Air locking in plastic pipe systems

Air locking is a problem in many poorly designed plastic pipe systems. The consequences of air locking, especially for continuous production lines and more specifically aquaculture, where the pipe is an integral part of the life support system, can be very costly.

Systems can air lock for a number of reasons but the phenomenon is more common in pipelines which are fed by a low pressure head and / or running at low water velocities. Pipelines that run at high pressures and high velocities have more mixing effect, and even if there is some air trapped in a part of the pipework, it will usually be pushed through the system and out at a valve or purge point.

Air locking can occur at various times of operation and all types of plastics such as pvc pipe, abs pipe etc. are just as prone. Some users install clear pipe systems so that they can observe the air collecting in the pipes and fittings, but this is often only short lived, as the internal bore of the pipe gets coated with particles or biological fouling, which limits the vision. System start up is a time when air locks often occur as parts of the pipe and fittings are full of air at the start of the process. The only way for an air lock to occur once a pipeline is fully flooded is for air to be drawn into the pipe system.

Air can be drawn in through a number of ways. An incorrectly assembled section of plastic pipe may contain a slight leak. If the water velocity, and angle past this leak are at certain values, then the hole will not leak water, but instead, air will be sucked into the plastic pipe in a venturi effect. This very often occurs at threaded joints, especially where there is a size reduction in the pipe, which results in a higher velocity and a lot of turbulence. Air can also get entrained into your plastic pipe systems through incorrectly seated seals in pumps, filters and other devices. Pumps are the most common source however, as there is true suction at some points in the system, assisting the draw in of air.

Once in the pipework, entrapped air rises to the top of the pipe, with small bubbles coalescing into large slugs of air. If the water velocity and pressure in the pipe are high enough the slugs will be drawn along the pipe and will not have a chance to collect. If however, the pipe system is poorly designed, water velocities are low, pressure is low and the pipe either undulates, or has high and low points in the system rather than a continuous gradient, the air can collect and form a barrier to water passing.

Once a system has been installed, if it is prone to air locking, there are a number of methods to alleviate this. In low head systems, plastic pipe fittings can be installed at the point of the air lock and rise up to a level higher than that of the head. Air will then escape through this route. Where this is not possible, special air release valves can be added to the pipe. These types of  valves use an internal float that opens a small valve in the presence of air and closes when all air has been released. Air release valves are only suitable for use in plastic pipe systems where there is a clean liquid (of the valve becomes blocked) and can only be used in a pressure line e.g.; NOT on the suction side of a pump or the fitting will act as a venturi and draw even more air into the pipe system.