1. Overview of the Legacy Siemens FTC 281 / EC 31 / 21.265 Chute Anti-Clogging System
The legacy chute anti-clogging assembly reported on jetty ship unloaders comprises three physically separate items:
- FTC 281 – a level controller that energises the probe circuit, evaluates the returned signal, and drives an output relay (the controller itself).
- 21.265 – a metal rod probe suspended vertically inside the transfer chute, with the lower tip positioned at a height corresponding to the acceptable head of bulk material.
- EC 31 – the electronic module mounted in the field, which conditions the probe signal before the line-run cable carries it to the FTC 281 controller in the MCC or control room.
Functionally, the chain is short and unambiguous: EC 31 powers the probe, the probe sees the bulk material, the controller interprets the resulting electrical change, and the controller's output relay de-energises the conveyor start permissive. When the controller can no longer be sourced as a spare, the whole string becomes a single point of failure. This reference walks through a field-proven replacement strategy that retains the existing chute mounting point, cable conduit, and relay interface wherever possible.
2. Operating Principle: Conductance-Based Blocked-Chute Detection
The original system is a conductance / earth-fault arrangement. The probe is electrically isolated from the chute structure, and the controller injects a low-voltage AC excitation between the probe tip and the steelwork of the chute (the local earth). While the chute is open and air surrounds the probe tip, the path between probe and earth is an open circuit – the controller sees a high impedance.
When bulk material (coal, iron ore, limestone, clinker, grain, etc.) accumulates high enough on the probe, the bulk itself becomes a conductive path to the chute wall. The controller senses the impedance collapse, the earth-fault detection circuit latches, and the output relay drops. That drop is wired into the conveyor starter as a permissive, so the conveyor is taken out of service the instant a build-up reaches the probe tip.
Three engineering consequences follow directly from this principle:
- The bulk material must be appreciably conductive (or at least damp/ionic). Dry, low-dielectric powders and very dry plastic pellets are poor candidates for any conductance-based system, legacy or replacement.
- The probe must be isolated from the chute steelwork with a non-conductive gland. Any moisture path down the mounting thread will mimic a real blockage and false-trip the conveyor.
- The controller's sensitivity must be set well above the worst-case wet-bulk resistance and well below the leakage resistance of the gland. A 2:1 margin at each end is field-standard.
3. Why the Legacy System Is Now a Liability
End-of-life for a Siemens industrial sensor is rarely a single date – it is a series of cascading obsolescence events:
| Obsolescence Stage | Indicator in the FTC 281 Chain | Impact on Maintenance |
|---|---|---|
| Active product | Spare stocked at OEM | 48 h replacement |
| Phase-out announced | Last-time-buy notice issued | Stock-pile or plan migration |
| Spare exhausted | Distributors out, OEM returns "no longer available" | 3PL brokers at 5–10× list price |
| Service discontinued | Repair returns rejected, manuals archived only | Forced system replacement |
Once the FTC 281 controller and the EC 31 field electronics reach stage 3, the cost of a single ship-unloader stoppage – demurrage, ship-shore-side renegotiation, manual chute rodding – dwarfs the cost of a complete modern replacement. Migrating on a planned shutdown is cheaper than migrating under duress.
4. Replacement Technology Options
Four modern sensing principles are credible substitutes for the FTC 281 / EC 31 / 21.265 chain. Each is shown against the constraints imposed by the original installation.
| Technology | Representative Siemens Model | Mounts in Existing 21.265 Port? | Bulk Conductivity Required? | Output | Best-Fit Application |
|---|---|---|---|---|---|
| Capacitance (RF admittance) | Pointek CLS300 | Yes, with new gland | Any, including low-dielectric dry bulk | Relay / 8/16 mA | Direct electrical substitute; chute blockage, high/low level |
| Ultrasonic, non-contact | Pointek ULS200 | No – requires top-of-chute window | None (acoustic reflection) | Relay, 2-wire | Choked chute detection through air column |
| Capacitance, compact | Pointek CLS100 / CLS200 | Yes, smaller footprint | Same as CLS300 | Relay, 2-wire | Tighter chutes, lower temperature |
| Rotating paddle (fallback) | SITRANS LVL100 / Pointek PRL200 | Mechanical adapter | Any dry bulk, no dust ignition sensitivity to conductivity | Microswitch / relay | Last-resort, very dusty or high-vibration chutes |
For an in-kind replacement that uses the same probe hole, the same cable, and the same relay interface, the Siemens Pointek CLS300 is the first choice. For a complete redesign that removes the probe intrusion into the material stream, the Pointek ULS200 ultrasonic is the field-preferred option for blocked-chute detection on conveyors and transfer points.
5. Selecting the Siemens Pointek CLS300 as Drop-In
The Pointek CLS300 is a two- or four-wire RF capacitance switch with integrated electronics in the housing – effectively an EC 31 and FTC 281 collapsed into a single IP65/68 device. It is the closest functional equivalent to the legacy chain.
| Parameter | CLS300 Specification | Notes vs. FTC 281 / EC 31 |
|---|---|---|
| Supply | 12–33 V DC (2-wire) or 100–230 V AC (4-wire) | Replaces EC 31 loop supply |
| Output | Solid-state relay, 30 V DC / 100 mA (2-wire) or 5 A DPDT relay (4-wire) | Drop-in to FTC 281 relay contact wiring |
| Process temperature | −40 to +400 °C, version dependent | Exceeds original FTC 281 range in most variants |
| Process pressure | Up to 35 bar (probe dependent) | Suits pressurised transfer chutes |
| Probe length | Up to 5.5 m rigid or 35 m cable | Matches 21.265 rod length |
| Sensitivity | 0.5 pF minimum, 1,000 pF maximum, trimmable | Set once at commissioning; no periodic recal needed |
| Approvals | ATEX, IECEx, FM, CSA dust/ignition-proof versions | Mandatory if the chute carries combustible bulk (coal, grain) |
Because the CLS300 carries its own smart electronics, the EC 31 field module is removed entirely and the cable from the chute gland to the MCC is re-terminated directly on the CLS300 terminals. The FTC 281 controller in the MCC is then either kept as a passive terminal block (if wiring layout benefits) or removed and replaced with a small marshalling strip – its 24 V DC supply and relay output are regenerated locally at the new probe.
6. Selecting the Siemens Pointek ULS200 for Non-Contact Replacement
Where chute geometry permits a downward-looking installation, the Pointek ULS200 ultrasonic switch removes the probe from the material stream entirely. There is no contact, no abrasion, no false trip from a wet probe gland, and no reliance on bulk conductivity.
| Parameter | ULS200 Specification | Field Implication |
|---|---|---|
| Detection range | 0.25 m to 3.0 m, configurable window | Set the lower threshold just above the free-flowing material surface |
| Supply | 18–30 V DC, 2-wire loop-powered | Reuses the 24 V DC rail that fed EC 31 |
| Output | Solid-state switch, 100 mA | Wires directly into the same conveyor permissive |
| Beam pattern | ~10° conical, 80 kHz | Mount clear of falling curtains; aim at a clean chute wall opposite the feed |
| Dust tolerance | Compensation built-in; loses signal in very dense dust cloud | Add air purge on the ULS200 face if the bulk is ultra-fine |
| Temperature | −20 to +70 °C process face | Use the high-temperature version with standoff if the chute surface exceeds 70 °C |
The ULS200 is the modern industry default for blocked-chute detection on conveyors and transfer points where the operator can install a 50 mm–100 mm NPT boss on the chute roof. It is also the safer choice for ignition-risk dusts because no energised conductive surface enters the dust cloud.
7. Conductance-to-Capacitance Signal Migration
The biggest engineering error when migrating from FTC 281 to a modern switch is assuming the two systems are signal-compatible. They are not. The original FTC 281 monitors the AC resistance to earth of the probe tip; the CLS300 monitors the change in capacitance between the probe and the surrounding environment. The wiring practice is therefore different.
Three rules cover most field wiring conversions:
- Earth the probe shield at the panel end only. A CLS300 probe shield that is bonded to the chute steelwork at both ends injects 50/60 Hz hum into the RF measurement, which looks like a partial blockage.
- Remove the FTC 281 earth-fault sensing toroid if it is in-line. It presents a 0 V reference and a CLS300 is not earth-referenced; leave the toroid in place and the CLS300 will see a degraded signal-to-noise ratio.
- Verify fail-safe direction in the PLC. The original FTC 281 relay most likely de-energises on a blocked chute (relay logic convention). Configure the new switch and PLC input so that a loss of 24 V supply to the probe produces the same permissive drop. Failure to do this converts a fail-safe system into a fail-to-stop system.
8. Wiring and Relay Integration
The 24 V DC control wiring that ran from EC 31 to FTC 281 in the MCC was almost certainly 2-core shielded (signal) + 2-core (24 V DC). That cable can be reused; the change is in what each core connects to.
| Conductor | Legacy Termination at EC 31 / FTC 281 | CLS300 Termination | ULS200 Termination |
|---|---|---|---|
| Core 1 (red, +24 V) | FTC 281 L+ | Terminal 1 (+) | Terminal 1 (+) |
| Core 2 (black, 0 V) | FTC 281 L− | Terminal 2 (−) | Terminal 2 (−) |
| Core 3 (white, NO) | FTC 281 NO contact | Terminal 3 (load+) | Terminal 3 (load+) |
| Core 4 (blue, COM) | FTC 281 COM contact | Terminal 4 (load−) | Terminal 4 (load−) |
| Shield | Earth bar at MCC only | Earth bar at MCC only | Earth bar at MCC only |
The PLC input card is fed from the new switch's load terminals. A typical SIMATIC S7-1200 / S7-1500 wiring uses a 24 V DC sourcing input; the CLS300 / ULS200 load terminal sinks to 0 V on alarm. The same wiring is valid for an Allen-Bradley CompactLogix or ControlLogix 24 V DC input module.
9. Mechanical Installation Considerations
The 21.265 probe was a rigid rod mounted through a gland on the chute wall. Reuse is possible but not always wise. Inspect the gland for:
- Thread galling – an old steel gland that has been in service for 15+ years may not seal cleanly to a new stainless CLS300 probe.
- Conductive creep – carbonaceous dust can track down the original insulator. Replace the gland with a new PTFE-jacketed unit.
- Chute wall thinning – the area around the gland may be eroded. A localised doubler plate is cheaper than chasing a hole at height.
For an ultrasonic ULS200, the new boss is welded on the top of the chute, ideally at a 30° angle away from the falling material curtain. The acoustic window must see a free-air path to the bulk surface, not a wall impact zone. A test with a portable laser distance meter during normal flow confirms the aiming.
10. Commissioning and Calibration
- Isolate the conveyor and lock out the starter. A nuisance trip of an in-service conveyor during commissioning will cost at least a shift of ship-side delays.
- Power the new switch and wait the manufacturer's warm-up period (typ. 5–15 s for CLS300, 30 s for ULS200 acoustic settling).
- CLS300: set the sensitivity potentiometer. With the probe exposed to air, turn sensitivity up until the output switches, then back off ¼ turn. Confirm with grounded plate that it still trips with margin.
- ULS200: configure the detection window. Set the lower threshold 100 mm above the maximum expected free-flowing material surface; set the upper threshold above the chute roof. Switch state should toggle when material crosses the lower threshold.
- Re-wire the conveyor permissive. Restore the 24 V DC to the PLC input and run a no-load conveyor start to confirm the permissives chain closes. The blocked-chute alarm should clear when the probe sees air again.
- Document the settings on the controller faceplate: sensitivity value, dip-switch positions, fail-safe mode, and the PLC tag number that the alarm drives.
11. Verification Tests
After commissioning, perform three independent verifications before returning the conveyor to production service:
| Test | Procedure | Pass Criterion |
|---|---|---|
| Forced alarm | Apply grounded metal to probe (CLS300) or obstruct acoustic path (ULS200) | PLC alarm registers within 500 ms; conveyor stop permissive drops |
| Loss-of-supply | Remove 24 V DC at the new switch terminals | PLC alarm registers; conveyor cannot start |
| Real blockage | Briefly dam material upstream while conveyor is running at low feed | System trips before the material reaches the discharge; no spill |
12. Troubleshooting Matrix
| Symptom | Likely Cause | Action |
|---|---|---|
| False trip with empty chute | Wet probe gland; sensitivity too high (CLS300); dust cloud (ULS200) | Replace gland PTFE; reduce sensitivity one step; add air purge |
| No trip on real blockage | Sensitivity too low; fail-safe inverted in PLC; shield earthed at both ends | Raise sensitivity; verify PLC tag inversion; bond shield at MCC only |
| Intermittent trip on conveyor start | Vibration coupling into probe; dust re-entering acoustic path | Stiffen probe mounting bracket; aim ULS200 away from impact zone |
| Switch never powers up | Reversed polarity; loop load too high; open circuit in field cable | Check polarity with multimeter; verify loop resistance; ring out cable |
| Switch powers but output always on | PLC input wiring reversed; load terminal shorted; solid-state output damaged | Swap NO/NC at PLC; measure load terminal voltage; replace device |
13. Frequently Asked Questions
What does the Siemens FTC 281 controller do in a chute anti-clogging system?
The FTC 281 is the controller that energises the suspended probe, monitors the change in electrical resistance or capacitance between the probe and the chute structure, and trips an output relay the moment material touches the probe tip. That relay is wired into the conveyor starter as a permissive.
What is the difference between a Pointek CLS300 and a Pointek ULS200 for chute anti-clogging?
The CLS300 is a contact RF capacitance switch installed through the chute wall – a direct electrical substitute for the FTC 281 chain. The ULS200 is a non-contact ultrasonic switch mounted on the chute roof; it does not enter the material stream and is preferred for very dusty or combustible bulk solids.
Can the existing 21.265 probe and EC 31 field electronics be reused with a new controller?
No, the EC 31 and FTC 281 are tuned to a specific conductance operating window. A modern replacement integrates both functions into a single device (CLS300) or uses a different sensing principle (ULS200). Spare the EC 31 enclosure only if it is repurposed as a cable junction box.
Do I need to reprogram the PLC when migrating from FTC 281 to CLS300 / ULS200?
Usually not. The new switch replicates the same fail-safe relay behaviour – output drops on alarm – so the existing alarm tag, conveyor stop permissive, and HMI indication all carry over unchanged. Verify the tag direction and any inversion logic in the PLC before energising the conveyor.
How quickly can the FTC 281 / EC 31 system be replaced with a modern Siemens equivalent?
On a typical ship-unloader transfer chute, mechanical work (gland inspection, weld-boss installation for the ULS200) takes one shift per chute. Electrical termination, loop check, and commissioning tests take a second shift. Plan for a 16–24 hour window per chute under a planned conveyor outage.