Steam Traps: Update on Fixed Orifice or Venturi Types

STEAM TRAPS: UPDATE ON FIXED ORIFICE OR VENTURI TYPES

ISSUE:

These steam traps are marketed as providing increased trap life and reducing energy

losses compared to the inverted bucket, closed float thermostatic, and thermostatic type

traps that are normally specified.

DISCUSSION:

Fixed orifice and venturi steam traps have no mechanisms to modulate and control the

capacity of the trap. Compared to selection procedures for other types of traps, the sizing

of the orifice and venturi traps must be done much more precisely to accommodate the

variable condensate flow rates, while limiting the steam loss rates. It is necessary to use

proprietary selection procedures.

Smaller size orifice and venturi traps fail closed due to plugging from debris. This is a

serious safety hazard on steam main drips; condensate back-ups in the steam main can

cause dangerous water hammers that can have the power to rupture valves and pipe

fittings. Also, this use is a major challenge to these types of traps, because of the normal

very low condensation rates at high differential pressure when the line is at normal

pressure, and the much higher condensation rate at low differential pressure when the

line is being warmed-up. Inverted bucket traps can normally accommodate these

variations in flow and pressure.

Another critical usage is with reheat coils. These are vulnerable to freezing if a trap

malfunctions and condensate backs-up in the coil.

RECOMMENDATIONS:

Fixed orifice and venturi traps should not be utilized for steam main drips, preheat coils,

or for any usage where the fail closed feature cannot be tolerated. They should be

applied only for other uses, when there is assurance that the trap manufacturer can

precisely determine the condensate flow and differential pressure and will also provide

proper engineering selection procedures and follow-up services.

LINKS TO FM TECHNICAL INFORMATION LIBRARY (TIL):

See "HVAC Design Manual" at http://www.va.gov/facmgt/standard/manuals_hvac.asp

See "Outside Steam Distribution Design Manual" at

http://www.va.gov/facmgt/standard/manuals_steam.asp

FOR ADDITIONAL INFORMATION: Contact Jim Shoemaker at 202-565-5176,

Consulting Support Office (183A).

13 May 2003

FM-181A-DA-117

Index