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The importance of the small Automatic orifice in Air valves

November 16,2022

By Peter Telle

Most modern combination air valves have at least two orifices, which serve three functions as follows:

●The large orifice expels large quantities of air when the pipeline is being filled. When the pipeline is full and under pressure the large orifice closes and will never open again until the pipeline is being drained and de-pressurized.

●The large orifice also allows air to enter the pipe when it is being drained thereby preventing a damaging vacuum forming.

●The small orifice lets out the pressurized bubbles of air that continuously come out of solution in any operating pipeline, and accumulate in the Air Valve.

The large orifice therefore only operates when the pipe is being filled and emptied – which in most installations is once in a few years! Even though the large orifice function is very important, the small orifice plays a much more critical role as it actually is the item which does all the work when the pipeline is under pressure – which is most of the time. If the small orifice does not work correctly,

accumulated air causes big problems resulting in reduced pipe capacity, increased Water Hammer and all the usual problems associated with air in pipes.

In most air valves the small orifice is really small ie between 0.5mm and 2mm depending on the size and pressure rating of the air valve. The higher the pressure rating and the smaller the size of the air valve the smaller the orifice. In the case of a 25mm or 50mm 16bar air valve the orifice size is no bigger than 0.5mm in all traditional air valves. In the case of 40bar rated valves the small orifice is so small as to be almost non-existent!

The smallest impurity in a valve with such a small orifice would cause blockage of this orifice resulting in an air valve without a small orifice at all. Traditional air valves, which rely on standard methods of a float sealing on an orifice and the weight of the float to release itself from the orifice, all suffer from this problem.

Reason for this small size of orifice is as follows: The pressure in the pipeline exerts a force on the float equal to the area of the orifice times the pipe pressure. So the total force that keeps the float on the orifice is the buoyancy force plus this pressure force. When enough air accumulates in the air valve, the buoyancy force disappears and it is only the pressure force that keeps the float attached to the orifice. It is then the weight of the float that has to “unlock” the float from this orifice.

AFC’s combination air valve series has bigger small orifice from 1.6 to 4mm based on air valve sizes. Different Pressure rating will have different small orifice. It’s been accurate calculated to ensure air venting will be functional even working pressure is more than valve’s pressure rating. The reliable air venting functions had been tested 2500 times in type test and still works very well after that. Eliminate the bubbles by using AFC’s air valve will be your best option.

Following video showed how the air venting working in field.