How to size a steam trap for its condensate load
Sizing a trap on the steady running load alone almost always produces a trap that is too small. On start-up, cold pipe and cold equipment dump several times the running load at once. Standard practice is to multiply the running load by a safety factor: about 2x for a steam main drip leg and 3x for a heat exchanger behind a modulating temperature control valve, where the differential swings.
The second number that matters is the working differential. Published trap capacities are listed by differential, so you have to read the column that matches supply pressure minus return line back pressure. With 100 psi supply and 7 psi back pressure that column is 93 psi. Pick the model whose capacity at that differential clears the required figure.
High back pressure stops a trap from clearing condensate even while it is open. Once back pressure passes roughly 80% of inlet pressure on an absolute basis, most trap types lose their discharge capability, and the fix is a larger return line or a pump trap. When the condensate load is unknown, estimate it from the heat duty divided by the latent heat of the steam, then add the warm-up load for the piping separately. Confirm the final selection with a mechanical engineer and the manufacturer's capacity tables.
Frequently Asked Questions
No. An oversized trap never fills at light load, so it passes live steam and cycles constantly. Staying inside the 2x to 3x range that suits the service works better.
Yes. Float and thermostatic traps discharge continuously and suit modulating equipment, thermodynamic disc traps are simple but weak against back pressure, and inverted bucket traps tolerate water hammer better.