Structure improvement and strength finite element analysis of VHP welded rotor of 700°C USC steam turbine

Abstract

The optimized structure strength design and finite element analysis method for very high pressure (VHP) rotors of the 700°C ultra-super-critical (USC) steam turbine are presented. The main parameters of steam and the steam thermal parameters of blade stages of VHP welded rotors as well as the start and shutdown curves of the steam turbine are determined. The structure design feature, the mechanical models and the typical position of stress analysis of the VHP welded rotors are introduced. The steady and transient finite element analysis are implemented for steady condition, start and shutdown process, including steady rated condition, 110% rated speed, 120% rated speed, cold start, warm start, hot start, very hot start, sliding-pressure shutdown, normal shutdown and emergency shutdown, to obtain the temperature and stress distribution as well as the stress ratio of the welded rotor. The strength design criteria and strength analysis results of the welded rotor are given. The results show that the strength design of improved structure of the VHP welded rotor of the 700°C USC steam turbine is safe at the steady condition and during the transient start or shutdown process.

Keywords

700°C ultra-super-critical unit / steam turbine / very high pressure rotor / structure strength design / strength design criteria / finite element analysis

Introduction

The European Union (EU) and the USA started the advanced ultra-super-critical (USC) coal fired units development plan, whose steam parameter is 34.5−37.5 MPa/700°C−760°C at the end of 1990s. The EU launched the “Thermie AD700 Plan” to develop future coal fired units with advanced steam parameters in 1998. The research and development target of the “Thermie AD700 Plan” is to develop advanced USC units with nickel based alloy for high temperature, whose steam parameters are 37.5 MPa/700°C/700°C with an efficiency of 52% to 55% [ 1]. In Ref. [ 2], the master cycle of AD700 innovation with an efficiency of 53% is presented. Moreover, the USA Department of Energy started the Vision 21 Plan to develop the unit with steam parameters of 34.5 MPa/760°C/760°C [ 3].

The National Energy Administration of the People’s Republic of China established the 700°C Coal Fired Generating Technology Innovation Alliance on July 23, 2010. The technical level for China USC units is improved by using the 700°C coal fired generating technology to open up a new way for the energy conservation and emission reduction of the electrical power industry [ 4]. In Ref. [ 5], the economic benefit of the 700°C ultra-super-critical technology was analyzed. In Ref. [ 6], research and development status and localizations of recommendations of the 700°C grade advanced USC power generation technology were introduced. In Ref. [ 7], the status quo of the design and development of the 700°C advanced USC power generator unit system with three advanced steam-water circulation systems and an optimized design version of a power plant and its modules were described. In Ref. [ 8], the exergy loss, exergy loss coefficient and exergy efficiency of a 1000 MW/700°C ultra-super-critical coal-fire power generating unit were calculated by using the exergy analysis method.

The 700°C USC steam turbine is one of the key equipment for 700°C USC coal fired generating units, and the 700°C very high pressure (VHP) rotor is one of the key component for 700°C USC steam turbines. In Ref. [ 9], the thermodynamics, materials and the design of the HP turbine, the IP turbine and the turbine valves of the Siemens steam turbine AD700 power plants was introduced. However, references on VHP rotor strength design of the 700°C USC steam turbine are few and far between. Therefore, attempt is made in this paper to study the temperature field, stress field and the structure strength of the 700°C VHP rotors, which is of great significance for the development of the 700°C USC steam turbine.

Steam parameters of 700°C USC steam turbine

Steam parameters of steam turbine

The rated power of the 700°C USC steam turbine is 1000 MW and covers the double reheat. The structure of the steam turbine is a single shaft arrangement, six casing and six exhaust outlets (TC6F) with one VHP casing, one HP casing, one IP casing and three LP casings. The steam parameters are 35 MPa/700°C/720°C/720°C, i.e., the pressure of the main steam is 35 MPa, the temperature of the main steam is 700°C, and the reheat and secondary reheat temperatures are 720°C.

Steam parameters of VHP casing

The pressure loss of the main stop valves, control valves and steam inlet pipes of the 700°C USC steam turbines is 2%, i.e., the steam pressure and temperature before the first stationary blade are respectively 34.3 MPa and 689.45°C. There are 12 stage blades in the VHP casing. The first extraction opening is located after the 9th blade. The extraction pressure and temperature are respectively 16.54 MPa and 561.90°C. The exhaust pressure and temperature of the VHP casing are respectively 12.33 MPa and 510.65°C. The steam pressure p₀ and temperature t₀ before stationary blades, the steam pressure p₁ and temperature t₁ before blades as well as the steam pressure p₂ and temperature t₂ after blades for the VHP casing of the type steam turbine under 100%TMCR (turbine maximum continuous rating) are given in Table 1. The steam parameters for the VHP casing of the 700°C USC steam turbine are used to calculate the surface heat transfer coefficient and steady temperature fields of the VHP rotors.

Steam turbine start and shutdown curves

The full-arc admission structure and sliding pressure operation are used for the 700°C USC steam turbine. The pressure and temperature of the main steam, and the speed and capacity curves of the turbine at the startup and shutdown process are tabulated in Figs. 1 to 6. These curves are used to calculate the varying duty parameters of the 700°C USC steam turbine, and the heat transfer coefficient and transient temperature fields of the VHP rotors.

Mechanical models of VHP rotor

Structure design of VHP welded rotor

The welded rotor structure is used for the VHP rotor of the 700°C USC steam turbine, whose structure is shown in Fig. 7. There are two welded seams in the VHP welded rotor. The first welded seam is located at the gland packing near the high pressure side balance piston while the second one is located at the first extraction opening after the 9th blade. The blades are not installed on the second welded seam and the heat affected zone to ensure the safety of the welded rotor. The nickel base alloy is used for the high temperature segment between the two welded seams. The length, diameter and weight of the high temperature segment with the Ni-based alloy are respectively 4621.4 mm, 900 mm and 7.2 t. The 12%Cr steel is used for the two non-high temperature segments outside the two seams.

According to Ref. [ 10], IN617 is used for the large forging of the high temperature rotor of the EU 700°C USC steam turbine. The diameter of the rotor forging is 1000 mm and IN617 has the potential weld ability. Similarly, IN617 is used for the high temperature segment material of the 700°C VHP welded rotor in this paper. The physical properties of the welded seams and heat affected zone are the same as the non-high temperature segment material (i.e., 12%Cr steel).

Mechanical model for VHP welded rotor

The axial symmetry mechanical model is used for the finite element analysis of the 700°C VHP welded rotor, as depicted in Fig. 7. The third type boundary condition with steam forced convection heat transfer is used for the wheel rim, wheel surface, steam seal gland, glossy surface of shafts and so on of the VHP rotor outside surfaces. The third type boundary condition with lubricating oil forced convection heat transfer is used for the outside surfaces of the VHP rotor bearings. The second type boundary condition whose heat flux is zero (i.e., adiabatic condition) is used for the inside surfaces of the VHP welded rotor. The mechanical model and boundary conditions are used for the finite element numerical calculation of the temperature field of the 700°C VHP welded rotor.

The centrifugal force load of the rotor and blades is taken into consideration in the calculation of the stress field of the 700°C VHP welded rotor. The centrifugal force of the rotor is loaded in the VHP welded rotor while the blade centrifugal force is loaded on the wheel rim of the rotor. The calculation results of the temperature of all nodes are input into the mechanical model nodes of the VHP welded rotor. The calculation results of the stress field are obtained by use of the finite element analysis commercial software.

Typical positions for stress analysis

In this paper, the calculation results of the stress of the VHP welded rotor are the composition of the thermal stress and mechanical stress caused by the centrifugal forces. The typical positions for the calculation results of the stress field of the VHP welded rotor of the 700°C USC steam turbine are displayed in Fig. 7, including the following three regions:

1) High temperature segment of using Ni-based alloy: Position A1 is in the fillet before the 5th stationary blade. Position A2 is in the fillet before the 2nd stationary blade. Position A3 is in the wheel root fillet before the 1st blade. Position A4 is in the high temperature side fillet on the inside chamber of the 1st welded seam. Position A5 is in the high temperature side fillet on the inside chamber of the 2nd welded seam. Position A6 is in the rotor center position corresponding to the 5th blade. Position A7 is in the rotor center position corresponding to the 2nd blade.

2) Welded seam and heat affected zone: Position B1 is in the outside surface on the high temperature side of the heat affected zone of the 1st welded seam. Position B2 is in the middle region on the high temperature side of the heat affected zone of the first welded seam. Position B3 is in the inside surface on the high temperature side of the heat affected zone of the 1st welded seam. Position B4 is in the outside surface on the 1st welded seam. Position B5 is in the middle region on the 1st welded seam. Position B6 is in the inside surface on the 1st welded seam. Position B7 is in the outside surface on the low temperature side of the heat affected zone of the first welded seam. Position B8 is in the middle region on the low temperature side of the heat affected zone of the 1st welded seam. Position B9 is in the inside surface on the low temperature side of the heat affected zone of the first welded seam. Position B10 is in the outside surface on the high temperature side of the heat affected zone of the 2nd welded seam. Position B11 is in the middle region on the high temperature side of the heat affected zone of the 2nd welded seam. Position B12 is in the inside surface on the high temperature side of the heat affected zone of the 2nd welded seam. Position B13 is in the outside surface on the 2nd welded seam. Position B14 is in the middle region on the 2nd welded seam. Position B15 is in the inside surface on the 2nd welded seam. Position B16 is in the outside surface on the low temperature side of the heat affected zone of the 2nd welded seam. Position B17 is in the middle region on the low temperature side of the heat affected zone of the 2nd welded seam. Position B18 is in the inside surface on the low temperature side of the heat affected zone of the 2nd welded seam.

3) Non-high temperature segment of using 12%Cr steel: Position C1 is in the wheel root fillet of the high pressure side balance piston. Position C2 is in the low temperature side fillet on the inside chamber of 1st welded seam. Position C3 is in the fillet before the 10th stationary blade gland. Position C4 is in the low temperature side fillet on the inside chamber of the 2nd welded seam. Position C5 is in the wheel root fillet on the exhaust steam side surface of the rotor. Position C6 is in the rotor center region corresponding to the high pressure side balance piston. Position C7 is in the rotor center region corresponding to the12th blade.

Strength design criteria for VHP welded rotor

Strength design criteria of steam turbine components

According to the design rules of steam turbine components [ 11, 12], the average stress of the high temperature component at steady-state is less than {\sigma }_{{\mathrm{10}}^{\mathrm{5}}}^t/\mathrm{1.8}\approx \mathrm{0.56}{\sigma }_{{\mathrm{10}}^{\mathrm{5}}}^t , the surface stress of the high temperature component at steady-state is less than {\sigma }_{\mathrm{0.2}}^t , the maximum stress of the stress concentration position at the transient process is less than \mathrm{2}{\sigma }_{\mathrm{0.2}}^t , and allowable stresses of the welded seam and heat affected zone are 0.8 times for allowable stresses of the parent material. Here, {\sigma }_{{\mathrm{10}}^{\mathrm{5}}}^t and {\sigma }_{\mathrm{0.2}}^t are respectively the material stress rupture limit and yield limit at the working temperature.

Strength design criteria for rotor parent material

The strength design criteria for the finite element analysis of the VHP welded rotor of the 700°C USC steam turbine are given according to Refs. [ 11, 12].

1) The strength design criteria for the rotor parent material at steady rated condition are

(1) {\sigma }_{\mathrm{eq}\mathrm{1}}\le \mathrm{0.56}{\sigma }_{{\mathrm{10}}^{\mathrm{5}}}^t \mathrm{or} {\sigma }_{\mathrm{eq}\mathrm{1}}/{\sigma }_{{\mathrm{10}}^{\mathrm{5}}}^t\le \mathrm{0.56},

(2) {\sigma }_{\mathrm{eq}\mathrm{2}}\le {\sigma }_{\mathrm{0.2}}^t \mathrm{or} {\sigma }_{\mathrm{eq}\mathrm{2}}/{\sigma }_{\mathrm{0.2}}^t\le \mathrm{1},

where σ_eq1 is the equivalent stress calculated by the average of each stress component along the stress classification line at the steady rated condition, and σ_eq2 is the surface equivalent stress at the steady rated condition.

2) The strength design criteria for the rotor parent material at the transient varying duty process are

(3) {\sigma }_{\mathrm{eq}\mathrm{3}}\le \mathrm{2}{\sigma }_{\mathrm{0.2}}^t \mathrm{or} {\sigma }_{\mathrm{eq}\mathrm{3}}/(\mathrm{2}{\sigma }_{\mathrm{0.2}}^t)\le \mathrm{1},

where σ_eq3 is the maximum equivalent stress at the transient varying duty.

Strength design criteria for rotor welded seam and heat affected zone

1) The strength design criteria for the rotor welded seam and heat affected zone at the steady rated condition are

(4) {\sigma }_{\mathrm{eq}\mathrm{1}}\le \mathrm{0.44}{\sigma }_{{\mathrm{10}}^{\mathrm{5}}}^t \mathrm{or} {\sigma }_{\mathrm{eq}\mathrm{1}}/{\sigma }_{{\mathrm{10}}^{\mathrm{5}}}^t\le \mathrm{0.44},

(5) {\sigma }_{\mathrm{eq}\mathrm{2}}\le \mathrm{0.8}{\sigma }_{\mathrm{0.2}}^t \mathrm{or} {\sigma }_{\mathrm{eq}\mathrm{2}}/{\sigma }_{\mathrm{0.2}}^t\le \mathrm{0.8.}

2) The strength design criteria for the rotor welded seam and heat affected zone at the transient varying duty process are

(6) {\sigma }_{\mathrm{eq}\mathrm{3}}\le \mathrm{1.6}{\sigma }_{\mathrm{0.2}}^t \mathrm{or} {\sigma }_{\mathrm{eq}\mathrm{3}}/(\mathrm{2}{\sigma }_{\mathrm{0.2}}^t)\le \mathrm{0.8}.

Calculation results of stress field and temperature field of VHP welded rotor

Calculation results of improvement and stress of original design structure

The calculation results of the stress field of the VHP welded rotor of the 700°C USC steam turbine is exhibited in Fig. 8 at the cold start process of 19200 s. The maximum transient stress of the VHP welded rotor emerges at Position A1 at the cold start process of 19200 s and the calculation results of the transient stress of the rotor high temperature segment are listed in the Table 2. Because the stress ratio {\sigma }_{\mathrm{eq}\mathrm{3}}/(\mathrm{2}{\sigma }_{\mathrm{0.2}}^t)>1.0 , the original structure strength design for the VHP welded rotor of the 700°C USC steam turbine is not safe during the cold start process. The basis of structure improvement decreases the stress concentration by increasing the surface fillet radius. The structure improvement measures of the 700°C VHP welded rotor are:

1) The fillet radius of position A1 is increased from 3 mm to 11 mm.

2) The fillet radius of position C3 is increased from 3 mm to 8 mm.

3) The fillet radiuses of position A4 and C2 are increased from 30 mm to 50 mm.

The calculation results of the steady stress at the rated condition and the transient stress at cold start process of 19200 s of the original and improved structures of the high temperature segment of the VHP welded rotor are presented in Tables 2 and 3. According to Tables 2 and 3, because {\sigma }_{\mathrm{eq}\mathrm{1}}/{\sigma }_{{\mathrm{10}}^{\mathrm{5}}}^t\le \mathrm{0.56} , {\sigma }_{\mathrm{eq}\mathrm{2}}/{\sigma }_{\mathrm{0.2}}^t\le \mathrm{1} and {\sigma }_{\mathrm{eq}\mathrm{3}}/(\mathrm{2}{\sigma }_{\mathrm{0.2}}^t)\le \mathrm{1} at the high temperature segment of the rotor improved structure, the strength design of the high temperature segment improved structure of the VHP welded rotor of the 700°C USC steam turbine is safe at the steady rated condition and cold start process of 19200 s.

Calculation results of temperature field of improved structure

The calculation result of steady temperature (°C) field of the improved structure of the VHP welded rotor of the 700°C USC steam turbine at rated condition is demonstrated in Fig. 9. The calculation result of transient temperature (°C) field of the improved structure of the VHP welded rotor at the cold start process of 19200 s is shown in Fig. 10. The VHP welded rotor at the warm start process of 10200 s is shown in Fig. 11. The improved structure of the VHP welded rotor at the hot start process of 3420 s is shown in Fig. 12, and the improved structure of the VHP welded rotor at the very hot start process 540 s is shown in Fig. 13. The calculation result of the transient temperature (°C) field of the improved structure of the VHP welded rotor at the sliding pressure shutdown process of 10500 s is shown in Fig. 14. The improved structure of the VHP welded rotor at the normal shutdown process of 7200 s is shown in Fig. 15, and the improved structure of the VHP welded rotor at the emergency shutdown process of 80 s is shown in Fig. 16.

Calculation results of stress field of improved structure

The calculation result steady stress (MPa) field of the improved structure of the VHP welded rotor of the 700°C USC steam turbine at rated condition is shown in Fig. 17. The improved structure of the VHP welded rotor of the 700°C USC steam turbine at 110% rated speed is shown in Fig. 18, and the improved structure of the VHP welded rotor of the 700°C USC steam turbine at 120% rated speed is shown in Fig. 19. The calculation result of the transient stress (MPa) field of the improved structure of the VHP welded rotor at the cold start process of 19200 s is shown in Fig. 20. The VHP welded rotor at the warm start process of 10200 s is shown in Fig. 21, the improved structure of the VHP welded rotor at the hot start process of 3420 s is shown in Fig. 22, and the improved structure of the VHP welded rotor at the very hot start process of 540 s is shown in Fig. 23. The calculation result of transient stress (MPa) field of the improved structure of the VHP welded rotor at the sliding pressure shutdown process of 10500 s is shown in Fig. 24, the improved structure of the VHP welded rotor at the normal shutdown process of 7200 s is shown in Fig. 25, and the improved structure of the VHP welded rotor at the emergency shutdown process of 80 s is shown in Fig. 26.

Calculation results of stress of improved structure at typical positions

The calculation results of stress of the high temperature segment typical positions of the improved structure of the VHP welded rotor of the 700°C USC steam turbine are listed in Table 4. The non-high temperature segment typical positions of the improved structure of the VHP welded rotor are given in Table 5, and the welded seam and heat affected zone typical positions of the improved structure of the VHP welded rotor are tabulated in Table 6.

Calculation results of stress ratio of improved structure at typical positions

The calculation results of stress ratio of the high temperature segment of the improved structure at typical positions of the VHP welded rotor of the 700°C USC steam turbine at the steady condition are shown in Fig. 27. The non-high temperature segment at typical positions of the improved structure of the VHP welded rotor at the steady condition is shown in Fig. 28, and the welded seam and heat affected zone at typical positions of the improved structure of the VHP welded rotor at the steady condition are shown in Fig. 29. The stress ratios at A2/A4 in Fig. 27 and those at B1/B2 in Fig. 29 decreases along with rotor speed increment. The reason for this is that the direction of the thermal stress and mechanical stress caused by centrifugal forces is opposite at A2/A4 and B1/B2 but the direction of the thermal stress and mechanical stress is the same at other positions.

The calculation results of stress ratio of the high temperature segment at typical positions of the improved structure of the VHP welded rotor of the 700°C USC steam turbines during the start process are shown in Fig. 30. The non-high temperature segment at typical positions of the improved structure of the VHP welded rotor during the start process is shown in Fig. 31, and the welded seam and heat affected zone at typical positions of the improved structure of the VHP welded rotor during the start process are shown in Fig. 32.

The calculation results of stress ratio of the high temperature segment at typical positions of improved structure of the VHP welded rotor of the 700°C USC steam turbine during the shutdown process are shown in Fig. 33. The non-high temperature segment at typical positions of improved structure of the VHP welded rotor during the shutdown process is shown in Fig. 34, and the welded seam and heat affected zone at typical positions of improved structure of the VHP welded rotor during the shutdown process are shown in Fig. 35.

Analysis results of stress of improved structure of rotor

According to Table 4, Figs. 27, 30 and 33, because {\sigma }_{\mathrm{eq}\mathrm{2}}/{\sigma }_{\mathrm{0.2}}^t\le \mathrm{1} and {\sigma }_{\mathrm{eq}\mathrm{3}}/(\mathrm{2}{\sigma }_{\mathrm{0.2}}^t)\le \mathrm{1} at the high temperature segment at typical positions of improved structure of the rotor, the strength design of the parent material’s high temperature segment of improved structure of the VHP welded rotor of the 700°C USC steam turbine is safe at the steady rated condition and during the transient varying duty.

According to Table 5, Figs. 28, 31 and 33, because {\sigma }_{\mathrm{eq}\mathrm{2}}/{\sigma }_{\mathrm{0.2}}^t\le \mathrm{1} and {\sigma }_{\mathrm{eq}\mathrm{3}}/(\mathrm{2}{\sigma }_{\mathrm{0.2}}^t)\le \mathrm{1} at the non-high temperature segment at typical positions of improved structure of the rotor, the strength design of the parent material’s non-high temperature segment of improved structure of the VHP welded rotor of the 700°C USC steam turbine is safe at the steady rated condition and during the transient varying duty.

According to Table 6, Figs. 29, 32 and 35, because {\sigma }_{\mathrm{eq}\mathrm{2}}/{\sigma }_{\mathrm{0.2}}^t\le \mathrm{0.8} and {\sigma }_{\mathrm{eq}\mathrm{3}}/(\mathrm{2}{\sigma }_{\mathrm{0.2}}^t)\le \mathrm{0.8} at the welded seam and heat affected zone at typical positions of improved structure of the rotor, the strength design of the welded seam and heat affected zone of improved structure of the VHP welded rotor of the 700°C USC steam turbines is safe at the steady rated condition and during the transient varying duty.

Conclusions

1) The finite element analysis results obtained by the mechanical and thermal coupling technology show that the original structure strength design for the VHP welded rotor of the 700°C USC steam turbine is not safe on the cold start process and finite element analysis results for the improved structure of the welded rotor are safe at the steady condition and during the transient start or shutdown process.

2) By use of the strength design criteria and methods of the rotor finite element analysis after the structure improvement measures, the strength design of the VHP welded rotor of the 700°C USC steam turbine is safe at the steady rated condition. The VHP welded rotor is safe during cold start, warm start, hot start, very hot start, sliding pressure shutdown, normal shutdown emergency shutdown, and so on during the transient varying duty process.

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