Basic knowledge of steam turbines
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Basic Knowledge of Steam Turbines – Section 1: OverviewI. Overview
A steam turbine is a rotary thermal power engine that uses steam to generate power. Its advantages include high power output, high efficiency, simple structure, few vulnerable components, safe and reliable operation, easy speed control, low vibration, and low noise.
Classification
| Name | Description |
|------|-------------|
| By working principle | Impulse steam turbine: Steam expands mainly within the nozzle cascade. Reaction steam turbine: Steam expands within both the stationary and moving cascades. |
| By number of stages | Single-stage turbine: The flow path consists of only one stage. Multi-stage turbine: The flow path has two or more stages. |
| By direction of steam flow | Axial-flow turbine: Steam flows in parallel to the axis. Radial-flow turbine: Steam flows perpendicular to the axis. Circumferential-flow turbine: Steam flows in a circular path. |
| By characteristics of the thermal system | Condensing turbine: Exhaust pressure is lower than atmospheric pressure. Extraction-backpressure turbine: Exhaust pressure is higher than atmospheric pressure, with steam extraction in between. Back-pressure turbine: Exhaust pressure is higher than atmospheric pressure. |
| By application | Power station turbines: Used for power generation. Industrial turbines: Used to drive pumps and compressors. Marine turbines: Used as power units for ships. |
| By inlet steam pressure | Low-pressure turbine: 1.2–1.5 MPa. Medium-pressure turbine: 2–4 MPa. Super-medium-pressure turbine: 5–6 MPa. High-pressure turbine: 6–12 MPa. Ultra-high-pressure turbine: 12–14 MPa. |
| By rotational speed | Low-speed turbine: n < 3000 revolutions per minute. Medium-speed turbine: n = 3000 revolutions per minute. High-speed turbine: n > 3000 revolutions per minute. |
In refineries, waste heat generated during the refining process can also be utilized to produce steam for powering machines and pumps, thereby enabling comprehensive utilization of thermal energy. It is because of these advantages that steam turbines are widely used in refineries. II. Types of Steam Turbines There are a wide variety of steam turbines, which can be classified based on their working principles, performance, structural features, and other aspects. Section 2: Working Principle of Turbines. The main components of a turbine consist of two parts: nozzles (also known as stator blades) and rotor blades (also known as blades). The nozzle is fixed to the casing or baffle, while the moving blade is fixed to the disc. As steam passes through the nozzle, its pressure drops and its volume expands to form a high-speed steam stream, which drives the impeller to rotate and generate power. If the pressure of the steam within the blades does not decrease any further, that is, if the flow velocity of the steam in the blade channels (i.e., the relative velocity) remains constant, and the rotor is driven to rotate solely by the force exerted by the steam flow on the blades, such turbines are known as impulse turbines; they are also referred to as pressure-stage turbines, and are widely used in industry. If the steam continues to expand within the blades (referred to as the relative velocity), and this velocity is greater than that at the inlet, the work produced by such a turbine results not only from the force exerted by the steam on the blades but also from the large reaction forces arising from changes in the steam’s relative velocity. Therefore, these turbines are known as reaction turbines, and they are less commonly used in refineries. A turbine that consists of only one row of nozzles and one row of moving blades is called a single-stage turbine. Several single stages connected in series is called a multi-stage turbine. Due to the extremely high velocity of the steam stream after a single pressure reduction, the rotor speed becomes very high, exceeding the strength limits allowed by current materials. Therefore, the pressure staging method is employed; the pressure drop in the nozzle is small each time, which results in a low steam flow velocity. As a result, the energy of the high-pressure steam is fully utilized after passing through multiple stages of impellers, and the rotation speed of the impellers remains within the limits permitted by the material strength. This is why multi-stage turbines are used. If the flow velocity of the steam leaving each stage of blades remains high, in order to make full use of the kinetic energy of the steam stream, guide vanes can be used to direct the steam into the second row of blades (each impeller can be equipped with two rows of blades) to further drive the shaft and generate work. This is known as speed staging, or simply a speed stage (also referred to as a multi-speed stage). Speed stages are commonly used in small steam turbines, or in the first stage of a steam turbine. Section 3: Composition of the Steam Turbine Body. A steam turbine consists of three main parts: the turbine body, the control and protection devices, and the auxiliary equipment. I. Steam turbine body The steam turbine body includes: the stationary parts (fixed components) – cylinder, nozzles, diaphragms, steam seals, etc ； Rotor (moving part) -- shaft, impeller, blades, etc ； Bearings (supporting parts) -- radial bearings and thrust bearings. 1. Cylinder: The cylinder itself is horizontally divided into upper and lower parts, and each of the upper and lower cylinders is further divided into front and rear sections. The front cylinder is made of cast steel due to high temperatures, while the rear cylinder is made of cast iron because of lower temperatures. When a steam turbine unit starts or stops, or when its load is increased or decreased, the temperature of the cylinder body rises or falls, resulting in thermal expansion and contraction. Due to changes in temperature differences, the extent of thermal expansion can range from a few millimeters to over ten millimeters. However, the temperature change of the plate connected to the cylinder is very small. To maintain the relative position between the cylinder and the rotor, a pin system with an appropriate gap is installed between the cylinder and the plate. Its functions are: a) to ensure that the centers of the cylinder and the rotor are aligned, preventing changes in center due to the expansion of the machine structure, which could otherwise cause vibration in the unit or friction between the moving and stationary parts ； b. Ensure that the cylinder can expand freely to prevent deformation caused by excessive stress ； c. Ensure that the axial and radial clearances between the stator and rotor are within the specified limits. Based on their structure, installation location, and various functions, slide pins can be classified as follows: a. Transverse pins (Figure 7-1): Their purpose is to allow the cylinder to expand freely in the transverse direction; they are generally installed on the transverse center line of the exhaust chamber of the low-pressure cylinder or at the rear end of the exhaust chamber, one on each side. Figure 7-1: Vertical and horizontal pins b, vertical pin (Figure 7-1): Its function is to allow the cylinder to expand freely along its longitudinal center while preventing lateral movement of the cylinder’s longitudinal center. The point where the longitudinal pin centerline and the transverse pin centerline intersect is called the \"dead center\"; this point remains fixed as the cylinder expands. The longitudinal pin is installed at the bottom of the rear bearing seat and the front bearing seat. c. Vertical pin (Figure 7-2): Its function is to ensure that the cylinder can expand freely in the vertical direction, and together with the longitudinal pin, it helps maintain the longitudinal center of the unit unchanged. The vertical pin is installed between the rear part of the exhaust chamber of the low-pressure cylinder and the platform, between the front end of the high-pressure cylinder and the front bearing seat, and between the front end of the low-pressure cylinder and the end of the high-pressure cylinder in a two-cylinder turbine, as well as between those components and the central bearing seat. All vertical pins are located on the longitudinal center line of the unit. Figure 7-2: Vertical pin d, cat’s claw cross-pin (Figure 7-3): Their function is to ensure that the cylinder can expand laterally; simultaneously, as the cylinder expands and contracts axially, they push the bearing housing forward or backward in order to maintain the axial relative position between the rotor and the cylinder. The cat’s paw cross-mount is installed on the horizontal joint surface between the front bearing housing and the intermediate bearing housing of the double-cylinder turbine. The cat claw cross pin and the vertical pin work together to keep the center of the cylinder aligned with the center of the bearing housing. Figure 7-3: Cat’s-claw cross pins e; corner pins (Figure 7-4): Installed on the left and right sides at the bottom of the front bearing housing and the intermediate bearing housing of the two-cylinder turbine, to replace the bolts that connect the bearing housings to the platform, while allowing the bearing housings to move longitudinally. Figure 5-7-5: Slant pin. Figure 7-4: Angle pin. f, Slant pin (Figure 7-5): It is a type of auxiliary sliding pin that serves as a guide for both vertical and horizontal pins. Installed between the support feet on the left and right sides at the front of the exhaust chamber and the platform. The front bearing housing is mounted on the front frame, which is fixed to the foundation via foot bolts. A longitudinal key guide is provided between the front bearing housing and the front frame, allowing the former to slide longitudinally along the latter. The front cylinder is connected to the slider fixed to the front bearing housing via cat claws, and a vertical key is used for positioning between the front cylinder and the front bearing housing to ensure that their longitudinal centers align. The rear cylinder is mounted on the rear frame, which is fixed to the foundation via foot bolts. The guide plate of the rear cylinder ensures that it is aligned with the longitudinal centerline. For the slide pin system of each turbine, there is a point whose relative position remains unchanged regardless of how the turbine’s cylinders expand in all directions; this point is known as the dead center of cylinder expansion. To ensure that the cylinder can expand freely in all directions—forward, backward, left, and right—it is necessary to have a certain amount of clearance between each slider and its groove. After precision machining, these components are carefully adjusted by fitters; the sliding surfaces must be smooth, free from rust spots and burrs. If there is a malfunction in the slider system, it can hinder the normal expansion of the unit, and in severe cases, it can cause vibration in the unit or even prevent it from operating properly. 2. Nozzle assembly and diaphragm: In impulse turbines, the conversion of steam’s thermal energy into kinetic energy takes place in the nozzles. After passing through the nozzle throat with a variable cross-section, the steam expands in volume, its pressure decreases, and its flow velocity increases; it then enters the moving blades at a certain injection angle to do work. The partition in the turbine cylinder is a circular plate-like assembly composed of the outer edge of the partition, nozzles, and the partition body; the first-stage partition within the cylinder, together with the first-stage impeller that follows it, forms a pressure stage. The partition is divided into upper and lower semicircles, with positioning keys on the mid-surface to ensure that the upper and lower partitions form a single unit. In a steam turbine, the nozzle assembly mounted on the control chamber is commonly referred to as the nozzle group; it consists of the outer edge of the nozzle group, the nozzles themselves, and the inner edge of the nozzle group. Based on the manufacturing method, turbine diaphragms can be divided into three types: cast diaphragms, welded diaphragms, and assembled diaphragms. 3. Steam seal: The shaft seal at the high-pressure end of a turbine is called the high-pressure shaft seal; in single-cylinder turbines, it is also referred to as the front shaft seal. The shaft seal on the low-pressure side is called the low-pressure shaft seal; in single-cylinder steam turbines, it is also referred to as the rear shaft seal. The steam seal installed in the baffle steam seal groove is called a baffle steam seal. The one installed on the partition to work together with the gasket to prevent steam leakage is also known as a gasket steam seal. Whether it is an shaft seal, a diaphragm steam seal, or a girth seal, their structure and shape are largely similar, and their principle of preventing steam leakage is the same; they are all collectively referred to as steam seals. (1) Function of the steam seal: There is a certain gap between the shaft holes at both ends of the turbine cylinder and the rotating shaft; as a result, during operation, a large amount of high-pressure steam leaks from the steam inlet side of the cylinder. Looking at the exhaust side, the exhaust pressure of a conventional condensing steam turbine is around 0.02 kg/cm2 (absolute); in other words, the exhaust side is in a state of high vacuum. Air from the atmosphere will leak into the exhaust pipes and condenser through the rear shaft holes, thereby disrupting the vacuum in the turbine. Therefore, in order to reduce steam leakage from the high-pressure side and air leakage into the exhaust side, it is necessary to install shaft seal devices (also known as shaft seals) at the shaft holes at both ends of the cylinder. (2) Structure of the shaft seal: The comb-tooth structure with upper and lower teeth is currently widely used, as shown in Figure 7-6. The shaft seal element is either directly formed on the shaft seal ring or pressed into the grooves of the shaft seal ring; the shaft seal ring generally consists of four or six arc segments. The thinnest part of the tooth tip has a thickness of 0.1 millimeters. A spring plate presses against the shaft seal ring, keeping it in close contact with the partition or steam seal body. The function of this spring plate is to tighten the shaft seal ring; when the shaft seal ring comes into contact with the main shaft, it can retract automatically to prevent damage to the shaft seal. When the pressure difference is small, flat teeth can be used instead of high and low teeth. To reduce steam leakage, it is necessary to keep the shaft seal gap as small as possible. However, to ensure the safe operation of the unit, it is essential that there is no rubbing at the shaft seal; therefore, certain requirements must be met regarding the shaft seal gap. 1- Spring plate 2- Shaft seal sleeve 3- Shaft seal ring 4- Shaft Figure 7-6 High and low toothed shaft seals. In addition to shaft seals at both ends of the turbine, shaft seal plates also need to be installed in the shaft holes of each stage partition in order to reduce steam leakage between stages. The structure of the partition steam seal is the same as that of the shaft-end steam seal; it simply has a lower pressure difference, which means fewer steam seal plates are required. Furthermore, some turbine blades are also equipped with steam seal devices. (3) Materials for the steam seal ring and steam seal plates: The steam seal ring in high-temperature areas is made of chromium stainless steel 1Cr13, or chromium-molybdenum-vanadium stainless steel Cr11MoV, while the steam seal plates are made of chromium-nickel-titanium stainless steel 1Cr18Ni9Ti. Tin bronze is used for the steam seal ring in the low-temperature operating area, while lead brass is used for the steam seal plates. (4) End shaft seal system: To ensure the safe operation of the turbine, make rational use of the steam leakage from the end shaft seals, and improve the economic efficiency of the turbine. Each turbine end shaft seal is equipped with a dedicated shaft seal piping system. Although an axial seal is installed at the high-pressure end, it is still not possible to prevent steam from leaking outward through this seal. To minimize this loss, the axial seal at the high-pressure end is divided into several sections, with spaces left between each section; the steam leaking from these spaces is directed to different locations for utilization, thereby improving the efficiency of the steam turbine. The high-pressure steam leakage from small steam turbines can be directed through pipes to the low-pressure end shaft seals for use as sealing steam, while the remaining small amount of leakage passes through several shaft seal plates and is discharged to the atmosphere via signal pipes. Operators can determine the performance of the end shaft seal by observing the steam emission from the signal tube. At the low-pressure end, in order to prevent air from leaking into the cylinder through the shaft seal, steam at a pressure slightly higher than atmospheric pressure must be used to seal the shaft seal passage. This portion of steam is drawn from the high-pressure end shaft seal; in the shaft seal chamber, some of it flows into the low-pressure cylinder after passing through the shaft seal plates, while the other part flows out along the shaft seal gap and is finally discharged to the atmosphere via a signal tube. When the turbine is operating normally, in addition to being introduced into the low-pressure end shaft seal, the steam leaking from the high-pressure end shaft seal can be routed through pipes to the condenser. When the turbine is started or stopped, if there is no steam at the high-pressure end shaft seals, new steam that has been throttled to reduce its pressure should be supplied to both the high-pressure and low-pressure end shaft seals. 4. Rotor: The assembly of all rotating components in a steam turbine is called the rotor. It mainly includes components such as the main shaft, impeller, and blades (Figure 7-7). In addition to being subjected to high-temperature and high-pressure steam, the turbine rotor operates at high speeds and is affected by centrifugal force; therefore, the issue of vibration must also be taken into account. The main rotor types used in domestically produced units in our country are: (1) assembled rotor ； (2) Solid-forged rotor ； (3) Combined rotor ； (4) Welded rotor. The rotors of domestic small and medium-sized units with parameters below medium level all adopt a modular structure. Before assembly, the inner diameter of the impeller in a set-type impeller should be 0.05–0.15% smaller than the shaft diameter. During assembly, the perimeter of the impeller’s inner diameter is heated until it becomes 0.10–0.20 millimeters larger than the shaft diameter; alternatively, the temperature of the heating element can be maintained at 250–270°C. The impeller is then fitted over the shaft, and once it cools down, a strong compressive force is generated between the inner diameter of the impeller and the shaft, ensuring safe and reliable operation of the impeller at high speeds. The advantage of modular impellers is their ease of manufacturing, but when operating under high temperatures, they tend to become loose between the impeller and the main shaft; therefore, rotors in high-temperature units usually adopt a monoblock design. 1. Structure of the impeller: The structure of the impeller is divided into three parts: ① The rim section, which is the part where the blades are mounted and has a shape that matches that of the blade root structure. ② Hub section: It is through this that the impeller ring is fitted onto the shaft. ③ The wheel body section is the intermediate part that connects the rim to the hub. Due to the high speed, ④ its forces and deformations are primarily determined by the centrifugal force generated by the rotation of the impeller itself; a design with equal stress distribution is preferred, but it is difficult to manufacture. A cone shape is commonly used. 45 steel is used at low pressures, while 35CrMoA or 34CrMoA is used at medium pressures. Generally, seven balance holes are also drilled in the wheel body, in order to reduce the axial thrust acting on the rotor due to the pressure difference before and after the impeller. 2. Structure and fixation of the blade The blade consists of a blade root, a working portion, and a tip. The blade root is used to secure the blade to the impeller; its connection to the impeller must be strong and reliable, ensuring that the blade does not loosen under any operating conditions. Meanwhile, the structure of the blade root should be as simple as possible while still meeting strength requirements, in order to facilitate manufacturing and installation, and to minimize the axial dimension of the impeller edge. Common types of blade roots include: T-type, mushroom-type blade root, forked blade root, and fir-tree type blade root. Working section: Steam passes through here, converting kinetic energy into mechanical work; the working section is the main part of the blades. The recessed part of the blade is called the blade face, while the protruding part is called the blade back. Generally, the blade profile of a steam turbine remains the same throughout its height. When the blade height is very large and the ratio of blade height to diameter is greater than 1/10, twisted blades are used. The blade tip is usually designed in a rivet shape, with a surrounding band attached to it; this enhances the strength of the blade while reducing steam leakage. There are several types of shrouds: ① Riveted shroud: The shroud is made of flat steel and then fixed to the top of the blade using riveting. ②Integrated shroud: The shroud and the blade form a single unit; they are machined together during blade fabrication, and once the blade is assembled, the shroud is welded to it. ③Elastic arch-shaped strap: It is made by bending spring steel sheets into an arch shape, which is then fixed to the top of the blade using rivets, with interlocking ring connections. When the blades are long, drag links are also installed to connect the blades into blade groups, thereby enhancing the strength of the blades and improving their vibration characteristics. Lacing is usually made in rod or tube form; in primary blades, there are generally 1 to 2 turns of lacing, with no more than 3 turns. The methods of connecting the lacing include grouped connection, full-turn connection, and inter-group connection. There are several methods for installing blades: ① Circumferential embedding method: The blades are inserted sequentially from the circumferential direction into the corresponding slots on the impeller rim; the last blade seals the gap and is then fastened with rivets. Both T-type and mushroom-type blade roots use this method. ② Axial insertion method: The blades are individually inserted axially into the corresponding slots in the impeller rim. This method is often used to install fir-type leaf roots. ③ Radial cross-mounting method: The forked blade roots are inserted radially into the forked grooves in the impeller rim, and then fixed with rivets. This method is only applicable to the installation of fork-shaped blade roots. 5. Bearings: Currently, most steam turbines use sliding bearings. In addition to radial bearings, turbines also have thrust bearings. Because, when the turbine is in operation, an axial thrust is generated on the rotor, pushing it from the high-pressure side toward the low-pressure side. Therefore, a thrust bearing is usually provided at the front end of the rotor to withstand axial thrust and to determine the axial clearance of the flow-through section. At present, most of the front bearings in domestically produced units in our country use radial-thrust combined bearings. Section 5: Speed Control Systems In the petroleum and chemical industry, turbines are primarily used as prime movers to drive compressors, pumps, etc. To save energy, the efficiency of these turbines is designed based on operation at specific speeds. When the rotational speed changes significantly, it causes the turbine to deviate greatly from its designed operating conditions, resulting in reduced efficiency. To this end, it is necessary to stabilize the turbine at a certain speed, and the purpose of the turbine control speed regulation system is to meet this requirement. It adjusts itself by using changes in speed as a signal, based on the corresponding relationship between the torque and speed of the turbine. When there is a slight change in speed, the speed control system can automatically adjust the amount of steam supplied to the turbine, so that the turbine’s power matches the load and thus prevents large fluctuations in speed. The speed control system of a steam turbine consists of a starting device, safety devices, protection devices, a governor, and monitoring devices. I. Starting device: The function of the starting device is to open the quick-shut valve. Pressure oil from the emergency shutdown device is used as starting oil to feed into this device; as the control lever moves, the spool also moves, thereby connecting the starting oil pressure with the quick-shut oil pressure and opening the quick-shut valve. II. Safety device – Quick shut-off valve. The quick shut-off valve is installed horizontally on the steam inlet pipeline of the turbine cylinder; it consists of components such as the valve body, filter screen, and hydraulic cylinder (Figure 7-8). The quick shut-off valve is the main closing mechanism between the new steam pipeline network and the turbine; in the event of an accident during operation, it can cut off the steam flowing into the turbine in the shortest possible time. Valve body section: Figures 7-8 show the structure of the quick-close valve. The fresh steam passes through the valve cone of the steam strainer; inside this cone there is a unloading cone. Since its area is much smaller than that of the valve cone, it helps to reduce the lifting force when the quick-close valve is opened. After the unloading valve is opened, the pressure difference behind the valve cone decreases, making it easier to open. in the valve sleeve