How to Size a Steam Trap: A Practical Guide
June 23, 2026
Steam traps are one of the most failure-prone components in a steam system — and incorrect sizing is the leading cause. An oversized trap cycles too infrequently, leaving condensate in the system. An undersized trap becomes a bottleneck that starves downstream equipment. Getting it right requires more than guessing: it requires a defined method.
What Is a Steam Trap?
A steam trap is an automatic valve that discharges condensate and non-condensable gases from a steam system without allowing live steam to escape. Every steam-heated piece of equipment — heat exchangers, tracing lines, jacketed vessels, heating coils — requires a trap downstream of the condensate outlet.
When a trap fails open, it passes live steam into the condensate return, wasting energy and raising back pressure. When it fails closed, condensate floods the equipment, reducing heat transfer and risking water hammer.
The Sizing Inputs
To size a steam trap, you need three pieces of information:
1. Operating Steam Pressure (PSIG) The gauge pressure of the steam supply to the equipment. This determines the differential pressure across the trap (upstream steam pressure minus back pressure on the condensate return side).
2. Condensate Load (lb/hr) The rate at which condensate is produced. For a heat exchanger, this is calculated from the heat duty:
Condensate (lb/hr) = Heat Duty (BTU/hr) ÷ Latent Heat of Steam (BTU/lb)
Latent heat depends on pressure — at 15 psig it's approximately 961 BTU/lb; at 100 psig it's approximately 880 BTU/lb. Use steam tables or a calculator for accuracy.
3. Safety Factor A multiplier applied to the calculated condensate load to account for startup conditions, fouling, and load variation. Typical safety factors:
| Application | Safety Factor |
|---|---|
| Heat exchanger (steady load) | 2× |
| Heating coil (variable load) | 3× |
| Tracer line | 2–3× |
| Jacketed vessel | 2× |
Applying the safety factor gives the design condensate load — the value you size the trap to handle.
Note: The Steam Trap Sizer tool does not apply a safety factor automatically. It sizes to the condensate load you enter. It is your responsibility to determine and apply the appropriate safety factor for your application before entering the load into the tool.
Trap Types
A selection chart maps the design condensate load and differential pressure to a recommended orifice size and trap type. The main trap types are:
- Thermostatic (TS) — discharges condensate at below-steam temperature; best for tracing and small heat exchangers
- Inverted Bucket (IB) — mechanical trap; robust, tolerates waterhammer, good for high-pressure process applications
- Float and Thermostatic (FT) — continuous discharge at steam temperature; best for large heat exchangers and any application where waterlogging must be avoided
- Bimetallic (BM) — discharge below steam temperature; good for superheat
- Thermodynamic (TD) — compact, simple; good for high-pressure mains drainage
The chart is organized so you enter from the differential pressure axis and the design condensate load axis, and the intersection falls into one of the selection zones.
Common Sizing Mistakes
Oversizing with excessive safety factors. Applying a 6× safety factor "to be safe" results in a trap that operates at a tiny fraction of its rated capacity. For inverted bucket and thermodynamic traps, this causes short cycling — rapid open-close cycling that accelerates wear.
Using supply pressure instead of differential pressure. The trap works against the differential between supply and back pressure. If you have 100 psig supply but 40 psig back pressure, the differential is 60 psig — not 100. Using supply pressure alone means the orifice will be undersized — the trap won't have enough differential to discharge condensate at the required rate.
Ignoring startup load. At startup, all equipment is cold. The initial condensate load (warming the equipment from ambient to steam temperature) can be 5–10× the steady-state load. Float and thermostatic traps handle this well because they're continuously open until the equipment is up to temperature. Inverted bucket traps may struggle if undersized for startup.
Wrong trap type for the application. A thermodynamic trap on a large heat exchanger will waterlog the unit — it's a batch-discharge device and can't keep up with continuous condensate loads at low differential pressures.
Worked Example
Given:
- Heat exchanger duty: 500,000 BTU/hr
- Steam pressure: 50 psig
- Back pressure: 5 psig
- Application: steady process heat
Step 1 — Calculate condensate load: Latent heat at 50 psig ≈ 912 BTU/lb
Condensate = 500,000 ÷ 912 = 548 lb/hr
Step 2 — Apply safety factor (2× for steady heat exchanger):
Design load = 548 × 2 = 1,096 lb/hr
Step 3 — Determine differential pressure:
ΔP = 50 − 5 = 45 psig
Step 4 — Select trap: At 45 psig ΔP and 1,096 lb/hr design load, the selection chart recommends a Float and Thermostatic trap — appropriate because this is a large heat exchanger with a continuous, steady condensate load. The FT trap discharges condensate continuously as it forms, preventing waterlogging.
Try the Steam Trap Sizer
Rather than reading the chart manually, you can use our free online Steam Trap Sizer to enter your inputs and instantly get a recommended trap selection. It filters by connection type and body material, and flags oversizing risks automatically.