Ensuring the reliability of the main elements of steam turbines. Selection of rotor design

Calculation of axial forces and methods of their compensation

The axial force acting on the rotor of a steam turbine is determined by summing the forces generated within each stage on the working blades, on the annular part of the disk web, in the rotor steps between the diameters of adjacent diaphragm seals, and on the seal projections.

The first component of the axial force is determined by the difference in the axial projections of the speeds and the pressure difference, which depends on the degree of reactivity of the stage.

The second component on the annular part of the disk web, located between the root diameter and the rotor diameter under the diaphragm seal.

The third component of the axial force is generated on the rotor step.

and the fourth (on the seal projections)

The axial forces in the turbine are perceived by an axial bearing, which is installed in the HPC area and is often performed in combination with radial bearing RVD (combined radial-axial bearing). In multi-cylinder turbines, they try to balance the axial forces. For this, for example, the directions of steam flows in the HPC and MPC are made in mutually opposite directions, and the LPC is made double-flow and, therefore, unloaded from axial loads.

Static strength of turbine stage working blades

In the processes of thermal and aerodynamic calculations of a turbine stage, it is mandatory to check its working blades for static strength. The working blades are loaded with centrifugal forces and forces arising from the expansion of water vapor. Depending on the design and operating conditions, centrifugal forces can stretch, bend and twist the working blades. The forces from the action of the steam environment mainly bend its body.

In a blade of arbitrary cross-section without a shroud, it is permissible to determine the maximum stresses taking into account the unloading factor krazgr, which shows how many times the stresses in the root section of a variable profile blade differ from those for a constant profile blade.

During the expansion process, water vapor acts on the working blades with a force representing a distributed specific load, which generally varies along the length of the blade.

The simplest analysis of the effect of specific axial and circumferential loads based on the corresponding tensile and compressive stress diagrams of the blade body shows that the tensile stresses at its leading edge are maximum (in this regard, it is made thicker).

Steam Turbine Rotor Designs

Rotors are the most loaded elements of a steam turbine and can be made: a) with shrouded disks; b) solid forged; c) welded. In addition, combined rotors are used, in which the disks of the first stages are forged together with the shaft, and the last stages are forged.

Rotors with shod disks consist of a shaft and disks mounted on it with tension. Torque is transmitted from the disks to the shaft by means of friction created by contact pressure from the tension. For guarantee, a longitudinal (axial) key is made on the disk hub, and in the low-pressure cylinder, end keys are installed between the disks. These rotors are distinguished by the simplicity of the manufacturing technology, but can only operate at moderate temperatures (not higher than 300 ... 3500C), since at high temperatures, due to stress relaxation, the disk fit on the shaft weakens. Large disk diameters can be obtained in such rotors.

Solid-forged rotors are used in the high-pressure and low-pressure cylinders of modern turbines. In such rotors, the disks and shaft are turned from a single forging. A through hole with a diameter of 100-120 mm is drilled in the central part of the solid forged rotor for periscopic quality control of the workpiece. Today, the technology for manufacturing such rotors allows for their workpieces to be made with a diameter of up to 2 m and a length of up to 10 m.

Welded rotor structures are made from individual forgings with their subsequent welding with ring seams. After welding, the rotor undergoes heat treatment. Its disadvantage is the higher cost of manufacture compared to assembled and solid forged rotors. They are used in the CMP and CLP of steam turbines.

Steam Turbine Seal Designs

A multistage turbine uses end seals, peripheral seals along the working grid bandage, and diaphragm seals of the labyrinth type. The end seals must ensure a minimum of steam leaks in the area where the HPC and MPC rotors exit their casings. In the LPC, the end seals prevent atmospheric air from entering the flow part, where a vacuum occurs. The basic principles of labyrinth seal operation were given earlier.

The sealing ridges of the seals can be installed directly on the turbine rotor shaft. In this case, a thin tape with a thickness of 0.2-0.3 mm is calked into the shaft grooves. The radial clearance in the seals is 0.5-0.65 mm. To prevent shaft deflection, which may occur when rubbing on the rotor surface, thermal (compensation) grooves are made after each segment. In the HPC seals located near the axial bearing, the axial clearance is 3.5-3.8 mm, and in the seal on the opposite side, the axial clearance reaches 7 mm. This difference is due to the relative thermal expansion of the rotor (its expansion occurs from the locking point located in the axial bearing of the turbine). The designs of diaphragm seals differ from the end seals in the number of ridges.

The flows of water vapor in the seals are combined by a pipeline system and are regulated using a seal regulator depending on the operating mode of the turbo plant. Fig. 10 shows a diagram in which saturated steam is supplied to the LPC seals from a deaerator with pd = 0.6 MPa. The suction of the steam-air medium from the extreme (chimney) chambers of the end seals is carried out using ejectors (EI) into the coolers (OU) of the thermal circuit of the TPP. Since steam consumption through end seals in modern turbines is high, the heat of leaks is used in the regenerative feedwater heating system.

Example of steam turbine design

Let's consider the design of a multi-cylinder turbine K-300-23.5 LMZ. This turbine consists of high (HP), medium (MP) and low (LP) pressure cylinders, and is operated with initial steam parameters p0=23.5 MPa, t0=540 0C. The turbine is installed in a block with a once-through power boiler with a capacity of G0=264 kg/s with intermediate superheating of steam to a temperature of tpp=540 0C after its expansion in the flow part of the HPC.

The main structural elements of the turbine are:

• the rotors of its cylinders (respectively, RVD, RSD and RND), which together with the rotors of the electric generator and exciter make up the shaft line of the turbo unit;
• the housings of the corresponding cylinders;
• the equipment of the steam distribution system;
• the bearings for perceiving the radial and axial loads formed in the rotors of the turbo unit {radial (support) and axial (thrust) bearings}.

From the boiler, water vapor is supplied through two steam pipes to the stop valves (the actuators of the emergency protection system of the turbo unit), which are connected by bypass pipes to seven control valves installed near the turbine in the form of separate blocks. The control valves are the executive bodies of the turbine power control system. Their sequential opening provides access of steam to four nozzle boxes welded into the internal casing of the HPC. Full opening of the first six valves, supplying steam to three nozzle boxes, allows the nominal power of the turbo unit to be realized. Maximum power is provided by opening the seventh valve with access of steam to the fourth nozzle box. In the left section of the HPC, the expansion of steam is carried out first in the control stage, and then in five stages, after which the water vapor makes a 1800 turn and moves between the internal and external casings of the cylinder. In the right section of the HPC for this turbine there are six turbine stages, after expansion in the flow part of which the water vapor with the parameters of 4 MPa and 330 0C is sent for intermediate superheating in the superheater path of the boiler.

After intermediate superheating, the water vapor is sent through two stop and control valves to the turbine stages of the MPC, of ??which there are 12. For the K-300-23.5 LMZ steam turbine, the medium pressure cylinder is combined with one part of the MPC. In total, the MPC has three identical flow parts and, accordingly, three outlet devices. Each part of the MPC consists of five turbine stages, the last of which has an average diameter of d2,cp = 2.48 m and a length of working blades of l2 = 960 mm. Thus, after the MPC, the water vapor is divided into two flows, with flow rates equal to 1/3 and 2/3 of the total flow rate. After separation, two thirds of the steam are sent through receiver pipes to a two-flow low-pressure cylinder with a steam pressure of 0.24 MPa and a temperature of 240 0C in front of it. After expansion in the flow parts of the low-pressure cylinder, the steam is sent to the condenser through the corresponding outlet pipes.

The high-pressure rotor is made of one piece and is connected to the medium-pressure rotor by a rigid coupling, the coupling halves of which are forged as a single piece with the high-pressure and medium-pressure shafts. The left part of the high-pressure shaft rests in a radial bearing, and a combined radial-axial bearing is located between the high-pressure and medium-pressure cylinders. The medium-pressure rotor is made of a combined type: the disks of the first 12 turbine stages are forged as a single piece with the shaft, and the disks of the last 5 stages (related to the low-pressure cylinder) are mounted on the shaft with interference. The rotors of the high-pressure cylinder and the dual-flow low-pressure cylinder are connected by a semi-rigid coupling, and the rotors of the low-pressure cylinder and the electric generator are connected by a rigid coupling with slip-on coupling halves. The radial bearings of the outlet part of the high-pressure and low-pressure cylinders are built into the outlet pipes.

All turbine housings have a horizontal flange connector. The HPC casing is made double, which allows, with a reduced thickness of the walls and flanges of the inner and outer casings, to improve the maneuverability of the turbine due to their faster and more uniform heating together with the high-pressure hose. The inner casing is made of 15Kh11MFBL steel. The diaphragms of the left section of the HPC are installed directly in the inner casing, and those of the right section are in cages fixed in the outer casing. The MPC casing consists of three parts connected by vertical technological connectors. The front part of the casing is made of 15Kh1M1FL steel, the middle part is made of 25L steel, and the rear part is welded from sheet carbon steel. All MPC diaphragms are welded. The MPC casing is welded, double-walled. Cast iron diaphragms of the first four stages are installed in the inner casing. The MPC casing (including the MPC outlet part) rests on the foundation frames by means of a support belt made along the perimeter near the flange horizontal connector.