How to Size a Steam Trap
Oversized traps cycle wastefully; undersized traps back up condensate. Learn the correct sizing procedure — from calculating condensate load to selecting the right orifice.
Why Sizing Matters
A correctly sized steam trap discharges condensate as fast as it forms, without passing live steam. An oversized trap opens and closes too frequently (wearing the seat) and may pass steam on each cycle. An undersized trap cannot keep up with the condensate load, causing waterlogging, reduced heat transfer, and water hammer.
The two things you need
Sizing a steam trap requires two numbers: (1) the condensate load in kg/hr, and (2) the differential pressure across the trap in bar. Get these right and the manufacturer's sizing chart does the rest.
Step 1: Determine the Condensate Load
The condensate load is the amount of steam that condenses in your application. Calculate it based on the equipment type:
| Application | How to calculate load |
|---|---|
| Heat exchanger | Q (kW) × 1.6 = kg/hr condensate (latent heat ≈ 2100 kJ/kg) |
| Pipe warm-up (startup) | Calculate from pipe mass, specific heat, and warm-up time |
| Tracing line | Use manufacturer's table — typically 0.3–1.0 kg/hr per metre |
| Drip leg on steam main | Typically 1–5% of steam flow under running conditions |
| Process heater | Q (kW) ÷ latent heat (kJ/kg) × 3600 = kg/hr |
Tip: Always check the manufacturer's data for the specific equipment. For warm-up loads (startup), the condensate load can be 3–5× the running load — size for the higher of the two, or use a trap with adequate startup capacity.
Step 2: Apply a Safety Factor
The calculated load is the minimum. Apply a safety factor to account for load variations, startup surges, and operating pressure fluctuations.
| Application | Safety factor |
|---|---|
| Continuous process (heat exchanger, heater) | 1.5× calculated load |
| Batch process (alternating on/off) | 2.0× calculated load |
| Steam main drip leg | 2.0–3.0× calculated load |
| Startup / warm-up loads | 3.0× calculated load |
Don't oversize without reason
A safety factor that is too high causes the trap to cycle too fast and wear prematurely. Use the appropriate factor for your application, not a blanket 3× on everything.
Step 3: Determine the Differential Pressure
The differential pressure (ΔP) is the pressure difference across the trap. It is the upstream steam pressure minus the downstream back pressure.
- Identify the upstream steam pressure at the trap inlet (Ps).
- Identify the downstream pressure (Pb) — this is the condensate return line pressure or atmospheric pressure if discharging to drain.
- Calculate ΔP = Ps − Pb.
- If the condensate is lifted vertically after the trap, subtract 0.1 bar per metre of lift from ΔP.
- Verify that ΔP is positive — if the back pressure exceeds the upstream pressure, the trap cannot discharge.
Startup vs running pressure
At startup, the steam pressure is lower than operating pressure because the line is still warming. If the trap needs to discharge condensate at startup (it usually does), verify that the trap can operate at the lower startup differential pressure.
Step 4: Select the Trap Type
| Application | Recommended trap type |
|---|---|
| Mainline drip legs | Thermodynamic (TD) |
| Steam tracing | Thermodynamic or thermostatic |
| Heat exchangers (steady load) | Float and Thermostatic (F&T) |
| Heat exchangers (variable load) | Float and Thermostatic (F&T) with modulating valve |
| Air venting at startup | Thermostatic (separate air vent) |
| Superheated steam lines | Thermodynamic (only type that handles superheat) |
Step 5: Select the Orifice / Capacity
Using the manufacturer's capacity chart for your chosen trap type, find the model whose discharge capacity at your ΔP exceeds the safety-factored condensate load.
- Find the manufacturer's capacity table for the trap type you selected.
- Locate your differential pressure (ΔP) in the pressure column.
- Read across to find the capacity in kg/hr at that ΔP.
- Select the trap model whose capacity exceeds your safety-factored load.
- If the capacity is well above your load, consider a smaller orifice to reduce cycling wear.
| Example | Value |
|---|---|
| Calculated condensate load | 200 kg/hr |
| Safety factor | 1.5× |
| Required capacity | 300 kg/hr |
| Differential pressure | 4 bar |
| Selected trap capacity at 4 bar ΔP | 380 kg/hr |
| Result | Adequate — trap exceeds required capacity |
Common Sizing Mistakes
- Sizing for startup only: Startup loads are much higher than running loads. If you size for startup, the trap will be oversized for normal operation and cycle too fast. Size for running load with startup capacity as a check.
- Ignoring back pressure: If the condensate return line has back pressure, the effective ΔP is reduced. Failing to account for this results in a trap that cannot discharge at full load.
- Ignoring lift: Lifting condensate vertically after the trap reduces the effective ΔP by 0.1 bar per metre. If the lift is too high, the trap will stall.
- Using the pipe size for the trap size: The trap connection size is NOT the same as the trap capacity. A 15mm trap can have a much larger capacity than a 25mm trap of a different type. Always size by capacity, not by pipe connection.