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May 30, 2008
4 Min Read
Selecting the correct slide gate or diverter valve for a conveying application is critical. Getting it wrong can mean the difference between system success and failure, especially when handling challenging materials. Several factors should be considered during the selection process, including material characteristics and their compatibility with valve seat and seal materials, conveying parameters, actuator requirements, valve maintenance, and environmental protection. Facilities should consider all of these factors to select the right valve for the application at hand.
Material characteristics determine which type of valve is required. Coarse materials require valves and diverters with greater clearances, while fine materials such as carbon black, calcium carbonate, or titanium dioxide require valves and diverters with tighter tolerances in order to ensure dust-free service, prevent material leakage, and guarantee a clean plant environment. Other material characteristics that are important when selecting a valve are whether the material is sticky, abrasive, friable, corrosive, or a combination of these characteristics. To ensure maximum service life, appropriate seat and seal materials must be selected to handle a range of material characteristics.
Conveying parameters—whether the slide gate or diverter is used in a gravity or pressure installation—must also be considered when selecting a valve for a particular application. Material bridging in gravity applications presents a big challenge. While aeration or vibration can be used to improve material flowability, these techniques sometimes have undesirable consequences. Slide gates that are used to control material flow can be adversely affected by the use of these flow aids, especially if the slide gates are designed strictly for gravity applications. A common misconception is that if aeration pressure of 5 psig works, 25 psig will work even better. This is not the case. When using aeration, the overall goal is to infuse the material with air to enhance product flowability. This procedure should be followed using lower pressures over a longer period of time rather than higher pressures over a shorter period, achieving the same goal while minimizing the deleterious effects of aeration and maximizing slide gate service life.
When choosing a valve for material-handling applications, a key consideration is to select the appropriate actuator. The most common actuators include pneumatic cylinders and electric, hydraulic, and manual actuators. A facility should choose the appropriate actuator based on its initial cost, duty cycle, travel speed, and cycle frequency. Pneumatic cylinders are the most common actuators used for process valves. Clean and easy to maintain, they have a continuous-duty rating. Electric actuators are desirable in low-temperature applications, in which compressed air is vulnerable to freezing. The downside of using electric actuators is their slow actuation speeds, limited duty cycle, and high initial cost.
In fast-paced process environments, there is a premium on time. Hence, maintenance considerations should be a significant part of selecting a slide gate or diverter valve. Important factors to consider include service life, ease of maintenance, accessibility, the downtime required to rebuild a unit, and the cost of spare parts. More-demanding applications require frequent maintenance. In such cases, it is advantageous to use a slide gate or diverter that can be serviced easily or serviced while it is still installed. If minimal downtime is critical, a spare or campaign valve should be kept in reserve, enabling the plant to reduce downtime, increase production, and lower the stress on maintenance personnel.
The choice of materials used in the construction of slide gates, diverter valves, and seats and seals can affect the environment. For example, when valves are intended for outdoor use, nylon should not be used because of its water absorption characteristics. Materials that are resistant to corrosion such as aluminum and stainless steel should be used in place of carbon steel.
It is clear that many factors should be weighed before selecting a slide gate or diverter valve for a material-handling application. If a facility views each installation as unique, especially in challenging material applications, it will successfully choose the right valve, at the right price, with the right features.
Shawn Werner is chief engineer for Vortex Valves North America, a division of Salina Vortex Corp. Werner received a BS in mechanical engineering technology from Kansas State University. A member of the Society of Manufacturing Engineers, he has 15 years of experience in the dry bulk solids industry.
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