Cyclone dipleg

The cyclone dipleg is the vertical pipe at the bottom of a cyclone separator that carries separated solids out of the cyclone - either back into a recirculation loop (in CFB boilers and cement preheaters) or into a discharge hopper.

Pluggage problems

Dipleg pluggage is one of the most operationally disruptive failures in any cyclone system. Once the dipleg blocks:

Separated solids back up into the cyclone cone
Re-entrainment into the gas stream rises
Collection efficiency collapses
Process flow imbalance follows immediately

A single plugged dipleg can knock out an entire multi-cyclone tube or a major CFB combustor.

Cleaning

Sonic horns mounted at the dipleg keep separated material flowing. On large industrial cyclones, multiple horns are sometimes distributed along the dipleg length to address pluggage at any elevation.

Function in the cyclone

The dipleg maintains a solids seal between the cyclone and the receiving hopper, loop seal or return leg. Separated particles spiral down the cyclone wall and leave through the cone into the dipleg. If the dipleg seal is lost, gas can flow upward through the discharge path and re-entrain solids that should have been removed.

In CFB boilers and cement preheaters, the dipleg is part of the process circulation loop, not just a drain pipe. Stable solids flow affects heat transfer, pressure balance and process control. A plugged or leaking dipleg can upset the whole cyclone stage.

Pluggage mechanisms

Diplegs plug when material bridges, sinters, gets wet, agglomerates or meets a mechanical restriction. Common causes include sticky ash, alkali salts, oversized debris, refractory fragments, poor cone geometry, air leakage, failed flap valves and hopper overfilling. Once solids back up into the cyclone cone, collection efficiency falls and re-entrainment rises.

Operators watch cyclone pressure drop, temperature split, solids return, hopper level and downstream dust load. In cement preheaters, a dipleg blockage can create a rapid build-up event and require immediate feed reduction.

Acoustic-cleaning context

Sonic horns are useful when pluggage is caused by cohesive powder or early wall build-up. Mounting near the cone, dipleg entry or receiving hopper can keep the material moving without manual rodding. They are less effective against large refractory chunks, welded metal debris or fully sintered plugs. The discharge path still needs correct geometry, sealing and access.

Field checks

Dipleg condition is watched through cyclone pressure drop, solids return temperature, loop-seal behaviour, downstream oxygen or carbon monoxide changes, and visual signs at poke ports or inspection doors. A partly plugged dipleg can still pass some solids, so the symptom may be unstable return flow rather than a complete blockage. In cement preheaters, this can upset meal distribution to lower stages. In CFB boilers, it can disturb solids circulation and combustion balance.

Maintenance inspections focus on build-up at the cyclone cone outlet, flap valves, expansion joints, refractory, air blasters and access ports. The risk is both process and safety related: a sudden release of hot solids can overload downstream equipment or expose workers during manual rodding. Acoustic cleaning can help keep dry deposits loose near the cone and dipleg entry, especially where conventional access is poor. It cannot clear a packed column of hot material once the dipleg has bridged solid.