Slurry valves are to the mining and other process industries what midfielders are to soccer teams: They are not often heralded, but they are hard working. For managers of such operations, victory means maintaining uptime in processes that often involve heavily abrasive and corrosive materials. Keys steps to winning any valve challenge include:
- Understanding the specifics of the application.
- Picking the right pumps and valves for the job.
- Considering total cost of ownership over the life of the equipment.
Defining Slurry ApplicationsSlurries are used to move solids through refining and other industrial processes by mixing them with water. Though slurries are most common in the mining industry, other process industries -- pulp and paper, chemical manufacturing and power generation -- also include applications for slurry valves.
Nickel mining, for example, involves digging and blasting ore and dirt out of a surface mine. Depending on the particular facility, the ore and dirt may be mixed with water to form slurry for transport. The fertile slurry travels from the surface mine to the processing facility. After the nickel is removed from the slurry at the processing facility, the leftovers, which are called tailings, are sent as a slurry to the tailings pond for water recovery. Heavy-duty slurry pumps and slurry valves are required to move the nickel-rich slurry and waste tailings where they need to go in the process.
The abrasiveness of the solids in the slurry is a key consideration in determining the right type of valve for each job. When a valve is in the open position, the solids will continuously scrape against a valve’s port like a metal file or a sandblaster. When the valve is closing, the gate will be exposed to the same abrasion as the port. Depending on the weight of the solid and particle size, the abrasiveness can be extreme.
Some mining applications also involve corrosive chemical processes. In copper mining, for example, sulfuric acid is used to attract particles of copper as part of the refining process. The acid has the potential to corrode metal valve parts.
Likewise, the production of acid itself creates the need for tough slurry valves because of these same issues. For instance, the manufacture of phosphoric acid creates a buildup of waste, called scaling, around the valve’s gate and port area that makes it harder to open and close the valve. In addition, the manufacturing process involves periodically flushing the system with sulfuric acid, which is highly corrosive and requires valves made of specially coated materials and special alloys. In fact, dense scaling in phosphoric acid production can cause the average valve to fail on its first closure.
Types of Slurry ValvesJust as each soccer coach customizes his own playbook, the managers of slurry processes can choose from an array of types, sizes and material options when selecting valves to find those that best suit his application. Most slurry valves can be classified into one of three categories based on the functioning of the valve’s final closure element.
- In a knife gate valve, a relatively sharp metal gate pushes down and cuts through the slurry to create closure.
- Somewhat similar to a knife gate, a ported slide gate has a hole or port in the gate for the media to flow through in the open position. When valve closes, the port slides out of the bottom of the valve, allowing the upper portion of the gate to stop the media flow.
- In a butterfly valve, the gate remains in the slurry, parallel to the piping, and allows the slurry to pass through when the valve is open. When it is closed, the gate turns perpendicular to shut off the flow. Because the abrasion against the butterfly gate is constant, this style of valve is not suited to many slurry applications.
The valve seats also include a range of choices. They include metal-to-metal seats, which are more durable but tend to allow some leakage, and other sealing materials, which can provide drip-tight shutoff but must be selected carefully for durability and chemical compatibility. Many knife gate valves with elastomer seating have seat material options that can withstand temperatures around 300 to 400°F (149 to 200°C) and pressures up to 400 psi. This makes them suited to most typical slurry applications in mining and paper processing. For applications involving higher pressures and temperatures, some knife gate valves and ported slide gate valves are designed to meet ANSI Class 300 pressure limits and exceed 1,800°F (982°C).
Ported slide gate valves most often are used with very heavy solid content media, both in wet slurries and dry applications. For instance, these valves are found in coal handling and high consistency stock in the pulp and paper industry. The sliding gate cuts through the thick slurry more easily than knife gates. These gates also cut through heavy scaling effectively, and the design lends itself to a long seat life because little stress is put on the seat from the way the gate closes.
Slurry applications require valve designers to think about the three primary forces that can cause valves to malfunction: wear, friction and pressure. Wear is caused by the abrasion or corrosion of the slurry itself. The valve gates and seats must be made of materials (and coated if necessary) to withstand the materials in the process. Friction is another force. Whether it is metal-to-metal, metal-to-rubber, or some other combination, the sliding of the gate against the valve seat creates friction that stresses the valve components. The third force is pressure. Closing a valve under pressure is more difficult and requires harder materials. In designing slurry valves, the impact of all of these competing forces must be considered together. In choosing a harder material for a valve seat to withstand abrasion, for example, engineers must recognize that harder materials are more brittle and likely to break under pressure.
Choosing a Valve for an ApplicationChoosing the appropriate valve requires matching the range of valve types, sizes and materials to the specific needs of each application. Fortunately, all valve-shoppers can consider a few common variables to help narrow the options. They include:
- Slurry solids composition, which is the size of particles and hardness/abrasiveness of the material being processed.
- Slurry chemical composition, which is the specific chemical and percent concentrations of the slurry. The specific concentrations greatly affect the material of construction choices.
- Percent solids, which is the percent of the slurry material by weight that is solids. Typical slurry applications are 30 to 60 percent solids; extremes can be 60 to 70 percent solids, or even higher.
- Pressure and temperature can be a factor to consider in some applications. For instance, bottom ash handling valves in the power generation industry must handle temperatures as high as 1,500°F (815°C), and high pressure tailings pipelines operate at pressures exceeding 650 psi.
- The type of seal required also can affect the selection process. For example, is a drip-tight seal required at the specific point in the process that the valve will be used, or is some small leakage around the valve seat acceptable?
The initial purchase price of the valve is, of course, another important consideration along with the needs of the process. It is important for valve purchasers to make financial decisions based on total cost of ownership rather than simply the initial purchase price of the valve assembly. Total costs include:
- Consumables (the gate, seat liner, seat/seals and packing).
- The expected life of the product.
- The risk of downtime in the event of catastrophic failure.
Just as soccer coaches must carefully choose midfielders to have winning teams, mining and other process industries managers must take special care in selecting slurry valves to meet production and profit goals. To keep the total lifetime cost of a valve to a minimum, valve shoppers need to search for a valve with the right characteristics to match the unique application.