Study of Steam Export Transients in a Combined Cycle ower lant Alfonso Junquera Delgado Departamento Mecánico, Empresarios Agrupados c\ Magallanes 3 Madrid 8003 ajd@empre.es Almudena Travesí de los Santos Departamento Mecánico, Empresarios Agrupados c\ Magallanes 3 Madrid 8003 atd@empre.es Abstract A Combined Cycle ower lant incorporates a cogeneration system wic supplies steam to a nearby industry. Because of te contractual need to ensure te supply of steam to industry, te Combined Cycle Steam Export System is connected to two auxiliary boilers wic will operate wen te cycle is not available. Using Ecosimro, we ave studied te transients produced in te face of uncontrolled events wic could give rise to an interruption in te supply. Te purpose of te study is to verify te suitability of te control system and determine te initial load of te auxiliary boilers wic guarantees te continuity of steam supply. Key Words: Steam transients, cogeneration, Ecosimro, combined cycles, control systems. 1 INTRODUCTION A Combined Cycle basically comprises tree components: a gas turbine, a eat recovery steam generator and a steam turbine. Te gas turbine burns natural gas to generate electrical power. Te gas turbine exaust gas is routed to a eat excanger wic is commonly known as te eat recovery steam generator. Here te residual eat from te exaust gas is used to eat te water and generate steam wic is expanded in te steam turbine, increasing te overall production of electrical energy. Te Combined Cycle under study comprises a eat recovery steam generator wit tree pressure levels, eac formed by an evaporator, a pressure drum and a supereater. Te main steam from te ig pressure supereater expands in a ig pressure section of te steam turbine. As it leaves, te cold reeat steam is combined wit te steam from te medium pressure supereater and te mix enters te reeating area of te eat recovery steam generator. Tis ot reeat steam is ten fed into te medium pressure turbine. Te steam produced from te second expansion is combined wit te steam from te low pressure supereater and tis mix is routed into te low pressure section of te turbine and ten discarged to te condenser. A Combined Cycle ower lant can produce 390 MW of electrical power wen all te steam produced in te eat recovery steam generator is passed troug te turbine. If part of te steam generated is exported, te electrical power decreases to 350 MW for a maximum of 150 t/ export steam. Te greater part of tis quantity (66 t/) is consumed by a single nearby industry. Table 1 contains te series of pressure and temperature requirements tat te export steam must fulfil at te point of delivery: Table 1: Export Steam Conditions arameter Minimum Average Maximum ressure (barg) 3 5 30 Temperature (ºC) 30 34 43 resent flow 0 45 Future flow 41 66 Tere are tree possible sources of export steam: main steam, cold reeat steam and medium pressure steam. Te source used depends on te steam flow exported and te cycle load used. Selection of te source to feed te supply is based on a pressure criterion. Te transient states produced in te system can be attributed to two different causes. Tey may be due to canges in te conditions of te cycle suc as a unit trip, load rejection, transfer to te auxiliary boilers, etc; or tey may be due to variations in te demand for export steam. Of all te possible cases, tose wic are most significant ave been cosen for te study because of teir effect on te operating conditions of te cycle Te main purpose of te study is to establis for eac plant operating state, auxiliary boiler load levels and control system parameters tat guarantee a continuous supply of export steam under te specified pressure and temperature conditions. C09-1
SYSTEM DESCRITION.1 EXTRACTION STEAM As indicated previously, combined cycle export steam can come from tree different sources: Main steam Cold reeat steam Medium pressure steam All tree supplies ave some components in common wic prevent te backflow of steam, suc as te isolation valves at te inlet of eac of te lines and te non-return valves. Te main steam extraction is located at te outlet of te ig pressure supereater. During normal cycle operation, te steam flow from te ig pressure supereater is divided and routed to te ig pressure turbine and te process steam extraction. Te main steam pressure and temperature are controlled by means of trottling and attemperation wit feedwater so tat tey matc te conditions of te cold reeat steam. Te cold reeat steam is extracted at te ig pressure turbine outlet upstream of te inlet to te reeater witout any adjustment to its termal caracteristics. Lastly, te intermediate pressure extraction steam is obtained at te outlet of te intermediate pressure supereater, downstream of te bypass connection, and is subjected only to pressure adjustment. Te auxiliary boiler startup curve is a datum provided by te manufacturer. It must be borne in mind tat operation of te auxiliary boilers below teir maximum tecnical output (approximately 0 t/) cannot be guaranteed..3 EXTRACTION STEAM CONTROL SYSTEM During normal operation, te combined cycle control system canges from one steam source to anoter depending on te cycle load level and steam export flow. Wen demand increases, steam is introduced from a source wit iger pressure until te steam from te lower pressure source is replaced. Tis replacement is automatic, maintaining te pressure upstream of te steam export control valve. Te possible extraction modes from te different sources are as follows: Cold reeat steam (CRH) Main steam and intermediate pressure steam (H+I) Main steam (H) Te Extraction Steam Flow Map (Figure 1) illustrates te different operating modes expected, depending on te cycle load level and te demand for process steam. Te steam from all tree sources is routed to te process steam export eader. Te conditions of te steam in tis line are adjusted to tose required by te process. Te adjustment is made in two stages; namely, pressure regulation and attemperation wit feedwater. Lastly, te pipe wic carries te steam to its final destination is connected inside te eader. A connection is made in tis line to te auxiliary boilers.. AUXILIARY BOILERS Te facility is equipped wit two auxiliary boilers, eac wit a capacity to produce a maximum of 75 t/. Tey are designed to guarantee te supply of process steam witout transients wic give rise to loss of supply in te event of unit trip or oter combined cycle unavailability. Te auxiliary boilers ave a dual control system wic can be used depending on weter or not te cycle is exporting steam. Figure 1 3 DESCRITION OF THE MODEL Te steam export transients ave been simulated wit a model built wit Ecosimro. Te model incorporates te following components: Boundary conditions ipes Control valves Non-return valves Attemperators I controllers C09-
ressure sensors Auxiliary boilers Tere are two types of ports wic connect te different components: fluid ports wose variables are pressure, temperature and flow; and control ports wose variables are analogue signals. Te model built wit Ecosimro is depicted in Figure. Te limits of te model are as follows: a. Input Outlet from te ig pressure supereater Outlet from te intermediate pressure supereater Cold reeat steam outlet b. Output Steam export supply Connection to oter consumers Main steam entering te turbine 3.1 BOUNDARY CONDITIONS Tere are two types of boundary conditions: Capacitive: were te flow, te pressure and te temperature are specified Resistive: were te pressure and te entalpy are specified 3. IES Te equations wic represent te beaviour of te pipes include te effects of load loss due to friction and te storage of steam inside tem. Additionally, te termal capacity of te metal is also taken into account. altoug te excange of eat between te metal and te outside is not considered because all te pipes are insulated. Eac pipe is considered divided into a number of equal elements, alternating resistive elements (pressurisers, type R) and capacitive elements (mass accumulators, accumulators, type S). Te number of elements depends on te volume of te pipe and te precision required. As a general rule tere is just one resistive element and one capacitive element for eac pipe, except for te connecting line wit te industrial consumer (some 00 m in lengt) wic is divided into ten elements. In addition, we avoid joining two elements of te same type to prevent te generation of algebraic loops in te equation system resolution. Eac pipe element must comply wit te mass and energy conservation principles: dρ V = min mout dt du dt in out (1) = q q q () Te boundary conditions ave to be defined for eac one of te model limits. In te case of ig and medium pressure extraction steam and cold reeat steam, tese conditions are te pressure and entalpy; for te remaining limits (export steam supply, supply to oter consumers, main steam to te turbine) te pressure, temperature and flow of eac point ave to be specified. V is te volume of te pipe element ρ is te density m is te inlet or outlet flow of te element considered q is te energy tat enters or leaves te element, and it is calculated by multiplying te inlet or outlet flow by te average Hsteam_to_turbine Set-point 9.5 bara s v I Cntrl_pi steam_h steam_i steam_cr pipe_h pipe_i1 pipe_cr1 H_eader valve_ceck_cr pipe_cr CV_IEX pipe_h1 pipe_i valve_ceck_i pipe_i3 CV_HEX 4 colector_eader pipe_h valve_ceck_h pipe_h3 attemperator_h Set-point 6 bara s I aux_boiler1 v Cntrl_pi pipe to Dow 4 1 p_sensor pipe to aux boilers steam_to_dow aux_boiler C09-3 1 HRSG Extraction Steam Header CV_ROCEX attemperator rocess Steam Export Header col_export_ eader steam_to_oters
entalpy in tat element U is te internal energy of te pipe element Heat transfer between te pipe and te steam is calculated wit formula 3: q c ( Tv T ) A = (3) c is te coefficient of eat excange due to convection, calculated using te etukov formula [Reference 1] T v is te temperature of te steam T is te temperature of te inner surface of te pipe A is te inner surface area of te pipe element Te evolution over time of te internal temperature of te metal is determined by equation 4: dt dt q = (4) M c M is te mass of te pipe element c is te specific eat of te metal Using te mass and energy conservation equations for eac pipe element (equations 1 and ) based on te pressure and te entalpy and te operand, we obtain: dρ dρ ρ + min mout ' d d = dρ dρ V ρ + d d (6) min mout dρ ' V d ' = dρ d ( q q q ) in out (7) is te pressure of te steam is te entalpy is te derivative of te pressure wit respect to time ' is te derivative of te entalpy wit respect to time dρ is te derivative of te density wit d respect to te entalpy at constant pressure dρ is te derivative of te density wit d respect to te pressure at constant entalpy Te pressure drop in eac pipe element is a function of te geometry and of te frictin coefficient K and is calculated wit te following formula: l A. m' =. m ρa ( in + avin ) ( out + avout ) K l is te lengt of te pipe element m is te steam flow is te inlet or outlet pressure av is te artificial viscosity A is te pipe section ρ is te density 3.3 CONTROL VALVES (8) Tere are tree pressure control valves; two are located in te ig and intermediate pressure extraction steam eaders (CV_HEX and CV_IEX) and te oter in te process steam export eader (CV_ROCEX). Using te model built wit Ecosimro and based on te most unfavourable steady state conditions, te capacity and type of te valves ave been calculated (see Table ). Tus te valves CV_ROCEX and CV_HEX are of te isopercentage type, wile te valve CV_IEX is of te linear type. On te oter and, an actuator as been incorporated into eac valve wic provides te opening times summarised in Table : Table : Control Valve Caracteristics Valve Size Cv Opening CV_ROCEX 10 160 5 s CV_IEX 6 400.5 s CV_HEX 8 875 7 s Valves CV_ROCEX and CV_HEX are positioned in accordance wit te signals received by teir I controllers to maintain pressure at te industrial supply point and in te eader top of te process steam export eader, respectively. Te position of valve CV_IEX is a boundary condition since its opening is controlled to maintain pressure in te intermediate pressure drum, wic is not included in te model. C09-4
Control valve operation is modelled in accordance wit Reference. 3.4 HEADER Te component denominated eader is used to connect te different elements. It is equivalent to an S type element except tat it can ave several inputs and outputs and it does not incorporate load loss. Te model includes four eaders: H_eader component: tis is located at te ig pressure extraction steam inlet and it divides te flow into two parts; one part is routed to te turbine and te oter part to te process steam export eader colector_eader component: tis represents te connection of te tree extraction steam lines in te eader top of te eader col_export_eader component: tis is located at te end of te eader and comprises one inlet from te eader and two outlets, one to te steam export pipe and te oter to future consumers col_aux_boiler component: tis is located in te steam export pipe and represents te connection of tis line wit te line from te auxiliary boilers 3.5 ATTEMERATORS Te system is equipped wit two attemperators, one located in te ig pressure extraction steam eader and te oter in te process steam export eader. Bot are positioned downstream of te associated pressure control valve. Eac attemperator comprises te following components: Water flow control valve Temperature sensor downstream of te attemperator I controller Header Te set-points of te ig pressure attemperators and te process steam export eader are 40ºC and 40ºC, respectively. 3.6 AUXILIARY BOILERS Te auxiliary boilers are controlled in two different ways: tere is flow control in te discarge of te boilers and pressure control at te point of steam export supply. Wen te combined cycle is exporting steam, te auxiliary boilers are producing a constant flow of steam commensurate wit te load and demand. However, wen te auxiliary boilers are supplying all te export steam, tey control te supply pressure. Wen tere is a trip in te combined cylce, te auxiliary boilers automatically begin to increase teir load to te steam level tat existed prior to te trip. During tis process, te combined cycle as to maintain te supply pressure until valve CV_ROCEX as completely closed or until te auxiliary boilers ave reaced te required steam level. Te Ecosimro model of te auxiliary boilers includes te following components: Boundary condition of te capacitive type, were te pressure and entalpy are establised Control valve Valve actuator Flow sensor I flow controller Valve actuator control signal selector Input signal delay to te eader Signal delay simulates te delay between te firing signal and steam production in te boiler. 3.7 NON-RETURN VALVES Te components wic represent te non-return valves prevent te backflow of steam during transients and canges in all operating modes. Teir formulation includes te corresponding load loss. 4 TRANSIENT ANALYSIS CASES Among te transients wic can give rise to variations in steam export operation are tose produced by te transfer of supply between te auxiliary boilers and te combined cycle, load rejection, an increase or reduction in te cycle load, etc. From te point of view of guaranteeing te supply of export steam, only te transients unleased by uncontrolled events are of interest; fundamentally, combined cycle trip and sudden variations in steam export demand. 4.1 COMBINED CYCLE TRI All te cases studied are based on an initial combined cycle steady state corresponding to te values C09-5
establised in te eat balances. After 300 seconds ave elapsed, te unit trips and te steam accumulated in te pipes, supereaters and ig pressure drum is routed to te process steam export eader. Te moment te trip occurs, te command is given to start one or bot auxiliary boilers to cover te demand for steam. Trougout te duration of limits. On te oter and, since reduction in demand is not a controlled process, a sudden stepped decrease is assumed. 5 RESULTS AND CONCLUSIONS Te following graps are an example of te results obtained in a combined cycle trip. Tis case is based 0 18 16 14 Flows (kg/s) 1 10 8 Auxiliary Boiler 1 Auxiliary Boiler rocess Steam Export Header H steam export I steam export Steam Export Header attemperation H attemperation Steam_to_export 6 4 0 0 100 00 300 400 500 600 700 800 Time (s) startup, te residual eat from te eat recovery steam generator is used to continue generating steam at a progressively lower pressure and temperature. Te evolution of tis pressure and temperature is a datum supplied by te boiler manufacturer. Wen te supply pressure cannot be maintained witin te limits specified in section 1, additional transients are calculated wit initial states different from tose of te auxiliary boilers in order to determine te minimum initial load of te boiler wic will enable te supply pressure to be maintained. 4. VARIATION IN EXORT STEAM DEMAND As in previous cases, tis case is based on an initial steady state wit te cycle operating wit a preestablised load level and demand After 300 seconds ave elapsed, a sudden variation in demand is produced due to sutdown or startup of one of te industrial consumers. Since increase in demand is a controlled process, te aim is to determine te maximum allowable rate of increase wic will not produce a variation in pressure beyond te specified on an initial situation wit 66 t/ export at 100% load and one of te boilers at te tecnical minimum. Wen trip occurs, te order is given for te auxiliary boiler to increase te load. Wile te boiler is working to obtain te steam export flow tat existed prior to te trip, lack of steam is compensated wit tat contained in te ig pressure area of te eat recovery steam generator. To tis effect te ig pressure control valve (CV_HEX) opens after te trip, diverting te steam -wic in normal conditions would go to te condenser- troug te turbine bypass towards te process steam export eader. Figures 3 and 4 illustrate te evolution of te system flows and pressures. We can see a dual transient, te first corresponds to te trip after 300 seconds and te second to te cange in auxiliary boiler control mode 700 seconds after steam input from te cycle is sut off. In te combined cycle under study, it is concluded tat for steam export levels below 66 t/ tere is no need to keep any auxiliary boiler in operation. For greater export levels, one or bot sould be kept at te tecnical minimum. Figure 3 C09-6
36 34 3 ressure (bar a) 30 8 6 Steam_to_export HRSG Extraction Steam Header rocess Steam Export Header Max_res Min_res 4 0 0 00 400 600 800 1000 100 Time (s) References Figure 4 [1] Capman, A., (1984) Heat Transfer 4 t Edition [] Instruments Society of America., (ANSI/ISA- S75-01-1985) Flow Equations for Sizing Control Valves C09-7