Auto Control Loops Presentation

Auto Control Loops of Large-sized Thermal Plants

Janmejay MahapatraHead (Instrumentation)

Avantha Power Infrastructure LTD.6th Nov 2011 – PMI, Noida

Structure of the Controller

Is my plant performing as per Design?

An eye on the various measured values of different processes:

Temperature Pressure Flow Level Water & Steam Parameters (e.g.

conductivity, pH, silica etc.)

Is my plant performing as per Design?

And, hands on the various means to regulate the different processes:

Valves Dampers Speed Regulators Stroke Controllers

The Onerous task of the plant operator

The huge number of processes involved Electrical Load has to be matched with the

fluctuating demand Creeping of inefficiencies as the process

deviates from the design Safety of the equipment at risk as the

process drifts beyond allowable limits Start-up & Shut down activities Two shift operation

The Basic Control Schemes

On-off Control Proportional Control Proportional-Integral ( P-I ) Control Proportional – Derivative ( P-D ) Control Proportional-Integral-Derivative ( PID ) Control Ratio Control Spilt Range Control Cascade Control Feed-forward Control Adaptive Control

The Basic Control Schemes : Responses

Let’s recognise these:

Roughly 75% of process control loops cause more variability running in the automatic mode than they do in the manual mode.

A third of them oscillate as a result of nonlinearities such as valve dead band.

Another third oscillate because of poor controller tuning.

The remaining loops oscillate because of deficiencies in the control strategy.

Some simple rules of Tuning the Auto Control Loops

Increase the gain to achieve tight control and to

speed up the set point response.

Decrease the gain to provide a smoother, slower,

and more stable response.

Whereas insufficient gain slows down the initial

approach, inadequate reset action slows down the

final approach to set point.

If the reset action is too large (reset time too

small), there is excessive overshoot.


If there is excessive overshoot and oscillation,

decrease the reset action (increase reset time).

For level loops with low controller gains (< 5.0),

reset action must be greatly restricted (reset

time increased to 50 or more minutes) to help

mitigate nearly sustained oscillations.

Rate is primarily used in temperature loops that

have narrow span transmitters and slow scan

times.


There is noise in every process variable.

If the noise is greater than the measurement

resolution limit, the loop sees it.

If it causes an output change greater than the

final element or control valve resolution limit,

loop variability will be worse in auto than in

manual, particularly if there is any rate action.

Tuning the Auto Control Loops

Test new tuning settings by changing the set

point in both directions.

Don’t leave these new settings over night

until you are sure they are right.

Before entering new settings, ask the

operator for permission to tune the loop.

Ask the operator what the maximum

allowable size of variation.

Put the controller in manual before tuning.

Feed Forward Control

Useful for processes with significant dead times. Controlled condition is affecting processes

other than that is being controlled. Controller is required to respond promptly but

the response of the measured value is slow. Usually it is the derivative of the external

measurement. The “Anticipatory” action is added to the output

of the controller. Normal feedback control after the passing off of

the transient.

Cascade Control

Cascade control is a type of control in which a secondary (slave or inner) loop is added.

Secondary Loop gets a set point from a primary (master or outer) loop.

The secondary loop response is normally faster than (about five times) the primary loop response.

The secondary loop can correct for upsets it can measure before they affect the primary loop.

BHEL - The Base Documents

Recommended Scheme of Auto Controls –

Trichy Document

APRDS Control Scheme – Trichy Document

Analog Control Schemes – PEM Document

HPBP Control Scheme – PEM Document

CMC Scheme – PEM Document

The Control Loops

Heater Level Controls LPH - 2 / 3 level controlHPH - 5A / 6A / 5B / 6B level control

Hotwell Level Control GSC minimum flow control Gland seal Water Header Pressure control Controls of Deaerator

Pressure control Level control

Dosing ControlAmmonia, Hydrazine, Phosphate

The Control Loops

Furnace Press Control Secondary Air Flow Control Drum level control Fuel Flow Control PA Header press Control Mill Air Flow Control Mill Air Temperature Control SH Steam Temp Control RH Steam Temp Control Burner Tilt Control AH Cold End Temp Control

The Control Loops

CBD Tank Level Control Atomising Steam Press Control HFO Flow Control LDO Flow Control HFO Heater – A O/L Temp Control HFO Pump Discharge Press Control LDO Pump Discharge Press Control

The Control Loops

HP Bypass Pressure Control HP Bypass Temp. Control APRDS Pressure Control APRDS Temperature Control

Co-ordinated Master Control (CMC)

Importance of Heater Level Control

Feed Water / Condensate Temp. Rise

Terminal Temp. Difference (TTD)

Tsat(steam) – TFW Outlet

Increase in TTD implies worse heat transfer

Drain Cooler Approach (DCA)

TDrain Outlet – TFW Inlet

Increase in DCA implies decrease in water

level.

Heater Performance Monitoring

LPH –2 LEVEL CONTROL

LPH –3 LEVEL CONTROL

HPH -5A LEVEL CONTROL

HPH -6A LEVEL CONTROL

HPH -5B LEVEL CONTROL

HPH -6B LEVEL CONTROL

HOTWELL LEVEL CONTROL

GSC MIN FLOW CONTROL

GLAND SEAL COND PR CONTROL

Why is Deaerator Pegging Required

Heat the incoming feed water to the saturation point to reduce the solubility of entrained gases.

The gases are basically O2, CO2 and NH3 - very corrosive at elevated temperatures.

Set of trays that separates the water into thin sheets from which the gases can easily escape.

The gases are vented out of the deaerator to the atmosphere.

To ensure mixing & elimination of corrosive gases, the FW level & steam pressure controls are important.

Why is Deaerator Pegging Required

DEAERATOR PRESS CONTROL

DEAERATOR LEVEL CONTROL

Hydrazine is used as oxygen scavenger. Hydrazine is one of the most costly chemical. Hydrazine dosing done at De-aerator outlet (BFP

suction)

Feed water has to be alkalized to a pH of 9 or higher, to reduce oxidation.

Also helps in forming of a stable layer of magnetite on the water-side surface of the boiler, protecting the material underneath from further corrosion.

Achieved by dosing alkaline agents into the feed water, like NaOH or volatile NH3.

Ammonia dosing done at CEP Discharge.

Dosing in Condensate & Feed Water

AMMONIA DOSING CONTROL

HYDRAZINE DOSING CONTROL

It is the Drum which speaks in a tube type boiler! Loose reddish powder – Boiler corrosion Loose Greyish powder – Economiser / pre-boiler

corrosion

Purpose is to reduce hardness and facilitate scale control.

Phosphate is a highly soluble, powder compound – significantly effective for hardness & scale control

Binds to calcium – prevents scaling but forms a sludge.

Phosphate dosing done at boiler drum.

Chemical Dosing in Drum

PHOSPHATE DOSING CONTROL

FURNACE PRESS CONTROL

AIR FLOW CONTROL

DRUM LEVEL CONTROL

Typical Drum parameters of 500/600MW sub-critical unit – 197 Kg/cm2 & 363 Deg C!

Density is proportional to Pressure. Density is inv. proportional to Temperature.

DP is affected by changing density of water & steam inside boiler.

Drum Level Measurement - Facts

DRUM LEVEL CONTROL

AIR FLOW CONTROL

FUEL FLOW CONTROL

PA HDR PRESS CONTROL

MILL-A AIR FLOW CONTROL

MILL –A AIR TEMP CONTROL

SH STM TEMP CONTROL

BURNER TILT CONTROL

RH STM TEMP CONTROL

Boiler Flue gas contains water vapour, sulfur & other chemicals.

Low Flue gas temp. (below dew point) at AH cold end may result in condensation of water vapour.

Combination with sulfur forms sulfuric acid.

Results in corrosion of the cold end metal.

Why AH Cold end needs to be protected?

AH COLD END TEMP CONTROL

CBD TANK LEVEL CONTROL

ATOMISING STEAM PRESS CONTROL

HFO FLOW CONTROL

LDO FLOW CONTROL

HFO HEATER – A O/L TEMP CONTROL

HFO PUMP DISCH PRESS CONTROL

LDO PUMP DISCH PRESS CONTROL

HP BYPASS SYSTEM PRESS CONTROL

SPRAY BLOCK VALVE LOGIC

HP BYPASS SYSTEM MISC ALARMS

HP BYPASS SYSTEM TEMP CONTROL

AUX STEAM HDR PRESS CONTROL

HT AUX STEAM HDR TEMP CONTROL

LT AUX STEAM HDR TEMP CONTROL

COORDINATED MASTER CONTROL SCHEME


Off design final FWH temperature at a 500MW Coal fired plant

Outlet Temperature Target 225◦ C

Actual 214◦ C

Difference -11 ◦ C

CHECKED PERFORMANCE PARAMETERS Temp Rise Target 27◦ C

Actual 18◦ C

DCA Target -12.22◦ C

Actual -16.1◦ C

TTD Target -12.22◦ C

Actual -6.94◦ C

Auto Control Loops Presentation

Documents

Transcript of Auto Control Loops Presentation