Two competing philosophies apply to valves used for abrasive slurry service. One philosophy says the valve trim should be hardened with special coatings, such as stellite, tungsten carbide or ceramics. The other philosophy suggests using a softer valve material, such as rubber.
Both approaches have strengths and weaknesses, depending on the application. For example, high temperature processes favor metal or ceramic materials. Most rubber materials have a maximum operating temperature of around 400 F. Although rubber in static seals can handle higher temperatures, the stretching and distortion of the rubber in abrasion-resistant valves limits its temperature range.
There are good reasons to match valve types to an application. For example, if the pressure drop across the valve is less than 50 percent of the inlet pressure, pinch valves may be the correct selection. If the pressure drop exceeds 50 percent of the inlet pressure, other valve types may be more appropriate. If the pressure drop is too high, cavitation is likely. Valves equipped with cavitation trim or multi-stage trim don't perform as well in slurries because of the narrow openings or holes required.
In general, a process with a turndown exceeding 10:1 would be well served by a V-ported control valve. An aggressive slurry, however, may shorten the lifetime of the valve trim. In certain slurries, trim can fail as frequently as every three to six months, even when coated with stellite or ceramic. If a plant uses many of these valves, the maintenance and repair costs can be staggering.
In many processes, valve selection isn't critical. However, in slurry systems, the financial impact can be significant. The one-year maintenance costs can vary from $11,000 to $48,000 per year based on the valve type. An abrasive slurry usually multiplies operating costs by 40 percent to 300 percent over non-abrasive processes. Appropriate equipment selection in these facilities is critical to minimizing costs.
Even relatively clean or non-abrasive processes have some abrasive elements. The financial consequence of modifying standard valves to function in slurry service can be expensive and performance may suffer.
Internal geometry
Even the smallest pocket or void volume in a valve becomes a natural collection point for slurry and suspended solids. If the void is near moving or rotating valve parts, it's likely to collect enough material to stop the rotation or movement. Then the operator can no longer turn the valve, or the actuator turns the stem of the valve so hard it breaks. The solids can attach themselves to the ball, which then shreds Teflon or metal seats, resulting in valve leakage.
Plug valves and ball valves with body cavity fillers have protective features that minimize the amount of material entering the void volumes. Although the design increases service life, it too can fail for the reasons cited above. Another consideration is operating torque. These valves usually have torque requirements 50 percent to 200 percent greater than standard ball valves and the actuators that rotate these valves can be quite large and expensive.
Knife gates are inexpensive and offer reasonable performance in slurry applications. In general, a knife or blade is pushed into some type of wedge that, when the valve is open, collects slurry particles. Some rubber-lined knife gates have improved wedge areas so that as the knife is pushed to the bottom, the wedge separates, allowing the rubber to seal against the knife. Rubber-lined knife gates, with its purge ports that release particles in the wedge area, offer abrasion resistance and a better sealing design compared to traditional metal knife gates. Unfortunately, the ports release material to the environment. Also, knife gates typically aren't used for control purposes. Most knife gates begin to leak over time because material that accumulates in the wedge won't permit full closure.
Scaling
Common processes prone to scaling involve lime, green liquor, brine, PVC, calcium carbonate and silica, all of which can build up on metal or ceramic surfaces. When the valve is closed, scale rips through and destroys the seats. Also, scale packs voids and pockets.
The pinch valve is appropriate for materials that scale. As the rubber sleeve flexes, it cracks and breaks the scale. Fluid velocity increases as the pinch valve sleeve closes, flushing material out. In general, pinch valves are self-cleaning.
Improved rubber
Rubber technology has advanced significantly during the last 30 years. Pinch valves have been on the market since the early 1950s, when they featured relatively primitive rubber technology. Many still do, so buyer, beware. Look for current rubber technology. A better grade of pinch valve brings improvements in process availability and reduced operating costs. Instead of adapting a standard valve, use one designed specifically for abrasive slurries. Pinch valves are used for abrasive slurries. They are self-cleaning in scaling service, they don't clog or jam. Remember, pinch valves are only as good as their rubber sleeves. They also can be used as control valves within their operating limits.
Todd Loudin is president of Larox Flowsys, Inc. He can be reached at 410-636-2250 or todd.l[email protected].
Figures: Larox Flowsys, Inc.