The trend to pump slurries over longer distances at higher solids concentrations is putting slurry valves to the test, as these severe applications have high pressures and are very abrasive, according to AR Controls Director Julien van Niekerk.
Another factor to be taken into consideration is that the media conveyed is often corrosive. In general, engineers designing for process efficiency find that material and equipment capabilities pose the most serious limitations on allowable process conditions.
The most abrasive media found in the mining industry are slurries resulting from hard ore types and ferrosilicon. The media and process properties vary in different parts of the extraction process. In metal-extraction mines and plants, for example, abrasive slurries are found in the milling, cyclone, and DMS circuits, again in concentration feed and tailings, and to a lesser degree in flotation and thickener areas.
Particle size, shape, hardness, and density all play an important role, as well as the angle of impact on the wear surface. Pumping the media at a high solids concentration, pressure, or velocity also leads to elevated levels of abrasion. These wear effects are exacerbated when throttling abrasive media.
“If the media transported is abrasive, it is best not to install valves on the bend of a pipe, or on a Y-joint,” van Niekerk cautions. It is also important to consider whether the application is bidirectional or unidirectional, and to ensure that valves are installed in accordance with the installation instructions provided.
Rubber-lined gaskets may be convenient to use, but can pose problems for certain gasket-free or integrated-gasket valve designs. It is best to investigate the gasket valve requirements in the early project phase to ensure that rubber lining is not applied on the mating pipe flange faces. When carrying out a valve selection, it is important to not only look at the process conditions, but also to consider the general installation environment. For example, the media might not be corrosive, but the environment may very well be.
“It is always best to use full port valves in slurry applications. Valves with a torturous flow path or with an element directly in the flow path, such as butterfly, globe and weir-type diaphragm valves, will wear prematurely in slurry applications. This refers back again to the importance of valve selection,” van Niekerk elaborates.
AR Controls offers unique products for when there is a requirement for the throttling of abrasive media. For example, DeZURIK RCV rotary control valves have been engineered specifically for high-precision throttling control in severe-service applications where high-pressure media contains entrained water vapour or suspended abrasive particles.
These combine the control accuracy of globe valves with the strength of severe-service ball valves. The valves are available with a tungsten carbide or heat-treated nickel overlay plug. Various other valves are available for abrasion isolation applications. “We also offer customers the option of using remote-mount electro-pneumatic positioners, rather than having the positioner mounted directly on the valve,” van Niekerk highlights.
The positioner and its complementing air-preparation equipment are supplied in a plug-and-play stainless steel pneumatic enclosure that can be custom-made according to individual requirements. Mounting the positioners remotely in enclosures helps protect the positioner from environmental damage, and is ideal when valves are in hazardous or hard-to-reach installations. This also prevents tampering and unauthorised entry to the equipment.
“Materials technology has a direct impact on the valve industry,” van Niekerk comments. Advanced materials and coatings are being developed continuously. However, even though innovation can be rapid, market readiness is important, as resistance to change can be a large obstacle to advancement.
Valve specialists can identify suitability issues early on, and hence it is best for them to become involved from the outset. “They will be able to offer the best solution for the most challenging applications. Working closely with a specialist valve supplier will certainly help to reduce cost of ownership and downtime,” van Niekerk elaborates.
It is also important to inspect valves regularly in order to detect signs of wear and failure early enough, so that valves can be maintained or repaired before they become irreparable.