Considerations in installing spouting
November 9, 2015
In my previous article that appeared in the September issue of World Grain, we examined the types of spouting and how to size them. In this article, we look at factors to consider when installing spouting.
Spouting must be assembled so the interior edges from one section to another are aligned to provide a smooth surface. This is necessary, whether using liners or just metal sections, to avoid creating exposed edges resulting in wear points and damaging soft products such as pellets. When passing from an unlined spout section into a lined section of spouting, the edges of the unlined section must be located such that the inside surface does not hit the liner edge. This can be done by using a larger diameter spout for the lined section (see sketch 1 on page 90 for a comparison of joining spout sections and their interior alignment).
A spout must be installed on a slope that is steep enough to carry the product, by gravity, from its inlet to outlet. This steepness is measured by an angle measured between a horizontal plane and the slope of the spout as shown in sketch 2 on page 90.
The slope angle must be greater than the angle of repose and coefficient of static friction of the material being handled. Although products can flow on slightly less slopes, I would recommend the slopes shown in Table 1, page 86.
Depending on the length and slope of a spout, the conveyed product reaches higher velocities the longer the spout. Information found in the Nolin Milling, Inc. catalog states that grain velocities exceeding 1,750 feet per minute can damage whole grain. For spouting installed on a 45-degree slope handling whole dry grain, the grain might enter the spout from a bucket elevator at a speed of 618 feet per minute. At 20 feet down the spout, the velocity doubles to approximately 1,235 feet per minute. At a point 40 feet from the top, the velocity is about 1,750 feet per minute, at 60 feet the velocity is 2,140 feet per minute and at 80 feet the velocity is 2,470 feet per minute.
If the material exiting the spout has not been retarded, it will enter the next piece of equipment or bin in a turbulent flow. This can affect loading uniformity and/or product continuity (by separating different particle sizes).
At every turn or termination point, the product in the spout is subjected to abrupt changes in direction or a break in the velocity. For long spouts, a cushion box, sometimes called a “dead head,” should be installed at the lower end of the spout to break the velocity of the product in the spout and direct it vertically into the bin or other piece of equipment.
These cushion boxes are available commercially or may be constructed in the field. Any cushion box has a thick impact plate installed to slow the product and allow it to fall off the plate and discharge vertically into the bin or equipment. These cushion boxes should be built so that a covering of the product is held against the impact plate to absorb additional product as it reaches the end of the spout. It should also be designed to be self-cleaning so no product is left in the cushion box when the spout is empty.
Many times the product entering the spout needs to be directed to more than one destination. This can be done using a two- or three-way valve. If more destinations are required, it is best to use a turn-head (distributor).
Valves used in-gravity spouting are typically of two types – flip-flop or bucket (pan) style. A flip-flop valve has a square body with a plate that can be flipped to direct flow to one outlet or the other. This plate needs to be of heavy or lined construction as it often takes impact as well as sliding loads from the material being diverted.
This type of valve will not seal well on the sides and top tip and is not recommended if soft or fine product is being handled. The pan valve has a chute, of heavy or lined construction, in it that rotates from one outlet to the other and contains the product as it passes through the valve. This valve can be designed to seal well. Either of these two types of valves can be purchased with outlets arranged as an upside down “y.” Valves can also be purchased in an offset configuration where one outlet goes straight out and the second outlet is an offset to the side.
A turn-head (distributor) can have one inlet and many outlets with the inner spout able to be rotated to a selected outlet. The slope of the inner spout and the outlets of the turn-head must have slopes at or greater than the minimum slope needed to convey the product. Turn-heads are standardly available in 45-, 50-, and 60-degree models. Turn-heads can have lined inlets and outlets and a heavy duty AR steel or lined inner spout for high wear applications. Turn-heads can also be designed in single, double or triple configurations. Please consult different turn-head manufacturers for the specific features of their products.
One thing that is often overlooked or not thought of when installing spouting is the amount of air carried with product as it passes through the spout. As the material in the spout travels it is mixed with air as the material does not fill the spout cross-section completely. The area not occupied by the material is air. Due to the air being mixed in the material, the whole mixture occupies the internal cross-section area of the spout. When this mixture of air reaches the discharge of the spout, both the product and the air with it exit into the next piece of equipment or into a bin wherever the spout discharge is directed.
If the product and air mixture is not discharged into a sealed area, the air will leak or blow out of the destination point as dust. This is due to the fact that the product-air mixture creates pressure at the spout discharge. One of the places this dust is most noticed is dust leaking out of vents or around manhole covers in bins. As the product fills a bin it displaces air already in the bin in addition to delivering added air into the bin. As an example, a system transferring 10,000 bushels of whole wheat into a bin is delivering 12,500 cubic feet of whole wheat into the bin. This is approximately 208 cubic feet per minute of air volume pushed out of the way for the grain. If the entrained air is an additional 500-600 cfm of air entering the bin with the grain, the bin internal pressure increases as the open volume of the bin decreases with filling.
This air has to go somewhere in order to keep the bin from being pressurized. It will seek any opening available. Depending on the total opening area available to relieve the air, the air velocity is usually high enough to carry fine material particles through the opening, resulting in dust being emitted from the bin. The air pressure from the product-air mixture discharged at the outlet of the spout must be relieved somewhere. This can be done with equipment open to the atmosphere, some type of sealed atmosphere to send the air further down the system, or the installation of some type of dust control system to collect and discharge this air. One of the most difficult places to control dust, and even avoid internal contamination between bins, is where a turn-head (distributor) is used to direct the product to various bins. If the bins are well sealed, the air pressure will relieve itself by flowing back up the spout to the distributor. This can cause cross contamination within the distributor by the air being vented into an adjacent spout opening in the distributor. To avoid cross contamination, a turn-head must have an inner spout that is designed with seals at both ends of the inner turn spout to avoid any leakage of product or air internally within the distributor.
An example of a method for venting air from bins that are being filled from a distributor may be found at the website for the Essmueller Company (www.essmueller.com). Open the website by clicking the “welcome” button and follow through by then clicking the “enter,” “turn-head distributors,” and “BV” 360-degree bin vent buttons. This website shows an attachment that can be attached to a distributor to vent return air from a bin being filled to a dust control system. Look at the “installation,” “components” and “selection help” sections to see how this type of system may be added. A dust relief system of this type can be used with various bands and types of turn-heads.
In summary, all spouting must be installed at or greater than the slope needed to convey the product it carries. All interior joints should be built so no product impacts the interior edge of the next section of spouting. Cushion boxes should be installed at the end of the spout to break the product speed and discharge it into the equipment or bin. All valves should be installed so that the outlets are on slopes meeting or exceeding recommended minimum spout slopes. All cushion boxes, valves and turn-heads should be large enough to avoid restricting flow of product passing through them, and should be lined or of heavy construction if required. Consideration needs to be made on how to handle the entrained air that is discharged from a spout.
Fred Fairchild is feed science professor emeritus in the Department of Grain Science at Kansas State University. Prior to coming to Kansas State in 1994, he worked in the industry designing, constructing and commissioning numerous mill facilities. He is a licensed professional engineer. He can be reached by e-mail at firstname.lastname@example.org