From the plastics in your smart phone to the fertilizer used to grow the fruits and vegetables you eat every day, dry bulk solids are the building blocks for many of the items that shape the world around us. The industries that create these products (the fertilizers, the plastics, the food products, the building supplies) all have the drive and desire to make their processes more efficient, more cost effective, and more reliable. The most effective way of accomplishing these goals is to understand what is occurring within a process so that it can be optimized. Ideally, that means measuring the input and output of each minor process in order to understand, for instance, the optimal setting for each piece of machinery, the best ratio for mixing ingredients, the best way of maximizing output while minimizing cost. Because dry bulk solids are such an important part of many of the products around us, being able to accurately measure them is critical to the efficiency of their production. The question, then, is “How do you measure dry bulk solids accurately?” The simple answer is “Don’t measure them inaccurately!”
    
Some of the most common causes of inaccurate bulk solids flow measurement include:

    •    An improper selection of measurement technology
    •    Measurement assumptions regarding key variables
    •    Inadequate space for installation
    •    Product that collects, builds up, or sticks to the measurement element
    •    Highly mechanical components that become unreliable due to wear

Choosing the Correct Measurement Technology
There are many ways of measuring solids flow, and picking the correct flow measurement device is crucial to the success of any flow measurement application. The flow meter you choose greatly depends on what your requirements for the flow measurement are (including the accuracy requirement) and where within your process you would like to perform the measurement.
    
For the measurement of flow being pneumatically conveyed, there are a variety of non-intrusive flow meter styles that use radar, Doppler, or nuclear technologies in order to calculate mass flow. These meter styles are non-intrusive, meaning that the measurement element does not penetrate into the flow stream. Their non-intrusive style makes these types of meters versatile. However, in certain circumstances, they can be quite inaccurate as well.  
    
For static product that is no longer in process (product in silos, trucks, railcars, etc.), static weigh scales or truck scales are certainly an option as they can be quite accurate. Static weigh scales are somewhat limited in their application however, as the process must halt completely during the measurement cycle and often, the measurement is only performed after product has been emptied into the measurement vessel (silo, railcar, truck, etc.), making overfilling a distinct concern.
    
As an alternative to the use of static weigh scales, dynamic flow meters measure the flow of product in-process. In the case of filling silos, trucks, or railcars, dynamic flow meters can measure the product as the fill is occurring, not afterwards which allows for better control of the fill and mitigates the effects of over filling and under filling.
Dynamic flow meters are great for filling applications, but also for a variety of other applications, ranging from short batching applications to continuous blending operations and everything in between. Coriolis meters are one type of dynamic flow meter that utilizes spinning mechanisms that change torque as product passes through it. This change in torque is then processed as mass flow. Coriolis based mass flow meters are generally quite accurate. However, special attention to maintenance and wear are required when utilizing these devices as they are highly mechanical in nature. As with all mechanical devices, continuous use without preventative maintenance and repair could lead to the damage of key components. Furthermore, if the rotating mechanism fails on some Coriolis-style meters, they will not allow product to flow through them, which means that if the meter does not have a bypass system, production could cease.
    
Another group of dynamic flow meters utilize a gravimetric method of measuring flow. Gravimetric flow meters are typically accurate and dependable flow meters that allow for an actual, real-time measurement of flow rather than a calculation based on assumed values for key components such as density, shape, or particle size. One type of gravimetric flow meter used for measuring small batches of product is the loss-in-weight meter which typically offers a reliable and accurate measurement. These loss-in-weight measurement devices utilize a vessel on load cells and as product is drawn out of the vessel, the load cells detect the difference, or loss, in weight in order to deduce mass flow. Because loss-in-weight devices experience measurement "blind spots" as the vessel is being filled with product, loss-in-weight measurement tends to be much more accurate for very low flow rates and for shorter batching runs where the hopper will not need to be refilled during a batching cycle.  
    
For larger batches or for continuous processes, belt scales or weigh belts can be used as they do not experience measurement blind spots. Weigh belts and belt scales utilize a belt conveyor in conjunction with load cells in order to measure the amount of product passing over the belt. Because the belt’s speed is integral to the measurement of mass flow, the belt’s tension is critical to the flow meter’s accuracy. Many things can affect the belt’s tension including changes in temperature and even normal wear. Thus, belt scales and weigh belts often require calibration checks to verify their calibration. Because they are highly mechanical in nature, these types of flow meters may require frequent replacement of parts or repairs and so maintenance costs can be high as well.
    
Some of the most accurate and versatile gravimetric style flow meters utilize a measurement that is based on centripetal force. As product slides across a curved measurement surface, the circular motion of the particles generates a centripetal force that is proportional to mass flow. As long as product is able to slide across the measurement surface, these types of meters are extremely accurate and the accuracy is typically not affected by the type of product that is being measured (granular, powdery, bulky lightweight materials, heavy dense materials, etc.). These meters typically have a small vertical footprint, can be installed nearly anywhere in a process, and are a great choice for nearly any “in process” installation where accuracy is paramount.
    
Choosing the correct measurement technology is crucial to the success of any flow measurement installation. Knowing where you want to measure product and how you intend to use the data offered by the flow meter is critical to the meter’s success. Know your process and know what each type of flow meter can bring to your particular application. It is also important to know the full cost of any flow measurement system. While the upfront cost of a flow meter may be quite low, low accuracy or high maintenance costs can make a “cheap” meter very expensive. Conversely, a seemingly expensive meter may be extremely accurate and have extremely low maintenance and repair costs and so may actually be quite inexpensive in the long run.

Assumption of Density or Other Variables
For many mass flow measurement devices, variables such as density, product size, or even product temperature are assumed to be constant. Of course, these variables are rarely ever constant and can vary greatly even in fairly consistent products like plastic pellets or rice. Some factors that can affect density are: changes in moisture content, changes in environmental temperature, or changes in the process itself. Some processes may require a "warming up" and/or "cooling down" period where the equipment within the process will behave differently (resulting in different "grades" of product) during different phases of production. If the changes in product quality are great enough to affect the density, then the accuracy of any flow measurement which assumes density could suffer.

In many cases, especially in those processes where the specifications of the products being measured can change, meters that actually measure flow will have a distinct accuracy advantage over meters that must assume as constants, process variables that will change over time. Some gravimetric flow meters utilize technologies that are based on true force based measurement, which does not depend on product density, particle size, product shape, or even particle elasticity. Thus, changes in these parameters will have no effect on flow measurement accuracy.

Inadequate Space for Installation
Space within today's modern industrial plants is often at a premium, and the space needed for the proper installation of a flow measurement device is, at times, simply non-existent. Some meters have a small footprint, but installing them adequately, requires several feet of additional drop into them in order to produce an accurate flow measurement. Other meters have a massive footprint and require a great deal of real estate in order to install (sometimes horizontal space, sometimes vertical space, and sometimes, both). Often, rather than installing the proper flow meter, which may involve the extensive rearrangement of existing equipment and can require the use of precious labor and resources, many plants elect to use inadequate measurement devices, or worse, elect to forgo flow measurement all together.

Finding the space to retrofit a flow meter into an existing process can be a cumbersome ordeal. However, selecting the right type of flow meter can make the search for space a whole lot easier. Some gravimetric flow meters require a consistent drop into the meter in order to maintain an accurate flow measurement. This consistent drop is often found at the discharge of existing feed devices, meaning that the closer the flow meter is to the discharge of a feed device (screw conveyor, belt conveyor, rotary valve, etc.), the more accurate it will be. These types of flow meters, therefore, do not require excessive amounts of additional space in order to maintain accuracy, but rather, require only the footprint taken up by the meter itself. In some cases this can be as little as 19 in. of vertical space. Often, the standard installation size of meters can be reduced even more by utilizing custom flanges, custom enclosures, and custom mounting to allow meters to be installed directly into a process without the use of space consuming transitions.

Product Buildup on the Measurement Element
If not addressed, product buildup on key components can cause a variety of difficulties for flow measurement devices, especially when measuring powdery materials such as gypsum or flours. Buildup on measurement surfaces can often cause zero shifts and will ultimately affect the accuracy of the measurement system if not properly attended to. Buildup can cause additional concerns such as undue wear or equipment failure, especially if the product being measured is corrosive in nature. For applications in the food industry, where equipment is required to be "food grade", buildup could cause contamination or sanitation concerns and clean-up can take hours, during which time production must be suspended. This routine clean-up and maintenance can often be both costly and time consuming.

Experience with products and application knowledge is a necessity, especially when measuring very powdery products or corrosive materials. Some meters employ flow enhancement devices including a variety of coatings and liners that reduce or eliminate buildup concerns on flow surfaces. Other options for buildup concerns include the use of vibration on key flow surfaces or the use of air as either a steady, continuous flow, or as a blast of compressed air across key flow surfaces. When the proper flow enhancement devices are utilized in conjunction with vast application knowledge, even the most difficult to measure products can be measured extremely accurately.

Wearing of Mechanical Components
Mechanical wear is one of the biggest contributors to the malfunction of a flow meter. Some metering devices utilize extensive mechanical designs with numerous moving components in order to measure mass flow. With extended use, the mechanical components within these devices can begin to wear or even break all together. If key components are allowed to wear to the point of malfunction, the result can be inaccuracy and unreliability in flow measurement. If choosing a measurement system that is highly mechanical in nature and that does require many moving parts, it is often necessary to maintain a painstaking preventative maintenance schedule which includes the replacement of worn parts before they impact the accuracy of the system and long before they fail.

Summary
Knowing your process and how a flow meter will incorporate into that process is integral to the success of any attempt to optimize your process with the addition of a flow measurement system. When determining the right flow meter for your application, make sure you consider the various types of flow meters that exist, but also be sure that you know what types of assumptions each meter makes, how much space is required to install them, how they might deal with buildup and how they are affected by and attempt to mitigate wear. When selected properly and when properly integrated into your system, a flow meter acts as a window into your process, showing you how your process is running, and allowing you to better understand and thus control your process.

    Christopher D. Lewis is in inside sales and marketing with Eastern Instruments, Wilmington, NC. The company has been engaged in the development of solid particle mass flow and airflow measurement and control systems since 1984.For more information, visit easterninstruments.com.
 
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