Lime is used by a large majority of the chemical process industries in either a powder or liquid form. Engineering and maintenance personnel often face a long ," and potentially expensive ," trial-and-error process to find the best process equipment to handle lime.
Limestone is mined from a quarry as calcium carbonate (CaCO3). It then is crushed and fed into a kiln at approximately 2,000 Degrees F, where the carbon dioxide is burned off (calcining) to make calcium oxide (CaO).
CaO is ground to a powder in either a tower mill, spiral classifier or slaker. Milk of lime is used for many industrial purposes, including pH control, power flue-gas cleansing, calcium extraction in pharmaceutical manufacturing and more. One of the most common uses is in the chemical process industry is pH control.
The reason lime is so difficult to handle in piping systems, instrumentation and valves is that lime particles are very jagged. Lime does not dissolve, but instead is merely suspended in solution. Any cracks, crevices or void areas will cause the lime particles to fall out of suspension and fill these voids. Lime further aggravates this situation when it hardens in these collection points. The lime changes its state to a solid mass of material ," commonly referred to as scaling. Scaling causes a pipeline's inner diameter to become smaller and smaller. Material buildup on valve seats and other surfaces can cause the valves to freeze in position.
This schematic shows the typical lime treatment process used in many process industries.
Tips for lime service
Process equipment, instrumentation and valves selected for use in lime slurry systems should limit cavities, cracks and void areas. Even a small collection point can cause equipment failure and countless hours of downtime and maintenance.
The ideal product for use in lime slurry should have the ability to clean itself or break apart scale and also should be completely free of void spaces or cavities. If any piping product is to be installed in a lime slurry system, it should be full port as well to limit obstruction or potential lime slurry buildup.
A common suggestion for increasing valve performance in lime slurry systems while using plug and ball valves is to use an actuator that is significantly oversized. The actuator size is increased, so the actuator output capability is roughly two times the normal manufacturer's recommended torque requirement in clean liquids.
Actuator oversizing might improve performance or decrease downtime resulting from sticking valves, but it will not solve all related maintenance problems. Because lime is very abrasive, many ball and plug valves will be affected severely by the lime. A stellite coating or another hardened material on the ball will help protect a valve against the abrasive nature of lime.
The seats also are a major concern. Again, hardened-steel seats with a scraping edge are probably your best alternative in lime applications. "Scraping" hard-coated metals will perform better in scaling substances because they have the ability to scrape built-up material off the ball and plug surfaces.
Limestone removed from caverns such as this one finds use in many industrial applications.
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 with scale buildup is turned through these seats, usually resulting in a short lifetime. Because ball and plug valves have cavity areas housing the ball or plug, a substantial amount of material will accumulate in this cavity area over time. If possible, you should install flushing ports in the valve so the body cavity area is flushed with water to clean out material accumulation after each cycle. This process will help minimize material buildup in the cavity area.
It can be expensive to build a ball or plug valve with all the above-mentioned features. The cost of a ball or plug valve equipped with these features can be five to six times that of a traditional Teflon-seated ball or plug valve. Unfortunately, however, most Teflon-seated ball or plug valves will not perform satisfactorily in lime slurry.
Gate and knife gate valves
Gate and knife gate valves can be used in many slurry services. With many gate valves, however, you must be willing to sacrifice tight shutoff in lime slurry service. Most gate valves force the gate into a wedge area 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 be a spot for material accumulation. The lime will accumulate in this area, cause difficulties in valve operation and could prevent sealing the valve completely against the line pressure.
The ideal knife gate valve for lime service features a hard-surfaced leading knife edge. For surfacing, stellite or some other material capable of protecting the softer steel blade is used. Actuator forces in knife gates also should be increased to give the valve the ability to cut through or close tightly against the lime buildup in the wedge.
The knife of the knife gate will be exposed to scaling. The scale buildup on the knife most likely will result in packing problems in knife gates. The scale will accumulate on the valve's knife. As the knife is opened, the scale buildup will be dragged through the packing, requiring increased forces to open the valve. The packing also will be affected severely when this material is dragged across it. With most knife gates, you will experience significant packing leaks.
If you are going to use knife gates in lime slurry service, you should incorporate a scraping packing material. This material would be a hardened substance that has the ability to scrape the knife clean with every operation. Also, the knife gate valve should have increased actuator forces capable of dragging this knife through the packing material.
Pinch valves are a good solution for lime slurry service because they have a straight-through design with no crevices or cavities for material collection. A pinch valve is a very effective device that has a self-cleaning effect on scaling materials.
A rubber tube or sleeve is pinched by steel bars on the centerline of the valve to close. To close the pinch valve, you begin by stretching the rubber sleeve. As you stretch the rubber sleeve, the material or scale buildup begins to flake. As you continue to close the valve, the flaking becomes greater, but the fluid velocity increases substantially. Thus, the flaked material or scale is pressure washed from the elastic surface of the rubber sleeve.
Shown here are 8-inch air-actuated pinch valves handling lime slurry in a power plant application.
Pinch valves also address abrasion concerns. In abrasive flows, you have two options. The first is to make the ball, plug or gate valve and piping materials much harder. The second approach is to make the valve or piping material softer. Softer materials allow the abrasive particles to bounce off the surface without destroying it.
For this reason, pinch valves have been used in mining applications on very coarse slurries for the past 30 years. With any mineral-based slurry, pinch valves are a very viable option for protecting against abrasion. A pinch valve also offers protection against clogging or jamming that can occur with other valves in lime slurry service. Many valves such as ball valves with stellite or harder coatings might be able to withstand the abrasiveness of lime slurry. However, they are subject to jamming or clogging because they have cavities that allow for material collection.
Pinch valve selection must be performed very carefully and with due diligence. Stainless steel or carbon steel ball valves and plug valves do not vary greatly from one reputable manufacturer to another. As long as you use one of the "more reputable" ball or plug valves, you most likely will have a valve free from porosity or imperfections. In addition, some ball or plug valves have modified designs to enhance performance in difficult services.
Pinch valves, however, can vary greatly from one another, and rubber quality and properties can differ drastically from one manufacturer to another. A good analogy is the purchase of automobile tires. Pretend you have the option to purchase either a 30,000-mile set of tires or an 80,000-mile set. Side by side, these tires look almost identical, but the 80,000-mile-rated tire certainly will cost more. For the extra money, it will give you two times the useful life of an inferior tire.
My point regarding pinch valves is that if you have tried them in the past and have been unhappy with product performance, then perhaps the make you selected was simply "inferior." You might wish to give pinch valves another try with a different make and manufacturer. Good designs are available, and reputable companies will stand behind their products after the initial sale. A high-quality pinch valve typically can handle lime slurries without any special product enhancements.
A pinch valve or diaphragm that has a preset weir could decrease valve performance quite substantially, however. This nonflexible weir will accumulate scale; because it does not flex, it will result in increased wear to the rubber sleeve. The nonflexible weir also defeats the self-cleaning effects of pinch valves.
Is it a fit?
I have noticed that many manufacturers and valve users try to force their standard valves into applications in which they do not belong. Obviously, a large process plant will want to standardize products as much as possible to cut costs in spare parts and personnel training.
However, what many of these large plants fail to recognize is that this practice might prove very costly. I remember a situation in which a company was using a standardized control valve that needed to be repaired every six months. By using a product that was more suited for a slurry process, the plant was able to double its mean time between failures.
The plant had 22 control valves in this process. The five-year operating cost of the previously used 22 control valves, excluding the cost of downtime, was $242,000. By replacing these valves with better-suited slurry control valves, the five-year cost was reduced to only $55,000.
The financial ramifications of improper valve, instrumentation and piping selection for processes such as lime slurry can have a long-term negative impact on most operations. Although a simple lime slurry control loop for pH control in many chemical plants is a very small portion of the process, it can be a large drain on operating costs. Plant decision-makers would be wise to choose process valves and instrumentation for this portion of the process carefully.
Chemical process plants can select from a wide array of valves for use in lime slurry service. I did not exclude any types of valves intentionally. I attempted to focus on the types of valves more commonly used in lime slurry, as well as to offer recommendations that might help improve valve performance, regardless of which type of valve is selected. Improper valve selection for lime slurry service can have a significant negative effect on continued operating costs. A larger up-front investment could result in quite substantial savings in the future.
Loudin is president of Larox Flowsys Inc., Linthicum, Md. Contact him at [email protected].