All rotary valves leak air (or gas) even where there is no pressure differential as even the product entering the valve displaces air into the inlet area.
This is significantly increased when operating under a pressure differential due to losses past the operational clearances and carry-over of pockets of high pressure decompressing into the inlet. The total of such losses are normally referred to as ‘leakage’ and given a single quantity value.
There are many factors that affect the rate of leakage and the ratio of the elements that make up the total. All are affected by the system conditions prevailing at any given time
In practical terms most manufacturers only offer leakage charts based on tests measuring leakage past valves in a static, product-free condition. Because the presence of product has a mitigating effect on leakage, known as the ‘Blocking Factor’, the manufacturer’s published rates are seen as having adequate contingency for providing allowances for sizing blowers, fans etc. Where leakage rates are considered critical then better analysis will be necessary, taking into all aspects of the duty conditions.
What is important is that upward leakage can have a significant effect on the system performance either positively or negatively especially where the valve is operating in a flooded condition.
The upward flow can reduce throughput by opposing product flow as well as aerating the product and reducing its bulk density. Allowance for this must be made when sizing the Rotary Valve. This is progressively less of a problem as valve sizes increase as the larger throat sizes make for better dispersion of leakage air. Valves should also be fitted direct to the outlet of any hopper so that leakage air can disperse easily. This is less of a problem when the valve is not controlling the throughput but care needs to be taken if the conditions could result in flood feeding at any time.