Email this pageTraps and Orifices Part 2
What is an Orifice No.?
The discussion of orifices in Traps and Orifices Part 1 focused on why the diameter of a trap orifice (valve seat) is much smaller than the diameter of the connected piping. In Part 2, we will discuss the significance of the trap orifice no.
At first glance, it may seem counter-intuitive that the higher the trap orifice number, the smaller the diameter of the orifice. It is understandable that this may seem odd at first, but a closer look reveals the reasoning behind this.
The orifice number indicates the maximum differential pressure in kg/cm² (bar) at which the steam trap will discharge condensate. For example, a No. 10 orifice would be rated for 10 kg/cm² (10 bar, 150 psi). The higher this number is, the higher the pressure at which the orifice can be used to drive condensate through it. In order to achieve higher operating pressure differentials, a smaller opening size would be required. However, a smaller orifice results in a lower discharge capacity for any given operating differential pressure.
Forces Acting to Close and Open the Valve
When we think about the size of an orifice, we must consider 2 of the forces at work inside the free float trap, namely; the force that acts to open the valve and the force that acts to close the valve.
The mechanism that causes the free float trap to operate is the force of buoyancy. Buoyancy causes the float—which is the valve itself—to rise, and after the float rises up off the valve seat, the valve is in the open position. In other words, the force of buoyancy is the force that acts to open the valve. If we assume that the specific weight of condensate is a constant, then the buoyancy of the float is determined by the volume of the submerged part of the float. The force of buoyancy is therefore it its highest value when the float is completely submerged, and given that the same float is being used, it is not possible to achieve a stronger force acting to open the valve than this.
In contrast, the force acting to close the valve is a force that is created by the diameter of the orifice and the difference between the pressures in front of and behind the orifice. An example of this that is familiar to many of us is when the water is draining out of a bathtub that uses a drain plug. If the plug gets too close to the drain as the water is going down, the plug is sometimes sucked back into place on the drain, closing off the flow of water. A force identical to this is at play inside a free float trap. The force is represented as pressure x surface area, so if the orifice diameter is a constant, then the larger the difference in pressures in front of and behind the orifice (the differential pressure), the stronger the force acting to close the valve. Conversely, if the differential pressure is fixed, then the larger the orifice diameter, the stronger the force acting to close the valve.
Orifice No. Selection
While the force acting to open the valve has a maximum value that is determined by the size of the float, how large the force acting to close the valve can grow is dependent on the operating pressure. The steam trapping function cannot operate if the valve remains in the closed position, so it is necessary to overcome this limitation imposed by the operating pressure.This is why there are different orifice no.’s with their variety of corresponding maximum operating pressures.
Relationship between Capacity, Pressure and Orifice No. for the Same Float/Trap
To Increase the Discharge Capacity
Increase the orifice diameter (smaller orifice number)
=> Reduce maximum operating pressure = smaller orifice no.
To Increase the Maximum Operating Pressure
Decrease the orifice diameter (larger orifice number)
=> Reduce discharge capacity = larger orifice no.
From this it can be seen that, even with the same trap model, there are selection options to be made between maximum operating differential pressure and discharge capacity. Reducing the maximum operating pressure allows us to achieve a greater discharge capacity at any given differential pressure, while reducing the maximum discharge capacity provides a higher maximum operating pressure.
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