3. Malfunctions and Transmitters

A training session is about things going wrong. The instructor injects malfunctions into the running plant: a valve that sticks, a pump that trips, a sensor that lies. This page covers the catalogue, the severity and ramp settings, and the Transmitter block that makes sensor failures possible.

What you will learn

• The malfunction catalogue and what each one does to the plant
• Severity and ramp time
• Activating, deactivating and clearing faults by hand
• Adding a Transmitter between a process variable and its controller, and failing it

Prerequisites

• Page 2: you can start and freeze a session

The Malfunctions tab

1. Object: pick the flowsheet object. The Malfunction list only shows the kinds that apply to it; the two plant-wide faults appear with no object selected.
2. Severity: 0 to 1. What it scales depends on the kind (table below).
3. Ramp (s): simulated seconds over which the fault builds up from zero to the chosen severity. Zero means a step change.
4. Activate. The fault appears in the grid, with its state, and the journal records it.
5. Deactivate clears it (the plant recovers as far as physics allows); Remove deletes it from the grid; Clear all removes everything.

Malfunctions can be activated while running or frozen. They are applied at the start of every step, before the controllers act, so a controller cannot undo them.

The catalogue

Malfunction	Applies to	What happens	Severity
Valve stuck	Valve	The stem stops where it is. The valve rejects what the controller writes, so the displayed opening is the real one.	ignored
Valve fails closed / fails open	Valve	The stem travels to 0 % or 100 % over the ramp and ignores the controller meanwhile.	ignored
Valve passing (leak through)	Valve	The valve no longer shuts tight: the opening cannot drop below severity x 100 %.	minimum opening
Pump trip	Pump	Target speed goes to zero; the pump coasts down at the rate its rotational inertia allows; the pressure rise follows the speed squared. Clearing restarts the motor.	ignored
Exchanger fouling	Heat exchanger	The fouling resistance grows at severity x rate per second.	rate
Utility loss (duty)	Heater, cooler in fixed-duty mode	The duty falls to (1 - severity) of what it was, over the ramp.	fraction lost
Sensor frozen	Transmitter	The reading stops updating.	ignored
Sensor drift	Transmitter	The reading drifts at severity x 0.1 display units per second.	drift rate
Sensor bias	Transmitter	The reading is offset by severity x 10 display units.	bias
Sensor full scale / zero	Transmitter	The reading pins at the top or the bottom of the span.	ignored
Power failure (plant-wide)	everything	Every pump trips and every heater and cooler loses its duty.	passed to each
Instrument air failure (plant-wide)	everything	Every control valve goes to its fail position (closed unless listed as fail-open).	ignored

Try it: a stuck level valve

1. Start the session at 10x and let the level settle around 0.3 m.
2. Malfunctions tab: object LV-001, malfunction Valve stuck, ramp 0, Activate.
3. Watch LIT-001: the feed keeps coming, the level rises, and PID-013's output climbs to 100 % without moving the valve. The journal shows the fault and, once the level crosses a limit, the alarm.
4. Deactivate. The valve jumps to where the controller wants it and the level comes back.

Try the same with Valve fails closed and a ramp of 30: the closure is gradual and a trainee who watches the trend can catch it before the alarm.

The Transmitter block

Controllers and gauges in DWSIM read the true process value. Real instruments lag, delay, add noise and fail. The Transmitter is a unit operation the OTS adds to the palette: it sits between a process variable and whoever reads it.

Adding one

1. Add a Transmitter from the object palette (it is listed with the other unit operations once the OTS is installed). Its tag starts with XT-.
2. Open its editor (click it on the flowsheet). Process value and alarms... opens the same editor a gauge has: choose the object and property to measure (for the sample, SG-01 / Operating Pressure), the units and the alarm limits.
3. Set the span (low and high, in display units), and the dynamics:
   • Time constant (s): first-order lag; 0 for an ideal sensor
   • Dead time (s): transport delay
   • Noise std. dev.: Gaussian noise added to every reading
4. Apply. The editor shows the true value and the measured value side by side while the session runs.

Making the controller read it

Open PID-012 and change its controlled variable from the separator pressure to the transmitter's Monitored Value. From now on the controller sees what the instrument reports, and every sensor malfunction in the catalogue becomes a lesson:

• Sensor frozen on a pressure transmitter: the controller thinks nothing changes, the real pressure drifts, the gauge that still reads the true value disagrees with the faceplate. The trainee has to notice the disagreement and switch to manual.
• Sensor drift: the controller slowly moves the real pressure away from the set point while its own display looks perfect.

The transmitter's alarms are computed on the measured value, like a real DCS.

Failure mode on the block itself

The editor's Failure mode field sets the same modes by hand, outside a session. Malfunctions restore the previous mode when cleared.

The next page packages malfunctions into scenarios with objectives and a score.