By Peter Telle
Surge tanks are one of the oldest of surge alleviation methods. In the past they were seen as the only reliable way to deal with surge in pipelines but air valves have come a long way since then and are now a viable alternative.
Surge tanks inject water into a pipeline where a surge event causes low pressure, or allow water out of a pipeline where a surge event causes in high pressure.
They’re mostly closed systems. The surge tank does not actually consume water. Water that is injected during a low pressure event is removed afterwards. Similarly water that is removed during a high pressure event is returned once the pressure drops.
Surge tanks generally are powered by air so that they can add or remove water fast enough to counter pressure changes.
The most common types are compressor surge tanks and bladder surge tanks.
Compressor surge tanks are closed units with an open interface between the water at the bottom of the tank and the air at the top of the tank. Air is supplied by a compressor. When the required volume of air is pumped into the tank the pressure of the air and water is equalized. Some air is absorbed into the water as it flows in and out of the tank. This means that the air volume and pressure needs to be monitored and controlled. The compressor has to be permanently connected to add air when needed. This means complicated controls. Injecting air into the pipeline is an undesirable side effect.
Bladder surge tanks overcome many of these disadvantages.
● They require almost no maintenance.
● The water and air is separated so no air gets absorbed. This means that no compressor or control system is needed to top it up. It also means you’re not injecting air into your pipeline.
The proper and reliable long life operation of these tanks is dependent on the quality of the bladder. Several firms have are now producing bladders that give long lives. A.R.I has a joint venture with one such company (Pauchard – part of the Fayat group) who has built a reputation worldwide for reliable bladder surge tanks.
Air valves have traditionally been used to remove air from water pipelines and inject air when a pipeline is being drained so that harmful vacuum conditions can be avoided. With the development of air valves and the design of fast intake and reduced velocity exhaust they have received more attention as potential water hammer reducing devices and are becoming more and more accepted for this function.
Such two–stage air valves have a large orifice and a smaller orifice, which is automatically activated by differential pressure and “switches” over when differential pressure reaches a level to cause the orifice to be activated.
Here’s how it a two-stage air valve works as a surge suppression device in a pump station.
● Column separation occurs when a pump trips. This causes a low pressure (or even a vacuum) downstream of the pumps. The fast reversing water column which fills this low pressure zone is what causes the high water hammer slam.
● A two-stage air valve on the delivery side of the pump would allow air into the pipeline quickly enough to replace the low pressure.
● On reversal of the water column the air valve would allow this air to be evacuated again.
● If this reversal occurs at too fast, the high differential pressure (at the air valve) causes the air valve to switch to the smaller orifice resulting in a lower exhaust flow rate and a reduced rate at which the water column reverses. This reduces the slam onto the check valve at the pump.
This means that correctly sized two-stage (or non-slam) air valves can fulfil the same function as expensive surge tanks in addition to their normal function of removing air from the pipeline.
These two-stage air valves can also be used at other locations on the pipeline:
● At low pressure points which are close to the hydraulic gradeline and where column separation easily occurs during even normal operation of the pipeline.
● During pipe filling operation which can sometimes be uncontrolled and too fast. Normal air valves would then evacuate the air at too high a velocity resulting in air valve slam (when the water arrives at the air valve) which can cause serious damage to the air valve itself as well as the pipeline and components in close vicinity.
So why don’t pipeline designers use air valves exclusively instead of more expensive surge tanks?
● In some cases the column separation volume is so large that too many air valves would be needed to overcome the low pressure scenario.
● In other cases a relief function has to be performed (instead of minimizing a low pressure event) which can only be performed by a surge relief valve or surge tank.
● In the past air valves were regarded as being unreliable and not to be depended on to fulfil the important task of water hammer reduction. This is probably justified because there are many poor quality air valves available. They are seen as high maintenance items. Evidence is the fact that most air valves start leaking after a short period of being in operation and operators often close the isolating valves under the air valve to stop the leak!
● And finally most 2-stage air valves do not switch at a low enough differential.
AFC’s KAR5 adjustable anti-shock air valves overcome these concerns.
● They are reliable. A superior seal design means that they come with a 10 year leak free guarantee.
● The switching point of their 2-stage air valves is also much lower than most others. The switch point is adjustable from outside of the valve based on differential air pressure
With the confidence of a reliable air valve switching at the correct differential pressure point and working maintenance free for many years water hammer reduction techniques can utilize air valves to a greater extent at great savings and increased reliability.