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Powder & Bulk Solids, June 2018

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TechnicalExclusive 10 J u n e 2 0 1 8 Lorenz slide gate – blind bolt hold A How to Properly Select Slide Gates At the most basic level, a gate is an object which permits passage into or through an enclosure. Gates as they apply to pneumatic conveying/material han- dling are no different. At the most basic level, gates open and close a path of material flow. The most ef- fective means of accomplishing this task is through the use of a sliding blade that moves across the gate opening to block the path of material flow. The most basic gates are simply just this. A basic shut off gate consists of a thin blade which is enclosed inside a body. The body creates an en- closed space that can be open and closed by the blade. This type of gate is simple to construct and operate. However it is not perfect and can only be used in certain applications. Other applications de- mand that the gate be able to provide seals against product leaks, seals to prevent pressure loss (or vac- uum loss), or special considerations to reduce wear. Leaks A simple shut off gate is prone to leaks in certain situations. It will adequately perform its function if the material is coarse (large enough to bridge the small gaps inside the gate), and is installed in a gravity feed line. If the material is fine and/or the line is under pres- sure (or vacuum), there will be leaks. This is the reason why a number of other gates exist for pneumatic conveying and material handling applications. Each type provides a different con- figuration to offer a various seals in order to match the requirements of the system in which they are to be installed. Around Blade (Top to Bottom) Seals are often necessary in order to prevent mate- rial or air leakage between the inlet of the gate or outlet of the gate. This is important especially in low-pressure pneumatic conveying systems, as the air pressure at the inlet, will tend to drive material through any available openings, resulting in potential product loss, as well as a loss of air pressure within the conveying line. In such cases, a gate with a tight seal around the blade opening would be required. Along Blade (Internal to External) As the blade slides back and forth, it could poten- tially drag material along with it, from inside the gate opening to the outside. This results in product loss. When inside a pressure system, the losses will be greater as material will be driven through any gaps between the blade and the slot it passes through in the gate body. To prevent these leaks, scrapers can be placed in advance of the blade opening to clean the blade and prevent material from being pulled out, while a tight seal on the top, bottom, and sides of the blade will eliminate any gaps through which material can pass under pressure. Air Purge In situations where the leakage of material from the gate orifice to the body (along the blade) may be es- pecially problematic, an air purge system can be in- corporated. This system, applies a positive pressure to the gate body. The pressure is set slightly higher than what the conveying system is run at. This cre- ates a differential pressure between the gate body and the gate orifice. Since air will tend to pass from the region of higher pressure (body) to the region of lower pressure (orifice), material will be prevented from leaking, as well as from being pulled through any seals along the blade as there will be a constant flow of air in the opposite direction. Through Body (Internal to Atmosphere) Material or air pressure leakage from the interior of the valve (orifice) to atmosphere also needs to be ad- dressed. This can be a concern since material leak- ing out often becomes waste (cannot be reintroduced into the process), create a potentially dangerous dust hazard or could contaminate the surrounding envi- ronment. This issue can be different with the vary- ing gate construction methods and the methods used to seal any joints or seams. Reactions to Material Flow Blade Deflection The most critical and most vulnerable part of the gate is the blade. If the gate is required to cut through a static or moving head of material, the blade can be subject to a variety of forces which Ricky Patel, BESc, Lorenz Conveying Products

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