Overview
EN 26704:1991 (identical to ISO 6704:1982) provides a simple, functional classification of automatic steam traps based on the mode of actuation of their obturation device. The standard’s scope is limited to grouping the main types-without prescribing detailed design, construction or performance requirements-and it is intended to standardize terminology and classification across industry and standards bodies.
Key topics
- Purpose: establish a clear, technology‑neutral steam trap classification according to actuation method.
- Main categories:
- Mechanical traps - actuated by condensate level (e.g., closed float, open-top/bucket float, inverted float).
- Thermostatic traps - actuated by condensate temperature (e.g., vapour‑pressure traps, bimetallic/thermoelastic traps, liquid/solid expansion traps).
- Thermodynamic traps - actuated by fluid dynamics and pressure differences (e.g., obturator traps, impulse/piston traps, labyrinth orifice traps).
- Key concepts and terms: automatic steam trap, obturator, seat, pressure chamber, orifice, fluid flow direction, condensate behaviour.
- References to related ISO standards: ISO 6552 (terminology), ISO 6553 (marking), ISO 6554 (flanged trap dimensions).
- Diagrams: the standard uses schematic sketches to illustrate each trap type and its operating principle.
Applications
EN 26704:1991 is primarily a classification and reference document that supports practical use cases such as:
- Specifying trap type in steam system design and piping & instrumentation diagrams (P&IDs).
- Communicating requirements between engineers, manufacturers and procurement teams when selecting traps for steam distribution, process heating, HVAC and industrial steam systems.
- Training and documentation for maintenance and reliability teams to recognize trap families (mechanical, thermostatic, thermodynamic) and their typical operating behavior.
- Harmonizing national and international standards references used by standards committees, certification bodies and technical authors.
Who uses this standard
- Steam system designers and process engineers
- Valve and steam trap manufacturers and product managers
- Maintenance, reliability and facility managers
- Technical writers, specifiers and standards committees
Related standards
- ISO 6704:1982 - Automatic steam traps - Classification (original international text)
- ISO 6552, ISO 6553, ISO 6554 - referenced for terminology, marking and dimensional guidance
- National adoptions (example: SIST EN 26704:2000) reflect CEN/ISO transposition
This classification-focused standard is a practical reference for correctly identifying and specifying automatic steam traps by actuation principle, improving clarity across procurement, design and maintenance workflows in steam-using industries.
