CFW supplies a full range of rotary valves for any application. Ask our sales engineers about the rotary valves that will best keep your pneumatic conveying system operating at peak efficiency.
Rotary valves are commonly used to introduce bulk solid material into a pneumatic conveying system. They include almost all rotary airlocks or rotary feeders and so are known by any of these names, and are also called rotary seals, cellular wheels and star feeders. They maintain both the flow of materials between components and the required pressure of a pneumatic system, protecting it from pressure drop. Rotary valves can be used with both negative and positive pressure systems and across a wide range of system pressures, depending on the design that is used. Although they are the most common feeding device, poor selection of the size and type means that they are often more troublesome than necessary. Correct selection is crucial for satisfactory performance and depends on the particular conditions of your conveying system.
The rotor in a rotary valve has several blades or vanes with spaces (pockets) between them. Material drops from a hopper above the valve into the pockets as they turn to face upwards in succession. In this way, bulk solids can be introduced into the conveying system against pressure. A rotary valve that is directly connected to a hopper which contains material can control the feed rate by changing its speed of rotation, and is characterised as “flood-fed”. They are also used downstream as airlocks, pressure seals (e.g. seals for negative-pressure cyclone and dust filter outlets) and flame or explosion barriers.
The choice of rotary valve depends on the application. Perhaps 60% or more of applications can use standard units, while the remainder will need specialised solutions.
The two major kinds of rotary valve are drop-through valves (mostly for free-flowing materials like grain, rice, sugar and polymer pellets) and blow-through valves (used for less free-flowing powders that tend to stick or compact in the pockets, such as milk powder, flour and cocoa powder). In the former, the material is entrained into the conveying air after passing through the valve, whereas in the latter the air actually passes through the valve, helping to prevent any build-up of material. Drop-through valves are more suited to abrasive materials, whereas blow-through valves have larger capacities.
There are also off-set valves with a side inlet that prevent the material from completely filling the pocket. This type of valve prevents shearing of the material, since it goes into the trough of the pocket. For this reason it is often used with pellet.
Some pressure losses are inevitable, since the pockets of the valve release pressurised air as they turn towards the outside of the system (a phenomenon known as “blowback”), and because a clearance needs to be maintained between the rotor vanes and the housing. This can lead to problems with the valve, including unstable material flows, line blockage, valve wear and low feed rates. The clearance between the housing and the rotor helps determine pressure drop and tends to deteriorate over time as the valve wears out. Pressure drop is also affected by operating temperatures that significantly exceed ambient temperatures, as well as the type of material, the use of venting, system pressure and the head of product above the valve. Poor leakage estimates can lead to wrongly sizing the blower or fan of the system.
The operating principle of the valve also makes it susceptible to wear from abrasive materials, especially material fed into positive-pressure systems. The leakage across the high pressure differential of the valve causes high-velocity air streams, which result in erosion of the valve when fine particles are entrained in the air. This erosion can be even more severe than the abrasion of the valve. The abrasion can be minimized with the right materials. Erosion and pressure drop can be eliminated when feeding into a vacuum conveying system, but not when off-loading from a hopper. Conversely, in positive-pressure systems, off-loading can be done without leakage or erosion, but not feeding.
High rotor speeds can lead to inconsistent filling efficiency. The feed problems caused by backflow of gas and biased intake can be mitigated by ensuring that the valve is well vented. The feed channel needs to be the right shape to ensure proper flow across the entire cross-section. In this regard, a short standpipe inlet can be useful.
The components of rotary valves can differ markedly in their suitability for the application. Some considerations include
- Seal type: Seals vary widely in cost, ease of maintenance and efficiency.
- Rotor vane type: Standard rotors tend to have a pulsed rather than a smooth output. Helical or staggered vane designs are available that mitigate this effect where it is a problem. Vanes may also be closed (constructed with a disc at both ends of the rotor) or open. The closed type gives significant problems and should only be used where heavy material shearing cannot be avoided.
- Number of vanes: The standard number is eight. A larger number improve sealing while reducing filling efficiency.
- Blades: Where practical, a replaceable design may be chosen. A variety of construction materials are possible, including bronze-tipped and hardened steel. Polished stainless steel rotor construction will resist abrasion and prevent contamination of the flow material.
- End covers: These can be mounted either with inboard bearings or outrigger extensions. The latter is generally used for more demanding purposes.
- Abrasion protection: The rotor and internal components can be protected from the inevitable problem of abrasion in various ways. Surfaces can be equipped with ceramic linings (an effective but costly solution) or be hardened with plasma-sprayed or electrolytically deposited materials.
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