Valve Won't Close
// Systematic troubleshooting — work through each step before moving on
Configuration assumption: This guide assumes a de-energize-to-close configuration with a 120 VAC solenoid and 24 V DC feedback. Verify your equipment's actual configuration before applying any step. Training aid only — follow your site's LOTO, permit, and SOP.
1
Is the PLC output (DO) actually turning OFF?
// Check HMI or force monitor — is the output bit 0?
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NO — Output still ON
PLC logic problem. The output is not de-energizing. Check: interlock conditions not met, sequence step not advancing, manual force left on from previous troubleshooting. Fix the logic/interlock first.
YES — Output is OFF
PLC is correctly commanding close. Problem is downstream. Move to Step 2.
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2
Is the solenoid de-energizing? (LED off?)
// Check solenoid LED — should be OFF when valve commanded closed
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NO — LED still ON
Voltage backfeed. Measure voltage at solenoid terminals — if 120VAC present when PLC output is off, there is a backfeed path. Check for stuck relay contact, parallel output, or shorted wiring.
YES — LED is OFF
Solenoid is correctly de-energized. Air should be exhausting. Move to Step 3.
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3
Is air exhausting from the actuator?
// Listen for exhaust at solenoid or actuator — feel for air movement
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NO — No exhaust air
Solenoid spool stuck. Air not exhausting means spool is not shifting inside the solenoid. Could be product buildup on spool, corroded spool, or damaged O-rings. Try manual override first — if that works, solenoid body needs cleaning or replacement.
YES — Air is exhausting
Pneumatics working correctly. Actuator is losing pressure. Move to Step 4.
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4
Does the valve physically move when air exhausts?
// Watch the stem or disc — does it move toward closed position?
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NO — Valve doesn't move
Mechanical problem. Spring may be broken or weak. Stem packing may be too tight. Crystallized product may be jamming the disc or stem. Try manual override — if it won't close manually either, it's a mechanical seizure. If manual closes it, spring is the problem.
YES — Valve moves to closed
Valve physically closed correctly. Problem is the closed confirmation signal (ZSC). Move to Step 5.
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5
Is ZSC (closed confirmation) reaching the PLC?
// Measure voltage at PLC DI input for ZSC — should be 24V when valve is closed
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NO — ZSC not reaching PLC (0V)
Westlock/limit switch problem. Valve closed but PLC doesn't know it. Check: Westlock cam slipped off stem, micro-switch inside Westlock failed, broken wire between Westlock and PLC DI card, terminal screw loose. Manually push cam — does ZSC change state?
YES — ZSC is ON at PLC
✓ Problem resolved. If ZSC is now showing correctly and the sequence can advance — the issue was intermittent. Document what you found and monitor. If sequence still not advancing, check PLC logic for additional interlocks.
// Possible causes to investigate (in no particular order):
Westlock Cam Slipped
Cam rotates on the valve stem over time. Valve moves but cam doesn't reach the switch. Fix: Loosen set screw, rotate cam to engage switch at closed position, retighten.
Spring Worn / Weak
Spring loses tension over time, especially in high-cycle applications. Valve partially closes but doesn't reach full closed position. Fix: Replace actuator spring, check spring force spec.
Product Fouling on Stem
Product crystallizes or builds up in the stem packing area, creating mechanical resistance. Valve opens but spring can't overcome the friction. Fix: Steam clean stem area, replace packing, increase cleaning frequency.
Spool Stuck in Solenoid
Internal spool doesn't shift when de-energized. Air stays in actuator. Manual override button on solenoid confirms if mechanical. Fix: Clean or replace solenoid valve body.
Broken Wire / Loose Terminal
ZSC wire broken or terminal screw loose. Valve physically closed, ZSC switch working, but signal not reaching PLC. Fix: Tug test all terminals on ZSC circuit, repair broken wire.
These are common possibilities, not a ranked statistical distribution. The actual likelihood depends on your specific equipment configuration and operating history.
// FAST FIELD TEST — DO THIS FIRST
| Test | How | Normal Result | If Abnormal |
|---|---|---|---|
| Manual override | Push button on solenoid or actuator | Valve moves to closed | Mechanical seizure — stem/disc issue |
| Solenoid LED | Visual — look at LED when PLC output is OFF | LED OFF | Backfeed voltage — check wiring |
| ZSC voltage | Measure VDC at PLC DI terminal for ZSC | 24VDC when closed | 0V = cam, switch, or wire problem |
| Westlock cam | Manually push cam toward switch | ZSC changes state in PLC | Switch failed — replace Westlock |
| Air exhaust | Listen/feel at solenoid exhaust port | Air exhausts when de-energized | Spool stuck — clean/replace solenoid |