Engineers, designers and steam users should consider the potential consequences to a steam system where an orifice drain fitting “orifice” is utilized for condensate discharge.
An orifice (sometimes incorrectly referred to as a venturi) drains condensate through a fixed hole (there may be a series of varying inside diameters before and after the fixed hole).
The orifice diameter is fixed and cannot vary during operation; therefore, it is used for a single flow rate at a specific differential pressure. The condensate flow rate within the steam system varies, sometimes significantly. The load changes can have dramatic effects on process temperature-controlled applications; therefore, an orifice is generally not recommended for this type service. Similar issues may be experienced on expected “steady state” applications, such as draining steam mains or tracing, or when changes in ambient temperature or system backpressure occur.
Weather conditions such as cold weather, wind, rain, or snow can increase system load requirements when these weather conditions contact un-insulated sections of pipe, valves, flanges or other fittings.
Sizing an orifice on expected “steady state” applications, such as steam main drip, must be performed carefully, so neither undersizing nor gross oversizing occurs. Undersizing must be avoided to prevent dangerous condensate back up and flooding of equipment or steam mains. So the orifice will normally be at least slightly to moderately oversized for the largest load condition. Undersizing can create dangerous water hammer conditions leading to safety and reliability issues, including serious personal injury and equipment damage. The oversizing is aggravated when the load decreases through the fixed orifice.
To keep the steam loss to a minimum, the orifice may be sized for the worst case running steam load condition and a small safety factor. Startup of the steam system cannot be accomplished with a running load orifice. A second orifice in a bypass line may be required. Some orifices can be closed manually after the startup phase has been completed. This procedure could help reduce the level of excessive steam loss after the warm-up is reached. This second orifice possibility requires additional expense and is generally not done. In most cases, when using an orifice for drainage, only a supervised startup procedure should be done. Supervised startup requires a manual valve be opened to remove both condensate and air during the warm-up process. When full steam pressure is achieved, the valves are closed.
For the full technical sheet…see FCI
http://www.fluidcontrolsinstitute.org/pdf/resource/steam/ST101Orifice.pdf