Automating electric valve actuators for tailings distribution networks

Tailings disposal is one of the more difficult aspects of a mining operation. The dense and abrasive material is thick and difficult to pump, and it must be efficiently transported over long distances to the disposal areas (Fig. 1). This is accomplished using strong pumps, a lengthy network of pipes, and many automated valves to divert the material to different areas in the tailings pond. Figure 1 Tailings networks must transport dense, abrasive and sometimes hazardous material from processing areas to remote disposal areas. Proper automated valve selection for this application is crucial for success. Normally, these tailings network valves take the brunt of the wear, and they can be expensive to keep working and operating. This article focuses on those automated valves and discusses design choices and technical features for new and existing tailings network applications. Dewatering increases challenges Composed of the unused components of mining ore, tailings are typically a highly abrasive mix of gravel, pulverized rock and dirt. Tailings used to be transported in a slurry, created using large quantities of water, but environmental recovery efforts at virtually every mine for dewatering have resulted in much more erosive and viscous tailings. The gooey, abrasive mix must be pumped over significant distances through a network of pipes to reach the tailings disposal areas, where it is diverted by automated valves to evenly spread the tailings across scattered storage locations. The application was never easy, but the dewatered tailings slurry is now much more difficult to pump and transport because it is even more viscous. Pressures at the head of the tailings network are significantly elevated, and the thickened material is much more punishing on the diversion valves. Government entities are also highly focused on effluent treatment and management, so valve status and remote monitoring are becoming increasingly necessary. Valve selection Knife gate valves are best suited for tailings valve applications (Fig. 2), but there are significant design differences among valve models. To ensure best operational performance, any valve selected for tailings service should have the following design features: High-pressure-class flange ratings — Water removal has significantly raised the pressures required to transport the material, and these pressures are further elevated when long-distance transport is involved. Typical applications require full American National Standards Institute (ANSI) class 150 flange ratings, and some applications may require class 300 flanges near the head of the network.Full port design — Any valve in slurry service should incorporate a full port design because any reduction or change in inside pipe diameter will create higher velocity areas that accelerate damage.Adjustable wear rings — Erosive slurries tend to wear the valve at the six o’clock position. Hardened wear rings on the face of the valve can extend service life, and rings that can be easily rotated and/or moved to the opposite side of the valve will last significantly longer.Advanced valve seal designs — Valve seals that utilize broad, soft seats will seal better and last longer, provided those seats are protected from erosion. Single O-ring seals tend to fail quickly, but larger, specialty polymer seals will perform much better, especially if they are located below the lip of the valve, where they are better protected.Advanced and adjustable gate seals — Recent gate seal improvements incorporate transverse seals that lock into the seat sealing surface to create a gateedge seal, minimizing leakage from the gate edges. These seals also allow for field adjustment of the packing material while the valve is installed and in service. Figure 2 This Clarkson KS1 valve with electric actuator is installed in a tailings application. It includes a bidirectional, full port design, advanced long-life seals, zero-leakage environmental seals, and reversable and rotatable wear rings. Proper valve selection is paramount to minimize operating cost. Poorly selected valves have been known to wear out in days and even hours in harsh tailings service, so it is typically wise to invest more initially for the right valve. Selecting an actuator Historically, the remote location of tailings diverting valves severely limited the options for valve actuation. Hydraulic actuators were often employed, and the mine operators drove to each valve and used a local hydraulic pump or hand jack to adjust the valve. This worked well for remote valves requiring only infrequent adjustment, but it created operator exposure concerns when dealing with hazardous tailings applications. It also provided no means for remote monitoring and operation. Valve actuators are generally limited to hydraulic, pneumatic or electric operators. Hydraulic actuators have traditionally been the preferred option for high torque applications, though they are prone to oil leaks and require significant maintenance to keep the hydraulic actuating system operational. Pneumatic actuators are ubiquitous and inexpensive, but running the required instrument air to remote locations is expensive, and maintaining a pressurized air system in the dusty and arid environments common to mines can be very challenging. The last choice, electric actuation, has generally not been utilized due to their expense, limited torque and maintenance problems, but newer designs are addressing these and other issues. While hydraulic and pneumatic actuator designs have remained largely the same for decades, electric actuators have seen dramatic improvements in design and equipment features (Fig. 3). Stronger, smaller motors with lifetime lubricated gear boxes have allowed electric actuators to achieve much higher torques, and with less required maintenance. The motors are also much more efficient, allowing smaller electric actuators to run on the power provided by solar panel/battery installations. Tailings piping systems use larger line sizes and valves upstream, and smaller line sizes and valves downstream. The upstream electric valve actuators can often be supplied with line power as they are close to the main mine areas, and this level of power is often needed due to the high torque and power requirements of large actuators. Downstream valves are smaller, as are their electric actuators, and these can often be operated with the reduced power available from a solar panel/battery installation. Wired and wireless digital communication protocols allow electric actuators to be controlled and monitored from remote locations, and modern electric actuators provide a host of diagnostic information that can be used to predict failures and speed troubleshooting. If a wired protocol is used, only a single pair of wires is needed, so the amount of conduit and wiring required from each electric actuator to a host, such as a distributed control and/or an asset management system, is minimal. In most installations, a wireless communication protocol can be used, eliminating the need for wiring to a host. Diagnostics ease maintenance, and less of it is required because newer electric actuators are much more reliable. Though initially more expensive, electric actuators usually have a significantly lower life cycle cost than other valve actuation options, especially in greenfield applications. All these improvements have altered the economic landscape for tailings network valve actuation. Depending on the size, location and criticality of the valve, an electric actuator is often the best choice for many tailings applications. This is especially true if valve monitoring and/or remote control is necessary. Advanced diagnostic and control software paired with a wireless network provides remote monitoring and control of the valve. Low power applications can employ a “sleep mode” to minimize power use when the valve is not in operation. Electric actuators in action The following use case shows how electric actuators installed on knife gate valves in a tailings system improve operation. Coal mine. A large coal mine in Australia uses a combination of open-cut and underground mining. A new project will extend its life by expanding the mining area, allowing overall production rates to be maintained, with mine life extended to 2039. The new mining area will need to extend through the existing tailings storage facility (TSF), which holds approximately 30 million m3 (39 million cu yd) of tailings. To enable continuity of mining, the mine operator will need to relocate these tailings to a new TSF. A tailings relocation system and water management system modifications have been designed to provide the infrastructure required for the relocation of this material. Figure 3 The latest generation of electric actuators (Bettis XTE3000 with rising gearbox from Emerson shown) are smaller, more reliable and generate far greater torque than older models, yet require far less power. The piping distribution system for the new TSF will include four electrically actuated and 30 manually operated DN500 Clarkson KS1 knife gate valves, with the electrically actuated valves using a BIFFI ICON3000 actuator running from line power. The electrically actuated valves will be operated up to 100 times per week, and the mine operator is confident the selected valves and actuators will perform as required during this difficult operation. The valves were selected based on the company’s existing relationship with Emerson and its ability to provide a total solution. Conclusion Automated valves installed in tailings systems are subject to punishing conditions, and in many installations they are required to operate frequently, often hundreds of times a month. These valves are often installed in locations that are difficult to access for maintenance, so they must operate as designed for long periods of time without failure. When maintenance is required, it needs to be scheduled proactively, instead of running to failure, to minimize repair costs and maximize production. Electric actuators with remote connectivity provide the information required to monitor valve conditions and inform staff of required maintenance, increasing uptime. Mine operators should evaluate these valves carefully, consulting with local vendor experts whenever possible to make sure the right type and model is selected to meet the specific requirements of their tailings systems.

Automating electric valve actuators for tailings distribution networks

Authored by:
Nick Farrand, Daniel Quintero

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