The major control loops of a thermal power plant, drawn as live block diagrams. Move the disturbance slider and watch the signals actually propagate — these loops are far easier to understand seeing them move than reading about them.
Verified against published standard
How to read the tag names (ISA-5.1)
1st letter = what's measured: Level, Flow, Pressure, TemperatureT = Transmitter (measures it) — e.g. LT = Level TransmitterIC = Indicating Controller — e.g. FIC = Flow Indicating ControllerCV = Control Valve, the final element — e.g. FCV = Flow Control ValveΣ = a compute block (sum, select, ratio) — not a physical instrument
Three-Element Drum Level Control
Combustion Control with Cross-Limiting
Superheater Steam Temperature (Cascade Attemperation)
Mill / Pulverizer Control
Furnace Draft Control
Coordinated Master Control (CMC) — Unit Load
HP / LP Turbine Bypass Control
Deaerator Level & Pressure Control
HP Heater Level Control (3-Element)
Condenser Hotwell Level Control
AVR / Excitation Control (Generator Voltage)
Once-Through Boiler Fire/Water Ratio Control
Sliding Pressure / Governor Valve Position
Condenser Vacuum (Backpressure) Control
Three-Element Drum Level Control
Feedwater · Core
The single most important — and most misunderstood — loop on a drum boiler. Level must be held tight: too low uncovers tubes and overheats them, too high carries water into the turbine.
60
drag to disturb · 1 s
0mm
ON TARGET
0 mm here means "at the normal operating level" (the setpoint), not empty — real level transmitters are ranged around normal level, not the full vessel. Positive = above normal, negative = below.
Process Value (PV) — the live measured trendSetpoint (SP) — the target
Level stable at setpoint. Feedwater is matching steam flow, with the master PI trimming only small residual errors.
▸ Show controller detail & step-by-step signal flow
Controller Outputs
LIC
MASTER — holds level
SP0 mmPV0 mmERR+0 mmOUT0 % trim
-4050% of range40
FIC
SLAVE — holds feedwater flow
SP60 %PV60 %ERR+0 %OUT60 % valve
060% of range100
How the signal flows right now
CASCADE + FEEDFORWARD
1
Steam flow (feedforward)60.0 %
The bulk demand. Feeding this FORWARD means feedwater tracks steam immediately instead of waiting for a level error.
2
Level master trim+0.00 %
The master only corrects the small leftover error — it does not set flow by itself.
3
Sum → slave setpoint60.0 %
feedforward 60.0 + trim +0.00 = 60.0. This is the CASCADE handover: the master output becomes the slave SETPOINT.
4
Slave flow controller60.0 → 60.0 %
The slave drives the valve until measured feedwater flow equals that setpoint.
5
Valve position60.0 % open
Final element.
The Problem
Shrink and swell. When steam demand suddenly rises, drum pressure drops, steam bubbles expand, and the level appears to RISE even though the boiler is actually losing water. A naive level controller reads "high level" and closes the feedwater valve — exactly the wrong action, at exactly the wrong moment.
The Solution
Feed the steam flow FORWARD into the level loop so the controller anticipates the demand change instead of reacting to a misleading level signal, and CASCADE the level controller onto a fast feedwater flow loop so supply-pressure disturbances are caught before they ever affect level.
Key Elements
▸Element 1 — Drum level (the controlled variable)
▸Element 2 — Steam flow (the disturbance, fed forward)
▸Element 3 — Feedwater flow (the fast inner loop)
Block Types
Measurement / transmitterPID controllerFunction block (sum, select, ratio, f(x))Final control elementPhysical process
WARE — 3-Element Steam Drum Level Control (shrink/swell explanation)
About This Calculation
A power plant runs on dozens of interacting control loops — boiler drum level (three-element), superheater/reheater temperature (via attemperation spray), combustion (fuel-air ratio), deaerator pressure and level, feedwater flow — each with its own dynamics, its own coupling to neighboring loops, and its own reason a naive single-loop PID tune can go wrong. This page runs live, interactive simulations of several of the most consequential ones, showing the actual process response (not just a static formula) so the effect of a tuning change, a disturbance, or a control-strategy choice (single-element vs. three-element level control, for instance) is genuinely visible rather than just described.
Frequently Asked Questions
Why does drum level control need three elements instead of one?
Single-element control (measuring only drum level) reacts too slowly to a sudden steam demand change — a load increase causes drum level to briefly rise (shrink-and-swell) before it falls, which a level-only controller misreads as “level is fine” or even “add less water” at exactly the wrong moment. Three-element control adds steam flow and feedwater flow as feedforward signals so the controller responds to the load change directly, not just to its delayed effect on level.
What is shrink and swell, and why does it matter for tuning?
A sudden increase in steam demand drops boiler pressure slightly, which causes existing steam bubbles in the drum water to expand — drum level rises even though water inventory is actually falling (swell). The reverse happens on a load decrease (shrink). A level controller tuned without accounting for this inverse response will tend to move the feedwater valve the wrong way immediately after a load change.
Why is superheater temperature control often the hardest loop on the unit?
It combines a long, variable process dead time (spray water takes real time to travel through the remaining superheater tubing before it affects outlet temperature) with a process gain and dead time that both change significantly with load — a PID tuned well at full load is commonly too aggressive or too sluggish at part load.
Can I use this to actually re-tune a real loop’s PID constants?
Use it to build tuning intuition and to compare control STRATEGIES (e.g. cascade vs. single-loop, feedforward vs. feedback-only) — not to lift Kp/Ti/Td values directly onto a real DCS. A real loop’s actual process gain, dead time, and time constant depend on that specific unit’s equipment and must come from an actual tuning study or step-test on the real process.