Air Release Valve Selection: Types, Sizing and Placement

Air in pressurised pipelines reduces available bore, creates differential pressure that triggers false readings on flow and pressure instrumentation, accelerates internal corrosion and, on pump rising mains, initiates column separation that produces far more severe surge events than the pump trip itself. Air release valves remove entrained air during and after commissioning, and release air pockets that accumulate over time at high points. Selecting the wrong type, or placing a valve incorrectly, will either fail to remove the air or cause the pressure rise it was installed to prevent.

The three types: kinetic, automatic and combination

A kinetic air release valve (large orifice type) is designed for one purpose: releasing the large volumes of air displaced during pipeline filling. The orifice is large enough to pass air at the rates that occur when a main is filled at normal velocity. When water arrives at the valve, the ball float rises and closes the orifice. Kinetic valves are not suitable for continuous duty: they cannot release the small quantities of entrained air that collect at high points during normal pressurised operation.

An automatic (small orifice) air release valve releases the small bubbles of entrained air that collect at high points during normal pressurised operation. The float sits below the body, held down by a lever mechanism. As a bubble of air accumulates at the valve, the reduced buoyancy allows the lever to open a small needle orifice through which the air bleeds out. The orifice is too small to pass significant flow if the valve is inadvertently submerged in a flooded chamber.

A combination air release valve incorporates both a large kinetic orifice and an automatic small orifice in a single body. This is the standard choice for most water distribution and rising main installations: it handles filling operations and provides continuous air release during normal pressurised service. The two functions are independently housed and neither affects the operation of the other.

Anti-shock models

Standard combination valves close the kinetic orifice abruptly when the water front arrives. On fast-filling mains or when air is expelled under high pressure, the water front arrives at the valve at significant velocity. Instantaneous closure creates a pressure spike as the momentum of the incoming water column is arrested.

Anti-shock combination valves include an adjustable needle mechanism in the kinetic orifice circuit that controls the rate at which the orifice closes as water approaches. Controlled closure over the last portion of float travel reduces the impact pressure to levels the pipeline can accommodate without damage.

Anti-shock valves are required on pump rising mains and high-pressure distribution mains where filling rates are high. On low-pressure gravity mains that fill slowly, a standard combination valve is usually adequate. If uncertain, specify anti-shock: the cost premium is small relative to the consequence of a pressure spike during commissioning.

Placement on the pipeline profile

Every high point on the pipeline profile requires a combination air release valve. Air naturally migrates to high points during operation, and any unvented high point will accumulate a pocket that restricts flow and can initiate column separation during pump deceleration.

On long horizontal runs without high points, install combination valves at intervals of 500 to 800 metres. Air bubbles that form during operation will otherwise migrate to the nearest high point and cannot pass through intermediate fittings: eventually they coalesce into pockets that affect hydraulic performance.

On pump rising mains, a kinetic or combination valve should also be installed on the pump discharge, immediately downstream of the pump control or non-return valve. During pump startup, the column above the pump is often partially air-filled; rapid expulsion of this air before the water front reaches intermediate valves prevents pressure peaks further along the profile.

Avoid installing air release valves in chambers that flood regularly. A submerged valve body prevents the float from closing correctly and can allow pipeline drainage during pump-off periods, introducing contamination risk and allowing air back into the main.

Sizing

For combination air release valves on water distribution mains, size the kinetic orifice for the expected filling rate: typically 0.3 to 1.0 m/s filling velocity in the main. Most combination valves for water mains are available in DN50 to DN100 body sizes covering this range for DN100 to DN600 mains.

Undersizing the kinetic orifice produces back-pressure during filling that slows the fill rate and, in severe cases, prevents complete filling. Oversizing increases the risk of slam on closure. Manufacturer filling rate tables for specific valve body sizes and pipe diameters should be used for final selection rather than general rules.

Sewage and wastewater applications

Standard air release valves are entirely unsuitable for sewage rising mains. The small orifices of the automatic chamber block within days in the presence of sewage, and fibrous material accumulates on any internal surface that provides a ledge. A blocked air release valve on a sewage main provides no surge protection and creates a contamination risk as sewage backs up into the valve body.

Sewage-specific air release valves use large unobstructed orifices sized to prevent solids accumulation, corrosion-resistant ball floats with smooth surfaces that do not retain fibrous material, and body materials that resist the chemical environment of raw sewage.

Three types are available to match the three standard duties: kinetic for filling, anti-shock for surge protection and combination for general-purpose rising main installations. Specify sewage-specific designs for any application where the fluid contains suspended solids, oils, fats or fibrous material, including effluent from food processing, stormwater with organic content and all sewage rising mains regardless of apparent cleanliness.