Steam Mains

Steam mains carry steam from the boiler or header to the branch piping. Inadequate trapping of steam mains will greatly reduce the efficiency of the entire system. Condensate retention in steam mains will inevitably lead to differential shock that may result in damage to steam equipment. Trapping mains is a challenge because, while very small loads are generated during normal operation, huge loads can be created at system startup. Steam traps should be sized for operating load.

To account for the difference between startup and running loads, two startup methods are commonly used: supervised and automatic. Supervised startups are widely used for initial heating of long and/or large-diameter mains. The suggested method is to fully open all manual drip valves on the line. Any air and retained condensate, as well as warm-up condensate, is freely discharged to the atmosphere or to the condensate return system. After the warm-up cycle is completed, the manual drip valves are closed and the steam traps take over the job. Automatic warm-ups occur when the system startup proceeds without supervision or manual assistance, using only the traps.

Regardless of the method used, sufficient time must be allowed during the warm-up cycle to minimize thermal stress and prevent any damage to the system. The recommended warm-up rate is not to exceed 100° F (38° C) per 8 minutes.

Installation Guidelines

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Table 18-1
Figure 18-2.
Trap draining drip leg on main.
Figure 18-3.
Trap draining leg at riser. Distance "H" in inches (meters) ÷ 28 (10) = psi (bar) static head for forcing water through the trap.

Trap Requirements

Traps are best suited to steam main applications if they:

  • Have a long service life.
  • Resist mechanical wear.
  • Resist corrosion.
  • Can endure hydraulic shock.
  • Vent non-condensables at steam temperature.
  • Perform well on very light loads.
  • Respond immediately to slugs of condensate.
  • Fail open.
  • Handle dirt.
  • Resist damage from freezing (some cases).

The inverted bucket trap meets all of these requirements best. In non-freezing environments below 30 psig (2 bar), the float and thermostatic trap is an acceptable alternative. An alternate choice in a freezing environment is the thermostatic trap or controlled disc trap.


To determine trap capacity necessary for installation:

Multiply the Load Calculation Value by the Safety Factor

Safety Factor

Select traps to discharge the condensate produced by radiation losses at running load.

• For traps installed between the boiler and the end of the main, use a safety factor of 2:1.

• For traps installed at the end of the main, or ahead of reducing or shutoff valves that are closed part of the time, use a safety factor of 3:1

Load Calculation

The following formulas are expressed as imperial values. To view this page in SI metric units, click here.

Calculate using a formula


C =
Condensate in lbs/hr-foot
A =
External area of pipe in square feet
(Table 17-1, Col. 2 below)
U =
Btu/sq ft/degree temperature difference/hr from Chart 17-1 below
t1 =
Steam temperature in °F
t2 =
Air temperature in °F
E =
1 minus efficiency of insulation
(Example: 75 percent efficient insulation:
1-.75 = .25 or E = .25)
H =
Latent heat of steam
(See Steam Table)
* Handbook N-101 (1.3 MB)

Table 17-1 Calculate using a table.

Chart 17-1 Btu Heat Loss Curves


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