Solid state

Measuring a bin of bulky materials can be a hit and miss affair. Whether they are being stored in silos or moving through the plant, myriad parameters will have a bearing on how well they can be quantified.

Unlike liquids, bulk solids can settle into unpredictable shapes, says Richard Farnish, a consultant at the Wolfson Centre for Bulk Solids Handling Technology at Greenwich University.

I think the biggest problem is when you have silos where significant material hang up occurs

Richard Farnish

The root of the problem, says Farnish, is that most level measurement methods are “most effective when the inventory surface is even and all the material is being drawn down as a complete cross-section”.

“I think the biggest problem is when you have silos where significant material hang up occurs,” he says.

“If you were using a level probe this could be stuck in material that is not going anywhere.”

The other problem operators can encounter, says Farnish, is when they measure level using load cells that indicate the product is flowing when in fact it is stuck inside.

There are various approaches to measuring bulk solids within processes, he adds.

Depending on the degree of sophistication required, a basic level probe will provide a ‘yes or no’ signal when it becomes submerged as a means of initiating a refill of material, to manage the process.

“Another approach is to mount process equipment onto load cells which gives you a reading of what you have at that moment in time,” he says.

“As the weight reduces, the measurement data can be used to control the process, provided that the equipment is correctly de-coupled of course.”

Measuring level is becoming increasingly important in the process sector because many industries are seeing a move from batch manufacturing to continuous processing, says Jamie Clayton, operations manager at Freeman Technology.

“This calls for more stringent control of raw materials in order to achieve a consistent final product,” he says.

“A common misconception is that a less cohesive powder will always provide better results, but free-flowing materials can present their own issues. For example, they may be more prone to flooding and segregation, both of which would have a detrimental impact on performance.”

The shape or character of the material, internal vessel structure and size, are just some of these challenges that emerge when applying levelmeasurement technologies.

The result is that there is a growing legion of contact and non-contact level measurement instruments to choose from, including radar, ultrasonic, laser, and nuclear technologies, to name a few.

The question is how to match a particular technology to the application at hand at the correct price point for the level of accuracy required?

The first things to consider, say experts, are whether you require a point or continuous measurement, and whether your process environment is best suited to a contact or non-contact solution.

While contact-based technologies such as guided wave radar might offer the best solution in some process environments, one of the key advantages of non-contact devices, such as ultrasonic, nuclear or radar, is that they can detect long, wide, as well as narrow areas of level measurement, says Frost & Sullivan analyst V Sankaranarayanan.

“Non-contact level measurements essentially ensure a contamination free-environment because there is no contact from the sensor, as found in other forms of level indication such as mechanical, capacitance or conductivity,” he says.

“The non-contact nature of the level indication is also attractive to industries where the medium being measured is of a corrosive nature as found in the chemical and petrochemical industries.”

According to industry executives from Emerson Process Management, it is about choosing the right tool for the job.

The company sells a wide range of instruments for measuring level and has recently launched a range of new products into the market including contact (guided wave radar) and non-contact radar level devices.

Emerson’s Rosemount 5300 Guided Wave Radar has been used as a tool for measuring solid products such as plastic granules for several years, says Ingemar Serneby, senior application specialist for radar, at Emerson Process Management.

The company also sells two non-contact radars, and the most recently released (the Rosemount 5402), has additional solids capability.

This is achieved with software that helps to control the feeding of a silo for solids such as granules of woodchip.

“There is definitely a need in the market for simple solutions that measure and don’t cause interruptions in the process,” says Serneby.

He sees the main growth opportunities for radar in small-to-mid-sized applications with small vessels containing substances like plastic granules.

Many such applications currently have no measurement capabilities or are at best using very unreliable electronic devices.

Another product that Emerson has recently launched for solids level measurement is the Rosemount 5708 3D Solids Scanner for dust and powder applications that are not suited to radar, such as fly ash commonly found in power-generation industries.

Because many solids have an irregular surface, solids scanners work by triangulating the shape of a pile, while radar works by measuring in the footprint, says Asael Sharabi, technical director for the Rosemount 3D Solids Scanner.

“This gives it a very wide application in silos, warehouses, big storage rooms and stockpiles,” he says.

“We have sold more than 8,000 scanners worldwide into the cement, food and beverage industries, as well as power plants,” says Sharabi.

“It scans the surface and takes a multiple point reflection from complex surface of material and from that calculates the quantity of what is in the bin or silo. It works in a low frequency to penetrate dust.”

Carmel Olefins (CAOL), a manufacturer of petrochemical products for the plastics industry, is a recent convert to scanning technology.

The company was using a host of level measurement technologies, but wanted to reduce maintenance costs, and improve reliability of readings and monitoring of raw materials during its process.

In order to measure the amount of materials in the tanks during filling from the production line and emptying into the trucks, it mounted Rosemount 3D Solids Scanners onto 20 of its silos.

Following installation, it says the self-cleaning scanners were able to provide volume measurements throughout the complete fill and empty cycle of each tank.

Berthold Technologies supplies non-contact bulk weighers and in-motion weighing systems that use radio-metrics to gauge the volume of what are often powdery substances, says Graeme Webb, technical sales manager.

“We specialise in radio-metrics for everything from mining, sugar industry to paper recycling,” says Webb.

Berthold’s measurement technology can be used with many types of moving equipment such as belt conveyers, screw conveyors and bucket elevators, with much greater accuracy and flexibility than a traditional load cell, which needs to maintain a flat surface, he says.

Although the capital cost isn’t the lowest on the market, the technology has been adopted in situations where greater process control is a priority, says Webb.

“It is used to control exactly what is coming in and going out of a plant so they can balance the books and see what efficiencies they can make. If they are bringing a certain amount of materials in and putting out less then they need to find out where they are losing out,” he says.

“Our equipment is non-contact so there is nothing to wear out or break. And calibration is not required to retain accuracy, unlike load cell systems. Radio-metrics is all based on physics, which doesn’t change over time.”

One of the obstacles to its more widespread adoption is the necessity to apply for licensing to use it on site, adds Webb, because it is based on nuclear technology.

Although fear of radiation is common, Webb says stringent regulations in the UK ensure the technology is safe to use.

“Although it contains gamma radiation, it doesn’t spread anywhere, and there is less radiation than you’d encounter on a flight to America,” he says.