Limitations of Fixed Orifice Devices

Why Steam Trap Technology is Better

Operation

The most common fixed orifice devices utilize a nozzle called a venturri and some basic laws of physics and thermodynamics in which to operate.

As hot condensate under pressure flows through the venturri nozzle, its rate of flow (speed) increases due to a decrease in volume within the venturri nozzle. As the speed of the liquid (hot condensate) increases, the pressure surrounding the liquid decreases and the result is an area of low pressure ( Bernoulli’s Theorem Principle ). The BTU’s that are entrained in the hot condensate will then escape to the area of low pressure surrounding the liquid and flash into steam, choking the throat of the nozzle, and intermittently restricting the flow of hot condensate and live steam within the nozzle. When saturated steam passes through the nozzle, its pressure and temperature increase due to the restricted volume within the nozzle. As the steam exits the nozzle into an area of lower pressure, a small amount of flashing occurs that intermittently restricts the flow of saturated steam in to the return lines.

Inability to Handle Varying Loads

Fixed Orifices by nature cannot adapt to handle the varying loads commonly associated with steam heating systems. Because the orifice is fixed and sized according to either the maximum or minimum load requirement, any fluctuation in ambient temperature will cause the steam heating load to change. Thus, any change in the load requirement will cause a backup of condensate or loss of live steam. Because one size does not fit all in a steam heating system, fixed orifices are typically oversized by 25%-30%, resulting in live steam loss.

Backup of Condensate

When a back-up of condensate occurs, the efficiency of the heating equipment is significantly reduced and susceptible to damage, due to water hammer and freezing of heating coils. When condensate temperatures are allowed to drop below saturated steam temperature and mix with carbon dioxide ( a by-product of condensed steam ), carbonic acid is formed which corrodes piping and steam heating coils. Any damage to heating equipment, as a result of the back-up of condensate, water hammer, and/or corrosion, is specifically excluded by the fixed orifice warranty.

Live Steam Loss

When a fixed orifice is sized to handle the maximum condensate load and the conditions are such that a lighter load is realized, the net effect is live steam loss which increases the back pressure within the return lines and carbonic acid formation in the return piping.

Susceptibility to Clogging

As with any type of mechanical system, dirt is the enemy. Even with the use of fine mesh strainers and orifice sizes of 5/16″, the steam trap is still susceptible to dirt and scale clogging and must be maintained. A fixed orifice is a hundred times more susceptible to clogging because of the extremely small orifice, usually 1/32″ – 1/64″. The use of fine mesh strainer screens will not prevent the passage of smaller oxides that gradually accumulate and plug the tiny hole. Because fixed orifices plug up so easily, the strainers and blow-down valves must be routinely maintained. Failure to do so typically voids the fixed orifice warranty.

Poor Air Venting Capabilities

Fixed orifices have very poor air venting capabilities that significantly reduce the efficiency of the entire system. The fixed orifice is much smaller as compared to a steam trap with a 1/4″or 5/16″ orifice. Poor air handling will increase warm-up periods and reduce system efficiency.

Difficult to Repair & Maintain

Because they are so susceptible to clogging and rely so heavily on strainers and blow-down valves, fixed orifices require constant attention and maintenance. When a fixed orifice becomes clogged, it must be removed from service, taken off-line, and cleaned. The notion that fixed orifices require no maintenance and are virtually trouble free is misleading and false.

Summary

Like any mechanical device, steam traps need to be maintained in order to provide peak performance. When they are neglected and left to operate improperly, they can, over time, cause the failure of other traps and components within the system. Converting a facility of failed steam traps over to fixed orifices in order to escape steam trap maintenance may create a sense of improvement, but this is only a temporary fix. The fixed orifices inability to properly vent air, remove water, and trap steam will eventually surface in the form of condensate back-ups, steam loss, orifice clogging, and damage to equipment.

The idea that these simple devices can effectively perform the same basic functions as a steam trap in a typical steam heating system is false. Steam heating systems have varying loads that can change month to month, day to day, and in some cases hour to hour. Only a properly operating steam trap can provide for the continuous removal of air and water while preventing steam from passing its point of use. The road to steam system efficiency starts and stops with a systematic approach to steam trap maintenance, not gimmicks.

You may download further information available from the U.S. Department of Energy.