Good hydraulic integrity in any pipeline is essential. The continual focus on development in the design of air valves is testament to the part they play in ensuring that integrity.
This essential piece of equipment needs to be expertly designed in order to do its job effectively: An air valve lets air in and out of a pipeline – which can either be filled from empty or working under pressure – and it must do this automatically and with perfect control.
When a pipeline is being filled from empty, and is being filled too quickly, water hammer can result. This occurs when the water arrives at the air valve, and the float closes at the same rate as the air being expelled. Designers realised this potential problem with air valves, which resulted in two “traditional” solutions:
- Two Stage Non-Slam Air valve. This traditional valve simply consists of a spring-loaded check valve, which is held in the closed position by the spring. It only allows air into the pipeline; a separate small orifice allows air out of the pipeline. The problem with this design is that it limits the discharge of air on pipe filling. This leads to the velocity of the water column being restrained as it moves towards the air valve, keeping the air in the pipeline for longer periods.
- Three-Stage Non-Slam Air valve. This second traditional design comprises a standard double orifice air valve, with a check valve in reverse on its inlet. The check valve is normally open allowing air to flow freely through the large orifice of the air valve. As soon as the water arrives, the check valve closes (due to the denser media flow) and the balance of the water flows through the smaller bypass. This allows the large float to close slowly without slam.
- This design was later incorporated into air valves as a ‘built-in” feature.
The 3–stage, non-slam air valve is equipped with an additional disc, which is approx. 20% of the large orifice area. The air valve is normally open with a large active orifice. This large discharge orifice switches over to the smaller discharge orifice (disc) when the air flow exceeds a preset value. The non-slam effect is achieved by the ability to slow down the water column before all the air is expelled from the pipeline.
Advantages over the two-stage air valve
- Discharge at low air pressure will release large quantities of air quickly.
- The switch over to the smaller orifice (disc), when the air discharge pressure exceeds a preset value, occurs while air is still in the line. This reduces the air slam pressure. The residual air provides a cushioning effect for the initial pressure slam (generated during the switchover from the larger to the smaller orifice).
Disadvantages of the three stage air valves
- Air discharge pressures are usually very small during bulk evacuation of air (.1 to .5bar). When discharging small quantities of air (total air volume), pressures may not reach the switch value.
- When the switchover to the smaller orifice does NOT take place, the valve loses its NONSLAM capability. All air goes out through the large orifice, leading to air slam conditions.
There are major differences between the various manufacturers when it comes to the switching pressure. The lower the switching point, the better the reaction of the air valve, and its ability to lock enough air in the pipeline. The ARI switch point is 0.02 bar (ideal) compared to Dorot and Vent-O-Mat 0.07.
When water columns are separated, it is critical to have large enough air intake. This creates a sufficient air pocket in the pipeline, with the necessary volume to slow down the colliding water columns. If the length of time of the vacuum condition is smaller than a few seconds, the air pocket will be too small to sufficiently slow down the colliding columns. It will not be able to prevent the surge.
The A.R.I Dynamic non-slam air valve
The ARI Dynamic air valve operates in a totally different manner. It discharges all the air in the pipeline through the large orifice, and when the water column arrives it closes the air valve slowly enough to prevent an abrupt change of velocity. This action prevents surge and water hammer. The unique rolling diaphragm sealing mechanism reduces changes in velocity thereby reducing the slam effect.
Advantages:
- The Dynamic air valve always ensures water flow after ALL the air is out of the pipeline. Unlike the 3-stage non-slam air valves where the non-slam effect is lost if the switch does NOT take place.
- Water flow starts immediately after air flow stops, i.e. it continues to maintain SOME flow velocity, which prevents sudden deceleration. When water flow stops through the air valve, it does so in a gradual fashion, thereby ensuring a non-slam effect. There is no need to build an air pocket in the pipeline.
This revolutionary concept developed by A.R.I. has been used in Global projects over the last 10 years. It has proven to be superior in solving water hammer problems in most applications.
