What is the residual velocity loss of a steam turbine

This involves the principle of steam turbines, and I'll talk about it in layman's terms

The steam turbine can do work, which is to convert the thermal energy of steam into mechanical energy. There are 2 processes in between. The first is the kinetic energy of the steam in the static vane grid of the steam turbine, which converts the thermal energy into steam (which can be understood as turning the stationary steam into steam with a speed of sound), and the second is the high-speed steam flow to push the moving blades to rotate and convert the kinetic energy into the mechanical energy of the blades.

Theoretically, we want to convert all the kinetic energy of the steam into mechanical energy, so that at the outlet of the moving blades, the velocity of the steam should be 0, in fact, this is not possible, the exhaust steam at the outlet of the moving blades still has a high velocity, and the kinetic energy carried is still very high. Fortunately, these kinetic energies will be carried to the next level to continue the work. When the steam flows through the last stage of the moving blade, the steam enters the condenser and transfers the heat energy to the cooling water.

Therefore, when the steam leaves the last stage of the moving blades, the energy carried by the steam is called the residual velocity loss.

When the steam leaves the moving blade, it still continues to rotate, that is, it has a certain kinetic energy, and this part of the kinetic energy that is not used up is called residual velocity loss.

Search term residual velocity loss sharing steam from the moving blade, the absolute velocity has a certain kinetic energy, this part of the kinetic energy is not utilized, it will be converted into heat energy again, so that the exhaust enthalpy value increases, resulting in the loss of work capacity.

The steam discharged from the final stage of the steam turbine also has a certain velocity. This loss of energy is called residual velocity loss.

There are mainly nozzle loss, moving blade loss, residual velocity loss, blade height loss, fan loss, partial steam inlet loss, friction blast loss, steam leakage loss, and wet steam loss in the steam turbine stage. 1) Nozzle loss and moving blade loss are formed by the friction between the steam flow through the nozzle and the moving vane and the friction between the steam flow and the blade surface. 2) Residual velocity loss refers to the fact that the steam still has a certain velocity when it leaves the moving blade, and this part of the velocity energy is not used at this level, so it is the loss of this level.

However, when the steam flow into the next stage, the steam flow energy can be partially used by the next stage. 3) Blade height loss refers to the loss caused by the formation of vortex currents at the root and top of the nozzle and moving blade grid. 4) Sector loss refers to the fact that the blade is arranged in a ring along the wheel rim to make the flow channel section fan-shaped, therefore, the pitch, circumferential velocity and steam inlet angle are changed along the blade height direction, which will cause the steam flow to hit the blade and produce energy loss, and the steam flow will also produce the flow in the radius direction and consume the steam flow energy.

5) Part of the inlet steam loss is due to the "blast" loss when the moving blade passes through the arc section without a nozzle, and the air repulsion loss occurs when the moving blade enters the working arc of the nozzle from the non-working arc section. 6) Friction blast loss refers to the friction between the high-speed rotating impeller and the surrounding steam and drive the steam to rotate, to consume a part of the useful work of the impeller, and the steam flow between the separator and the nozzle to form a vortex under the action of centrifugal force also consumes the useful work of the impeller. 7) Steam leakage loss refers to the fact that due to the pressure difference in the steam turbine, a part of the steam will leak away through various dynamic and static gaps without passing through the flow channel of the nozzle and moving blades, and will not participate in the main stream work, thus forming a loss.

8) Wet steam loss refers to the fact that the steam in the low pressure area of the steam turbine is in a state of wet steam, and the water in the wet steam can not only expand and accelerate the work, but also consume the steam flow energy, and also have a braking effect on the movement of the blades, consume useful work, and erode the blades.

The so-called ultimate vacuum refers to the vacuum when the differential pressure between the front and rear of the final blade of the low-pressure cylinder reaches the critical pressure ratio, at this time, if the vacuum rises again, the steam can only expand outside the outlet of the final blade, and the free expansion outside the exhaust port forms steam resistance, which affects the economy! It is difficult to achieve in actual operation, so it is rarely mentioned.

In detail: when the steam has reached the expansion limit in the beveled part of the moving blade at the end of the steam turbine, the power of the steam turbine will not increase due to the increase of vacuum. Further, even if the final stage of the steam turbine has not yet reached the expansion limit, but because with the decrease of back pressure, the specific volume of exhaust steam continues to increase, and the final exhaust area is constant, so the residual velocity loss of the final exhaust steam will continue to increase, when the effective heat drop increased due to the decrease of back pressure is equal to the increment of residual velocity loss, the vacuum reached at this time is called the ultimate vacuum.

What is the ultimate vacuum of a condenser.

In the operation of condensing equipment, measures should be taken from all aspects to obtain a good vacuum. But the higher the vacuum, the better, but there is a limit. The limit of this vacuum is determined by the expansion limit of the outlet section of the last blade of the turbine.

When the steam passing through the last blade has reached the expansion limit, if the vacuum is continued to be raised, it is not possible to obtain economic benefits, but will reduce economic benefits.

To put it simply, when the expansion of steam in the final blades reaches its limit, the corresponding vacuum is called the ultimate vacuum, and some are called the critical vacuum.

The losses of the steam turbine are divided into internal losses and external losses. The internal loss includes the throttling loss of the steam inlet mechanism, the pressure loss of the exhaust pipe, and the intra-stage loss, and the intra-stage loss mainly includes the loss of blade height, the fan loss, the loss of the blade grid, the loss of residual velocity, the friction loss of the impeller, the impact loss, the loss of part of the inlet steam, the loss of wet steam, and the loss of air leakage. There are two types of external losses: mechanical losses and external losses.

Nozzle loss. When the steam flows through the nozzle, part of the steam produces disturbance and vortex, and there is friction between the steam and the nozzle surface, causing the loss of work ability.

Loss of moving blades. When the steam flows through the moving blade, the friction and vortex between the steam flow and the surface of the moving blade will also cause a loss of work ability.

Residual speed loss. When the steam is discharged from the moving blade, the absolute velocity has a certain kinetic energy, and this part of the kinetic energy is not utilized, it will be converted into heat energy again, so that the enthalpy of the exhaust steam increases, causing the loss of work ability.

Steam leakage loss. It consists of two parts: one part is the leakage of steam from the shaft seal at the end of the cylinder. The other part is the steam leakage loss in the stage, including the steam leakage loss in the separator seal, moving vane and cylinder clearance.

Friction blast loss. Friction loss refers to the loss caused by friction with steam when the impeller rotates, and the power consumed by the steam on both sides of the impeller to form a steam vortex when it is carried to rotate. Blower loss refers to the friction loss between the two sides of the blade grid and the steam, and the additional loss caused by the steam drumming from one side of the moving blade to the other side when the part of the moving blade is rotated without steam flowing in the partial inlet stage.

Friction loss and blast loss are collectively referred to as friction blast loss.

Vapor rejection loss. In the partial inlet stage, the steam from the nozzle passes through only some of the flow channels of the vanes, while the other vanes are filled with stagnant steam. When this part of the moving blade rotates to the nozzle and aligns it again, the main steam flow from the nozzle first expels this part of the trapped steam, which reduces the steam velocity.

There was a loss of energy.

Moisture loss. The velocity of water droplets in wet steam is smaller than that of steam, and the vapor molecules consume a part of the energy to accelerate the water droplets, causing energy loss. At the same time, due to the low flow velocity of the water droplets, when entering the moving blade, it just impacts the back at the inlet of the moving blade, which has a braking effect on the impeller and consumes a part of the useful work.

In addition, there are losses such as steam extraction, stem leakage, and hydrophobic.

2, 3, 4, 5, 7 are intra-level losses.

Internal losses of the steam turbine include:

1) Throttling loss of the steam inlet mechanism; (2) pressure loss in the exhaust pipeline;

3) Intra-stage losses of steam turbines.

The losses within its intermediate are:

1) Blade grid loss; (2) loss of residual velocity; (3) Blower friction loss of impeller;

4) impact losses; (5) loss of leaf height; (6) sector loss;

7) Partial inlet steam loss; (8) Wet vapor loss; (9) Steam leakage loss.

The nozzle is an immovable steam duct composed of two adjacent static blades, which is a structural element that converts the thermal energy of steam into kinetic energy. The nozzles installed in front of the first stage of the turbine are divided into several groups, each of which is controlled by an adjusting valve.

The partition is the wall of the turbine stage to hold the static blades.

A static blade is a blade that is fixed to a partition and is stationary.

The most basic working unit of a steam turbine, which consists of a row of nozzles and a row of moving blades, is called the stage of the steam turbine.

When the steam turbine is adjusted by nozzle, the inlet cross-sectional area of the first stage changes accordingly with the change of load, so the first stage of the nozzle regulating steam turbine is usually called the regulating stage. The other levels are collectively referred to as non-regulated or pressure grades. The pressure level is a grade based on the pressure drop or enthalpy drop that is reasonably distributed in the utilization class group, and it is a single column of impulsive or reactionary grade.

In order to increase the enthalpy drop of the regulating stage, the residual velocity of the first row of moving vanes is used to reduce the residual velocity loss, so that the steam flow at the outlet of the first row of moving vanes changes the flow direction through the guide vanes fixed on the cylinder, and then enters the second row of moving vanes to continue to do work. At this time, the stage with one row of nozzles and two rows of moving blades on the first-stage impeller is called a double-row speed stage.

Upstairs is very comprehensive, but if it's easy to remember, you might as well simulate a process of steaming in your head.

First, steam enters the turbine through a control door. Generally speaking, the door will not be fully opened, and there will be a throttling loss at all.

After that, the steam enters the moving and static blades to do work, because it is impossible to seal perfectly between the moving and static blades, there will always be a gap, and the steam may leak through this gap, so there is an interstage loss or called steam leakage loss.

Since the temperature of the steam is generally higher than that of the blades and cylinders, some of the steam turns into water (very few small droplets). Because of the presence of superheat, this part is very small, which is called moisture vapor loss.

In addition, there are inevitable mechanical losses such as mechanical friction and friction of the relative movement of the blades and steam (blast friction) during the entire rotation process of the steam turbine.

The last and biggest loss comes from the exhaust loss of steam that has not been completely done, that is, after the steam has completed the work in the steam turbine, it enters the condenser and becomes water. The energy cups released by this process are completely wasted.

1. Throttling loss.

2. Nozzle loss; The surface is not smooth at different velocities while the molecules rub.

3. Loss of moving blades; Eddy current loss, collision loss, friction loss, sub-current loss, 4, residual velocity loss.

5. Friction blast loss.

6. Steam leakage loss.

7. Loss of moisture.