which reduces the speed of the steam stream and results in energy loss.

In some steam-inlet stages, the steam exiting the nozzles passes only through the flow channels of some of the rotor blades, while the flow channels of the other rotor blades are filled with stagnant steam. When this section of the moving blades rotates back into alignment with the nozzle, the main steam stream emerging from the nozzle must first push out this stagnant steam, which reduces the speed of the steam stream and results in energy loss.

Steam rejection loss occurs in stages with partial steam inlet; when the moving blades pass from the arc section without nozzles to the arc section with nozzles, there is stagnant steam between the blade rows. The steam emerging from the nozzles must first displace this stagnant steam before it can push on the moving blades, which inevitably results in a decrease in the speed of the steam acting on the blades and thus causes loss. Since these two streams of steam appear to exert a repulsive force on each other, this phenomenon is known as steam rejection loss.

In some steam-inlet stages, the steam exiting the nozzles passes only through the flow channels of some of the rotor blades, while the flow channels of the other rotor blades are filled with stagnant steam. When this section of the moving blades rotates back into alignment with the nozzle, the main steam stream emerging from the nozzle must first push out this stagnant steam, which reduces the speed of the steam stream and results in energy loss. It is called gas rejection loss.

In some steam-inlet stages, the steam exiting the nozzles passes only through the flow channels of some of the rotor blades, while the flow channels of the other rotor blades are filled with stagnant steam. When this section of the moving blades rotates back into alignment with the nozzle, the main steam stream emerging from the nozzle must first push out this stagnant steam, which reduces the speed of the steam stream and results in energy loss.