Wear in Rotary Valves

Alumina ceramic lined rotary valve

When rotary valves, or blowing seals, begin to wear they leak causing a reduction in line pressure and the overall output of a pneumatic conveying system, to the point where it becomes necessary to change the valve in order to avoid blockages. If there are multiple valves feeding the conveying system, wear can become a serious and expensive problem very quickly.

Figure 1 shows a 300mm NB rotary valve fitted with a diamond ground 95% alumina ceramic wear resistant lining system and adjustable hardened steel vane sealing blades. The valve is to feed silica sand into a pneumatic conveying system.

Figures 2,3 & 4 show wear to the ceramic rotor bore lining system caused by high velocity hard abrasive particles leaking past the side of worn steel vanes (see Fig.6). The grey colour around the edges of the vane is were grit blast erosion is most prominent.

Wear to the ceramic rotor bore lining system
Non-product side of rotor bore

Figure 2A shows again the non-product side of the rotor bore with the narrow band of wear close to the end plate. The wear extends aproximately on third of the way around this side of the lining system. The maximum wear of the bore lining, between the end plates and not including the narrow bands of wear close to the end plates, is around 0.15mm (0.006"). The wear on the product side of the ceramic lining system is negligible.

Figure 3 shows how the leakage past the rotor vane ends has cut through the 14mm thick removable ni-hard housing for the ceramic rotor bore lining system. The abrasive leakage has removed approximately 1.5mm of ceramic at the corner of the rotor bore and a 14mm thickness of the cast ni-hard housing, in line with the ceramic's predicted wear life of around 10 times that of cast ni-hard.

Wear from leakage past rotor vane ends
Wear due to high velocity air leakage

Figure 4 shows the narrow band of wear at the edge of the ceramic rotor bore and the end plate lining. This wear is due soley to high velocity air leakage between the rotor vane ends and the ceramic end plate lining, on the non-product side of the rotor bore. If the ends of the rotor blades can be made more wear resistant, the narrow wear bands at the extreme edges of the rotor bore can be dramatically reduced.

Traditional methods of protecting the internals of a rotary valve, such as athin tungsten carbide coating, can improve the operating life of the valve by around 3 times, when used as protection against hard and sharp bulk minerals. Alumina ceramic can improve the operational life of a rotary valve by more than 20 times that of thin tungsten carbide coatings, before regrinding of the ceramic lining is necessary. Reaction bonded silicon carbide ceramics can provide further increases in wear resistance of up to 200 times that of cast iron.

Rotary valves working in positive or negative pressure environments suffer wear from air, laden with sharp abrasive particles, leaking at high velocity past the rotary vane sealing blades (see Figs.2, 3 & 4) and also through the rotor shaft stuffing box seals. Most of the wear occurs on the non product side of the valve rotor bore. The weight of the free flowing powder being carried through the product side of the valve tends to seal the air pressure by blocking the gaps between the vane sealing blades and the rotor bore. As the powder falls in to the air flow of pneumatic conveying system, and the now empty pockets rotate through the non product side of the valve, air and grits pass between the vane sealing blades and the rotor bore, usually at high velocity, wearing both the sealing blades and the rotor bore. The greater the pressure and volume of air and grits passing the vane sealing blades and through the rotor shaft stuffing box seals (see Figs. 7 & 8), the greater will be the wear in the valve and the shorter will be the operational life of the valve.

Worn steel vanes
Wear on secondary ceramic rotary seal face

Figure 7 shows 1.5mm wear on the whole secondary ceramic rotary seal face on each end of the rotor vane shaft.

Figure 8 shows the minimal wear on the stationary ceramic end plate lining system that it seals against. The rotary secondary seal always wears at a greater rate than the stationary end plate. When primary rotary ceramic seals are used on closed vane rotors, they can wear 4mm on diameter without any appreciable wear occuring on the stationary ceramic rotor bore lining.

Minimal wear on stationary ceramic end plate

Diamond ground ceramic valve lining technology has been thoroughly tested and continuously refined over the past 13 years as a retrofit system, requiring modifications to be made to the valve rotor and the valve body to facilitate the lining components. These advanced systems are used to protect valves against wear and corrosion caused through handling all manner of abrasive minerals and metals, such as silica sand, ferrous oxides, pearlite, sinter, china clay, grain, mill scale, slate dust, petro-coke, coal, fly ash, fertiliser, cast iron swarf, granite dust and chippings and many other abrasive materials.

Precision advanced ceramics have proved to be the most cost effective materials available capable of dramatically reducing extreme wear in valves that are used to control the flow of bulk minerals and they can be fitted to worn and new valves.

Omegaslate SuperSeal Systems Limited, 2 Chirk Close,
Forest Gate, Kidderminster, Worcs. DY10 1YG. United Kingdom.
Tel: +44 (0) 1562 755 824 Fax: +44 (0) 1562 742 979
Email: info@omegaslate.com