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-pressurised.
- 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 pressurised 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 bouancy force plus this pressure force. When enough air accumulates in the air valve, the bouancy 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.
ARI have since many years ago patented a design of small orifice, which works on the “rolling seal” principle as follows: Because the “Rolling Seal” mechanism opens a very small area in the beginning to “crack” the seal of the pressure force, it does not suffer the problems of traditional small orifices, which are very limited to the size they can be designed for. This is also the reason why the ARI range of air valves, have a small orifice, which is the same (large) size throughout the size and pressure range.
An additional advantage of the rolling seal mechanism is that it has a self-cleaning action allowing any small particle of dirt to be blown clear of the valve.
Many companies have unsuccessfully attempted to copy the ARI patent of “rolling seal” design, as it is the only method that overcomes the age-old problem of small orifices. These small orifices have been the “Achilles heel” of air valves and a problem few are aware of. After all, nobody is aware of whether a small orifice in an air valve works or not. It is only when a pipe burst happens that sometimes it is discovered that the small orifice did not do it’s job and allowed accumulated air to magnify a water-hammer event into a catastrophic pipe burst.
This article (with diagrams) is available to download here: The importance of the small Automatic orifice in Air valves