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Research & Application of Steam Admission Optimization

for Ultra Supercritical Steam Turbines

BangkokThailand, October 2016

1  Introduc*on

2  Principleofsteamadmissionop*miza*on

3  Typicaladmissionanditsop*miza*on

4  Conclusion

CONTENTS

1 Introduction

Steam admission of steam turbine can be divided into two categories: full arc admission partial arc admission.

partial arc admission full arc admission

1 Introduction

For USC steam turbine, according to different steam admission mode there are mainly three typical types in china: full arc admission with single valve throttling control (type A), partial arc admission with sequence valve control (type B) partial arc admission with composite valve control (type C)..

typeAsinglevalvethro1lecontrol

typeBsequencevalvecontrol

typeCcompositevalvecontrol

1 Introduction

After manufacture and installation, the design efficiency of steam turbine have been fixed. While the operation efficiency is not the same, it still need to be improved by optimizing operation mode. Steam admission optimization is a very effective way.

Design Efficiency

Operation Efficiency

The basic cycle of thermal power plant is the Rankine cycle. with

reheat system or regenerative heat system

Two effective way to improve the cycle efficiency:

Increasing initial parameters, such as initial pressure, initial

temperature, reheat temperature

Reducing terminal parameter, such as exhaust back pressure

2 Principle of steam admission optimization

mcp t ri b p gη η η η η η η= g g g g g

Steam admission optimization: adjust HP steam turbine

governing valve to obtain higher initial pressure, not at the

expense of HP cylinder efficiency.

Improve cycle thermal efficiency and turbine efficiency

2 Principle of steam admission optimization

Generation thermal efficiency ηcp

Closing the HP control valve always can raise the main steam pressure. But if the HP CV opening is unsuitable small, the throttling loss is large, which will cause HP cylinder efficiency decreased more. So, the benefits brought by improving initial pressure will be less than the loss caused by turbine efficiency decline. the essence of the steam admission optimization is to find the best balance between the main steam pressure and the HP cylinder efficiency to achieve the optimized operating efficiency.

2 Principle of steam admission optimization

3.1 Type A (full arc admission with single valve throttle regulation) USC steam turbine units

3 typical steam admission and its optimization

3.2 Type B (partial arc admission with sequence valve regulation ) USC steam turbine units

3.3 Type C (partial arc admission with composite valve regulation ) USC steam turbine units

This type of ultra supercritical units usually have two HP main stop valve and two HP control valve, some with a overload control valve.

3.1 Type A --full arc admission with single valve

the most economical operation mode for type A is fully opening the HP control valve. But lose the ability to quickly respond to load changing. Most of this types of units use the way of valve throttling, to improve the performance of changing load at the expense of operation efficiency.

3.1 Type A --full arc admission with single valve

For a 1000MW ultra supercritical steam turbine, test result shows that heat consumption rate at HP control valve 30% opening is higher than that of CV fully open by 35kJ/ (kW.h) on average. Heat consumption rate at HP control valve opening 50% is higher than that of CV fully open by 15kJ/ (kW.h) on average.

3.1 Type A --full arc admission with single valve

30% 100% 35( / . )HR HR kJ kW h− =

50% 100% 15( / . )HR HR kJ kW h− =

The condensate throttling is introduced to address the above problem. It use storage energy in condensate system as a supplementary way for load changing.

3.1 Type A --full arc admission with single valve

Condensate

Extrac;onSteamintoLPHeater

SteaminLPcylinder

Genera;onload

With the condensate throttling, the HP control valves can be fully open to achieve a new energy-saving coordination control system. The load changing delay caused by control valve full opening can be compensated by condensate throttling. Meanwhile with the optimization of boiler coal and feed water control , unit load changing ability will be improved overall.

3.1 Type A --full arc admission with single valve

Schematic diagram of CCS based on condensate throttling

So under the premise of basically meeting the requirement of load changing performance from the power grid, high operation efficiency and energy-saving have been achieved for type A steam turbine.

3.1 Type A --full arc admission with single valve

AGC load changing curve (650MW - 1000MW) while HP CV full open

some new technology for primary frequency control function have been applied when frequency of power grid is unusually low. Such as cutting off HP or LP heater in regenerative system, or cutting off extraction steam of heat supply system as a supplemental method for PFC.

3.1 Type A --full arc admission with single valve

PFC load increasing curve at 49.9Hz (2994rpm) while HP CV full open for a 1000MW unit Increasing about 40MW load in 30s

Type B steam turbine are usually equipped with 4 or 6 HP control valves. In large capacity steam turbine dual flow regulating stage are applied in this type of steam admission.

3.2 Type B --partial arc admission with sequence valve

schemeof1000MWunitHPnozzlesarrangement

This type B steam turbine uses 2 level management for the control valve by the original design, i.e. GV1 ,GV2 and GV3 HP control valve open at the same time, then GV4 HP valve open in sequences control mode.

3.2 Type B --partial arc admission with sequence valve

But the main steam pressure is low under two level management during low load, Therefore, it is necessary to change DEH steam admission function to three level valve management to improve operation efficiency. The valve opening sequence is changed to first GV2 and GV3 (at the same time), then GV1, last GV4.

3.2 Type B --partial arc admission with sequence valve

Tests show the new valve sequence can improve the thermal efficiency of the power generation by about 1% at the 50% load.

The steam turbine adaptability to new valve sequence should be fully considered during optimization. We should pay attention to the regulation blade strength and the stability of rotor system.

3.2 Type B --partial arc admission with sequence valve

Composite valve admission means that in high load the admission is similar with sequence valve nozzle control, while in low load the admission is similar with single valve throttling control. The advantages of this admission is that the heat shock during start is small, because it’s similar with single valve throttling control. It is beneficial to the thermal stress control for steam turbine. In high load range (about more than 90% rate load), steam turbine operation efficiency is also high due to the similar sequence valve nozzle control.

3.3 Type C --partial arc admission with composite valve

In the start-up and low load using similar throttling control and four control valve will open with different opening. After certain load, some valve will close, turn to similar sequence valve control.

3.3 Type C --partial arc admission with composite valve

HP nozzle arrangement steam admission function curve

of the composite valve

So we need to change the valve open sequence, If possible. The control logic of valve management in steam turbine DEH will be modified to add sequence control mode and switching function between original composite mode and new mode.

3.3 Type C --partial arc admission with composite valve

For a 600MW unit, coal consumption rate will decrease about 2.2 g/kW.h in sequence valve control at 60% load, and about 0.9g/kW.h at 80% load.

We should properly optimize the PID and other parameters of turbine master for the new mode. Also we should pay attention to the safety of regulation blade, bearing vibration, temperature after mode switching.

3.3 Type C --partial arc admission with composite valve

After manufacture and installation, the steam turbine design efficiency have been fixed. But the operation efficiency is not the same. Therefore, it is very necessary to optimize steam admission, to adjust HP control valve, to improve the operation efficiency as much as possible close to or even exceed the design efficiency.

4 Conclusion

DesignEfficiency

Opera*onEfficiency

Steam admission optimization is an effective work. After optimization, the steam turbine operation efficiency usually can be increased by about 0.2% to 1%, achieving the result of energy-saving and emission reduction. In addition, these benefits can be get only by optimizing operation mode, with no additional hardware investment.

4 Conclusion

Zhu Jianfei Email: zhujf@mhdlshanghaipower.com