Email this pageTraps and Orifices Part 1
What is an Orifice?
The word ‘orifice’ literally means an opening. In the context of TLV steam traps, the term orifice is used to refer to the opening or passage through the valve seat. The size of the orifice depends on the steam trap’s body size and the operating pressure differential. In the case of the J3X free float steam trap, for example, the orifice options for different pressure ranges are approximately 2-3 mm in diameter or less. Note that the diameter of an orifice is much smaller than the inner diameter of the connected piping.
Why is the Diameter of an Orifice So Small?
While a free float steam trap with a nominal connection size of 15mm would be connected to piping with an inner diameter of 15mm, the orifice may have a diameter of around 2-3mm or less. Why is the diameter of the orifice so much smaller than the inner diameter of the piping?
Although piping is generally sized for two-phase flow (condensate with steam vapor space), the orifice only needs to be sized for the condensate volume. A 2-3mm orifice with a pressure differential of 0.2 MPaG can discharge approximately 350 kg/h of condensate. This would be sufficiently large for condensate drainage based on the estimated steam consumption of small-scale equipment that would have a condensate outlet of 15mm. The J3X discharge capacity is slightly larger as condensate can also be discharged through its thermostatic air vent.
It can be seen from this that an orifice with a diameter much smaller than the diameter of the connected piping is sufficient to meet the discharge capacity needs of the steam trap. It follows, of course, that a larger size orifice would allow the trap to have a greater discharge capacity. However, for the trap to operate at the same pressure differential, this would require a proportionally larger float, which would in turn increase the size of the trap body.
Discharge Capacity and Nominal Connection Size
In the case of most mechanical type traps, it is the size of the orifice, not the size of the connection port, that determines the discharge capacity. There is no direct relationship between connection size and discharge capacity. An example of this is the J3X, in which the models with connection piping sizes 15 mm, 20 mm, and 25 mm all have the same discharge capacity for any given orifice size.
A larger size orifice allows a trap to have a greater discharge capacity. However, this requires a proportionally larger float for the same pressure differential, which increases the size of the trap body. In order to design a trap with sufficient capacity, the appropriate orifice size and float diameter must be determined.
Valve opening and closing forces play a role in determining the size of the orifice and float. For a more detailed explanation of this, see Traps and Orifices Part 2.
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