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

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

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 Function Symbol 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 Variable Function A = Analysis** C = Conductivity D = Density F = Flow L = Level M = Moisture P = Pressure S = Speed T = Temperature V = Viscosity W = Weight Z = Position FRT **Self-defining symbols such as O 2, FR R = Recording I = Indicating T = Transmitter RT = Recording Transmitter IT = Indicating Transmitter p. H, etc. , can be used in place of A.

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

SAMA Legend Measuring or Readout 1. CONTROLLER Automatic Signal Processing A Manual Signal Processing SET POINT GENERATOR Final Controlling RESET ∫ K PROPORTIONAL MANUAL SIGNAL GENERATOR T AUTO. /MAN. TRANSFER SWITCH A | Signal Repeater ∑ Summing d/d 1 Derivative K, -K ∆ Difference ∫ Integral Proportional, Reverse Proportional X Multiplying 2. AUTO MANUAL + BIAS STATION ∑/h Summing ÷ Dividing √ Root Extracting f(x) Non Linear or Unspecified Function f(t) Time Function AUTO. /MAN. TRANSFER SWITCH ± T A MANUAL SIGNAL GENERATOR < Low Selecting High Limiting | > Low|Limiting > BIAS ADJUSTMENT A > High Selecting 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 INPUT + BOILER FEEDWATER CONTROL FUEL DEMAND AIR DEMAND FIRING RATE DEMAND STEAM TEMPERATURE CONTROL OUTPUT

Furnace Pressure Control

Block Diagram of Boiler Control INPUT + BOILER FEEDWATER CONTROL FUEL DEMAND AIR DEMAND FIRING RATE DEMAND STEAM TEMPERATURE CONTROL OUTPUT

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 (K) (a) +K (b) + K (c) etc. + bias = output Typical single-drive control system. For simplicity, redundant transmitters have not been shown on this typical control drawing. See Figure 4 A for ANSI/ISA-5. 1 -1984 format.

Performance Three Element

Block Diagram of Boiler Control INPUT + BOILER FEEDWATER CONTROL FUEL DEMAND AIR DEMAND FIRING RATE DEMAND STEAM TEMPERATURE CONTROL OUTPUT

Firing Single Fuel/Cross Limiting Fuel FLOW STEAM HEADER PRESSURE FT PT AIR FLOW FT A f(x) P A I T SP P A I T LO SELECT Fuel CONTROL SP P HI SELECT A I T AIR CONTROL f(x) Fuel Feed AIR DAMPER

Cross Limit Control With O 2 Trim

Flow Characteristics

Non Linear Flow

Characterization

Control Valve Sizing Calculations • Cv = 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 Cv 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 = 600 1/3 = 200 valve drop

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

Control Valve Sizing Calculations (cont’d) • Valve sizing water 450 o. F, SG = 0. 827 • The calculation 600, 000 pph is: Cv at 60 o = 84. 85 Cv at 450 o = 77. 16

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

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 span • Final elements Valves etc – Valve Sizing

Questions and Discussion

Related Resources from ISA • • Boiler Control Systems Engineering Jerry Gilman The Control of Boilers Sam G. Dukelow ISA Courses ES 15 Boiler Control System Engineering – Three day course – One day overview – WEB introduction course • ES 16 Burner Management System Engineering – Three day course – One day overview Phone: (919) 549 -8411 E-mail Address: info@isa. org