01 VG Series air release and vacuum break valves Operation and benefits The topic of sizing and placement is significant and complex the purpose of this document is simply to point out important considerations when choosing an Air Release valve and specifically Sewage air valves. Vacuum Breaking ( on draining pipeline ) The most important Air Release valve function is during Vacuum. Ultimately the end user pays for area of protection. The drawings show the inlet and outlet of the valve. During vacuum the air moves from the outlet back into the valve and pipeline. The ratio between inlet and outlet is important for three reasons. a) The cost of protection is directly related to the ratio between the inlet and the outlet. If the outlet is 50% (area ) of the isolating valve you are effectively paying the same price but getting half the protection b) Unlike during discharge the pressure of the atmosphere is fixed and it is not possible to compress it as is done during exhaust. So the flow volume into the valve is limited by the valve area. c) A further complication is that at approximately 0.5 atmospheric differential pressure the air goes sonic and effectively any increase differential pressure will not increase the flow volume into the pipeline. Air Release ( during filling ) The outlet orifice sizing is important during filling. Essentially the water velocity behind the air is traveling towards the valve at the same speed. The valve is designed to close when water gets to it. If the velocity of the water is very high when it closes, it can induce a pressure rise in the pipeline in excess of 2 x the working pressure of the pipeline. Pressure rises of this type can easily be 30 times the working pressure! Reducing the outlet to about 1 % of the inlet allows the air back pressure to build up in the valve and start to slow down the approaching water. This type of orifice is generally termed the anti shock orifice. The results are predictable and can be modeled with hydraulic analysis software. When these type of orifices finally close the water velocity is going sufficiently slow enough not to induce a very big pressure rise in the system ( generally less than 1.5 times the working pressure )
02 Air Release options ( historically ) One previous option was to have a closing mechanism that could change the outlet size from 100% to 1 % at a certain air velocity. At that time this was a significant improvement over the older type of valves. It was thought that allowing the bulk of the air to escape before switching to the anti shock orifice would speed up the filling rates but this is not the case. What actually happens is a large part of the vital air is lost and therefore less air is available for backpressure to assist the deceleration of the water when the valve closes Holding that closing mechanism closed during filling has some significant benefits. In actual fact the presence of some air during filling reduces the friction loss and so increases the filling velocity initially but is slowed down on closing by the backpressure. Filling rates are generally set out by the design and operational engineers and high filling rates are not encouraged. Orifice options Often a spring is used as an option to hold the closing mechanism.this is usually called a Bias mechanism ( as shown on left ). Generally a valve can consist of 3 orifices. a) Outlet which should be equal to the inlet = 100 % area b) A closing mechanism with the anti shock orifice in it = about 1% area c) A small orifice = about 0.025% area which is simply to open and vent and small bleed air that build up in the valve d) The lower float ( shown as round ) which floats up and down with the water level and controls the small orifice opening.
03 Significant differences of the air valves Body shapes for Sewage valves Any splashing during filling will and can close the anti shock orifice during filling leaving only the small orifice to try to get rid of large volumes of trapped air. In conventional air valves once the anti shock is lifted and pressurised the anti shock orifice will not open again until the pipeline is de-pressurised The valve is able to fully open the anti shock orifice at any time during pipeline operation. The uniqueness of this operation is such that even if the small orifice is functioning and a sudden large volume of air arrives at the valve the full anti shock orifice is opened. The valve is able to open either the small orifice or the anti shock orifice depending of the volume of the air in the valve irrespective of pressure This is a significant benefit. For Sewerage valve bodies the attempt to increase the clearance around the float and increase the compression zone by tapering the upper shape. A = operating fluid level, B = Max compression level ( during filling or surge ) C = final closing mechanism. The design objective is to keep the sewage effluent away from C. One way to do this is to taper the body so there is more volume at A and as the fluid moves up, the area ( B ) gets smaller and that assists in increasing the back pressure and keeps the effluent away from C. A more efficient method is to use a stepped body like ( far right ). This has more compression which allows the distance BC = almost 2 times the BC distance on the taper type. Also the use of a inverted cup type float which has less tendency to allow the build up of effluent or slurry on the float. This can generally be a problem on round floats which collect rags and other debris. The distance between the operating level ( A ) and the closing mechanism ( C ) is decided by the working pressure ( previous drawing ) and natural law, be cautious about sewage air release valves that are much shorter.
04 Service benefits series VG sewage air release valves The valve is split into 2 parts. A wetted area and a dry upper head which houses all the mechanisms. The upper head can be removed by removing the bolts. The use of an o ring ensures that very little torque needs to be applied to the bolts during service. The lower body has no drainage port because of the ease of cleaning ( but is an option if required ). The head is lighter and can be lifted out of the manhole and the lower float inspected and washed. Generally when the valve is operational the effluent level is as shown on the left. Having a spring loaded ( bias ) closure mechanism reduces splashing and the shape of the lower float further reduces effluent moving up into the head. The materials are all stainless steel and plastic so little or no corrosion. Benefits 100% ratio between inlet and outlet ensure max intake capacity Controlled discharge via anti shock orifice as standard ( most valves this is an option ) Full discharge via the anti shock and small orifice according to demand under pressure ( as standard ) Sealing system and discharge orifices protected from effluent during operation ( separate head ) Cup type lower float is better at keeping debris from building up on float Significant increase in size between the bottom of the float and the barrel wall ( new stepped barrel design ) Easy maintenance by removing head Spring loaded closing mechanism ( bias ) as standard ( most valves this is an option ) When comparing series VG air release valves with other valves check the costs against, function, materials and all options like inlet outlet ratio, bias mechanism and the inclusion of an anti shock orifice that functions under pressure. For further information see
05 Controlled venting via anti shock during initial filling and at any other time during pressurised operation according to demand. Anti shock is able to open fully under pressure.
06 Valve under pressure - fluid level at operating level Tubular float is efficient at reducing build up as it has no upper surface like a sphere
07 Valve under pressure - fluid level at operating level Small bubbles of pressurised vented by small orifice as they arrive at the valve
08 Pipeline pressure goes below atmospheric Valve responds by opening the large orifice fully and the atmospheric air flows into the pipeline to balance the pressure