Impact of the HP Preheater Bypass on the Economizer Inlet Header

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1 Impact of the HP Preheater Bypass on the Economizer Inlet Header Dr.-Ing. Henning Zindler E.ON Kraftwerke Tresckowstrasse Hannover Germany henning.zindler@eon-energie.com Dipl.-Ing. Andreas Hauschke and Prof. Dr. techn. Reinhard Leithner TU Braunschweig Institute for Heat- and Fuel Technology Franz-Liszt-Strasse Braunschweig Germany a.hauschke@tu-bs.de and r.leithner@tu-bs.de Abstract: In the case of high pressure preheater tube leakages feed water can flow to the turbine and cause damages. To protect the turbine a hp (high pressure) preheater bypass is installed. When this bypass opens the feed-water inlet temperature will drop by approximately 120K in modern super-critical steam generators causing fatigue due to additional thermal stresses in the economizer inlet header. To get a better understanding of the impact and the numbers of the possible load cycles the results of a transient power plant-simulation by the program ENBIPRO are used as input for the simplified method for calculating fatigue of the EN Tubes, pipes and heat exchangers are simulated dynamically by using FVM (Finite Volume Method) and solving the resulting equation system by the SIMPLER- Algorithm (Semi Implicit Momentum and Pressure Linked Equations Revised). The FVM calculates all data needed for calculating the temperature distribution over a thick walled pipe like a header. This includes information like fluid temperatures and heat transfer coefficients. The temperature distribution in the pipe wall itself is calculated in a post processing by using the numerical method of Crank and Nicolson. Based on the transient temperature distribution in the wall pipe the thermal stresses and the fatigue is estimated by using the EN Key Words: FVM, SIMPLER, Fatigue, High pressure preheater bypass, EN 12952, Crank and Nicolson 1 Introduction In the case that a high pressure preheater tube has a leakage, e.g., a weld fails, feed water of high pressure flows through the leakage to the extraction line of the turbine. If the damper in the extraction line also fails the turbine can be flooded and damaged. For safety reasons a HP preheater bypass is installed to protect the turbine. A flow diagram with the bypass is shown in figure 1. When opening the HP preheater bypass the feed water inlet temperature will drop by approximately 120 K. This temperature difference is much higher in modern super-critical steam generators than in older ones with lower pressure. Therefore, the thermal stresses in the thick walled components especially in the economizer inlet header have to be checked more accurately. The question is raised how often the HP preheater bypass can be opened before the life time of the economizer inlet header is consumed by fatigue. Figure 1: HP Preheater Bypass ISSN: ISBN:

2 2 Approach A fatigue analysis can be performed by either using FEM (Finite Element Method) analysis or by a simplified method like it is described in the EN In both cases simulation data of fluid temperatures and heat fluxes to the pipe wall are needed. This data can be generated by transient simulation programs like ENBIPRO that is developed at the Institute for Heat and Fuel Technology at the TU Braunschweig. ENBIPRO is described in [1], [2] and [3] and based on [4], [5] and [6]. ENBIPRO doesn t calculate the temperature distribution in the wall of the pipes. Therefore, in a post processing the fluid temperatures and the heat transfer coefficients calculated by ENBIPRO are used as input data for a Crank- Nicolson-Algorithm as described in [7] that solves the transient energy balance inside the pipe wall. The result is the transient temperature distribution in the pipe wall. Based on the temperature distribution a simplified fatigue analysis is performed as described in the EN The energy balance of the pipe wall at a certain cross section is: 0 = c R ρ R dϑ R dt + Q 0 Q in A R (7) ENBIPRO discretizes the partial differential equations system by using a FVM described in [5]. The FVM is solved by using the SIMPLER algorithm described in [4]. The ability of EN- BIPRO to couple partial and ordinary differential equation systems is described in [1]. 3.2 Calculation of the Temperature Distribution in the Pipe Wall The heat flux in the pipe wall of each volume element is modelled by ϑ c R ρ R = 1 ( rλ ϑ ) R t r r r Figure 2 shows a discretised pipe wall. (8) 3 Mathematical Model 3.1 ENBIPRO For calculating the water and steam flow inside the tubes and pipes ENBIPRO uses the following numerical models for the momentum, mass and energy balance: ri ra dr 0 = ρw t + ρw2 0 = ρ t + ρw p S w (1) (2) 0 = ρh t + ρhw Q in (3) A st The mathematical model of the momentum, mass and energy balance of the flue gas side is simplified to: 0 = dp dx + λ 1 R d 0 = dṁ dx ρ 2 w2 (4) (5) 0 = dṁh dx Q 0 (6) Figure 2: Discretizised pipe wall of a thick walled component λ is regarded as constant. The boundary conditions at the inner side and outer side of the pipe are modelled as dϑ R = α d r λ (ϑ R ϑ u ) (9) The discretization of the partial differential equation is done by using the implicit finite difference method of Crank and Nicolson described in [7]. The algorithm becomes implicit by discretising the storage term over the half time step: ( ) ϑ t i,k+ 1 2 ϑ i,k+1 ϑ i,k Δt (10) ISSN: ISBN:

3 For all other terms arithmetic mean values of time step k and k + 1 have to be calculated. The result is an implicit equation system for each time step of a tridiagonal structure that can be solved easily by using the TDM-algorithm as it is describedin[4]. 3.3 Fatigue Analysis The fatigue analysis is performed for a bore hole in the first (inner) discretised volume element of the pipe wall by using EN12952 [8] part three chapter thirteen and appendix B and D. 4 Simulated System and Boundary Conditions The simulated system is shown in figure 1. It shows the feed water pump, the HP preheater, the bypass, the three way valve, the non return valve and the economizer inlet header. The HP preheater is simulated by only one heat exchanger. The bypass is kept warm by a small mass flow. The temperature distribution and the fatigue is calculated in the first cross section of the economizer inlet header because the thermal stresses are highest due to the fastest temperature change and highest mass flow. The economizer inlet header is overflown by flue gas like in current two pass boiler designs. During the simulation the three way valve stops the mass flow through the HP preheater within five seconds. The geometries of the components of the original design have been slightly modified. During the simulation the feed water temperature upstream of the economizer will drop by 117 K. The mass flow of 325 kg/s is constant, the inner diameter of the economizer is 114 mm and the wall thickness 55 mm. The connected economizer heat exchanger tubes have an outer diameter of 44.5 mm and a wall thickness of 6.3 mm. The pipe material is 13CrMo45. The feed water has an inlet temperature of 315 C at the beginning and a constant pressure of 320 bar. The flue gas has an inlet temperature of 357 C and a mass flow of kg/s. 5 Results Figure 3 shows the temperature distribution over the time and the wall of the first cross section of the header wall. It can be seen that the temperature drops very fast at the inner surface of the header. Temperature in C over the Header Wall and Time Time in s Radius in m Figure 3: Temperature distribution in the pipe wall ISSN: ISBN:

4 Figure 4 shows the most important temperature curves over the time. For calculating the fatigue the temperature of the inner surface and the integral average pipe wall temperature are needed. Temperature in C Temperature over the pipe wall Water Temperatur in C Inner Header Wall Temperature in C Integral Header Temperature in C Outer Header Wall Temperatur in C Flue Gas Temperature in C Time in s Figure 4: Wall and Fluid Temperatures by The integral average temperature is defined 2 ϑ m = ro 2 r2 in N Δrϑ i r i (11) i=1 destroy the magetite film. This will have an impact on the wall thickness and the life time of the turbine. But it should not be necessary to have an emergency shut down of the boiler because the frequency of a failure of a HP preheater is very low. To protect the magnetite film the maximum thermal tension at the inner side of the bore hole has to be less than 200 N. The protection of mm the magnetite film stipulates 2 the maximum temperature differences in the pipe wall. In the simulated case the maximum temperature difference as defined by EN is 39K and the maximum temperature decrease velocity is 0,35 K/s. The simulated maximum temperature difference is 63 K and the simulated maximum temperature transient is 12 K/s. The number of allowable cycles till the starting of crack is approx Conclusion and Outlook That means that even with new super-critical boilers and high temperature rise in the HP preheaters the bypass can be used, but each use will ISSN: ISBN:

5 Symbols Symbol Unit Description A m 2 Cross section area c J/(kgK) Specific heat capacity d m Diameter h J/kg Enthalpy l m Pipe length ṁ kg/s Mass flow p Pa Pressure Q W/m Heat flux per unit length r m Radius S w N/m 3 Source term t s Time w m/s Velocity x m Axial coordinate α W/(m 2 K) Heat transfer coefficient λ R Friction coefficient λ W/(mK) Heat conductivity ρ kg/m 3 Density Index Description in Inside i, k Counting indices m Mean N Maximum number o Outside u Fluid R Tube st Steam w Wall References: [1] H. Zindler, Dynamische Kraftwerkssimulation durch Kopplung von FVM und PECE Verfahren mit Hilfe von Adjungiertenverfahren, Fortschritt-Berichte VDI Reihe 6 Energietechnik, Nr. 573 [2] G. Stamatelopoulos, Berechnung und Optimierung von Kraftwerkskreisläufen, Fortschritt-Berichte VDI Reihe 6 Energietechnik, Nr. 340 [3] Epple; Leithner; Linzer; Walter; Simulation von Kraftwerken und wärmetechnischen Anlagen, 2009, ISBN [4] S. Patankar, Numerical Heat Transfer and Fluid Flow, Hemisphere Publishing Corporation, 1980 [5] H. Walter, Modellbildung und numerische Simulation von Naturumlaufdampferzeugern, Fortschritt-Berichte VDI Reihe 6 Energietechnik Nr.: 457, Wien 2001 [6] K. Brenan, S. Campbell, L. Petzold, Numerical Solutions of Initial-Value Problems in Differential-Algebraic Equations, siam, 1995 [7] Stephan/Bähr, Wärme- und Stoffübertragung, Springer Verlag [8] EN Wasserrohrkessel und Anlagenkomponenten [9] J. Bausa, Dynamische Optimierung energie- und verfahrentechnischer Prozesse, Fortschritt-Berichte VDI ISSN: ISBN:

The Research of Heat Transfer Area for 55/19 Steam Generator

Journal of Power and Energy Engineering, 205, 3, 47-422 Published Online April 205 in SciRes. http://www.scirp.org/journal/jpee http://dx.doi.org/0.4236/jpee.205.34056 The Research of Heat Transfer Area