HP - LP BYPASS AND

APRDS SYSTEM

HP BYPASS SYSTEM

LP BYPASS SYSTEM

AUXILIARY PRDS SYSTEM

INTRODUCTION

The HP bypass system in co-ordination with LP bypass system enables boiler operation and loading independent of turbine.

This allows quick raising of steam parameters to a level acceptable to turbine for rolling during start-up.

Steam is bypassed from the main steam line to CRH line through HP bypass and from the Hot Reheat line to Condenser through LP bypass.

The HP bypass valve can handle a maximum of 60% of the full load turbine steam flow (i.e. 408 tph steam flow).

SINGLE LINE SCHEMATIC

LP TURBINE IP TURBINE HP TURBINE

HP BYPASS

LP BYPASS

CRH to

BOILER

HRH from

BOILER

TO CONDENSER

MAIN STEAM from BOILER

HP BYPASS The possible phases of operation of HP bypass

station can broadly be classified as follows :-

Boiler start up with T.G. set at standstill. Raising of steam parameters to a level

acceptable for TG rolling at a relatively faster rate than otherwise is possible.

Turbine loading while steam flow gets transferred to the turbine.

Parallel operation with turbine on load rejection. Allowing boiler operation following turbine trip-

out provided boiler load is less than 60%.

Preventing safety valve opening at raised steam pressure.

SYSTEM DESCRIPTION

The HP bypass system consists of two parallel branches that divert steam flow from MS line to CRH line. The steam pressure on the valve upstream side can be maintained at the desired level. The steam is de-superheated in order to keep the steam temp. in CRH line within limits, below 325 degC.

The MS pressure ahead of turbine is maintained by two nos. of pressure reducing valves BP1 & BP2 with valve mounted electro-hydraulic actuator.

SYSTEM DESCRIPTION Contd..

The steam temperature downstream of the HP bypass station is maintained by two nos. of

spray water temp. control valves BPE1 & BPE2

They have valve mounted electro-hydraulic actuators.

The spray water is available from the BFP discharge line.

There is also one spray water pressure control valve (BD) with valve mounted electro-hydraulic

actuator.

CONSTRUCTION - HPBP

HP Bypass station consists of the following element per unit :-

Two combined pressure reducing and desuperheating valves (BP1 & BP2) complete with valve mounted electro-hydraulic actuators. Each valve has been sized to pass 204 TPH of steam at inlet condition of 150 kg/cm2 & 5370C.

Two breakdown orifices (OP4 & OP5). One spray water isolating valve (BD1) complete

with valve mounted electro-hydraulic actuator.

Two spray water control valves (BPE1 & BPE2) with valve mounted electro-hydraulic actuators for temperature control.

Two oil supply systems

OIL SUPPLY UNIT

The oil supply unit provides the hydraulic actuation energy for the complete actuation system.

An electric motor driven oil pump sucks the hydraulic fluid through a suction strainer and pumps it through a pressure filter and via a non-return valve into the accumulator.

A safety relief valve protects the system against over pressure. The accumulator supplies the system with pressurized oil and covers all the peak supply requirement.

OIL SUPPLY UNIT Contd..

The oil pump is switched off when the accumulator is fully charged.

From the accumulator the oil is fed through the supply manifold with the pressure reducing valve and the pressure is set and controlled.

OIL SUPPLY UNIT Contd..

The pressure switch monitors the oil pressure in the accumulator and provides the signals to switch on the oil pump motor.

From the supply manifold the oil is fed through the pipe work and the 3 micron pressure filters to the appropriate control valves and the actuators.

Four micro switches (pressure switches) are mounted on the OSU :-

Pump motor ON

Pump motor OFF

Pressure too low

Pressure too high

SERVO VALVE

Servo valve ST is the part of the SULZER electro hydraulic actuator system. It converts the electrical signals of the control electronics into hydraulic signals and uses these to control the actuator.

The two stage servo valve is actuated by the torque motor which is controlled from an analogue positioning amplifier or from a manual desk control.

The torque motor moves the control fork of the servo valve and operate the pilot stage (1st stage) which controls the position of the control piston (2nd stage).

A mechanical override acting directly on the control piston permits local manual operation of the valve.

SERVO VALVE

Servo valve ST is produced for three flow ratings :-

ST10-4 & ST10-05 for low flow rates

ST10-63 & ST10-65 for medium flow rates

ST20 & ST20-200 for high flow rate

BLOCKING UNIT

The electro hydraulically pilot operated blocking unit is mounted between the servo valve and the actuator. It

closes off both ports to the actuator if electrically de-

energized or with insufficient oil pressure, and holds the

piston of the actuator in its last position. A mechanical

override on the blocking unit permits also local manual

de-blocking.

Blocking Unit BL .2 is produced in two versions :-

BL10-2 for low flow rates

BL20-2 for high flow rates

PRESSURE & TEMP. CONTROL

The operation of HP bypass station is manipulated by the pressure and temperature set points and is

independent of LP bypass operation.

The control system is designed to maintain the steam pressure ahead of bypass valve to the given set value.

The steam temperature at downstream of the valve is automatically controlled to the given set value.

The pressure set point and temperature set point can be adjusted from remote manually.

Pressure control

The pressure signal is sensed from the main steam line by pressure transmitters

Depending upon the initial pressure condition at the time of boiler firing, the pressure set point is to be adjusted to a value equal to steam pressure ahead of bypass valve plus a negative bias pressure.

This would result in opening the valves. The pressure controller will then try to maintain the set pressure by allowing a flow matching with the firing rate.

As the firing rate increases, the set point needs to be manipulated in the same manner to allow matching flows. This however shall be possible till the maximum flow capacity of the valves are reached.

Pressure control Contd..

Upon reaching the target steam parameters for turbine rolling, the boiler firing rate can be maintained at that

level.

Consequent upon steam admission into the turbine, the pressure ahead of bypass valves shall tend to fall in view

of constant firing rate.

This will result in closing of bypass valves due to pressure controller action. The process will continue till

the HP bypass valves are fully closed

Pressure control Contd..

After this further loading of the turbine, HPBP pressure set point can be increased to a value equal to rated

turbine throttle pressure plus a bias of pressure 5 ata

This maximum limit of set point should be below the safety valve set pressure for super heater.

With this pressure set point, the HPBP station will automatically open to balance the discrepancy between

steam generation and consumption arising out of load

rejection under constant pressure operation.

Temperature Control

The temperature measuring signal for the HPBP station is sensed by temperature transmitters at the down stream of BP1, and BP2 and spray control valves are modulated.

The control loop for the steam temperature at the downstream of HPBP valves can be operated by incorporating a fixed set point which is slightly higher than the rated cold reheat steam temperature (i.e. rated temp. plus bias temp.).

Should this be not acceptable by the boiler, the temperature set point needs to be manipulated accordingly.

Priority Valve Open Cmds.

During large load rejections, the bypass station will open in about 3 seconds after initiation of signal from the followings :-

Generator Circuit Breaker open

Turbine Trip

Turbine load Shedding Relay (LSR) operates

Rapid opening of the HPBP valves under above conditions is achieved by activating a fast opening device (SSB Unit) which is incorporated in the bypass valve actuators.

Basic Interlocks

HP Bypass valves can be opened at a vacuum > 400 mm Hg

Opening of the main Control valves >2%, The corresponding Attemperation Valves will move to force

auto and will start opening.

The BD valve will open after getting open feedback of > 2% of any control valve.

Down stream Temperature V. High trip will initiate closing of corresponding Control valve and its

Attemperation Valve.

Alarm is generated for deviation from set pressure and temperature.

Quick Acting Device

The quick stroking device SSB is mounted directly on the

actuator of BP1 & BP2. This device may be used for two

purposes :-

Fast operation of the actuator faster than normal control path.

Emergency function, such as fail to open or close by electric power, acting independently of control operation

system.

Quick Acting Device Contd..

Quick stroking device gets its emergency oil from an additional accumulator which is always fully charged from the OSU during normal operation.

This accumulator is positioned near to the actuator to reduce the pressure losses caused by friction which provides a safe function of at least one stroke even after failure of the OSU.

A check valve in the charging line of the accumulator prevents the oil from flowing back.

During operation with the normal control path, a pilot operated check valve in the SSB unit prevents the oil from flowing out of the accumulator into the actuator.

Quick Acting Device Contd..

A solenoid operated 3/2 way poppet valve is used as pilot valve.

Because the pilot valve is of poppet construction, the accumulator can be kept charged over a long period after the failure of the OSU. The SSB units are operated by this pilot valve which can be selected to trip de-energized or energized.

Simultaneously with the tripping of the SSB unit, the normal control path becomes de-energized i.e. the blocking unit closes or the solenoid valve moves to central position t prevent any unwanted oil circulation.

LP BYPASS

The Purpose of operation of LP Bypass valves are similar to that of operation of HP Bypass valves.

Low pressure bypass system enables to establish an alternative pass for dumping the steam from the

Reheater outlet directly into condenser at suitable steam

parameters.

LP BP System Description

LPBP system consists of two parallel branches

Each branches consists of a Stop Valve with valve mounted electro-hydraulic actuator, a pressure reducing

control valve and a temperature control valve with valve

mounted electro-hydraulic actuator.

Steam pressure at valve upstream side can be maintained as desired level.

HP turbine chamber pressure or hot reheat steam pressure before intercept valves is the

controlling variable for the LPBP control system.

LP BP System Description

The steam is de-superheated in order to keep the downstream steam temperature within allowable limits

(i.e. hotwell temp.) by the spray control valve and spray

nozzles (STD) in de-superheater.

This spray water is taken from CEP discharge line.

The double shut off arrangement of LPBP system separates the reheater from the condenser during

normal operation.

Controlling of these parameters can be done by Manual or Auto mode. So, LPBP system maintains the upstream pressure (HRH pressure) and downstream

temperature (condenser temperature).

Oil Supply Unit

The hydraulic power unit supplies the hydraulic energy necessary for the actuators. One single unit can supply several actuators.

The hydraulic power unit consists mainly of two internal gear pump (1W + 1S) which draws the hydraulic fluid from the tank and pumps it into the accumulators.

There are several control and safety equipments to guarantee the supply of hydraulic fluid from the accumulator. As soon as the pressure in the accumulators rises to 18 Mpa, the accumulator charging valve switches the pump over to circulation,

Oil Supply Unit Contd..

Each Oil Unit has the following Instruments:

1. Pressure Switches: 3 Nos.: 90Kg, 110 Kg and 190 Kg

2. Temperature Switches: 2 Nos.: 50 deg and 55 deg C

3. Flow Switch: 2 Nos.

4. Level Switch: 1 No. (common for both)

Oil Supply Unit Contd..

Interlocks:

Man Mode:

1. Individual pump selection> Start

2. It will run for 2 mins.

3. If by this time the flow switch resets, the system runs

otherwise it goes to shut down.

Auto Mode:

1. Power On> both pumps take starts

2. It will run for 2 mins.

3. If the 110 Kg pressure switch resets it runs, otherwise

goes to shut down.

Oil Supply Unit Contd..

Interlocks Contd.:

Auto Mode:

1. If the pressure switch resets, the pump not selected

will stop.

2. In case of pressure low

Oil Supply Unit Contd..

Interlocks Contd.:

1. At temperature 50 deg C, the Fan takes start. The Fan

stops at a temperature 45 deg C.

2. At temperature 55 deg C the system trips.

3. At tank Level low, the system trips.

4. There are some common alarms, which are for anti

coincident happenings.

LP Bypass Interlocks

Conditions for Emergency Trip Commands:

1. Hotwell Level Very High.

2. Hotwell temperature High

3. Vacuum Low (-304 mmHg)

4. LP Bypass Attemperation Flow Deviation (Demand-

Actual) > 17 %

5. Turbine Axial Shift V. Hi/V.Lo (+1.1 and -1.1)

6. Turbine Lube oil Pressure low (1.2)

7. All CW Pump Trip

LP Bypass Valves

LP Bypass valves in Bakreswar are of CCI make PV model valves.

The control unit converts the electrical signals of the control system into hydraulic fluid flow. It is mounted on

the actuator of the LPBP control valves.

It controls intake and discharge of hydraulic fluid to and form the piston chambers of the actuator at a continuous

control mode, thus permitting highly precise positioning

of the actuator. Positioning times of less than three seconds can be

dynamically achieved without difficulties, and this in

connection with high response sensitivity and stability.

LP Bypass Valves

The blocking function of the control unit ensures that the actuator is held more or less drift-free in its position at manual mode or at the event of malfunctions.

Furthermore, the blocking function is taken care of by two electro-hydraulically pilot controlled check valves. In the event of power failure or loss of hydraulic pressure, the check valve close the ducts of the actuator.

The solenoid of the control unit is excited by a current signal being proportional to the deviation of set value to actual value. With higher current at the solenoids the flow through the directional valve increases.

Control units are produced in different versions :- such as PV4, PV6, PV10/1 & PV10/2.

LP Bypass Stop Valve

This is a Solenoid operated On-Off Valve.

One single demand is generated to the LP Bypass Stop Valve along with the LP Bypass Control Valves.

When there is no Emergency trip condition, this valve can be operated any time.

Normally the Stop Valve remains in open condition.

If the Valve is in closed condition, then with 1% opening of any LP Bypass Control Valve, this valve opens

LP Bypass Pressure Control Valve

Unlike HP Bypass, in case of LP Bypass, Demand is Unique for both the Control Valves.

The Control Valves maintain the pressure of HRH line.

In Manual Mode:

The Maximum Pressure Set Point is 38 Kg

The Minimum Pressure Set Point is 3 Kg

In Auto Mode:

The Pressure Set Point starts with 3 Kg

It integrates in every 5 seconds for new set point

It goes up to 11 Kg

Upto Control Valve Position f/B 25 %, this set point will remain. This is called Fixed pressure set Point

LPBP Pressure Control Valve Contd..

Beyond that, a sliding pressure Set Point is generated depending on the HP Chamber Pressure.

The pressure set point is generated as per a functional relationship between the HPT Chamber

pressure and HRH Pressure.

The Set point generation Logic is as follows:

MAX

Fixed Pressure Set Point

Sliding Pressure Set Point

LPBP Pressure Set Point

Max 38 Kg

LPBP Pressure Control Valve Contd..

In case of Stop Valve Emergency trip command, the LP Bypass pressure controller demand goes to zero through

a switching.

In case the Load Shedding relay (LSR) operates, the control valve demand goes to 100 %.

LSR operating time is typically 800 m secs. But the control valve opening time is approx. 10 secs.

In case the valves are gone out of control (such as LSR operation), they will come into control as below:

Attemp. v/v < 1% open, Pr. Cont. v/v will track from 25%

Attemp. v/v >1% open, Pr. Cont. v/v will track from 63%

Emergency trip overrides the Opening interlocks.

LPBP Temperature Control Valve

The spray water is taken from CEP discharge.

The Attemperation valves in LPBP system get demand from down stream pressure transmitters of individual pressure control valves.

The Pressure transmitters generate a pressure feedback.

This pressure feedback is converted to an attemperation flow demand through a function generation card.

Flow transmitters are mounted in the CEP Discharge line.

The demand of flow and the flow feedback is always calculated.

In case the deviation.17 %, the LPBP system trips

Auxiliary PRDS System

Auxiliary PRDS system consists of a Turbine PRDS control system as well as Boiler PRDS control system.

The controller of these system are pneumatically

controlled. Both the systems are identical in nature.

Auxiliary steam is tapped from the main steam line and its pressure is reduced and de-superheated to the

required temperature and pressure.

The spray water for de-superheating is supplied from the CEP discharge header.

In order to obtain better operational flexibility and controllability range, both the system have been split into

two identical PRDS system such as low capacity PRDS

and high capacity PRDS.

Auxiliary PRDS System Contd..

The low capacity is of 30% of that of the high capacity line.

Each system comprises of the pressure controller, one

desuperheater and one spray water flow controller. We shall henceforth call the Turbine PRDS system as

TAS and the Boiler PRDS system as BAS.

System Description- High Capacity

High capacity PRDS comprises of steam pressure reducing control valve, motorized upstream isolating

valve, downstream manual isolating valve, one spray

water temperature control valve and one desuperheater.

A motorized regulating globe valve is provided in bypass line of, which can be operated remote manually in case

of mal-operation of the pressure control valve.

A motorized bypass globe valve is provided for spray water control valve , which can be operated remote

manually in case of mal-operation of spray water control

valve.

System Description- Low Capacity

Low capacity PRDS comprises of steam pressure reducing control valve, motorized upstream isolating

valve, downstream manual isolating valve, one spray

water temperature control valve and one desuperheater.

A motorized regulating globe valve is provided in bypass line of, which can be operated remote manually in case

of mal-operation of pressure reducing control valve.

A motorized bypass globe valve is provided for spray water control valve, which can be operated remote

manually in case of mal-operation of spray water control

valve.

TAS/BAS Pressure/ Temperature

Control

The auxiliary steam pressure control valve supplies steam to TAS/BAS header by maintaining TAS/BAS line

pressure constant at the set point (11 kg/cm2).

Downstream pressure of PRDS valve is taken as

controlled pressure.

The auxiliary PRDS spray water flow control valve (CD) maintains the TAS / BAS line temperature constant at

the set point (2600C for TAS & 2200C for BAS). The

downstream temperature of PRDS valve is taken as

controlled temperature.

Selection of HC & LC TAS PRDS

During unit start up auxiliary steam for turbine is supplied to the Starting & Main Ejectors, Deaerator initial heating & pegging,

Turbine Gland Sealing. To meet this high demand of auxiliary steam

HC PRDS should be put into service.

After required condenser vacuum reached, starting ejector is withdrawn, so the aux. steam supply to starting

ejector is cut off.

After unit synchronization and at about 30 to 40% of unit load deaerator pegging is supplied from CRH / turbine

extraction line (CAP) and pegging from TRDS is cut out.

At about 40% unit load also, turbine gland sealing supply

valve from TPRDS closes as turbine becomes self-

sealing condition.

Selection of HC & LC TAS PRDS

At that position, auxiliary steam from PRDS is supplied to main ejector only. To meet this very low demand of

auxiliary steam , LC PRDS should be put into service,

because it will be very difficult to control TPRDS steam

pressure by HC PRDS valve which may cause of safety

valve popping of TPRDS header.

Good Bye