Mission 44 · PRO

The pump that howled: PLC fault finding exercise

Find why the pump is wrecking its seals, and stop it happening.

The transfer pump has had two new seals this month. The program has run for years: something on the skid has failed. Find it with the and list, then make the program detect it, light the right fault lamp and protect the pump.

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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: +300 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 pump that howled: what your code must do

Fitter's report: 'P-101 has had its mechanical seal replaced twice this month. Operators say it howls and gets hot, and no water moves.' This program opens valve XV-201, waits two seconds, then starts the pump. 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 and the pump is protected. The pump may only run while the valve is proved open. If the valve has not proved open within 3 seconds of being told to: the lamp for the failed device ON (SignalFault, ValveFault or PumpFault, and only that one), PumpOn and ValveCmd OFF. The lamp stays ON until Reset is pressed, and Start does nothing while it is ON. A healthy skid must still transfer to 80% with no lamp lit, even with a valve that takes two and a half seconds to open.

Transfer: Start opens XV-201, then the pump runs. Both stop at 80%.
Find the fault: One device has failed. The input and output list shows what each signal should read.
Protect the pump: PumpOn only while ValveOpen is ON.
Catch it: No ValveOpen within 3 s of ValveCmd: the failed device's lamp ON, and no other. PumpOn and ValveCmd OFF.
Reset: The lamp holds until Reset. Start does nothing while it is ON.
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: DeadheadSeconds (how long the pump really ran against a shut valve), PumpedLitres, and the real level in TK-202.

Tag table

The pump that howled: inputs and outputs

Your program reads 4 inputs and sets 6 outputs. One more input drives the simulation or shows a reading, 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
`Reset`	`BOOL`	ReadsAn input your code needs	Reset pushbutton S2: clear a fault
`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
`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 pump that howled: what the tests check

A healthy skid transfers to 80% and stops, with no fault lamp.
The pump never runs against a valve that isn't proved open.
A valve not proved open within 3 seconds lights its lamp, and only its lamp, and the transfer stops.
A slow but healthy valve is not a fault.
The lamp holds until Reset, and Start does nothing while it is ON.

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

Actuated valves seize: scale on the ball, a failed air supply, a sheared actuator coupling. The solenoid clicks, the lamp on the screen says open, and the pipe is shut.

A pump running against a closed valve moves no water, so nothing carries its heat away. The water in the casing boils, the seal runs dry and fails, and the bill is a new pump.

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.

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