How to Select the Right Steam Trap for Your Application

A steam trap is one of the most common — and most often misunderstood — items in an industrial steam system. Get the selection right and you have efficient condensate drainage, dry steam at your process, and minimal energy loss.

This guide is written for plant engineers and maintenance teams working with steam systems in India. We cover what a steam trap actually does, the three principal operating types, how to select the right one for a given application, and the mistakes that cause premature failure or system inefficiency.

What a Steam Trap Does — and Why It Matters

Steam traps remove condensate and non-condensable gases from a steam system without allowing live steam to pass through. If condensate is not removed promptly, heat transfer drops, water hammer risk increases, and corrosion accelerates.

The Three Principal Steam Trap Types

TYPE 1

Ball Float (Mechanical) Trap

A hollow stainless steel float senses condensate level. As condensate accumulates the float rises and opens the main valve. When steam reaches the trap the float falls and the valve closes. A separate thermostatic air vent handles non-condensable gases on startup.

Best for: Shell-and-tube heat exchangers, jacketed vessels, applications with large variable or continuous condensate loads.

Not well-suited to superheated steam service. Relatively large and heavy.

TYPE 2

Thermodynamic Disc (TD) Trap

Uses kinetic energy and pressure difference between live steam and condensate to operate a disc valve. Condensate passes easily. Hot flash steam under the disc builds pressure, snaps the disc shut, holds it until the steam condenses, then the cycle repeats.

Best for: Steam mains drip legs, turbine drain pots, distribution headers. Compact, robust, wide pressure range.

Key limit: backpressure must not exceed 50% of inlet absolute pressure.

TYPE 3

Bimetallic (Temperature-Actuated) Trap

Two bonded metals with different thermal expansion coefficients bend with temperature. At steam temperature the element closes the valve. As condensate cools below steam temperature the element relaxes and the valve opens — discharging subcooled condensate.

Best for: Steam tracer lines, instrument air lines, outdoor steam trace applications.

Retains condensate longer — useful for tracing, not ideal for process heat exchangers.

Steam Trap Selection Criteria

Selecting the right trap requires answering six questions before specifying a trap.

Operating Pressure

Rate above the maximum allowable operating pressure — not just normal working pressure. A 10 bar(g) header needs traps rated above 10 bar(g).

Condensate Load (kg/h)

Calculate from heat transfer equations. Size at 2–3× actual load at minimum differential pressure. Never use a 10× safety factor — that leads to oversizing and premature disc wear.

Continuous vs Intermittent Load

Continuous steady-state load → ball float trap. Intermittent batch process → TD trap may suffice.

Backpressure Ratio

Backpressure (absolute) ÷ inlet pressure (absolute). TD traps: keep below 0.5. Ball float traps tolerate up to ~0.8.

Superheat

Superheated steam → TD traps or inverted bucket traps. Avoid ball float types on superheated service.

Non-Condensable Gases

Large initial air loads at startup require a trap that can purge air. Ball float traps have an integral thermostatic air vent.

Application Examples

DRIP LEGS

Thermodynamic disc traps. Wide pressure range, compact, modest condensate loads. Specify at 2–3 m intervals on outdoor mains and at every low point.

HEAT EXCHANGERS

Ball float (F&T) traps — Spirax Sarco FT range. Continuous, often heavy condensate load. A TD trap on a large heat exchanger will waterlog under full load.

STEAM TRACING

Bimetallic or small TD traps. Bimetallic types suit systems where you want the tracer to run warmer.

TURBINE DRAINS

TD or inverted bucket traps. Superheated service, moderate condensate load, robust trap required.

What Happens When a Trap Fails

FAIL OPEN (BLOWING)

Passes live steam continuously — the most common failure mode. Energy bills rise and back pressure increases downstream.

FAIL CLOSED (LOCKED SHUT)

Condensate cannot drain. Heat transfer drops and water hammer may occur.

In a typical industrial plant, 20–30% of steam traps are in a failed condition at any given time.

P.C. Engineering Corporation conducts steam system energy audits that include trap-by-trap surveys. See our contact page to request a survey.