How the Right Rotary Valves can Eliminate the Need for Leakage Gas Venting

In the food industry, rotary valves are usually used to feed bulk materials into pneumatic conveying systems. In those processes, the rotary valve fulfills two decisive tasks: First, the turning rotor continuously carries the bulk material, such as starch, milk powder or other powders, from a silo or container into the conveying system. Second, the rotary valve seals off the overpressure area (e.g.+0.5 bar(g)) of the pneumatic conveying from the atmospheric area above the rotary valve. To ensure safe and efficient operation of the system in the long term, special attention must be paid to the leakage gas which is generated during operation.

What causes leakage gas and how does it affect the product intake?
Leakage gas is caused by the pressure differential between the pressurized conveying line and the inlet of the rotary valve. On the one hand, conveying gas flows through the function-related gap between the rotating rotor and the housing (gap gas). At the same time, the rotary valve chamber fills with conveying gas (scoop gas) after the conveying product has been discharged. Leakage gas is the umbrella term covering both gap gas and scoop gas. It flows upwards towards the downward-flowing conveying product from the silo / container, and as a result impedes the product intake. The fill level of the individual chambers of the rotor and consequently the throughput rate is not only determined by the product properties of the material being conveyed, but also by the leakage gas buildup within the rotary valve.

With conventional rotary valves, leakage gas is usually discharged via a lateral nozzle on the valve housing to improve the product intake at the valve inlet. However, this solution has the disadvantage that only scoop gas and not gap gas can be discharged. Additionally, part of the conveyed material might be removed together with the leakage gas through the vent nozzle, particularly when conveying powdery materials. As a result, not only is bulk material lost, but there is also a risk of older product residues collecting in the leakage gas pipe and later breaking off and falling into the valve. When conveying sensitive bulk materials such as milk powder, product residues can have a negative impact on the hygiene of the plant. Due to dead spaces in the leakage vent, potential contamination of entire product batches is a very real risk - even with careful cleaning. Furthermore, the installation of an external leakage gas vent involves additional investment and maintenance costs. Coperion's experts have decades of experience in conveying bulk materials and are familiar with the pitfalls of leakage gas removal for powders. To counteract these disadvantages, an innovative new approach was taken in the development of Coperion rotary valves.

How do Coperion rotary valves for powders achieve high throughput rates without leakage gas venting?
The rotary valves developed by Coperion for powders differ significantly from the usual units available on the market. They do not come with an external leakage gas vent nozzle on the housing and can thus effectively circumvent the problems mentioned above. In addition, they can be cleaned more easily and still achieve the highest throughputs. This is thanks to a rotary valve design with a particularly wide inlet without cross-sectional constriction. The inlet geometry, which is directly matched to the rotor volume, provides sufficient space to allow the leakage gas to expand at the rotary valve inlet and flow through the product column in the silo/container. As a result, material intake is not unduly influenced in the process. In addition, a high fill level is ensured, which still allows for high product throughputs.

Which conditions must be met in order to omit a separate leakage gas vent?
Installation without separate leakage gas discharge is subject to the following conditions:

1. The cross-section of the inlet geometry of the rotary valve must be appropriately large and optimally matched to the rotor volume, as with Coperion ZXD and ZRD rotary valves
2. The gap dimensions of the rotary valve must be accurately constructed in order to keep the gap gas to a minimum
3. The rotary valve must be installed directly under the silo
4. The silo has a discharge aid, e.g. a vibrating floor
5. There should be no cross-sectional constriction above the valve

Expert summary
Most conventional rotary valves for powders come with a separate leakage gas discharge vent on the housing or directly above the rotary valve. However, such external piping carries the risk of product residues and potential contamination, which can render entire batches unusable, especially in the food industry. At the same time, the conventional setup incurs additional investment and maintenance costs, which can be avoided with Coperion rotary valves without separate leakage gas venting.

In short, dispensing with an external leakage gas discharge vent has the advantage of lower investment costs, maximum operational reliability, better cleanability and increased hygiene.

Coperion's rotary valve solutions without separate leakage gas venting has been successfully used for many years in a large number of plants. Our experts are happy to advise you or your plant contractor on the benefits of using Coperion technology for your material handling applications.