HRSG Modeling for Process Control

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HRSG Process Control Modeling Tom McAuliffe
Dec 3rd 2008 POWERGEN Gas Turbine
Technology HRSG

• Valve Linearization
• IP Bypass Feedforward Algorithm
• HRSG Superheater Model

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Control Valve Linearization

• Most PIDs Control Valves
• PID Control Assumes Process Linearity
• Cv DP are Non-Linear
• Transition Between Valves in Parallel

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Control Valve Linearization - Example
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Small Valve Open Desired CV
Cv Curvefits
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Then, Big Vv Open Desired Cv
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Valve linearization In Practice
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IP Bypass Feedforward Feedback

• Plenty of Superheat lt 40 Flow
• Poor Attemperation Flow Accuracy
• Not Enough Superheat gt 40 Flow
• Good Attemperation Flow Accuracy
• Feedback to Feedforward Switch
• _at_ 40 Sparger Pressure (40 Flow)

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IP Bypass Problem Too Little Superheat
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Feedforward Algorithm
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IP Bypass Feed-forward Equation

• Condenser Requires
• hstm lt 2777 kJ/kg (1194 BTU/lbm)

Attemperator Feed-forward Spray Setpoint
Sparger Mass Flow Rate
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Bypass Flow, Only Need One Data Point
Need Sparger Flow at a Pressure, _at_ Required
Enthalpy
Draw a Straight Line Through Zero
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HRSG Superheater/Reheater Temperature

• Critical to Efficiency/Reliability
• Dearth of Model Info
• Complex but Simple
• Thermal Energy Storage (Heat Capacity)
• Lumped Capacitance Does Not Apply
• Biot Not lt 0.1 on Steam Side
• Mass of Fins Mass of Tubes

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HRSG Model Physical Data
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Segments, Subsegments, Nodes
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Example of Nodal Equation Development
Tube wall innermost nodes adjacent to outer
sub-segment boundaries experiences convection on
one surface and conduction on two surfaces.
Becomes Conceptually in Matrix Form
thermal diffusivity
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Matrix Math is Key (Matlab)

• 1000 seconds x 1000 ms/s 1,000,000 updates
  (for i 1 to)

• About 8 hrs per test run (about 17 minutes of
  real time)

• Could be worse
• 60,722 nodal temperatures
• 60,722 x 8 hrs 55 years

• C matrices are diagonal
• NSegments 120 for 60 foot duct, 137 for 67.5
  foot duct

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HP Attemperation
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RH Attemperation
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HP Steam Flow Step Change
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RH Steam Flow Step Change
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CTG Exhaust Flow Step Change
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CTG Exhaust Temperature Step Change
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HRSG Model Observations

• First Order Plus Deadtime (FODT)
• Multi-Input Multi-Output (MIMO)
• Inherently Stable (of course! )
• MIMO IMC (Internal Model Control)
• Gas Path Temperature Changes
• Missing Steam Flow vs Heat Input

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HRSG Model Future Additions

• Incorporate Pressure Drop
• Incorporate Evaporator Model Astrom Bell
  Drum Boiler Dynamics
• MIMO IMC Manfred Morari Robust Process Control

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Questions?

• For Follow up Go To
• www.powereng .com
• Contact Hailey office
• Ask for Tom (McAuliffe)
• Or
• Stop by our booth and ask for Tom
• Booth 4332 Power Engineers