Not all valves are made for every application, but some valves are best at handling the worst conditions. For this reason, a standard process valve may not function well in difficult, abrasive or harsh applications. Finding the best valve for more difficult applications may be a time-consuming and costly exercise.
In many process plants, standard valves may be installed in challenging applications, and they may perform horribly, needing frequent repair. Those same valves may be modified to improve performance, or perhaps plant personnel should search for a better designed valve specifically for the application.
Heavy slurries present challenges in many process industries. Slurries typically involve minerals that can be abrasive to traditional valves. Abrasive slurries are difficult for conventional valves.
Lime slurry is common in many industries—such as chemical processing, steel manufacturing, pulp and paper, mining, and power generation. It can be found in a powder or liquid form. Lime has different characteristics compared to clean or pure chemicals. It is abrasive and tends to scale in pipelines, valve components and instruments.
Lime Slurry Characteristics
Limestone is mined from quarries as calcium carbonate (CaCO3). It is then crushed and fed into a kiln at approximately 2,000 F, where the carbon dioxide is burned off (calcining) to make calcium oxide (CaO).
The CaO is ground into a powder in either a tower mill, spiral classifier or a slaker. Milk of lime is used for many industrial purposes, such as:
- pH control
- Paper filler
- Power flue gas cleansing
- Calcium extraction in food and
- Glass manufacturing
Lime particles do not dissolve but are suspended in solution. The particles are jagged and abrasive. Because lime is suspended in solution, any cracks, crevices or void areas in valves and piping systems allow for lime particle accumulation. Lime further aggravates this situation because it hardens in these collection points. The lime changes to a solid mass of material commonly referred to as scaling. Scaling causes a pipeline’s inner diameter to become progressively smaller and often causes valves to freeze in position because of material buildup on the seats and other surfaces. Because of these characteristics, the use of lime often leads to a long and potentially expensive trial and error approach to find the best process equipment to handle this substance.
Valves and instrumentation that will be used in lime slurry applications should have limited cavities, cracks and void areas. Even a small collection point can cause certain equipment to seize and become inoperable, which creates downtime and maintenance issues. Some plant operators oversize the actuator of plug and ball valves with the hope that this will improve the problem. They may increase the actuator size so it is capable of having an output that is roughly two times the normal manufacturer’s recommended torque requirement in clean liquids. This size increase may improve performance and downtime that results from sticking valves, but it will not solve all the maintenance problems.
Ball & Plug Valves
Many ball and plug valves will be severely affected by lime. Using a cobalt-alloy-coated ball or other hardened materials can help protect valves against abrasion. Valve seats can also be a major concern, and hardened steel seats with a scraping edge are the best alternative in lime applications. Scraping hard-coated metals will improve the equipment’s performance and reduce scaling by removing material buildup from the ball and plug surfaces.
Image 1. A pneumatically actuated, 10-inch, dual pinch design on a power plant FGD scrubber system
Most polymeric seats will not stand up over time in lime slurry. The scaling that occurs in these valves is like a wrecking ball to most polymeric seats. The ball or plug is turned with the scale buildup through these seats, and their lifetime is usually short because of this wear. Because these valves have cavities in which the ball or plug is housed, substantial material may accumulate there with time. If possible, flushing ports should be installed in the valve so that water cleans the accumulated material out of the body cavity after each cycle.
To summarize, if ball or plug valves are oversized to double the actuator torque, end users should include cobalt-alloy seats and all ball or plug surfaces should be made of a cobalt-alloy as well. The use of effective scraper seats and flushing ports would also improve the valve’s function.
Building a ball or plug valve including all these features can be expensive. The cost of a ball or plug valve equipped with these features may be five to six times more expensive than a traditional polytetrafluoroethylene (PTFE) seated ball or plug valve. Unfortunately, most PTFE seated ball or plug valves will not perform satisfactorily in lime slurry.
Gate & Knife Gate Valves
Gate valves or knife gates can be used in slurry service. With many gate valves, end users must be willing to sacrifice tight shut-off when they are used in lime slurry. Most gate valves have a wedge in which the gate is forced to close the valve. Knife gate valves have a sharpened edge to improve the ability to cut through solid particles. In lime service, the seating area will accumulate material/scale, making valve operation and sealing completely against line pressure difficult.