Bypass Controller [PDF]

Zitiervorschau

Bypass Controller Type AV6+ Application

Function

30 years experience with Sulzer-Bypass-Systems, including controls and safety functions, show that the bypasscontroller is neither an integrated part of the boiler controls nor the turbine controls. The Sulzer-Bypass-System is an independent system which serves to coordinate the boiler and turbine operation during startup, shutdown and load disturbances. The necessary signal exchange between the bypass-controller and other systems is small. Much more important is a bypass-controller which exactly fulfills the requirements of the bypass operation in the plant and allows the easy implementation of required safety functions. The life conserving operation of the heavily stressed valves and pipework, especially by accurate steam desuperheating in all operating modes and transients, poses a special challenge for the bypasscontroller.

HP Bypass

Features Standardised n Proven solutions from more than 30 years of experience with HP & LP-bypass automation n System responsibility in one hand Flexible n State Controller with Observer (SCO) reduces temperature transients for valves and pipes

Controller The HP-bypass controller consists of a pressure controller, controlling superheater pressure through the position of the steam valve, a temperature controller, controlling downstream temperature through the position of the spray control valve and a Op/ClController for the spraywater isolation valve. The key part of the pressure controller is the setpoint generator. It has to produce the correct setpoint for all the different operating modes during start-up, load operation, load rejection and shutdown. The heart of the setpoint generator is a rate limiter, which limits the gradient of any pressure increase during all operating modes, therefore protecting components from pressure/temperature transients.

n For any type of boiler plant n With or without safety function n Degree of automation selectable Open n Clear and simple interface to boiler and turbine controls n Hundreds of bypass controllers installed n Satisfies highest automation requirements

Fig.1

HP Bypass with Controller

For the case of large load rejections or turbine trips the bypass controller contains the necessary functions to initiate a quick opening, in order to keep the pressure excursion as small and short as possible. For a hot start, where the boiler is already under pressure, the functions min. position and minimum pressure are not active. The pressure controller will automatically select on basis of the superheater pressure the type of startup. Temperature control

Fig. 2

Cold Start-up with HP-Bypass

The Operating modes are mainly determined by process signals, namely the superheater pressure and the valve position. Therefore the bypass controller does not depend on many signals from the boiler or turbine controller. Start up The functions of the pressure setpoint generator are shown at the example of a plant cold start. First the Bypass valve is opened to minimum position (Ymin). This ensures steam flow through the superheater and reheater immediately after ignition. When there is enough steam production to reach a minimum pressure (pmin) the controller begins to control the live steam pressure by opening the bypass valves. When the Bypass valve reaches the position Ym (defined by the desired steam flow during boiler start up) the setpoint generator begins to increase the pressure setpoint in accordance with the steam production of the boiler, but limited by the maximum gradient. Once the target pressure for starting the turbine (psynch) is reached, the setpoint generator switches to (fixed) pressure control.

The HP-spraywater controller, has to deal with large variations of process conditions as gain, time delay, and disturbances. CCI/Sulzer state controllers with observer (SCO) are using a dynamic model of the process to reduce temperature deviations and especially temperature gradients during transient plant operating conditions. Possible reductions are by a factor of 2- 4 over conventional PI(D)-controllers. This is an important life conserving factor for the heavily stressed walls of valves and piping. Safety Function Regulations in various countries allow the use of the HP-bypass valves as safety valves against overpressure in the superheater. CCI/Sulzer HP-Bypass valves can be equipped with additional safe opening devices. The necessary safety control cabinet is completely independent of the HP-Bypass control, but for smooth transfer between safe opening and control the two functions are coordinated by a number of signals.

LP Bypass Controller Although independent in operation from the HP-bypass controller the LP-bypass controller must operate well coordinated with the HP-Bypass system. Pressure Control The LP-bypass pressure controller controls during start-up and load rejections the reheat pressure. Similar to the HP-pressure controller, it contains a setpoint generator producing the correct pressure setpoint during all operating modes.

As the turbine starts to accept steam the bypass will start to close until the turbine takes over all the steam produced by the boiler and the bypass is fully closed. Load operation As soon as the bypass is closed the pressure setpoint tracks the actual pressure plus a threshold dp which keeps the bypass closed (follow mode). The maximum gradient of the pressure setpoint is still limited. If the life steam pressure exceeds this gradient, e.g. due to a load rejection, the bypass will start to open and the controller returns to pressure control mode. The pressure is controlled by the bypass until normal operation has been restored and the bypass is closed again. Fig. 3

LP-Bypass with Controller

Boiler start-up The controller has to control the steam pressure in the reheater system. The bypass is either closed until the desired reheat pressure is reached and starts then to open and control the pressure, or the pressure is increased in a controlled manner similar to the HP-bypass. Both operating modes are available in the LP-bypass controller. Load operation The bypass is closed but the controller monitors the reheat steam pressure in order to open and control the pressure whenever an unacceptable pressure increase is recognised. The first stage pressure of the turbine is used to generate the sliding pressure setpoint. Condenser protection Whenever the condenser is not able to accept steam or the injection water system is unavailable, the bypass is closed through a separate safe channel in order to protect the condenser. This emergency closing function can close either through a separate safe closing device the LP-Bypass steam control valve or a separate LPbypass steam isolation valve. With large bypass valves, their flow capacity at high reheater pressure can exceed the absorption capacity of the condenser. For such cases the steam flow to the condenser is limited by the bypass controller. If power operated reheater safety valves are used (e.g. CCI/Sulzer MSV-valves), coordinated operation of the reheater safety valves with the LP-bypass can further improve plant operation for the case of turbine trip or load rejection at high load. The Sulzer-LP-bypass controller can provide the necessary signals for operation of the reheater safety valves. Spray water control Because the steam conditions after the LP-bypass desuperheater are usually very near or at saturation condition, the temperature after the desuperheater cannot be used as control signal. The necessary injection water flow and related valve position of the injection valve is therefore calculated from the steam flow and the steam conditions. The steam flow is in turn calculated as a function of the steam conditions and the valve position of the bypass valve.

Fig. 4

Bypass Controller Interfaces

output signals to the positioners are 4-20 mA signals. Emergency and trip input signals as well as alarm output signals are usually hardwired binary signals. Outputs are dry switchover contacts (SPDT), Input signals usually contacts from other systems. As standard connection to the plant DCS the bypass controller provides a serial MODBUS-interface. The MODBUS carries all input and output signals of the ManMachine Interface (MMI). It allows manual control of all valves as well as setpoints as defined in the detailed controller configuration. MODBUS is the de facto world standard for datalinks between different control systems and available on most DCS. A detailed interface signal list is usually worked out during engineering of the individual bypass controller. If a redundant manual control is required, a touch screen panel can be provided. The touch screen panel provides all process visualisation and complete manual control. Additionally through the touch screen panel all control parameters can be adjusted. The change of control parameters is protected by a password. If no touch screen panel is used the parameters can be changed through the programming interface of the controller with the help of a PC. The programming language for the bypass controller corresponds to the international IEC 1131 standard. The subset “Function Block Diagram” (FDB) is used which has the advantage that it provides directly an easy readable documentation of the controller functions.

Interfacing The field signals are usually connected as 4-20 mA signals to the bypass controller. The transmitter have individual circuit breakers. The positioners for hydraulic actuators are usually directly installed in the cabinet, so that output signals are directly driving proportional valves and solenoid valves. The

Fig. 5

Touch Panel with Process Picture of HP/LP-Bypass

Technical Data Cabinet

2200mm x 1000mm x 600mm Hx W x D 270 kg Double front and back doors

Ambient Conditions

0 – 32 ºC, max. 90 % R. H.

Electromagnetic compatibility (EMC)

– RFI IEC 801-3 27-500 MHz: 10 V/m

Standard functions HP-bypass

– Startup type selection (Cold start, Hot Start, Hot Restart)

(Not all functions

– Min Position

might be used in

– Min Pressure

specific application)

– Pressure increase – Fixed Pressure – Follow operation

– Ground Cont. IEC 801-5 2 kV shield to ground

– Shutdown – External Pressure Setpoint

– ESD IEC 801-2 8 kV air / 4 kV contact Power Supply

110 – 230 VAC redundant or 24 VDC redundant

– Max gradient can be variable, depending on any other signal value

Power Dissipation

max. 360 W

– Temperature Control

Transmitter Inputs

4– 20 mA Transmitter supply with individual circuit breakers

– Spraywater Isolation Valve Control

Binary Inputs

24 VDC Interrogation voltage for dry contacts

Binary Inputs

24 VDC interrogation voltage for dry contacts Dry contact SPDT 250 V / 1 A AC, 30 V / 1 A DC

Binary Signal Outputs

– State Controller with Observer (SCO) used for pressure and temperature Standard functions

– Pressure Increase

LP-bypass

– Fixed pressure – Load operation

(Not all functions might be used in specific application)

– Emergency closing of steam isolation valve – Spraywater control (Calculated)

DCS-Interface

MODBUS or MODBUS Plus (for other Datalinks consult CCI)

Analog Outputs (Optional)

4-20mA isolated 500V

Optional

– Control of third stage desuperheater spray valves

Terminals

Compression type Signals AWG 12 Power AWG 10 – 8

Options

– Functions can be adapted to satisfy specific plant requirements

Hydraulic Positioners

PVR10-10 mounted in the cabinet

CCI World Headquarters— California Telephone: (949) 858-1877 Fax: (949) 858-1878 22591 Avenida Empresa Rancho Santa Margarita, California 92688 USA CCI Sweden (BTG Valves) Telephone: 46 533 426 00 Fax: 46 533 125 00 Box 602 661 29 Säffle Sweden

– Signal connections to hardwired M/A-stations

CCI Switzerland (Sulzer Valves) Telephone: 41 52 264 95 00 Fax: 41 52 264 95 01 Im Link 11 CH-8404 Winterthur Switzerland

CCI Korea Telephone: 82 31 985 9430 Fax: 82 31 985 0552 26-17, Pungmu-Dong Kimpo City, Kyunggi-Do 415-070 South Korea

CCI Austria (Spectris Components GmbH) Telephone: 43 1 869 27 40 Fax: 43 1 865 36 03 Lembockgasse 63, 1230 Wien AT-1233 Vienna Austria

CCI Japan Telephone: 81 726 41 7197 Fax: 81 726 41 7198 4-15-20 Shukunosho Ibaraki-City, Osaka 567-0051 Japan

CCI China Telephone: 86 10 6501 0350 Fax: 86 10 6501 0286 Room 567/569 Office Tower Poly Plaza 14 Dongzhimen South Avenue Beijing 100027 China CCI Italy (STI) Telephone: 39 035 29282 Fax: 39 035 2928247 Via G. Pascoli 10A-B 24020 Gozle, Bergamo Italy 336 ■ 02/04