Overview
ISO 21905:2020 defines requirements and recommendations for the design, materials, modelling, controlling, fabrication, inspection, testing, installation, start‑up and operation of industrial gas turbine exhaust systems with or without a waste heat recovery unit (WHRU). It applies to on‑shore and off‑shore gas turbines used in oil & gas, chemical and process industries, utilities and other intensive energy users. For the standard, the exhaust system covers all items in the turbine exhaust gas stream between the GT exhaust gas collector outlet flange and the atmosphere termination(s).
The standard explicitly does not cover: heat recovery steam generators (HRSG), supplementary/auxiliary fired systems, exhaust gas collectors (exhaust plenums), fire detection/extinguishing, emissions control equipment, or WHRUs of the firetube type.
Key Topics and Requirements
ISO 21905 addresses engineering and practical requirements across the exhaust system lifecycle, including:
- Exhaust system design and configuration - service life, operating envelope, equipment specification and provision for future WHRU addition
- Materials and fabrication - casing and ducting (hot and cold designs), flange bolts, surface preparation, welding and preservation
- WHRU engineering - process design, tube bundle mechanical and thermal design, material selection, corrosion allowance and tube supports
- Modelling and analysis - computational fluid dynamics (CFD) guidance and thermal/structural analytical techniques (informative annexes)
- Vibration and dynamics - TEG (turbine exhaust gas) flow‑induced vibration assessment for ducting and tube bundles
- Auxiliary components - dampers, expansion joints, insulation, noise emission and stacks, steel structures and access
- Controls, instrumentation and commissioning - WHRU control philosophy, signals, wiring and protection, pre‑commissioning and performance testing
- Inspection, testing and maintenance - access, inspection intervals, spare parts and field assembly considerations
- Supporting annexes provide purchaser information, fabrication/welding guidance and recommended data sheets.
Practical Applications
ISO 21905 is used to ensure safe, reliable and fit‑for‑purpose exhaust systems that protect downstream equipment, enable waste heat recovery and support regulatory and contractual requirements. Typical applications include:
- Designing exhaust ducting and WHRUs for power generation and process plants
- Specifying materials, insulation and expansion arrangements for high‑temperature exhaust streams
- Developing procurement documents, supplier responsibilities and factory/site testing protocols
- Performing CFD and structural evaluations to mitigate vibration and thermal stresses
Who should use this standard
- Mechanical, piping and thermal engineers
- WHRU and HRSG designers and OEMs
- Project engineers, procurement specialists and asset owners in oil & gas, petrochemical, utility and industrial power sectors
- Fabricators, weld inspectors, commissioning teams and maintenance planners
Related standards
ISO 21905 complements other international and industry codes covering pressure equipment, welding, emissions and turbine performance. Project teams commonly use ISO 21905 alongside applicable national regulations and industry practices to achieve compliant exhaust and WHRU systems.