Standards Certification Education & Training Publishing Conferences & Exhibits Automation Connections ISA EXPO 2006

Standards Certification Education & Training Publishing Conferences & Exhibits Boiler Control Systems Engineering Jerry Gilman

Control System Functions A boiler is a process Regardless of what control technology is used all boilers require certain control functions. –Furnace Draft –Drum Level Feedwater –Fuel Air –Steam Temperature

Control System Considerations Draft Drum level feedwater –Single element –Two element –Three element Fuel air ratio Final elements Valves etc –Valve Sizing

Basic Diagram of a Boiler

Steam & Mud Drum / Circulation

SAMA Symbols Scientific Apparatus Makers Association ENCLOSURE SYMBOLS Table 1 FunctionSymbol Measuring or Readout Manual Signal Processing Automatic Signal Processing Final Controlling Within a circle use a letter symbol from Table II Within other enclosures us a symbol from Table III

SAMA Symbols (cont’d) MEASURING/READOUT LETTERS Table II Process VariableFunction A = Analysis** C = Conductivity D = Density F = Flow L = Level M = Moisture P = Pressure S = Speed T = Temperature V = Viscosity W = Weight Z = Position R = Recording I = Indicating T = Transmitter RT = Recording Transmitter IT = Indicating Transmitter FRT FR **Self-defining symbols such as O 2, pH, etc., can be used in place of A.

SAMA Symbols (Table III) SIGNAL FUNCTION PROCESSING SYMBOL FUNCTIONPROCESSING SYMBOL SUMMING  or + INTEGRATE OR TOTALIZE Q AVERAGING  /n HIGH SELECTING DIFFERENCE  or - LOW SELECTING PROPORTIONAL K or P HIGH LIMITING INTEGRAL  or I LOW LIMITING DERIVATIVE d/dt or D REVERSE PROPORTIONAL -K or -P MULTIPLYING X VELOCITY LIMITING V DIVIDING  BIAS  ROOT EXTRACTION  TIME FUNCTION f(t) EXPONENTIAL X n VARIABLE SIGNAL GENERATION A NON-LINEAR FUNCTION f(x) TRANSFER T TRI-STATE SIGNAL (RAISE, HOLD, LOWER)  SIGNAL MONITOR H/, H/L, /L

SAMA Legend T A A K ∫ 1. CONTROLLER SET POINT GENERATOR PROPORTIONAL RESET MANUAL SIGNAL GENERATOR AUTO./MAN. TRANSFER SWITCH T A A 2. AUTO MANUAL + BIAS STATION BIAS ADJUSTMENT MANUAL SIGNAL GENERATOR AUTO./MAN. TRANSFER SWITCH ± Measuring or Readout Automatic Signal Processing Manual Signal Processing Final Controlling Signal Repeater ∑ Summing ∑/ h Summing d/ d1 Derivative ∆ Difference ∫ Integral K, -K Proportional, Reverse Proportional X Multiplying ÷ Dividing √ Root Extracting f(x) Non Linear or Unspecified Function f(t) Time Function > High Selecting < Low Selecting High Limiting Low Limiting Velocity or Rate Limiter +, -, ± Bias T Transfer A Analog Signal Generator > | > | |

Simple Feedback Control

Feedforward Plus Feedback Control

Cascade Control

Ratio Control

Block Diagram of Boiler Control

Furnace Pressure Control

Block Diagram of Boiler Control

Boiler Steam Drum Swell / Shrink

Single Element Feedwater Control

Single Element Control Action

Two Element Feedwater Control

Performance Two Element (Ideal Conditions)

Performance Two Element ( Effect of Feedwater Variation)

Three Element Feedwater Control Typical single-drive control system. For simplicity, redundant transmitters have not been shown on this typical control drawing. See Figure 4A for ANSI/ISA format. (K) (a) +K (b) + K (c) etc. + bias = output

Performance Three Element

Block Diagram of Boiler Control

Firing Single Fuel/Cross Limiting AIR CONTROL FTPTFT A AT PI AT PI AT PI f(x) Fuel FLOW STEAM HEADER PRESSURE AIR FLOW SP LO SELECTHI SELECT Fuel CONTROL Fuel Feed AIR DAMPER

Cross Limit Control With O 2 Trim

Flow Characteristics

Non Linear Flow

Characterization

Control Valve Sizing Calculations C v = Number of US gallons of water at 60 o F that flows through a valve in one minute when the pressure differential across the valve is one pound per square inch –Valve sizing water no density consideration: –The term C v is generally used in industry for calculating the relative flow capacity in valves

Control Valve Sizing Calculations (cont’d) Valve sizing water no density consideration:  P = pressure drop across valve typical third of drop 600,000 pph water = 600,000  8.34 lb/gal = 1200 gpm  P = pressure drop across valve, typical a third of drop Pump pressure = 2000 psi, Drum pressure = 1400 psi 2000  1400 =  1/3 = 200 valve drop

Control Valve Sizing Calculations (cont’d) The calculation 600,000 pph is: The calculation for 250,000 pph is:

Control Valve Sizing Calculations (cont’d) Valve sizing water 450 o F, SG = The calculation 600,000 pph is: C v at 60 o = C v at 450 o = 77.16

Control Valve Sizing Calculations (cont’d) Valve sizing steam:

Control Valve Sizing Calculations (cont’d) Valve sizing steam:

Summary Regardless of the hardware and or software used certain control functions are required. These controls are –Furnace Draft –Drum Level Feedwater –Fuel Air –Steam Temperature Measurement / transmitter specifications –Calibration –Calibration span Final elements Valves etc –Valve Sizing

Questions and Discussion

Related Resources from ISA Phone: (919) Address: Boiler Control Systems Engineering Jerry Gilman The Control of Boilers Sam G. Dukelow ISA Courses ES15 Boiler Control System Engineering –Three day course –One day overview –WEB introduction course ES16 Burner Management System Engineering –Three day course –One day overview