Bri+-+Turbines

Bloch, Heinz P., and Murari Singh. //Steam Turbines//. 2 ed. New York: McGraw-Hill Professional, 2008. Print.

textbook, has designs for casing, rotor, blades. Rerates, upgrades and efficiency section.

Introduction to Steam Turbines The general process for a steam turbine begins at the steam inlet. The ** steam inlet ** holds steam piped from boiler, which is at a high pressure and temperature. ** Nozzles ** direct steam flow into the turbine. As the steam travels through the nozzles it expands to exhaust pressure, forming high-speed jets. These steam jet strikes a moving row of **blades** that are mounted on the ** rotor **. They cause rotor to turn; creating kinetic energy. At the end of the turbine excess steam is sent out the ** exhaust **. The **casing** houses all the components of the turbine and the **governing valves** control speed/load, inlet pressures, extraction pressures, and exhaust pressure. // Classifications of Turbines // There are two ways that the steam can make the blades turn, and these are known as reaction and impulse. Reaction turbines have nozzles mounted on the moving wheels. Impulse turbines have stationary nozzles and the blades move due to impact of a jet stream. Reaction turbines must have small internal clearances to minimize leakages between blades and a balance piston because of the large thrust loads generated. Impulse turbines can have larger clearances and are more rugged, therefore more often used. A turbine can also be a combination of both reaction and impulse. Turbines can have a number of blade wheels in sequence and this is known as ‘staging’ (i.e. single stage has just one wheel of blades). The different stages are separated by the diaphragm, which is stationary and houses the nozzles. Each stage has a successively lower temperature pressure as you move further downstream of the initial steam inlet. There are two types of special staging setups that are important to note. Velocity compounded staging (or Curtis staging) has one nozzle for every two rows of moving blades. The nozzle directs the steam at the first moving row and an intermediate row of stationary reversing blades directs it at a second moving row. This staging makes use of the high velocity steam jet with two rows of blades at a constant pressure. It has a small wheel diameter and tip speed, fewer stages, more is a more rugged turbine. Pressure compounded staging (Rateau staging) has one nozzle for every single row of blades. The pressure drops as steam travels out of each nozzle.

Another classification of steam turbines is multivalve or single valve. Single valve turbines are not applicable to industrial operations so they are of no concern for this report. Multivalve turbines have many points of entry for steam. Each valve opens sequentially only if previous valve is wide open. This allows the valves to react to load changes and high inlet volume flows. One valve feeds many nozzles, forming a short arc that allows for a better velocity ratio. This limits the pressure drop across governing valves, in turn minimizing throttle loss. Steam balance is an important consideration for turbines. A ‘straight’ steam balance means no steam is removed between inlet and exhaust. Straight turbines can be noncondensing or condensing, depending on desired removal. Almost all industrial turbines are extraction turbines. This means there are headers throughout the turbine that take off excess steam. Noncondensing extraction takes the steam off to supply it to a low pressure (LP) turbine further downstream. Condensing turbines take steam off at the lowest pressure possible using a heat sink to be taken off to process. The number of headers is reflected in the name of the turbine (i.e. Double Automatic Extraction Noncondensing) // Non-moving Parts // Shell · Must accommodate transient thermal stresses and provide a design that resists shell cracking and alignment changes · Double-shell design prevents initial steam from contacting outer casing joint · horizontally split and provides reliable, leak-free operation with metal-to-metal joints and moisture drainage ability Bearings · Supported on adjusting devices · Move independently of each other · On fixed bearing pedestals completely separate from steam-carrying turbine casing Steam admission · One or more governing valves · Bar lift or cam lift Diaphragm · Stationary partition between each rotating wheel that separates turbine into many pressure stages (successively lower pressures). Different sizes and designs for different pressures. Contains nozzles that accelerate steam to next stage · Conventionally Cast: preformed nozzle blades of cold rolled 405 stainless steel. Individual nozzle sections are positioned and ductile cast iron material (ASTM A-536, Gr. 80-55-06) is poured to form diaphragm. Only good for low temp and pressure drops of 25 psig. Being phased out. · Investment Cast: Individual nozzle blocks (17% chrome)assembled on inner and outer steel rings (ASTM A-283 Gr. D) by submerged arc welding. 7500F · Milled and welded: nozzles milled from solid blocks of stainless steel (hot rolled 405 ASTM A-276) and welded to inner and outer steel rings. Temp can be increased from 7500F with proper inner and outer material · Spoke type: used for large nozzle heights (>83 mm). Blades made of 405 steel or 17% chrome (sand cast, milled, or preformed). Fitted by ridges and pins, tack-welded, ridges removed, and welded by manual shielded electric arc method. · Diaphragms are horizontally split and keyed up the middle. Key is a seal and an axial locating device for two halves. Lower half is screwed to casing. · Various kinds of packing (see pg 45-47) o Labyrinth seals or brush seals // Moving Parts // Rotor · Determining factors for type of construction are long-term operation, pitch diameter, Max operating speed, steam temperature · Built-up: wheels are shrunk onto shaft. If a positive temperature difference between the wheel and shaft develops the shrink is lost and wheel is constrained from moving. · Solid: wheels and shaft are machined from single forging. At temperatures above 7500C integral wheel forging is used. o St 460 TS and 461 TS steels. See table on pg 99 for chemical composition o Tensile strength, yield point, and notch rupture are tested before rotor is approved · Combination: High temperature and pressure end is solid while low temp end can be built-up · For more materials see table on page 87 · Welded rotors: avoid solid machining. o Large turbines are usually welded o ADVANTAGES: Individual rotor components can be thoroughly forged guaranteeing large metallurgical quality, large flexural rigidity, low stress levels since there is no central bore, simple inspection before welding, favorable heat flow with no appreciable axial stress. o For highest temps and water droplet erosion, X20CrMoV121 steel is used. 21CrMoV511 can be used for high temp region and CrMo steel for lower temps o See table on page 105 for welded turbine material information Turbine Blades · Rotating blades are called “buckets” · Unsteady steam forces result in vibration resonance. Minimum resonant vibration is desired for optimal reliability · Drawn blades: less expensive because of less machining steps · Milled blades: machined from a piece of bar stock · Materials o 403 stainless steel (industry standard). High yield strength, endurance limit, ductility, toughness, erosion and corrosion resistance, and damping. Used within Brinell hardness of 207-248 to maximize damping and corrosion resistance o 422 stainless steel (used for high temp regions 700-9000F) higher yield, endurance, creep and rupture strengths o A-286 (used in hot gas expanders 900-11500F) nickel-based super alloy o Haynes Stellite Alloy Number 31 and titanium alloy (used where precision cast blades are needed 900-12000F) o Titanium (high speeds or long-lasting blades) high strength, low density, good erosion resistance · Each blade is individually attached to the wheel with a root. Standard: dovetail root, single or double tooth. See page 113. Final blade in a row is always secured with a high-strength fastener · “shroud” is the strip of metal connecting each blade · Due to the non-homogeneous nature of the steam flow, dynamic blade stresses are experienced. Poorly designed blades will experience flow separation · Gaps can be caused when blade and shaft materials have different thermal expansion coeff. (martensitic blade and ferritic shaft). Correct root and shroud geometry can prevent this with counteracting torrsional moment from each blade to its respective shroud · Transition zones o stages where the conditions vary. For example: salts dissolved in the steam are deposited on blades, but sometimes the stage admits temporary wet-steam due to changes in operating conditions (salt is dissolved again by condensing steam). On the other hand, superheated steam could pass through stages that normally have wet steam conditions o Range of stages where this occurs is determined through systematic investigation o Upper limit of design condition is minimum superheat, lower limit is maximum steam content o Upstream of transition zone is superheated steam, downstream is wet steam · Low pressure blades are longer