Cycle controller and sequencer

A cycle controller (or sequencer) programmes the firing pattern of one or more sonic horns - pulse duration, pulse interval, firing sequence across multiple horns, zone grouping, response to plant DCS signals. It can be either a dedicated standalone hardware unit or a subroutine inside the plant PLC or DCS.

Programmable parameters

Pulse duration - typically 5-15 s per burst
Pulse interval - typically 3-15 minutes between pulses on the same horn
Multi-horn sequencing - fire one horn at a time to manage compressed-air demand
Zone grouping - separate cycles for hopper, plate area and penthouse zones
Response to operator override - manual fire, manual silence, mode switching
Alarm output - to flag a stuck valve, low air pressure or controller fault

Standalone vs PLC-integrated

A standalone cycle controller is simple, cheap and adequate for small installations. Larger multi-horn systems benefit from PLC integration with the plant DCS so horn sequencing can respond to operator commands and process conditions in real time.

What it controls

The sequencer defines which horn fires, for how long, in what order and after what rest interval. A typical setup groups horns by boiler elevation, SCR layer, baghouse compartment or hopper line. The goal is to cover the vessel without firing too many horns at once and without starving the compressed-air header.

Important parameters include pulse duration, pause time, zone order, manual override, alarm reset, low-pressure permissive, maintenance lockout and integration with plant operating modes. A horn that is mechanically healthy can still perform poorly if the sequence is too short, too infrequent or disabled by an unnoticed permissive.

Integration with plant controls

Small systems may use a standalone timer panel. Larger plants often integrate horn control into a PLC or DCS so firing can respond to load, differential pressure, opacity, hopper level, SCR temperature or maintenance states. The controller should report horn status, last-fire time, valve fault, low pressure and local isolator position to the operator.

Good sequencing also avoids process conflicts. Horns should not fire into opened access doors, isolated compartments with personnel present, or equipment in a purge state where sound or pressure pulses are undesirable. These are control-design issues, not afterthoughts.

Maintenance and optimisation

The best sequence is adjusted after commissioning. Operators compare pressure-drop trends, deposit inspections, air consumption and noise constraints. If one zone fouls faster, it may need more frequent firing. If the air header dips, the sequence should be staggered or the receiver capacity improved. The sequencer is therefore a tuning tool for acoustic-cleaning performance, not merely an on-off timer.

Field checks

A sequencer is verified by observing the equipment it controls, not only by checking the display. For sonic horns, technicians confirm horn order, blast duration, rest period, solenoid response, air pressure recovery and permissive status. For baghouse pulse systems, they verify row order, pulse width, differential-pressure trigger points and offline-cleaning logic. A wrong sequence can waste compressed air, clean the wrong zone, or create pressure waves during an unsafe access state.

Maintenance checks include output relays, valve coils, cable terminations, enclosure sealing, backup settings and time synchronisation with plant controls. In a DCS or PLC-integrated system, the local controller should fail to a known state on loss of air, power or communications. Optimisation is practical: operators adjust intervals around deposit growth, fuel changes, load and pressure-drop trends. The controller is therefore part of the cleaning strategy, not just a timer in a box.