Steam Trap Selection — Float, Thermodynamic, Thermostatic, Bucket

Selection guide compiled by cBallast. 4-page PDF; the full content is on this page.

Compiled by

cBallast

Subject

Steam trap

Document type

Selection guide

Reference

TI-P002-XX / Spirax Sarco Steam Utilisation

Issued

2026-07-14

Pages

Format

Web page and PDF

Steam trap selection reference. Covers the four dominant working principles: float (mechanical, density-based), thermodynamic (velocity / disc trap), thermostatic (temperature-actuated, balanced-pressure or bimetallic), and inverted-bucket (mechanical). Includes selection matrix by application (steam main drip, process heater, tracer line, condensate return), sizing procedure (condensate load + differential pressure), safe operating discharge (SOD) capacity, and life-cycle failure modes.

The four working principles

Principle	What actuates it	Best for	Not for
Float (ball float steam trap, FT)	Density difference — a hollow ball floats on rising condensate and lifts a valve seat off its port	Process heat exchangers with variable load	Systems with heavy freeze risk (mechanism can freeze solid)
Thermodynamic (TD, disc trap)	Velocity difference — flash-steam under a stainless disc snaps it shut	Steam mains, superheated steam, small size, freeze-tolerant	Low differential pressure (< 0.3 bar)
Thermostatic (balanced-pressure or bimetallic)	Temperature difference — bellows or bimetal opens when condensate cools below sat temp	Radiator air vent, tracer lines, low condensate loads	Applications requiring immediate condensate discharge
Inverted-bucket (IB)	Buoyancy — bucket sinks when full of condensate, pulls valve open	Corrosive systems, dirty steam, freeze-tolerant	Low differential, superheated steam (loses prime)

Selection matrix

Application	First choice	Second choice
Steam main drip pocket	Thermodynamic (TD)	Inverted bucket
Process heat exchanger (variable load)	Float (FT)	Inverted bucket
Steam tracer line	Balanced-pressure thermostatic	Bimetallic
Radiator or convector heating	Balanced-pressure thermostatic	Float
Autoclave / sterilizer	Bimetallic	Float
Turbine drip	Thermodynamic	Inverted bucket
Superheated steam separator	Thermodynamic	Bimetallic
Condensate lift (against back-pressure)	Float with air vent (F&T)	Inverted bucket

Sizing procedure

Calculate the condensate load — for a heater the peak steady-state load is Q = m × c_p × ΔT / h_fg where m is the process fluid mass flow, ΔT the heat-up range, h_fg the latent heat at trap pressure. For start-up loading multiply the steady-state by 2 to 3.
Determine the differential pressure — supply pressure at the trap inlet minus the back-pressure in the condensate return line.
Look up SOD capacity — from the trap manufacturer's capacity curve at the actual differential pressure. Choose a trap whose Safe Operating Discharge exceeds the maximum condensate load with a safety factor of 1.5-2.
Verify pipe size compatibility — the trap inlet size must not throttle the condensate; typically pipe DN matches trap DN.
Check freeze / hammer conditions — outdoor traps need thermodynamic or bucket types with freeze-jacket or the trap installed below grade.

Failure modes

Failure	Symptom	Cause
Blown-through (leaking live steam)	Trap discharge continuously venting live steam, condensate return abnormally hot	Worn seat, stuck-open valve, debris on seat
Waterlogged (cold condensate build-up)	Upstream equipment cold, low heat transfer, condensate visible in sight glass	Failed-closed valve, plugged strainer, wrong-size trap
Cycling	Rapid open-close cycling of TD or IB trap	Undersized trap, high differential pressure oscillation
Water hammer	Loud banging in condensate return line	Steam pockets in condensate line, wrong lift geometry

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Compiled by cBallast for identification and cross-reference. Part numbers, designations and trademarks belong to their respective manufacturers. Confirm ratings and dimensions against the manufacturer's current documentation and the nameplate of the installed equipment before ordering or installation.