---
title: "Third-stage separator (TSS)"
description: "A third-stage separator (TSS) is the high-efficiency cyclone vessel installed downstream of the FCC regenerator to recover very fine catalyst fines that escape the regenerator's primary and secondary cyclones. The TSS protects downstream equipment — particularly the power-recovery expander — from catalyst erosion."
canonical_url: "https://sylio.co/glossary/third-stage-separator"
last_updated: "2026-06-28T02:29:40.988Z"
---

A **third-stage separator (TSS)** is the high-efficiency cyclone vessel installed downstream of the [FCC regenerator](/glossary/fcc-regenerator) to recover very fine catalyst fines that escape the regenerator's primary and secondary cyclones. The TSS protects downstream equipment — particularly the power-recovery expander — from catalyst erosion.

## Construction

A TSS typically contains 50–150 small high-efficiency [cyclone separators](/glossary/cyclone-separator) in parallel inside a common vessel. Each cyclone has its own [dipleg](/glossary/cyclone-dipleg) that drains into a common collection hopper.

## Failure modes

- **Individual cyclone pluggage** — fines bridge in a single dipleg, that cyclone bypasses gas to neighbours
- **Common hopper bridging** — the collection hopper plugs, backing up all the diplegs above
- **Cyclone erosion** — the high catalyst-fines velocity wears cyclone walls

## Sonic-horn duty

[Sonic horns](/glossary/sonic-horn) on the TSS common hopper prevent fines bridging. This protects the entire TSS-and-expander train from the cascading consequences of single-point hopper failures.

## Related terms

- [FCC regenerator](/glossary/fcc-regenerator)
- [Fluid catalytic cracking (FCC)](/glossary/fluid-catalytic-cracking)
- [Cyclone separator](/glossary/cyclone-separator)