Hp & Lp Bypass System - Steam Turbine Power Plant

B F

Bypass Controller Type AV6+

Application 30 years experience with Sulzer-Bypass-Systems, including controls and safety functions, show that the bypass-controller is neither a 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 fulfils exactly 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.

Features Standardised · Proven solutions from more than 30 years of experience with HP & LP-bypass automation · System responsibility in one hand

Flexible · State Controller with Observer (SCO) reduces temperature transients for valves and pipes · For any type of boiler plant · With or without safety function · Degree of automation selectable

Open · Clear and simple interface to boiler and turbine controls · Hundreds of bypass controllers installed · Satisfies highest automation requirements

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.

Function HP Bypass 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/Cl-Controller 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.

Fig.1

HP Bypass with Controller

automatically select on basis of the superheater pressure the type of startup. Temperature 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 a important life conserving factor for the heavily stressed walls of valves and piping.

Fig. 2

Cold Start-up with HP-Bypass

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.

Safety Function Regulations in various countries allow the use of the HPbypass 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 startup 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.

Once the target pressure for starting the turbine (psynch) is reached, the setpoint generator switches to (fixed) pressure control. 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. 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

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 HPbypass. Both operating modes are available in the LPbypass 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 LP-bypass 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

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 Man-Machine 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 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.

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

Standard functions HP-bypass

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

(Not all functions

– Min Position

Ambient Conditions 0 – 32 ºC, max. 90 % R. H.

might be used in

– Min Pressure

Electromagnetic compatibility (EMC)

specific application) – Pressure increase

– RFI IEC 801-3 27-500 MHz: 10 V/m

– Follow operation

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

– External Pressure Setpoint

Power Supply

110 – 230 VAC redundant or 24 VDC redundant

Power Dissipation

max. 360 W

Transmitter Inputs

4– 20 mA Transmitter supply with individual circuit breakers

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

– Fixed Pressure

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

– Shutdown – Max gradient can be variable, depending on any other signal value – Temperature Control – Spraywater Isolation Valve Control – State Controller with Observer (SCO) used for pressure and temperature Standard functions

– Pressure Increase

LP-bypass

– Fixed pressure – Load operation

DCS-Interface

MODBUS or MODBUS Plus (for other Datalinks consult CCI)

(Not all functions – Emergency closing of might be used in steam isolation valve specific application) – Spraywater control (Calculated)

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 Switzerland P.O. Box 65 Hegifeldstrasse 41 8404 Winterthur, Switzerland Telephone ++41 52 262 11 66 Telefax ++41 52 262 01 65

CCI World Headquarter 22591 Avenida Empresa Rancho Santa Margarita CA 92688, U.S.A Telephone ++1 949 858 18 77 Telefax ++1 949 858 18 78

CCI Japan 194-2 Shukunosho Ibaraki-City, Osaka 567 Japan Telephone ++81 726 41 71 97 Telefax ++81 726 41 71 98

CCI Korea 26-17, 26 Pungmu-Ri Kimpo-Eup, Kimpo Gun Kyungi-Do, South Korea Telephone ++82 341 85 94 30 Telefax ++82 341 85 05 52

– Signal connections to hardwired M/A-stations