Steam Trap Sizing Calculator — Condensate Load

Short answerRequired trap capacity = condensate load (lb/h) × safety factor (2–3×), matched to the trap’s rating at the actual pressure differential (inlet minus back pressure).

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How it's calculated

A trap must pass the condensate the equipment makes, plus a margin for cold-start loads that can be several times the running rate. Manufacturers rate traps in lb/h at a given differential pressure, and capacity falls as differential drops — so size at the lowest expected differential, not the boiler pressure.

Trap capacity ≥ Condensate load × Safety factor rate at ΔP = P_inlet − P_backpressure

Worked example

Load 300 lb/h, safety factor 3×.
Required = 300 × 3 = 900 lb/h at operating ΔP.

Reference tables

Typical safety factors

Application	Factor
Steam main drip	2×
Process / heat exchanger	2–3×
Modulating / low ΔP	3×

A number the tables do not give: sizing on running load alone starves the trap at startup, when condensate can spike to 2–3× the running rate.

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FAQ

Why a safety factor?

Startup and modulating loads produce far more condensate than steady state; the factor keeps the trap from backing up and water-logging the equipment.

Which differential do I use?

The lowest expected inlet-minus-backpressure differential, because trap capacity drops as ΔP falls — sizing at high ΔP undersizes the trap.

Which trap type?

Float & thermostatic for heat exchangers, thermodynamic or inverted bucket for mains — match the type to the load pattern, then size the capacity.

Does back pressure matter?

Yes — a lifted or pressurized return raises back pressure, lowering ΔP and required-capacity headroom; include it in the differential.

This page is a reference aid. Always verify against the trap manufacturer data and steam-system references. Figures shown are for orientation, not a stamped design.

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