Quick answer
A crusher‑feed hopper level switch that remains in the high‑level state while the hopper is empty or only partially filled is most often caused by material buildup on the sensing element, an unsuitable mounting location, mechanical sticking of the moving part, an incorrect sensitivity setting, or a wiring/PLC input fault. The first step is to verify the switch condition locally and compare its indication with the actual material level before any interlock is bypassed or the sensor sensitivity is altered.
Problem / job
The crusher feed line stops, an alarm is generated, or the interlock prevents crusher start because the level switch continues to signal a high level even after the material has been discharged from the hopper. The objective is to determine whether the signal is a true indication of retained material or a false high caused by sensor impairment.
Evidence
Collect the following data points before any corrective action:
- Visual confirmation of the actual hopper level from a safe observation point.
- Local indicator (LED, flag, or audible tone) on the switch versus the PLC/DCS input status.
- Physical condition of the sensing face, paddle, probe, or diaphragm – note any limestone, clay, or wet feed residue.
- Freedom of movement for rotary‑paddle or diaphragm devices.
- Current sensitivity or calibration setting and the exact mounting location relative to the hopper geometry.
- Continuity and insulation of wiring, connector integrity, and PLC input module health.
- Operational pattern – does the fault appear only with wet or sticky feed, after a prolonged run, or immediately after a shutdown?
Decision
Separate the possible causes into three broad categories: (1) sensor contamination or coating, (2) mechanical or mounting issues, and (3) electrical or control‑system faults. The diagnostic path proceeds from the simplest, most observable condition (visible buildup) to the more complex (PLC input diagnostics).
Practical 7‑step workflow
- Secure a safe observation point. Before any physical contact, confirm that the crusher is fully stopped, power to the feed system is isolated, and a lock‑out/tag‑out (LOTO) procedure is applied in accordance with plant safety policy. Use a designated viewing window, camera, or remote sensor to verify that the hopper is truly empty or at the expected low level.
- Compare local and remote indications. Observe the switch’s built‑in LED, flag, or audible alarm. Record whether it shows “high” or “low.” Simultaneously, read the PLC/DCS input status on the control panel. A mismatch (local low, PLC high) points to wiring or input‑module problems; a match (both high) suggests a sensor‑related issue.
- Isolate the switch and perform a visual inspection. With the LOTO in place, disconnect the power supply to the switch. Remove any protective covers and inspect the sensing element for adhered material. Pay special attention to the underside of rotary paddles, the tip of probe‑type switches, and the diaphragm surface of pressure‑type devices. Document any wet, sticky, or hardened buildup.
- Check mechanical freedom. Manually move the paddle, probe, or diaphragm through its full travel range. Resistance, binding, or a “stuck” feeling indicates mechanical obstruction, possibly caused by accumulated material or a seized bearing. If movement is impeded, clean the area and re‑lubricate any moving parts according to the OEM recommendation.
- Evaluate sensor placement and sensitivity. Review the installation drawing and compare the actual mounting location with the manufacturer’s recommended “active zone.” Sensors placed too deep, too shallow, or directly in a dead‑zone where material can cling after discharge are prone to false highs. Verify the current sensitivity setting; if the switch is set to a very low threshold, even a thin film of material can keep it in the high state.
- Inspect wiring and PLC input. If the local indicator cleared but the PLC still reads high, perform a continuity test on the cable, inspect connectors for corrosion, and verify that the input module is powered and correctly configured. Replace suspect wiring or swap the input channel with a known good one to isolate the fault.
- Re‑commission and verify. After cleaning, mechanical adjustment, or wiring repair, restore power and remove the LOTO only after confirming that the area is clear. Conduct at least three full fill‑and‑empty cycles, recording the switch state at each transition. Both the local indicator and PLC input must change synchronously and remain stable for the duration of each cycle. Document the results before returning the crusher to production.
Diagnostic matrix
| Evidence | Likely direction | Next check |
|---|---|---|
| Sensor face coated with wet or hardened material | False high caused by buildup | Clean sensor, verify location, re‑test |
| Local LED shows low but PLC input stays high | Electrical path fault (cable, connector, input module) | Perform continuity test, inspect connectors, swap input channel |
| Paddle or probe cannot be moved manually | Mechanical sticking or seized bearing | Disassemble if required, clean, lubricate, re‑assemble |
| Fault appears only when feed is wet or contains fine clay | Material adhesion or unsuitable sensor technology | Review sensor type, consider a non‑contact ultrasonic or radar level device, adjust mounting |
| Sensor mounted in a dead‑zone where material can pool after discharge | Improper placement leading to residual material triggering the switch | Relocate sensor to a verified active zone, verify with test runs |
| Sensitivity set to an overly low threshold | Switch trips on minimal film of material | Adjust sensitivity per OEM guidance, re‑validate |
Verification and acceptance criteria
Acceptance of the repaired level switch is achieved when all of the following measurable conditions are satisfied:
- During at least three consecutive fill‑and‑empty cycles, the local indicator and the PLC/DCS input change state in unison at the defined high‑level and low‑level points.
- After each cycle, the switch remains stable for a minimum dwell time specified by the OEM (verify the OEM or site‑specific setting) before any subsequent transition.
- No visible material remains on the sensing surface after the cleaning step, and the mechanical travel is free of obstruction.
- All wiring continuity tests read within the resistance limits recommended by the cable manufacturer; insulation resistance meets the plant’s standard for low‑voltage instrumentation.
- The interlock that prevents crusher start when the hopper is high operates correctly – i.e., the crusher will not start while the switch is in the high state, and will start promptly once the switch returns low.
- All safety devices (LOTO tags, lock‑out points, isolation switches) are restored to their normal operational status, and a written sign‑off is completed by the responsible maintenance technician and the control‑system engineer.
Common mistakes and repeat‑failure traps
- Bypassing the interlock to keep production running. Temporarily disabling the high‑level alarm removes a critical safety barrier and can lead to over‑filling, spillage, or equipment damage.
- Cleaning without proper isolation. Removing material from the sensor while power is present exposes personnel to moving parts and stored‑energy hazards. Always apply LOTO and verify zero voltage before reaching into the hopper.
- Re‑installing the switch in the same dead‑zone. If the original mounting location promotes material buildup, verify the manufacturer’s mounting clearance, process geometry and switching duty. Consider relocation only after engineering review confirms a representative high-level point and preserves the required protective function.
- Adjusting sensitivity without consulting OEM limits. Setting the switch too sensitive can cause nuisance trips; setting it too insensitive can miss genuine high‑level conditions. Use the OEM‑specified range as the baseline.
- Neglecting wiring inspection when the local indicator matches the PLC. Even when the local indication and PLC agree, intermittent wiring faults can remain. Inspect terminals and cable condition under approved isolation; perform only the electrical tests permitted by the switch and input-module manufacturers, with connected electronics disconnected where required.
- Using the wrong type of level switch for the feed material. Sensor suitability depends on bulk density, buildup, dust, vibration, temperature and mounting geometry. Compare rotary-paddle, capacitance and non-contact options against the actual material and manufacturer specifications; do not assume ultrasonic measurement will be reliable in heavy dust.
- Skipping the verification cycles after repair. Without documented repeatability, the root cause may not be fully eliminated, leading to recurring alarms.
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