Trends in weighing technology

The world of weighing is vast, with devices including solids flow meters, weigh feeders, PLC modules, load cells, belt scales and continuous weighing integrators. This article takes a look at the past and today--how products have changed, markets have expanded, and the approval process has become more challenging.

Why weigh?

Every industry uses weighing in one way or another. Industrial uses include static weighing, where material does not move as it is weighed by load cells, and dynamic weighing, where material is weighed in motion either by belt scale, weigh feeder, or solids flow meter. A few examples:

In the mining, aggregates and cement industry: belt scales are used for custody transfer as material is sold by weight; weigh feeders control the flow of raw material for recipe blending; belt scales monitor the amount of material being added to stockpiles. In the chemical industry: load cells support bins or hoppers to indicate material level for filling and dosing. In the food and beverage industry: solids flow meters indicate the amount of material added to a mixture. In the water/wastewater industry: weigh feeders control the rate of additives during water purification.

Selecting a weighing system

There are many different weighing solutions based on application requirements. Belt scales are the easiest solution to find – anywhere there is a conveyor belt transporting material, may be a belt scale. If there is moving material and a need to know its rate or speed on the conveyor, a belt scale is the answer.

Weigh feeders are a bit more complex than a belt scale in that they also control the flow of the material. A weigh feeder is a complete weighing solution designed for weighing accuracy and repeatability. Typically, weigh feeders are used to introduce materials into a process out of a bin or hopper.

Solids flow meters, in comparison, can be more of a challenge to apply. They are used where material falls from one process to another by gravity. The key here is falling material. The solids flow meters are placed in pipe and duct systems to indicate flow rates and totalize material flow.

Check weighers monitor the weight of a product to ensure it meets quality or production standards. Platform scales are used for inventory control. Dosing and batching machines ensure that materials are mixed at the right proportions to make food as delicious as possible. These are most of the weighing technology you will find in today’s automation plants worldwide. But how did some of these technologies evolve? And what are a few real-world applications?

That was then…

Forty years ago, belt scale demand came largely from North America. This market had long-established industries that were well suited for belt scale applications. Conveyors were built to Conveyor Equipment Manufacturers Association (CEMA) standards and the goal was clear: target the mining, aggregate, and cement business, otherwise known as the “heavy” industries.

Over time, other industries such as food and beverage, chemical, water/wastewater, and more, joined heavy industries in importance. Those industries brought new lists of requirements, which were difficult to meet with products focused on “heavy” industries. From a single belt scale offering all those years ago, Siemens, for example now has eight different versions of belt scales to meet applications: from food-grade wash down to 12,000 tph (13,220 stph) flow rates to hazardous environments.

Solids flow meters

Simply put, solids flow meters monitor the rate of bulk material flow in a process. They continuously measure the impact force of the material under gravity feed conditions and convert this signal into a flow rate used to control the material into a process or blending operation [Figure 1].

Figure 1: Sitrans WF250 solids flowmeters only detect horizontal movement from the material impact.

Over the years, solids flow meter options have expanded: different products for different applications, different materials of construction for material compatibility, ASME and DIN flanges. All of this adds up to more options, more info, and more solids flow meter applications.

Typical applications include:

• Cement

• Mineral

• Plastics

• Food processing

With the introduction of new flow meter models, these traditional applications have expanded, For example:

The design of a finish mill most commonly consists of a very large diameter steel tube filled with a designated quantity of steel grinding balls. As the mill is rotated at an optimum rpm (revolutions per minute), the grinding balls crush the clinker/gypsum mixture into a fine powder [Figure 2]. For peak efficiency, the mill should run with an optimum load. Acoustic sensors may be used to measure the frequency of noise coming from the mill, as there is a relationship between the frequency and amplitude of the sound generated and the loading of the mill. This reading must be instantaneous and not disrupt the material flow.

Figure 2: The cement finishing mill reduces the size of the material to make the end product.

Load cell systems can weigh the entire mass of the mill and provide loading readings by weight. It is also important to measure the rate of materials being fed into the mill, which is usually done with a belt scale monitoring feed rates, but there is also a need to measure the rate of flow of the coarse rejects being returned to the mill from the separator system. This reading from the recirculating load, or regrind circuit must be instantaneous and not disrupt the material flow.

Another option is specifically designed for use with gravity air slide conveyors. The unique in-feed section separates the flow of material and air so that the airflow is not totalized as material impact on the sensing plate. The solids flow meters only detect horizontal movement from the material impact. This ensures that material buildup on the sensing plate has no negative effect on accuracy. With these flow meters, plant operators receive precise rate-of-flow measurements instantly without having to interrupt the flow of material.

Weighfeeders

Early weigh feeder product lines were based entirely on an “engineered to order” concept. Whatever was needed for the application would be built to suit. There were no catalogue sheets, pricing structure, or outline drawings – and it took effort to ensure that end users received what they needed.

An engineered to order weigh feeder is designed for a specific application; it provides exactly what the customer needs and has asked for. Length, width, height, components, approvals and color are all up to the customer. As you can imagine, designing and creating these individualized solutions is very time consuming and labor intensive

Forty years later and the world is smaller than ever – information is available at the press of a button, allowing customers to view complete catalog offerings and the many styles of weighing products. Just one version of Siemens’ weigh feeders features 1,440,270,720 individual configurations. From the catalog, customers can access part numbers and prices with the click of their mouse – a far cry from having to visit a customer with a tape measure and sending a hand-drawn sketch for approval!

The world has gotten smaller, but customers’ requirements have not. Application knowledge and challenges are shared in blogs and during webinars (web-based seminars). One such shared weigh feeder application in the food and beverage industry is from Sloten, which produces milk powder for calves and young animals. Raw materials are processed into semi-finished products using a computer-controlled process that involves mixing of raw materials in liquid form, pasteurization, homogenization and spray drying. After this process the semi-finished goods are transported to the Deventer, Netherlands manufacturing unit where they are processed into the end products.

Sloten wanted to increase production capacity and have the ability to make smaller batches. Traditionally the company makes large batches up to 300 tons, but there is a growing demand for smaller quantities. Sloten therefore defined a need for a new blending and batching system with a higher accuracy. The weigh feeders are the core of the production plant in Deventer, so downtime while replacing the system needed to be as short as possible to reduce productivity losses. Sloten now has a freely programmable weighing and proportioning system that it can adapt to changing operational requirements.

Figure 3: At Sloten, Sitrans WW200 weighfeeders are fed with raw ingredients and weigh the batches with an accuracy of 0.5 %.

Weighing electronics

Eventually, customers demanded a board that would support all crucial functions of a weighing application such as set point, coarse and fine feed, and tolerance check. In the past, customers were using stand=alone equipment. For customers who don’t want their instruments connected through a PLC, integrators are still ideal. An example comes from the Czech Republic. A company that produces raw materials for refineries, petrochemicals and agrochemicals has five granulating plants, which were previously controlled with a system using relays and PLCs. With the modernization of the entire control system, the aim was to improve the reliability of the plant and increase the system’s ease of use.

To better control the dosing process, the company installed an electronic weighing systems for differential dosing [Figure 4]. During production, weighing sensors continuously weigh the entire mixing vessel, together with the product. As product is constantly flowing out, the weight decreases continually. The loss in weight is recorded over a certain period and the material flow is calculated on the basis of the data obtained. The highly precise dosing and corresponding increase in product quality proved to be a significant advantage for the customer.

Figure 4: Siwarex FTC has improved the accuracy and product quality of the petrochemical company's differential dosing operations.

A world of approvals

With all of these products, one trend that has become challenging to manage is the local requirement for hazardous approvals and metrological certification. To launch a global product that meets each of these different approvals requires quite a bit of work, as there are multiple approvals and different ways to meet each approval.

Every country has its own regulations and requirements, which are often quite different. For example, in Europe the method for protecting encapsulation requires almost double the thickness of North America. As well, in Europe, a belt scale can be approved at 0.5 %, while the US offers 0.25 %.

Each weighing product is typically approved for use in a number of countries worldwide, each certified by local authorities:

• Measurement Canada and Canadian Standards Association (CSA) in Canada
• National Type Evaluation Program (NTEP) and FM in the United States
• Measuring Instruments Directive (MID) and Atex in Europe
• Chinese Measurement Certification (CMC) and National Supervision and Inspection Center for Explosion (NEPSI) in China
• Gosudarstvennyy Standart, meaning “state standard,” (GOST) and GOST Ex in Russia

Summary

Decades later, we know it is not simply about having the right product, but the right solution with the global support to ensure material is weighed accurately all the time.

About the Author

Matt Morrissey is a product marketing manager at Siemens AG Industry Automation Division in the Sensors and Communication Business Unit.