Mission 43 · PRO

The tank that overflowed: PLC fault finding exercise

Trace a fault to a failed device, then make the program catch it.

The receiving tank overflowed last night. The program is sound: something on the skid has failed. Find it with the and list, then make the program detect it, light the right fault lamp and stop the transfer.

This mission is part of ProCreate a free account: every account gets a free Pro trial until 5 December.

No card. Nothing to pay. Nothing to cancel.

Official release: 5 December. Top the leaderboard on the day and win 3 months of Pro, plus 2 weeks of Pro to give a friend. 2nd place wins 4 weeks. 3rd place wins 2 weeks. Rules

Every Coaching lesson, from zero
Every mission and lesson, as you level up
A practical test for every title, with a certificate
Rewind any test, one scan at a time
Worked solutions when you're stuck
The code converter, as much as you like

The 3D two-tank transfer rig this mission runs on

Level: Applied
Time: About 25 min
Reward: +250 XP
Rig: 3D two-tank transfer rig
Siemens editors: Ladder (LAD), SCL
Rockwell editors: Ladder, Structured Text
Beckhoff editors: Structured Text
Stage: 26. Find the fault: it isn’t always the program

Required behaviour

The tank that overflowed: what your code must do

Night shift report: 'TK-202 overflowed during a transfer. The screen showed it empty the whole time, and the pump just kept going.' The program transfers water from TK-201 to TK-202: Start opens valve XV-201, the pump runs once the valve is proved open, and everything stops at 80%. Press Run, then Start, and watch. Compare each line of the input and output list with its Healthy column: one device is not doing what the list says. Then change the program so this failure is caught. Within a quarter of a second of the fault showing: the lamp for the failed device ON (SignalFault, ValveFault or PumpFault, and only that one), PumpOn and ValveCmd OFF, and LevelPV left at its last good value. No transfer may start while the fault is there. A healthy skid must still transfer to 80% with no lamp lit.

Transfer: Start opens XV-201. The pump runs once ZSO-201 proves it open. Both stop at 80%.
Find the fault: One device has failed. The input and output list shows what each signal should read.
Catch it: The failed device's lamp ON within 0.25 s, and no other. PumpOn and ValveCmd OFF.
Level shown: LevelPV keeps its last good value while the fault is there.
How it's checked: The checks press Start and Reset, nothing else, on a skid with the same fault as yours and on a healthy one. They read the skid itself: SpilledLitres (water on the floor), whether the pump is really running, and the real level in TK-202.

Tag table

The tank that overflowed: inputs and outputs

Your program reads 3 inputs and sets 6 outputs. 2 more inputs drive the simulation or show readings, for you to test with. The names are the same in every maker’s editor.

Tag	Data type	Your code	What it means on the machine
`Start`	`BOOL`	ReadsAn input your code needs	Start pushbutton S1: begin a transfer
`LevelRaw`	`INT`	ReadsAn input your code needs	LT-101 on the receiving tank, 4 to 20 mA input counts: 0 = 4 mA (empty), 27648 = 20 mA (full)
`ValveOpen`	`BOOL`	ReadsAn input your code needs	ZSO-201: discharge valve XV-201 is fully open
`Reset`	`BOOL`	Not neededFor testing or watching; your code can leave it out	Reset pushbutton S2: clear a fault
`FlowOK`	`BOOL`	Not neededFor testing or watching; your code can leave it out	FS-201 flow switch: ON while more than 2 L/s is flowing
`ValveCmd`	`BOOL`	WritesAn output your code sets	Open discharge valve XV-201
`PumpOn`	`BOOL`	WritesAn output your code sets	Run transfer pump P-101
`LevelPV`	`REAL`	WritesAn output your code sets	Receiving tank level for the operator, %
`SignalFault`	`BOOL`	WritesAn output your code sets	Red lamp H1: the level signal has failed
`ValveFault`	`BOOL`	WritesAn output your code sets	Red lamp H2: the discharge valve has failed
`PumpFault`	`BOOL`	WritesAn output your code sets	Red lamp H3: the pump has failed

In Siemens SCL, write a block’s own tags with # in front, as TIA Portal does: #Start. Rockwell and TwinCAT use the name on its own: Start.

Automatic tests

The tank that overflowed: what the tests check

A healthy skid transfers to 80% and stops, with no fault lamp.
When the device fails, its lamp, and only its lamp, comes ON at once.
The transfer stops: the tank does not overflow.
LevelPV holds its last good value while the fault is there.
Start does nothing while the fault is there.

Try it Use the buttons below the model and watch the rig respond.
Test solution Every check runs on your code in one press.
Rewind PRO (needs Pro) If one fails, step back through it one scan at a time to see why.

Stuck on this mission? Ask the members in the Locker Room.

Why it matters

Where you’d see this on site

A level transmitter's cable runs across the plant to the panel. It can be crushed in a tray, corroded in a junction box or pulled out by a contractor.

A broken wire doesn't look like a fault to a program. It looks like an empty tank, and a program that fills an empty tank will fill it for ever.

Where you'd find thisMove water without emptying one tank or flooding the other

At a food factory, a big storage tank refills a smaller wash tank. A pump pushes the water through a valve between them. Before pumping, the engineer checks both levels and that the valve has really opened. If the storage tank empties, the pump can run dry and damage its seal, the part that stops leaks.

Source tankHolds 400 litres when full
Transfer pumpA spinning motor does not prove water is flowing
Discharge valveA switch confirms the valve is openYou are here
Receiving tankHolds 100 litres when full

On a real site: places, parts and safety

Food factory — refill a wash tank
Brewery — move water from storage to a batch tank
Water-treatment works — move water between holding tanks

The model shows spills and a rough idea of pump wear. Do not use its timings as safe limits for real equipment.

A real site needs separate safety devices and a design made for that site.

The sizes, timings and cutaways are teaching choices, not a drawing of any one real installation.

Keep going