Guidelines on the Effects of Cycling Operation on Maintenance Activities

Guidelines on the Effects of Cycling Operation on Maintenance Activities Technical Report Remember to replace artwork and delete this box Guideline on the Effects of Cycling Operation on Maintenance Activities 1004017 Final Report, December 2001 EPRI Project Managers M Perakis M DeCoster EPRI • 3412 Hillview Avenue, Palo Alto, California 94304 • PO Box 10412, Palo Alto, California 94303 • USA 800.313.3774 • 650.855.2121 • askepri@epri.com • www.epri.com DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC (EPRI) NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM: (A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITH RESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT, 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EPRI ELECTRIFY THE WORLD is a service mark of the Electric Power Research Institute, Inc Copyright © 2001 Electric Power Research Institute, Inc All rights reserved CITATIONS This report was prepared by European Technology Development Limited Warwick Gardens Ashtead Surrey KT21 2HR UK Principal Investigator A Shibli This report describes research sponsored by EPRI The report is a corporate document that should be cited in the literature in the following manner: Guideline on the Effects of Cycling Operation on Maintenance Activities, EPRI, Palo Alto, CA: 2001 1004017 iii REPORT SUMMARY Cyclic operation can result in an increase in forced outage rates, higher operation and maintenance (O&M) costs, and further wear and tear on components due to additional overhauls and maintenance Such operation may also increase unforeseen costs due to additional personnel training requirements and the use of more sophisticated evaluation and inspection techniques These guidelines are intended to help plant operators and engineers assess the impact of cycling on maintenance activities and take appropriate preventive measures when operating a plant in cyclic mode While the guidelines provide important direction on O&M practices when shifting from baseload to intermittent operation, they not set out to be a comprehensive listing of individual plant maintenance activities Background The severity of cyclic operation affects boiler, turbine, electrical, and auxiliary components The effect is largely design dependent, and older plants originally designed for baseload usage fall into the less tolerant category Such units were designed with heavy section headers and piping with a poor response to thermal fatigue, which basically results from temperature changes during startup and shutdown Cycling tends to exacerbate such problems and will lead to an increased incidence of stress corrosion and corrosion fatigue of feedwater heaters, economizers, and turbine units Clearly, cycling utilities need to understand that events which are tolerable on an occasional basis during baseload operation—in terms of damage to the plant, risk to staff, or impact on the local environment—would be quite intolerable if occurring daily EPRI sponsored development of these guidelines to provide direction for all cycling plants, but particularly those shifting from baseload operation to intermittent operation Objective To develop guidelines addressing the effects of cycling operation on maintenance activities Approach Because of the range of technical issues involved, EPRI engaged a multi-disciplinary team to develop these guidelines Investigators based the guidelines primarily on United Kingdom maintenance experience for fossil steam plants with drum boilers, as revealed in a survey of plant operators and R&D organizations Information from other countries—including Ireland, Italy, France, Portugal, South Africa, Hong Kong, and the United States—was also acquired through surveys Included in the survey demographics was a large population of once-through boiler owners Finally, in developing the guidelines, investigators also relied on their own knowledge and understanding of plant issues as well as published literature v Results These guidelines focus on a number of key issues connected with O&M activities which are critical to successful cyclic operation These issues include • Minimization of plant damage and optimization of operation, with emphasis on turbines and boilers • Operability enhancements • Problems experienced under two-shifting duty • Engineering modifications to facilitate flexible operation • The impact of cycling on maintenance practices • Industry practices as they relate to regulatory codes Also featured in the guidelines are responses to the survey questionnaire along with interviews concerning routine maintenance practices, condition monitoring, and maintenance scheduling The appendices to the guidelines document several new and interesting developments in repair techniques In all, the guidelines emphasize that cycling mode presents challenges in the way utilities view O&M procedures Under cyclic operation, nearly every O&M procedure must be analyzed in detail and almost certainly modified to ensure safety, cost-effectiveness and economy, reliability/repeatability, and minimization of plant damage EPRI Perspective These guidelines are part of EPRI’s development efforts under Target 69, Plant Maintenance Optimization (PMO) The PMO mission is to lead the industry by developing and demonstrating products and services that improve the use of power plant maintenance resources and increase profitability for generation businesses Flexible cyclic operation of large coal-fired units has been successfully carried out while maintaining high plant availability, without excessive additional costs These guidelines will help utilities increase plant availability by taking a cost-effective, systematic approach to preventive maintenance activities when operating units in cyclic mode Keywords Cycling operation Plant maintenance optimization Preventive maintenance vi ABSTRACT This overview is mainly based on the UK experience for fossil steam plants with drum boilers However, information from other countries including Ireland, Italy, France, Portugal, South Africa, Hong Kong and the United States has also been utilized with this data including a large population of once through boilers Some new and interesting developments in repair techniques have also been included in the form of Appendices vii Vent Silencers Increased wear and tear due to frequent operation Vent valves Increased usage may require acoustic improvements to reduce noise levels and to contain emissions Increased corrosion due to frequent operation Increased wear and tear due to frequent operation Potential for excessive use due to poor boiler temperature and pressure control during rapid start up, shut down and load changes Quench cracking at valve by-pass and drain connections Cracking of valve seat and disk hard facing Thermal fatigue cracking of thick walled valve bodies especially at changes in section Cracking at welded roof and wall seals Thermo-mechanical fatigue of attachments and supports due to temperature differentials during start up and shut down Drain valves Safety valves Stop valves Boiler Valves Structure tube attachments resulting in tube misalignment 0 L B B R B R M N N L N N E H N 0 M M Review design with respect to acoustic performance and local noise constraints Modify or replace as required Review and monitor valve performance with view to improved maintenance, modifications or replacement Consider live loading of gland packings Review and monitor valve performance with view to improved maintenance, modifications or replacement Review boiler operability, control and instrumentation R Selective visual/MPI inspection of valve bodies and drainage ports at outages (4) Consider replacement of valves with modern design, possibly with P91 bodies Carry out local repairs or replacement with rolling spares Selective visual/MPI inspection of valve bodies and drainage ports at outages (4) Review design to ensure adequate flexibility to accommodate relative expansion Monitor and review tube attachments and alignment and consider design options OH E-9 MOD MOD MOD R OP OH MOD OH OH MOD Cycling Effects on Maintenance Activities in the UK – Summary of Typical Cycling Problems and their Potential Impact M Increased cyclic stresses from restrained or defective pipe support system leading to over-stressing at welds E-10 Stop/Throttle valves Turbine Steam Chest Increased wear and tear due to frequent usage Cracking of hard surfacing on valve head and valve seats Wear of valve spindles and seals Thermal fatigue & quench type cracking at valve body drains and by-pass portals Over stressing and cyclic stressing of terminal welds at turbine, steam chest, manifolds etc H Tendency for pipework to migrate to limit of travel on constant load supports Supports L Thermal fatigue and creep fatigue of welds Welds R 0 M M M 0 E S E M H M H Pipework L Potential for local thermal quenching of main steam pipework during boiler shut down Thermal quenching at drains due to suction from drains system Thermal stressing due to temperature mismatch during rapid admission of steam on start-up Main Steam Pipework Planned maintenance of valve operating gear and valve seats at outages (4) Repair and replace as required Selective visual/MPI inspection of valve bodies at outages (4) Machine out shallow cracks and re-profile to reduce stress concentrations Monitor crack growth Consider need for additional pipe movement restraints Consider need for detailed pipe stressing assessment under cycling conditions Examine pipe supports and pipe movements and compare with design intent to confirm correct functioning of pipework supports Consider need for metallographic replication where creep and type IV cracking identified as a threat Monitor welds especially terminal welds, with visual inspections and NDE as appropriate at outages (4) Reduce pressure in steam pipework to reduce saturation temperature (sliding pressure operation) Optimize start-up procedures to match steam to metal temperatures Monitor steam pipework and drain temperatures and assess implications Maintain pipework (and boiler) above saturation temperature as long as possible Cycling Effects on Maintenance Activities in the UK – Summary of Typical Cycling Problems and their Potential Impact OH OH OH MOD I OH OH OH OP OP OP I Casing Rotor Inlet nozzles HP & IP Turbine Chest Body Thermal fatigue and creep fatigue of thick sections, section changes eg flanges and at stress concentration features eg grooves Solid particle erosion from spalling oxide layers Increased erosion of HP blades under wet steam conditions on start up and sliding pressure operation Potential for alignment and axial expansion problems during rapid temperature changes Threat of low cycle fatigue not a major problem Low cycle fatigue due to rapid local temperature rise on start up Thermal fatigue cracking of thick walled sections especially at section changes and stress concentrations M E R R M M R E E L L M Install thermocouples and monitor temperature gradients and ramp rates Assess design by finite element analysis and operational procedures to optimize stresses Inspect and optimize thermal insulation Consider flange warming or heating elements to balance steam to metal temperatures Consider options for replacement with modern optimized design Consider variable pressure (sliding pressure) control to reduce temperature variations Monitor situation and modify operating procedures to minimize problems Install stop valve by-pass to facilitate prewarming Modify nozzle housing to accommodate differential expansion, eg include expansion slots Consider options for replacement with modern optimized design Consider valve warming to balance steam to metal temperatures Inspect and optimize thermal insulation Assess design by finite element analysis and operational procedures to optimize stresses Install thermocouples and monitor temperature gradients and ramp rates R I I E-11 MOD MOD R I I MOD OP MOD MOD MOD MOD Cycling Effects on Maintenance Activities in the UK – Summary of Typical Cycling Problems and their Potential Impact E-12 Condenser Tubes Vacuum raising Condenser Casing Rotor LP Turbine Seals Bearings Possible thermal cycling and water chemistry problems No significant problems attributed to low load or thermal cycling Requirement for increased performance to accelerate start up Increased wear and tear of start up vacuum raising plant due to increased number of starts No significant problems attributed to low load or thermal cycling Blade erosion at low loads when exhaust spraying used Fatigue and corrosion failure of final row blades Fatigue cracking at keyways Increased threat of transverse fatigue cracking of shaft Stress corrosion cracking of rotor discs and key ways Potential for seal damage due to differential expansion during start-up and shut down Increased air in-leakage during turbine start up and shut down Possible increased wear and tear on bearings and lub oil system but no significant evidence of problems arising from cycling have been identified M L M M M M H H L R R R E E R R Monitor situation Increase maintenance routines and frequency Consider increased capacity of start up equipment Consider blade replacement (titanium blade option) Machine out minor cracking OP R REN REN OH OH MOD Carry out opportunity inspection of shafts and blading at outages OP Replace discs with design less susceptible to SCC OH OP R MOD R Review water/steam chemistry Routine inspection and repairs at outages (4) Optimize procedures to minimize risk of gland rub occurring Monitor gland performance Consider use of off load gland steam system Routine inspection and repair Cycling Effects on Maintenance Activities in the UK – Summary of Typical Cycling Problems and their Potential Impact Stator Rotor Generators Chemical dosing Make up water Water Treatment Plant Ratchetting effect of differential expansion between copper windings and stator casing leading to breakdown of insulation and movement of windings Fretting of wedge bars induced by thermal cycling Potential for thermal fatigue of rotor forgings especially at section changes eg winding slots Thermal fatigue induced failures of windings Ratchetting effect of differential expansion between copper windings and end rings L M E E E E L L E E R R R L M H Variable conditions lead to instability of chemical monitoring and dosing Potential for stress corrosion cracking in end rings due to moisture when unit cold H N Start up of several units within short period limited by avaiability of make up water Increased consumption of water, especially at start up put additional burden on WTP Monitor operating temperatures and review control of stator cooling system to minimize temperature fluctuations Install generator condition monitoring system Consider need for rotor rewind or rotor renewal Consider change of end rings to 18%Cr 18% Mn alloy material Monitor condition of rotor and windings at outages Consider chemical control equipment and consider upgrading and/or replacement Review water quality under variable load conditions Consider off load storage requirements for additional capacity Review water treatment capability and consider additional streams OP E-13 OP MOD REN MOD OH MOD OP MOD Cycling Effects on Maintenance Activities in the UK – Summary of Typical Cycling Problems and their Potential Impact E-14 Controllability Control Systems Motors Electric Motors Transformers Switchgear Switchgear Old proportional, Integral and Derivative (PID) control systems not suited to part load and variable load operation Moisture and dust ingress during shut down periods Increased duty on windings during motor start operations Increased wear and tear due to stop/start operation Increased wear and tear due to stop/start operation Moisture and dust ingress during shut down periods Increased switching operations give rise to increased wear and tear H N M M H M M H R R R R 0 B Upgrade and/or install additional thermocouples and instrumentation Consider upgrading motors Install variable speed controller gear Increase inspection (condition monitoring) and maintenance of motors Apply testing and analysis procedures at regular intervals (oil analysis, frequency response, winding insulation, capacitance etc.) Increase inspection (condition monitoring) and maintenance of switchgear MOD REN MOD R R R OP OH Regular visual inspection and electrical testing during outages Monitor and minimize ingress of moisture R Monitor condition of stator on load Cycling Effects on Maintenance Activities in the UK – Summary of Typical Cycling Problems and their Potential Impact Pumps Storage tanks Fuel Oil Plant Coal Bunkers No significant problems attributed to low load or thermal cycling Conveyors No significant problems attributed to low load or thermal cycling No significant problems attributed to low load or thermal cycling Potential for fires in bunkers due to spontaneous combustion of un-compacted standing coal Potential for coal blockage due to compaction of standing coal No significant problems attributed to low load or thermal cycling Instability of unit at very low loads Coal stocks Coal Plant Low load operation Unable to operate at low loads (