How to Screen Frac Sand

Aug 22, 2014

"Frac sand" is high-purity quartz sand used by the petroleum industry during the hydraulic fracturing process (a.k.a. "fracking") to extract oil and natural gases trapped between rock formations. In the extraction process, the frac sand is mixed with water treated with chemicals and thickeners and pumped down a drilled well. The pressure of the water causes the rocks to fracture. The water and frac sand are then forced deep into the fractures, propagating them even further. When the pressure is released, the water recedes, leaving the frac sand behind to "prop" open the fractures. The openings produced facilitate the release of the oil and natural gases out of the rock and into the well where they can be extracted. The petroleum present in each formation will have different flow characteristics, so the frac sand must be tailored to the well. In order to be able to optimize the effectiveness of the sand, it must be supplied with a uniform size, composition and shape. It must also have the compressive strength to resist crushing by the rock formations as they close when pressure is reduced. To assure that the sand meets these requirements, it is evaluated for compressive strength and shape, after which it must be washed, dried, and screened to size. The final processing of frac sand requires precise screening in accordance with API Spec 56. Generally, there are two types of screeners that are used in the production of frac sand: inclined vibrating screeners and gyratory screeners or sifters. Most inclined vibrating screeners use a linear or elliptical motion to convey the material down the screen surface. Gyratory sifters use a more gentle sifting motion, utilizing primarily horizontal motion to convey the material.Inclined Vibrating ScreenersLet's start with the inclined vibrating screeners. There are both advantages and disadvantages to using an inclined screener. The first advantage is that they are relatively inexpensive. The other advantage is that inclined screeners can handle high tonnages of product. Industrial screening equipment can get rather costly, but inclined screeners will allow the processing of high amounts of material for a relatively low cost. However, along with those advantages come some disadvantages. The first disadvantage is that inclined screeners allow for poor spreading of material as it is fed to the screen. Inclined screens do not have any side-to-side motion. Thus, when material is dropped onto the screen it tends to run in a straight line down the length of the screen. Consequently these machines often require the use of a separate feeding device that spreads the material so that it can be dropped into a full width inlet. This feeder adds cost, complexity, and vertical height to the overall height of the machine. Without the spreader, the material won't spread properly and much of the screen surface area may be underutilized. Additionally, the screens will wear out much quicker in the highly used areas, resulting in early screen failure. The other disadvantage to inclined screeners is that they produce inaccurate separations. Because of the incline of the screen deck, the material is exposed to two different openings – the "actual measured opening" and the "effective opening". The "actual measured opening" is the true opening size of the screen mesh. For example, a 30 U.S. standard sieve has an "actual measured opening" of 600 microns. The "effective opening" results from the effect of gravity influencing how particles impact the screen. Therefore, if you put a 30 US screen on an inclined screener, the effective opening size of the mesh is smaller than the actual opening size due to the screen angle (see exhibit A). Accordingly manufacturers of inclined screeners have to "cheat" on the screen selection, using a larger opening than the particle size distribution requires. To give you a couple of examples, in a fines removal application an inclined screener may use an 18 market grade mesh (980µ) screen to make a 30 US (600µ) separation. In the process of travelling down the screen, some of the "good" near-size product (600-980µ) is lost to the fines fraction. If they try to prevent that product loss by going to a finer mesh, they will not be able to achieve adequate fines removal performance. In a scalping application, you typically cannot "cheat" on the opening. A 600µ scalp requires the use of a 600µ screen opening. Use of such a screen on an inclined vibrating machine will result in significant tail-over of good product as the ~400µ-600µ particles race down the screen and are lost to the oversize fraction.Gyratory SiftersAs with inclined screeners, gyratory sifters have their advantages and disadvantages. Gyratory sifters have a tendency to be more expensive than your typical inclined screener, and they have lower throughput in the same basic footprint as compared to inclined screeners. However, gyratory sifters are more efficient, producing cleaner, more accurate cuts. Additionally, the gyratory motion of the sifter tends to spread the material out on the screen for more effective use of the screen area. There are several attributes that must be considered when choosing a gyratory sifter. First, the motion of the sifter needs to be considered. Keep in mind that gyratory sifters utilize only horizontal motion. Some sifters have a fully gyratory circular motion; others have a gyratory-reciprocating motion. The gyratory circular motion produces the same motion across the entire screen area, creating equal efficiency throughout the screen deck. This motion is possible because the drive system is located at the center of gravity of the machine. It's this type of motion that helps to spread the material evenly on the screen. The motion also helps to produce more effective ball action on the self-cleaning ball decks. The gyratory-reciprocating motion is a combination of circular and straight-line motions. This occurs when the drive is located at the feed end of the machine. The screen deck is moving in a circular path at the feed end of the machine and moves in a linear motion at the discharge end. The circular motion allows for even spreading of the material, but the linear motion is not as effective for the ball action in the ball decks, causing screen blinding at the discharge end, which in turn lessens efficiency and throughput. Another attribute that must be considered is stroke and speed of the sifter. Some sifters have a short stroke and higher speed motion, which is optimal for fine particle separation. Other sifters have a longer stroke with a low speed motion, which produces a motion that is too active for fine particle separation. This type of motion is better suited for coarser applications like grain screening. Both motions can be effective in different applications. It's just a matter of what works best in your process. Finally, screen deck length needs to be considered when choosing a gyratory sifter. Screen deck lengths vary from manufacturer to manufacturer and are typically available in screen sizes of 7 to 12 ft in length. Sifters with shorter screen decks have a more compact footprint, but they also have shorter retention time for the material on the screen (see exhibit B). More retention time for the material results in better exposure of near-size particles to the screen apertures and more accurate screening. Additionally, longer screen decks are more beneficial when you’re screening with multiple screen deck configurations (see exhibit C). As the material travels down the length of the top screen deck, the smaller material falls through to the next screen deck and begins its journey down that screen. This continues to occur until the smallest of the material reaches the bottom-most screen and falls through to the bottom pan. On shorter screen decks, the smaller material often does not have enough retention time on the screen to fall to the bottom pan. This leads to screening inefficiencies or undersized product mixing in with the larger material fractions. The only way to remedy this would be to slow the rate of the feed, which leads to a reduction in capacity. The longer deck lengths can handle this more easily since they have longer screens and more screen area. As you can see, there are many factors to consider and options from which to choose when purchasing screening equipment for your process. When determining which screening motion is best for your process, keep in mind that gyratory motion is far superior to linear or elliptical motion when efficiency and accurate cuts are a requirement. Additionally, the gyratory circular motion will aid in the spread of the material over the screen area while promoting superior screen cleaning. When choosing which gyratory sifter is right for you, keep in mind that the stroke and speed of the machine impacts fine particle separation, and the deck length is critical when dealing with multi-deck applications. To make your life easier and to aid your equipment manufacturer in supplying the right equipment for the application, there are three rules to consider. One, know your process. What are you trying to accomplish? What is your rate requirement? What is your product specification? What happens before and after the screening operation? Two, know your product. Is it free flowing? What is the moisture content? What is the bulk density? If possible, know the particle size distribution (PSD) of the feed material. And three, require and participate in laboratory testing. Having all of the process and product information along with testing will ensure that you are on the right track when selecting screening equipment. Involve your screening equipment supplier as early as you can in the process and follow these rules, and you will greatly improve the probability that you will end up with a successful installation. Jeff Dierig is global marketing manager at SWECO (Florence, KY), a business unit of M-I LLC. For more information, call 800-807-9326 or visit www.sweco.com.

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