The production of knife gate valves for a gold company.

Long-life knife-gate valves for slurries

Vesconite – a specialized bearing material made from internallylubricated low friction polymers – is being put to good use in knife-gate valves used in the gold mining sector, reports Phillip de Villiers.
^ The production of knife gate valves for a gold company.

Article by David Sear

When Vesconite Bearings’ team visited knife-gate valve manufacturer A.C.Valves to fully understand how its client was using low-friction wear-resistant Vesconite polymer guide strips in knife gates, the company was manufacturing two valves for a South African gold mine that had experienced a break-down.

The 20” valves had been requested the day before and the mine’s supplier was due to pick them up on the day of Vesconite’s visit to the company, which can manufacture 200 - 300 knife gate valves a month depending on the size of the valve.

A.C.Valves director Collin Scott notes that the gold mine required two narrow-design wafer ring flange (RF) bi-directional valves, which are A.C.Valves’ most popular design and the design that uses Vesconite as a guide strip. The valves, which handle light and medium slurries, are relatively thin and are designed in keeping with mining company plant designs that require small compact plants.

The wafer RF valves consist of two cast iron pieces with a stainless steel gate that lies sandwiched between the two. A PTFE rod with an O-ring acts as a scraper at the bottom of cast-iron valve halves and a rubber seal fits onto the blade. Three Vesconite strips, meanwhile, are fitted on to the valve casing and act as guide strips, describes Scott. “The Vesconite gives a longer life to the blade. It prevents metal-on-metal wear, which would otherwise result in scratches on the blade which, when it was retracted into the stuffing box, would cause leakages,” he elaborates.

Scott notes that he manufactures quality products that will not breakdown and cause downtime and unscheduled shut downs for unexpected replacements and maintenance. His clients are aware that he has developed a product that its known for its quality, for its improved sealing, repairability and parts availability. “We have had no complaints about the Vesconite guide strips and there has not been a case where the valve has failed due to the guide strips,” Scott continues. “I have been using it for 10 years and it has worked perfectly,” he says.

Many of A.C.Valves’ customers are in the mining industry, and these companies would require robust knife-gate valves of different sizes depending on the size of their plants. They would need reliable valves that do not result in the loss of any commodity-containing slurry. Other customers include those in the water industry. These also have demanding requirements since valves are specified to remain in situ and have no maintenance requirements for twenty years.

A.C.Valves was established in 1996 and specialises in knife-gate valves that are supplied to many countries in Africa, as well as Canada, Argentina and other countries.

Meet Phillip de Villiers

Phillip is a technical consultant and sales representative with Vesconite Bearings in Johannesburg, South Africa.

He can be reached on +27 11 616 1111, via, or emailed at

Vesconite key features

Internal lubrication: Vesconite has built-in lubricants that translate into low friction and long life, even where no lubrication is applied.

High wear resistance: Vesconite provides outstanding wear resistance, resulting in extended service life compared to bronze bushings. Unlike many traditional materials, Vesconite is less affected by poor lubrication and dirty conditions.

High load bearing strength: Vesconite has very low creep rates and can therefore operate under tight tolerances. It has a much higher load capacity than nylon, and its compressive strength and clearances are not affected by water absorption.

Dimensional stability: Vesconite does not swell when exposed to water or humid conditions, while nylon can swell up to 3% when immersed. In most applications, little allowance needs to be made for moisture changes.

Low friction: Vesconite has outstanding dynamic friction, with a friction coefficient of less than half that of bronze or nylon. This allows for a higher PV ratio (load x speed).

Temperature limits: Vesconite is suitable for continuous use up to 65° C in wet conditions.

Shaft friendly: With Vesconite, the wear rate of corresponding metal pins and shafts can be reduced by as much as 90%. The resultant cost-saving benefits can alone justify a switch to Vesconite in many applications.

Chemical resistance: Vesconite is resistant to dilute and moderate acids, organic solvents, oils and petroleum. It has limited resistance to strong acids and alkalis.

Easy to machine – easy to fit: Vesconite machines easily on standard wood- and metal-working equipment, such as lathes, milling machines, bandsaws, drilling machines, planers, spindle molders and routers. As many customers around the world will testify, Vesconite is a both a pleasure to machine and fit.

Cost saving: Combining an exceptionally low wear rate and shaft-friendly features, Vesconite and its corresponding parts require little maintenance – resulting in significant cost savings.

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