SAFETY IN DESIGN AND INSTALLATION OF HEAT RECOVERY STEAM GENERATOR Ir. KWAN Ying Leung, The Hongkong Electric Co., Ltd., (852)-31433833,
[email protected] Ir. HO Cheung On, The Hongkong Electric Co., Ltd., (852)-31433831,
[email protected] Ir. KWONG Kwing Lam Johnny, The Hongkong Electric Co. Ltd., (852)-31433708,
[email protected] ABSTRACT The paper outlines the risk-based safety considerations and practices in design, engineering, shop fabrication, site erection and testing & commissioning of Heat Recovery Steam Generator (HRSG) in Lamma Power Station (LPS). Basic design elements such as statutory requirements, design codes, materials, geometry, operating principles and conditions, common failure mechanisms, and quality control inspections and controls for shop fabrication and site erection, risk assessment for erection works, works, etc., are discussed in the paper. Trends of technology development for utility-scale HRSGs such as once-through design of high pressure section, cyclic capability, supplementary firing, Selective Catalytic Reduction (SCR) system, water chemistry and comparison between vertical and horizontal flow HRSGs, etc., are highlighted. Safety practices adopted in testing and commissioning works of HRSG are also discussed. KEYWORDS: HRSG, WATER CHEMISTRY, FAILURE MECHANISM, SCR, RISK ASSESSMENT
1.
INTRODUCTION
1.1. HRSG Function and Features Utility-scale HRSGs are adopted widely over the past two decades due to rapid growth of CCGTs (Combined Cycle Gas Turbines) in the period. HRSG captures residual heat in flue gas at high temperature typically from 540 to 650oC and generates steam to steam turbine for power generation and if required for district heating as well in cold regions. The amount of residual heat input to HRSG is typically within 40-70% of the initial fuel input to an F-class gas turbine. HRSG can be of vertical or horizontal flow type according to its flue gas flow direction. For either type, it can be further classified based on the number of pressure stages (HP/IP/LP), drum type or once-through (HP section), with or without reheater (RH), with or without add-on features such as SCR (Selective Catalytic Reduction) / Carbon Monoxide (CO) catalyst and supplementary duct firing system. 1.2. HRSGs in Lamma Power Station
Figure 1 Lamma Power Station (LPS) and Lamma Power Station Extension (LMX)
Currently, the total installed capacity at LPS is 3,737MW, of which there are two(2) natural gas fired CCGT units in operation (680MW, namely GT57 and L9) and one(1) natural gas fired CCGT unit at the detail design stage
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(380MW, namely L10), while the remaining power generation units are coal-fired units and peak-looping oil-fired simple cycle gas turbines. GT57 is a 2-on-1 multi-shaft CCGT which was converted from two simple cycle D-class oil-fired gas turbines by retrofitting a common bottoming cycle in 2000, while L9 is a 1-on-1 single shaft table-mounted CCGT commissioned in 2006. L10, currently at the detail design stage, is basically of similar design as L9 except that L10’s power train is of a floor-mounted design. All existing HRSGs in LPS (two for GT57 and one for L9) are of vertical flow type. In contrast, a horizontal flow type HRSG will be adopted for L10 taking account of the floormounted design with lower centerline and relatively easier O&M access for SCR catalyst replacement. 1.3. Risks-based Approach for Safety To keep pace with CCGT technology in pursuing higher thermal efficiency and environmental performance, HRSGs are required to handle flue gas at high temperature and generate steam with increasing temperature and pressure. When combining with cyclic operation and stringent emission requirements, more operational risks will be imposed on not only the existing fleet of HRSGs but also the new HRSGs to be built. More and more add-on features are also adopted for post-combustion emission control in HRSG. For L10 HRSG, it will have SCR catalyst inside the flue gas path situated downstream of the HP evaporator. Ammonia, converted from urea solution via Pyrosis chemical reaction facilitated by flue gas tapped from the upstream of HRSG, will be injected back to HRSG at the upstream of the SCR catalyst for NOx reduction down to single-digit ppm level. To meet these challenges, this paper highlights the risk-based safety considerations and approaches adopted in design, engineering and fabrication, site construction, testing & commissioning of HRSGs in Lamma Power Station and also discusses other related issues including trend of technology development. 2.
DESIGN FOR SAFETY
To ensure the operational safety for a brand new HRSG to deal with the potential risks as mentioned above, the safety features and margin should be effectively built in during its design stage. 2.1. Compliance of Statutory Requirements and Design Code All pressure vessels operated in Hong Kong shall comply with Chapter 56 Boiler and Pressure Vessel Ordinance (Cap.56) and associated Code of Practice. The majority of HRSGs are designed and constructed to ASME Boiler & Pressure Vessel Code Section I: Power Boiler and the external piping is designed to ANSI/ASME B31.1: Power Piping. Some users prefer design to British Standards, EN Standards or TRD (German Technical Rules for Boilers) for HRSG because more thermal fatigue issues have been addressed in these standards. All statutory requirements as stipulated in the Boilers and Pressure Vessels Ordinance, Cap.56 have been followed. The maker certificates, shop inspection certificate, certificate of inspection during construction and associated technical information as well as the “Certificate of Fitness” issued by independent boiler inspector for GT57 and L9 HRSGs were submitted to the Labour Department of the HKSAR for registration. A recognized inspection body and boiler inspector will be employed for L10 HRSG. 2.2. Operating Conditions and Modes Apart from the statutory rules and design codes, the designer and end-user shall take a prudent approach to anticipate any particular risks which are generally beyond the scope of statutory requirements or design code. The operating conditions of HRSG should be carefully accessed so that the potential weakness on the steam side or gas side of the HRSG could be avoided, such as the combustion fuel (gas or oil), flow rate of steam and flue gas, density, temperature and gas composition. These design input parameters shall form the essential basis for materials selection, sizing and geometry design later. Designer/end-user shall also define and review the operation mode of its corresponding CCGT unit in terms of number of starts/stops, frequency of cyclic operation, unit start-up and shutdown curves, etc. Experience had shown that most of the operating HRSGs were originally designed for base load operation only and later suffered from new
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operation mode deviated from its design. For example, for the same flue gas/steam output condition, changing a HRSG from base load to frequent cyclic operation could induce excessive thermal stress which leads to accelerated consumption of fatigue life within short time or even damages of parts if there is no adequate margin reserved in the original design. In LPS, GT57 and L9 are base-load units while L10 is expected to be put on relatively more frequent cyclic operation to cope with the potential fluctuations of power output from the Offshore Wind Farm to be built in future. L9 and L10 CCGTs are designed to meet the operation mode as summarized below for reference: L9 CCGT L10 CCGT Operation Modes (Base Load) (Base Load and with more cyclic operation relatively) Operating Life 30 Years 30 Years Number of Cold Start 100 250 Number of Warm Start 800 1,200 Number of Hot Start 3,600 4,500 Number of Very Hot Start 100 150 Number of Unit Trip 100 Number of Normal Shutdown 4,600 6,000 2.3. Materials Selection All materials used in a HRSG are required to be designed and fabricated in accordance with ASME B&PV Code and the relevant ASTM requirements. HRSGs are in general operate at lower temperatures and pressures than conventional coal-fired boilers [1] (except some HP and RH sections) and the materials used for HRSG can be classified as follows:Materials Carbon Steel Alloy Steel
Service Temperature Upto 800°F/427°C 800°F/427°C to 1200°F/648°C
Modified 2%Cr or 9%Cr material has been commonly used in HRSG high-pressure superheater and reheater circuits. Beyond 1200°F/648°C, austenitic, nickel-chrome, or nickel base stainless steels are required. The ASME Boiler and Pressure Vessel Code lists 1500°F/815°C as the temperature limit for these stainless steels. Typical materials for various parts of HRSG include: SA-192 or 210 (low temperature sections such as economizer, pre-heater, etc,), SA-213 Gr. T11 or T22 (evaporator), SA-213 T91 or Code Case 2199 T23 (superheater), SA-106 Gr.B/C or P11 (low temperature header and piping), SA-335 P22 or P91 (high temperature header and piping), SA-515/516 Gr.60/70 (drums/deaerator) During the materials selection process, the designer should consider not only the materials’ physical strength (tensile, impact, fatigue and creep, etc.) or chemical composition at steady state (e.g. base load operation), but also the possible variances of these properties during transient/start-up/shutdown or emergency operation. The materials selection, sizing and geometry design for all components of HRSG should also be closely coordinated with the water/steam cycle chemistry for life-time optimization in order to keep each material operated with its design boundaries. More detailed failure mechanisms through interactions between the materials and water/steam cycle will be discussed in Section 2.5 “Common Failure Mechanism”. 2.4. Geometry Geometry is probably by far the most easily ignored but important factor in HRSG design. Thermal stress could be induced by rapid temperature change during transient operation for a single component, or differential temperature during normal operation between two connecting components with different wall thickness and/or variance of heat transfer rate between water/steam side and flue gas side. Apart from geometry optimization at component level to
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mitigate excessive thermal stress, the connection points such as tube-to-header, header-to-connection pipe, and pipeto-drum, etc. shall be reviewed as to whether the geometry is capable of eliminating or reducing thermal stress by allowing the concerned component to expand/contract as freely as possible during all operating conditions. For locations vulnerable to Flow Accelerated Corrosion (i.e. FAC), the designer shall adopt gentle change in geometry at the connection points such that rapid change in flow direction and/or flow speed could be minimized. 2.5. Common Failure Mechanisms The risks involved in the operation of a utility-scale HRSG could be classified as steam/water side and gas side risks. If not properly addressed in design or operation stages, these risks could lead to pre-mature or long-term failure depending on which failure mechanism dominates the most for the concerned component. Common risks associated with steam/water side of HRSG include Pitting, Flow Accelerated Corrosion (FAC), Stress Corrosion Cracking (SCC), Corrosion Fatigue, Overheating, Exfoliation, Under Deposit Corrosion (UDC), etc., while common risks associated with gas side include High Cycle Fatigue (HCF) and general corrosion in particular Cold-End Corrosion due to the presence of moisture and acidic gas [2]. It should be noted that risks of Creep, Low Cycle Fatigue (LCF) and Weld Failures could exist at both steam/water side and gas side which must be addressed at the same time. Cycle Chemistry could only control some of risks with corrosion nature at steam/water side, such as Pitting, FAC, SCC, UDC, etc. Based on previous experience, technical reference from OEM and professional bodies such as ASME, EPRI, etc., common design and operation considerations for mitigation of these failure mechanisms are reviewed below in the order of probability specific to L10 HRSG which is undergoing engineering process: 2.5.1. Flow Accelerated Corrosion (FAC) FAC is the localized thinning of a pipe or component as a result of dissolution of the protective oxide film and underlying base metal in a flowing media. In a power plant environment, carbon steels within operating temperature between 90-250oC with single or two-phase flow are the most vulnerable sections to FAC, such as economizer/evaporator of LP or IP sections. Research found that materials selection and cycle chemistry could be relatively more effective than geometry optimization in mitigation of FAC.
For L10 HRSG, all pressure parts with operating temperature between 90-250 oC will adopt alloy steel of grade P11/T11 or above to tackle FAC. Comparing with L9 HRSG, L10 LP or IP circuits materials will be upgraded from SA210 to SA335 T11.
Figure 2 Wearing Rate of Materials versus Temperature
As for cycle chemistry, High All Volatile Treatment (Oxidizing) (i.e. High AVT(O)) will be adopted for L10 to keep the pH value of LP feed water and HP/IP feed water always >9.7 and >9.4 respectively. Under AVT(O), oxygen scavenger (e.g. hydrazine) will be eliminated such that a relatively higher oxygen level could be preserved at water side which is favorable for formation of protective ferrous oxides, in particular, hematite. Figure 3 AVT (R) (Left) and AVT(O) (Right)
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2.5.2. Overheating and Exfoliation Overheating could be caused by flow blockage (short-term, such as abnormal valve operation) or design problem (long term, such as improperly designed wall temperature due to material selection or thickness, or improper cycle chemistry, etc.). As recognized as “high temperature oxidation”, corrosion rate under high temperature could be increased, which causes thickening of oxide scale on the internal surface of pressure parts. The thickened oxide could be fractionally exfoliated and those left behind will interrupt the heat transfer efficiency or causes underdeposit corrosion, or it could be fully exfoliated leading to direct exposure of base metal followed by another cycle of corrosion and then exfoliation. Some cases reported that excessive steam oxidation of T91 could start at 600650oC for HRSG application while there were some cases of excessive oxidation or leakage found in 9Cr-made boiler tubes of coal fired boilers, including L6-8 of LPS. For L10 HRSG, the HP and RH steam temperature is 582oC [3] (which is even higher than steam temperature in all existing coal fired units in LPS) at an average wall temperature of 594 oC under external flue gas temperature of 602oC. A detailed review will be conducted to study the pinch point, flue gas temperature profile, heat flux of the HP/IP Sections as well as the proven track record in using 9Cr materials for high steam temperature applications. The main purpose of the review is to ensure the overheating or exfoliation problems of 9Cr materials encountered in the coal-fired units would not be repeated in the operation of L10 HRSG. 2.5.3. Welding Failure The welding quality is always one of the top priorities of QA/QC system during design, fabrication and construction of HRSG. All Welding Procedure Specification, Procedure Qualification Record and Welder ’s Qualification shall be carefully reviewed during design stage according to ASME B&PV Code Section VIII. For L10 HRSG which is a modularized design, site welding is minimized at the greatest extent by fully utilizing the capability and convenience of shop welding. The HRSG modules in shop will be subject to non-destructive testing (NDT) as per B&PV Section V and followed by 100% hydrostatic test to be witnessed by independent boiler inspector under the Cap. 56. For site welding, the detailed risk control measures for welding quality will be discussed in Section 3.2. 2.5.4. Fatigues Fatigue is the passive weakening of a material caused by repeatedly applied load, i.e. cyclic stress. Cyclic stresses could come from pressure, constrained thermal expansion, thermal stresses across tube, pipe or drum walls. Therefore, fatigue affects both water side and gas side of the pressure parts and it is the major cause of failure of HRSG, especially under cyclic operation. Interestingly, the stress level which causes fatigue may be much less than the material’s yield strength. Damage due to fatigue normally occurs in localized high stress region. In general, fatigue due to cyclic can be classified as Low Cycle Fatigue (LCF) and High Cycle Fatigue (HCF) while Corrosion Fatigue is an enhanced growth of fatigue cracks under a corrosive environment with water/steam touched surfaces. It is worth to mention that some materials (e.g. most of ferrous alloys and titanium alloy) have infinite fatigue life if the cyclic stress is lower than fatigue limit which generally falls within the range of 35-60% of tensile stress. In contrast, most nonferrous alloys do not have a fatigue limit and thus the fatigue strength is defined as the stress level at which failure will occur at certain number of cycles. a) Low Cycle Fatigue (LCF) In general, a material’s stress and strain shall be the factors in determination of fatigue life under LCF. This is because high stress level causes both elastic and plastic deformation and thus the fatigue life is relatively short. Fatigue failure caused by cyclic load of 10 4 cycles or less is generally defined as LCF. Geometry design is the top priority in controll ing free expansion of the concerned attachment, bends, branches etc. For L10 HRSG, LCF will be examined by computer simulation combined with a review of fatigue life of the parts at locations prone to LCF. b) High Cycle Fatigue (HCF) In general, fatigue caused by cyclic load over 10 4 cycles is defined as HCF. For HCF, only the stress level will be the factor in determination of fatigue life since the strain level is totally elastic (and thus requiring higher number of cycles to cause failure). Again, geometry design is the top priority in controlling HCF by avoiding flow-induced excitation for HRSG application. For L10 HRSG (similar to previous ones), special attention shall be paid to the design of structural support of gas path nearest to GT and tube bundle support for accommodation of turbulent gas flow. Computer simulation of the concerned parts will be reviewed to ensure the HCF effects will be minimized.
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c) Corrosion Fatigue (CF) CF is the reduction in fatigue resistance due to the presence of a corrosive environment. Cycle chemistry and materials selection are the keys to minimize corrosive environment to tackle the global effect of corrosion under cyclic stress. 2.5.5. Cold-End Corrosion Cold-end refers to the low temperature section of HRSG where the flue gas is leaving HRSG such as LP economizer or pre-heater. Moisture content or acidic content in flue gas such as sulfur content of natural gas or light oil could condense and form sulfuric acid which will corrode the external surfaces of cold end section. For LPS, the natural gas is sourced from LNG terminal and the natural gas is basically sulfur free while the light oil (namely Ultra Low Sulphur Diesel) will have sulfur content ≤0.005%wt. Therefore, HK Electric would design cold end water recirculation (i.e. LP Economizer Recirculation) during oil firing to keep the cold end section warm enough and thus raise the cold end temperature always above the acidic dew point. Upgrading the materials at cold end section to material more resistant to sulfuric acid (such as stainless steel) is ruled out due to: i) all economizer materials has been upgraded from SA210 to SA335 T11 for L10 HRSG for tackling FAC at water side, and ii) the annual oil-firing hours is limited to 800 hours to avoid poisoning of SCR catalyst; iii) as back-up fuel for natural gas, oil-firing will only be operated during emergency situation and thus chance of oil firing is remote for L10. 2.5.6. Stress Corrosion Cracking (SCC) SCC is caused by the simultaneous presence of both tensile stress and a corrosive environment. Once a crack is initiated, it can further propagate and eventually penetrate the wall thickness along perpendicular direction to the applied tensile stress. It can be prevented by better water chemistry, reduced tensile stress and high grade of materials selection. For L10, the stress level will be reviewed similar to that conducted for mitigation of fatigues. The cycle chemistry will also be strictly followed to prevent corrosive environment. Upon ingression of impurity such as sea water, different action levels have been designed to cater to different degrees of seawater ingression during emergency, such as blowdown of HP/IP Drum water, phosphate injection to HP/IP Drums and ammonia injection to condensate discharge. 2.5.7. Pitting Pitting is extremely localized attack characterized as a hole or cavity with a diameter equal to or less than its depth. It is one of the more insidious forms of corrosion and difficult to measure quantitatively, predict, or even detect. Pits begin by the localized microscopic breakdown of a passive layer on the metal surface due to surface discontinuity, local variation in alloy concentration, scratch, gap in a coating, etc. Electrolytic cell forms with an extremely small anode to cathode ratio, producing a rapid corrosion rate. Dissolved oxygen drives cathodic reaction. Pitting could be controlled by oxygen content, pH value, minimizing regions of stagnant flow and surface crevices, reducing temperature, etc. For L10 HRSG, oxygen content and pH valve will be controlled under High AVT(O) during operation. A Dry Lay-up System will generate dehumidified air to the system and keep the residual humidity lower than 40% for suppression of corrosion rate to a practical minimum level so as to mitigate general corrosion including pitting on both steam/water sides. 2.5.8. Creep Creep is the deformation of solid materials under mechanical stress and its effect increases with temperature. Under creeping, the concerned materials will have change in microstructure and thus lowering of strength. The creeping effect is a progressive process but not at constant rate. At initial stage, the rapid strain slows due to work hardening; at intermediate stage while the hardening and annealing effects are virtually balanced, the stain grows with stress almost linearly; and at the final stage while the micro-voids are linked up, the strain grows rapidly again with accelerating stress level (due to reduced sample dimension) until material ruptures eventually. Generally, creep becomes significant at about 1/3 of the melting temperature of metals. The creep rupture strength of metals is controlled at “stress to rupture” at the end of 100,000 operating hours under ASME code. For L10 HRSG, creep life of metals will be carefully reviewed for the required operating life. Requirements under EN13445 shall be used as reference which contains more detailed design requirements on creep and fatigue. In the detailed study for using 9Cr materials at high steam temperature sections, the creep rupture strength of 9Cr materials
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at 594oC above the temperature limit of 565oC as specified in EPRI literatures will also reviewed based on experience gained from the coal-fired units. 2.5.9. Under Deposit Corrosion Deposits could be caused by improper feed water/make-up water treatment, condenser leakage (outside HRSG) or condenser, condensing and feed water piping or water chemistry additives (inside HRSG). Under deposit corrosion is caused by the thermally insulating deposits which shield the bare metal from water and create crevice when NaOH concentrates and thus it is also known as caustic corrosion. Generally, UDC is more often to occur at horizontal or slanted tubes (e.g. evaporator waterlines) which are partially full (rare in HRSG). NaOH could come from seawater leakage or overdosing of phosphate or caustic. Therefore, it can be prevented by reducing the free NaOH (e.g. preventing over-dosing of phosphate / caustic, preventing condenser leakage, keeping the boiler clean by chemical cleaning when required (i.e. deposit removal ), preventing leakage of caustic and polisher regeneration chemicals in case condensate polisher is used). 2.6. Cycle Chemistry As discussed above, some of the water-side failure mechanisms with corrosion nature could be controlled by proper cycle chemistry, such as CF, FAC, SCC, Pitting etc. The common controlled parameters and the associated measures are: Oxygen (by mechanical removal/deaeration and/or hydrazine dosing); Carbon dioxide (by ammonia/neutralizing amine dosing); Insolubles (by phosphate/chelants/polymers dosing); pH value (by phosphate/ammonia/caustic/amines dosing); Conductivity (by water treatment/polishing), Carryover (by drum separation efficiency); and Total Dissolved Solid (TDS) (by blowdown control). There are mainly three main streams of cycle chemistry programs, namely the Phosphate Treatment, All Volatile Treatment (AVT) and Oxygenated Treatment [4]. For CCGTs in LPS, AVT (R), i.e. All Volatile Treatment with a reducing environment, was initially adopted. Under AVT(R), ammonia, phosphate and hydrazine are dosed to the designated locations of the HRSG. As oxygen scavenger, hydrazine keeps oxygen content at a very low level less than 10ppb to facilitate formation of magnetite (Fe3O4) as the protective layer, which is however easy to be destroyed by impurities. It was then found in the industry that at a relatively higher oxygen environment, more hematite (Fe2O3) can be formed and thus the protective layer composed of both iron oxides is more stable than a single layer of magnetite. In addition, due to its potential carcinogenic risks, hydrazine is gradually phased out as far as practical. In LPS, AVT(R) was then converted to AVT(O), which is also All Volatile Treatment but with a naturally oxidizing environment. Under AVT(O), no hydrazine is dosed and ammonia and phosphate are dosed to the water cycle of HRSG. L10 will continue to adopt AVT(O) but with relatively higher pH value control as recommended by the OEM and thus it is so-called High AVT(O). For L10 HRSG, in addition to wet lay-up practice during shutdown (and under nitrogen blanketing), a dry lay-up system utilizing dehumidified air will be installed to keep the relative humidity inside the pressure parts lower than 40% during pro-longed outage of the HRSG. 3.
SAFETY IN FABRICATION AND INSTALLATION
To ensure the overall quality of HRSG is up to the satisfactory level as designed, HK Electric also followed safety approach for shop fabrication and site installation of HSRG. To reduce risks involved in the shop fabrication and site installation works, one should consider the overall constructability, extent of modularization, welding quality control, shop and site inspection & testing procedure, NDT & hydrostatic test, detailed lifting sequence/procedure of modules as well as detailed safety practices during testing and commissioning works. Coordination with other heavy move-in works of power train equipment shall also be reviewed.
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3.1. Overall Constructability and Modularization Nowadays, all OEMs of HRSG could adapt flexible modularization to varying extent to suit specific needs of different clients. Some manufacturers could even deliver a fully assembled HRSG from the manufacturing yard to the site directly. The constructability of HRSG mainly depends on key factors such as the size of available ground assembly area, maximum size/weight of module for marine shipment and unloading, limitations of in-land transportation and cost of local labours (especially qualified welders) as well as the available site construction time. Majority of the HRSG manufacturers could modularize utility scale HRSG into 10~15 nos. of modules. Subject to OEM’s design, each module weighs 90-160 tons (without casing) or 90-260 tons (with casing) typically. By adopting modularization concept, more synergy between shop fabrication and site installation works could be utilized at initial design stage or even tendering stage (for cost control). Figure 4 L10 HRSG For L10 HRSG with 15 nos. of modules (same as L9), it was reviewed that it could bring more benefits to reduce site welding quantity to a minimum level and thus shorten the overall construction time. The volume of site works and thus the associated construction risks could be minimized. It was also reviewed from safety point of view that 15 nos. of modules would be the optimum degree of modularization for LPS when taking into account the berthing capacity of 2,000 tons at LMX, the available ground assembly area of ~8,000 m2 and local ground bearing capacity limit of 30 ton/m2 (for seating a 750-ton crawler crane for lifting of the heaviest module). Typically, it would require 3 months from the first-column installation to drum lifting and another 6 months from drum lifting to completion of all hydrostatic tests of the HRSG. For L10, the SCR catalyst modules (in an array of 3 (W) x10 (H)=30 nos. of modules) will be inserted to its pre-installed inner frame from the top of HRSG. To avoid contamination of SCR catalyst before performance test, it is planned to install these catalyst modules after completion of oil-firing test and thus a small outage period of approximate 3 weeks will be required. 3.2. ITPs for Shop Fabrication and Site Installation To warrant quality of works at shop and at site, the specific Inspection and Testing Plan (ITP) should be strictly followed during the course of shop fabrication and site installation respectively. Among all the key inspection and testing items, welding shall be the most important factor affecting the integrity of HRSG. As mentioned in Section 2.5.3 above, all the WPS/PQR shall be reviewed and approved as per ASME B&PC Code and substantiated with Welder ’s Certificate and associated welding test records. Special attention shall be paid to the high-alloy steels such as 9Cr materials which demand high precision in pre-heat and post weld heat treatment (PWHT) as well as highly skillful welding techniques, in order to avoid alteration of microstructure and potential pre-mature cracking of weldments. For L10, it is required to maximize welding of high-alloy steels or thick wall materials at shop with better welding workmanship and working environment. HK Electric will witness the inspection hold point activities as per the Shop ITP and contract requirements. An internationally third party boiler inspector will also conduct witness check during the course of shop fabrication of the HRSG, such as review of mill certificates, welding inspection (NDT) and 100% hydrostatic test at shop before delivery. As far as site welding is concerned, the site inspection and testing plan would cover key site welding activities such as piping alignment check, site weld end preparation, welding machine set-up, welding materials inspection, site welder qualification test and weldment materials laboratory test, pre-heat and PWHT monitoring, etc. After site welding is completed, 100% RT shall be conducted for the weld joints and followed by hydrostatically tested at site except “golden” welds. For all major installation events such as heavy unloading, lifting, welding, pressure test, etc, Job Safety Analysis will be conducted to ensure the residual risks of the anticipated risks will be controlled at an acceptable level with mitigation measures /contingency plan in place, if it is so required.
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3.3. Safety Practices during Testing and Commissioning Works For pressure test of HRSG, a HRSG Pressure Test Package will be prepared to ensure everything concerning the work quality of concerned pressure parts / equipment will be recorded in compliance with the contract requirements. Major information of the Package shall include the Pressure Test Plan, P&ID mark-up, isometric drawings, valves, fitting & support details, piping welding records, installation record, field inspection reports, calibration certificates, related Engineering Sheet & Non-conformity Works Reports and Job Safety Analysis Sheet as previously discussed. After completion of cold commissioning activities, the HRSG shall be subjected to hot commissioning after firstfiring of the gas turbine. Two major hot commissioning activities for the HRSG shall be carefully planned and conducted due to the associated high risk nature, i.e. the alkaline boiling test (for cleaning of the water circuits such as economizer, evaporators and drums) and steam blowing out test (for cleaning of steam circuits such as superheaters and reheaters). Due to the complexity of the tests and wide range of permanent/temporary piping or jumpers involved, a special commissioning team led by an experienced Commissioning Change Engineer shall be formed to ensure that all detailed steps stipulated in the approved test procedure be implemented, in particular the walk down inspection and risk assessment before test, access control during the test and the required contingency plan. 4.
DISCUSSIONS
As gas turbine capacities become larger and exhaust gas temperatures increase progressively, supercritical HRSG is becoming possible, allowing higher steam temperatures and cycle efficiencies. HRSG technology is relatively mature, and large or revolutionary advances are not expected, though incremental improvements will continue. Some once-through HRSGs equipped with a once-through HP section instead of a HP Steam Drum (which is the thickest component hindering the start-up time) and without forced circulation pumps have been put into commercial operation. Supplementary firing system is sometimes adopted in cold regions for increased steam output for district heating during winter when the power demand is low. The supplementary firing system is normally located at the inlet duct of HRSG and thus it is also called duct-firing system. The duct firing system would introduce hotter exhaust gas to the pressure parts as well as casing materials at the downstream which must be taken into consideration on top of the normal design requirements for un-fired HRSGs. Environmental requirements such as NOx are generally tightened all over the world. SCR catalyst is a vanadium oxide-based materials which could catalyze the chemical reaction of NOx and ammonia at flue gas temperature of 280-350 oC typically [5], depending on OEM recommendation. Normally, the SCR catalyst is situated at the immediate downstream of HP evaporator. In some regions where CO emission is stringently regulated, CO catalyst is also installed for CO reduction at further upstream of the flue gas path since it requires a higher flue gas temperature to catalyze the chemical reaction. Given HK Electric’s system is relatively small and the CCGT units will be part of HK Electric’s core assets in electricity generation for the future, plant availability and reliability are the top priorities. In this respect, HK Electric has opted for the mature, reliable and conventional design adopted for L9 HRSG but with more enhanced features adopted for L10 HRSG to cater for more frequent cyclic operation and tightened NOx emission limit, such as FAC-resistant materials, High AVT(O) cycle chemistry and SCR system. 5.
CONCLUSION
To meet the increasing demand on higher thermal efficiency as well as fast-response/cyclic capability of CCGTs together with more add-on features such as SCR/CO catalyst, supplementary firing, fast-start capability, etc., it is reckoned that design-code compliance in HRSG design is the minimum criteria only. New HRSGs shall be built in with more forward-looking safety considerations for the entire design life (normally from 25 to 30 years) covering design, engineering, fabrication, construction, testing and commissioning and O&M stages. More attention shall be paid to the HRSG failure mechanisms and the associated prevention measures which can be timely implemented during design stage or operation stage to eliminate unnecessary hurdles or even failures. Live monitoring of key
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operation parameters and good maintenance practices with lay-up arrangement are required in order to ensure HRSG operation is always within the design boundaries as originally expected. ACKNOWLEDGEMENT The Authors wish to thank the management of The Hongkong Electric Co., Ltd. for the permission to publish this paper. REFERENCE 1. 2. 3. 4. 5.
Design & Construction of Heat Recovery Steam Generator – Hongkong Electric’s Experience, 19th Boiler and Pressure Vessels Seminar, CW Tso and L Wong of HK Electric HRSG Design, Operations & Understanding, Controlling of HRSG Damage Mechanisms , PowerEdge of Tetra Engineering Training, Singapore, June 2014 System Description of HRSG for Lamma Unit 10 , MHPS, 2015 In-house Design Manual for HRSG and Water Chemistry , HK Electric Study on Selective Catalytic Reduction (SCR) System for Unit 9 CCGT and New CCGT Units for Lamma Power Station , YL Kwan, SN Li and KL Kwong of HK Electric, CEPSI 2012
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