Operation optimization modefor nozzle governing steam turbine unit

Xu Jianqun Ma Lin Lü Xiaoming Li Ling

(1Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education， Southeast University， Nanjing 210096， China)

(2China Datang Corporation Anhui Branch Company， Hefei 230071， China)

(3China United Engineering Corporation， Hangzhou 310022， China)

I n China，the structure of the electricity market has changed significantly.Many 600 MW units are running into operation.These units often operate under partload conditions，which will lead to lower economy and more security risks.How to ensure the economy and safety is the focus of manufacturers and operators.

Nozzle governing is the most popular admission mode of the unit.However， risk problems are exposed more than before.For example， the action of the valve always changes the state of steam flow，which will cause steamexcited vibration.

At present， many studies have been done on this issue.Ref.［1］showed the influence of the admission mode on the units'rotor systems and gave the admission optimization methods.Ref.［2］calculated the destabilizing force acting on the rotor blades of the control stage.Refs.［3-4］also showed the influence of the admission on the stability of the rotor.An optimum mode of the steam distribution system was proposed.In Refs.［5-8］， the numerical calculation method was employed to analyze the amplitude response of steam-excited vibration.The results show the correctness of the catastrophe analysis method based on the catastrophe theory.

1 Steam Admission Optimization

1.1 Theoretical analysis

Steam forceFbappears when steam flow acts on the blades.Fbusually can be divided into axial forceFaand tangential forceFt， as shown in Fig.1.The two forces affect the bearing load，which indicates that the partial admission has an impact on the stability of the high pressure(HP)rotor， as shown in Fig.2.

Fig.1 Flow diagram of steam in regulation rotor cascade

Fig.2 Rotor and shaft bearing system.(a)Governing stage;(b)Left view of governing stage

1.1.1 Tangential force Ft

Ftcan be divided into horizontal forceFxand vertical forceFyas

where α0is the starting angle and α1is the finishing angle of the nozzle arc.

It is indicated from Eq.(1)that， the integration is zero during full admission and the integration is not zero dur-ing partial admission.It means that different admission modes have different impacts on rotor vibration and bearing loads.

1.1.2 Axial force Fa

As shown in Fig.1，Facan be calculated as

Fa=G(w1sinβ1－w2sinβ2)+Az(p1－p2) (2)

whereGis the steam flow that passes through the blade cascade per unit time;Az(p1－p2)is the force generated by pressure difference;w1，w2are the relative velocities.

It can be seen from Eq.(2)thatFais proportional toG.Setk=(w1sinβ1－w2sinβ2)，R=Az(p1－p2)， and Eq.(2)is simplified as

SetG1，G2，G3andG4as the steam quantities corresponding to nozzle groups 1， 2， 3 and 4， respectively.Fa1，Fa2，Fa3andFa4are the axial forces that affect the control stage of the rotor， respectively.

Taking bearing 1 in Fig.2 as an example， it can be known that bearing 1 is not only affected by the rotor，but also by the lateral force ΔFof the control stage.

Set bearing 2 as a fulcrum and bearing 1 as a free point.According to the torque balance， the following equations can be obtained:

It can be seen that bearing 1 is affected by ΔFx， together with ΔFy; ΔFxand ΔFycan be reduced by the optimum steam admission mode.

1.2 Solution

A 600 MW steam turbine after flow reconstruction is taken as an example.The designed valve open order is 3+4→1→2.The valve arrangement is shown in Fig.3.

Fig.3 Valve arrangement

According to the theoretical analysis above，the optimum admission mode is 3+2→4→1.The bearing loadFcan be calculated by

whereFrais the radial force of bearing pada;Φais the position angle of bearing pada.The comparison of original design and optimum design are shown in Figs.4 and 5.

Monitor results are shown in Tab.1.Bearing vibration and bearing pad temperature reach the safe value after optimization，and the steam-excited vibration caused by the imbalanced steam force is solved.The optimum design can effectively avoid the steam-excited vibration.

Fig.4 Original design scheme 3+4→1→2.(a)Relationship between bearing load and amount of admission;(b)Relationship between bearing load direction and amount of admission

Fig.5 Optimization scheme 3+2→4→1.(a)Relationship between bearing load and amount of admission;(b)Relationship between bearing load direction and amount of admission

2 Operation Optimization

A new operation curve is obtained after sliding pressure operation tests based on steam admission optimization.On the basis of the concept of valve position，the hybrid sliding operation is proposed.It means that we can keep two or three valves fully open and one partly.The relationship between the heat rate and the load is shown in Fig.6.The economic operation is shown in Tab.2.

Fig.6 Heat rate vs.electric power

Tab.2 Optimum operation and the valve opening situation of different loads

3 Conclusion

To solve the steam-excited vibration，the optimum admission mode is proposed.The hybrid sliding condition is added and the optimum sliding pressure curve is obtained，which provides a theoretical guide to the operation optimization.

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