Instrumental in maximising potential

Stephen Howes discusses the relative merits of control and instrumentation equipment within the solids and bulk handling industry.

The continuing role of control and instrumentation equipment is vital within UK industry and society as a whole, underpinning the basic principals of industrial processes throughout the market place. Remarkably however, despite being a crucial element within both domestic and commercial appliances, it largely remains to be a hidden technology, unknown to the discerning user. 

Yet, with the increasing threat of energy shortages and the very real effects of global warming, instrumentation and control technology can offer some respite to UK industry in the form of energy savings and production enhancement. 

The term ‘instrumentation' is essentially the use of tools or instruments for the measurement or control of a given variable. Operating alongside effective controls, the partnership provides a valuable source of information within a variety of working environments. In simple terms, in order for a site to be able to control any number of variables it requires accurate and detailed measurements, gained through instrumentation equipment. 

Potentially, depending on the industry, the variables to be measured could be any number of things from powders, liquids or solids all of varying dielectric ranges. With this in mind, there are a number of different sensors available within the market place designed with the specific needs of each variable in mind, and that incorporate within the design a number of challenges that may be experienced on site such as moisture, dust and build-up.

It is possible that effective measurement and control at each stage of the production process, bearing these challenges in mind, can increase availability, reduce waste, save time and increase output. 

In a typical aggregate process for example, instrumentation equipment can be installed throughout the production process, from level and weighing measurement at the primary crusher, through to the screening process, then on to the secondary crusher before being stockpiled. This equipment can then be further utilised before the tertiary crushers and on through the blending process and beyond into load out.

Furthermore, one of the most common procedures for industrial instrumentation is the measurement and control of the level of materials contained with storage and process vessels such as bins and hoppers or silos.  The use of level measurement here is ideal for reducing spillage and overflow and well as quality control and inventory management.  

Choosing the most suitable technology for a site therefore is dependent upon a number of factors such as the characteristics of the material to be measured, environmental conditions, and the properties of the container as well as the specific needs of the customers by way of cost, maintenance needs and accuracy. 

For example, level technologies suitable for the solids and bulk handling industries are ultrasonic, radar, guided wave radar, Inverse frequency shift capacitance and electro-mechanical:

Ultrasonic

Operational through crusher control, sand and gravel material levels, and ultrasonic technology links a non-contacting transducer to a controller, resulting in a highly cost effective solution.

The non-contacting technology is ideal for the harsh rugged environment seen in the bulk solids environment, the transducers are impervious to moisture, build up, corrosion and vibration and are virtually maintenance free. Ultrasonic technology is successfully used in a multitude of solid bulk material level applications from aggregate stockpiles to crusher control and provides a cost effective solution for the challenges of multiple levels required in screen house applications.

Radar

Operational through electromagnetic waves, non-contacting radar technology is also a low maintenance technology and unaffected by process atmosphere. The technology is suitable for liquids, slurries and solids such as fly ash silos, (pulverised) coal or finished cement and able to handle extreme temperatures and dust.  With the decreasing price of Radar measurement devices many sites are standardising on Radar devices for all their level measurement needs, trying to decrease the amount of spares required for any particular site.  However application choices still need to be made, as higher frequency devices will offer better functionality for some environments.

One of the latest products to be introduced into the Siemens range is the Sitrans LR260, available for use on virtually any stored solid medium. As the new two-wire continuous level radar based transmitter it can transmit reliably up to a distance of 30 m even in environments with extreme dust load and high temperature of up to 200 degrees.

Guided wave radar

Guided wave radar uses pulses of electromagnetic (EM) energy to measure material levels. When a pulse reaches a dielectric discontinuity (created by media surface), part of the energy is reflected. The greater the dielectric difference, the greater the amplitude (strength) of the reflection. 

This technology is a contact device that is unaffected by changes in process conditioning, high temperatures, pressure and steam, ensuring accurate measurement in level, volume and interface applications. The guided wave radar can automatically ignore obstructions and is able to apply algorithms to raw echoes in order to produce accurate and reliable measurements. This makes the technology ideal for the measurement of liquids or slurry applications of corrosive vapours, foam, saturated steam, high viscosity, quick fill/empty rates, low levels and varying dielectrics and product densities; such as filler silo and Fly Ash.

Inverse frequency shift capacitance

Inverse frequency shift capacitance devices incorporate a frequency-based approach to level measurement. The capacitance units monitor the effect of capacitance based on frequency change. The relationship between capacitance and frequency is inverse. Because a very small change in level will result in a large frequency change, the result is excellent resolution and accuracy.  

The capacitance device has been widely used as a level point switch, offering reduced maintenance costs when compared to conventional electro mechanical devices, particularly in applications with build-up.  However, due to the high accuracy and repeatability of the Inverse Frequency shift circuitry, capacitance probes are seeing resurgence within the market place as a continual level measurement device.  This technology is commonly used as safety devices for applications such as, blocked chute, High / Low level, and alternative failsafe devices for hazardous applications such as Bitumen measurement.

Electro-mechanical 

Electro-Mechanical switches have been the traditional technology employed for simple level detection. The most common being the vibrating fork.  A signal from the electronic circuit excites a crystal in the probe causing the fork to vibrate. If the fork is covered by material, the change in vibration is detected by the electronic circuitry, which causes the relay to change state after a one second delay. When the fork is free from material pressure, full vibration resumes and the relay reverts to its normal condition. This technology ranges from simple / cheap devices that detect liquid level, to the more robust device used to measure dry bulk solids in bins, silos, hoppers or settled solids within liquids (interface version).

Overview

The concept of continuous level measurement for bulk solids handling is based predominantly around non-contact technologies, providing accurate and repeatable measurement of solids, liquids, slurries and foam from within some of the harshest environments. Siemens instruments are designed to work in these environments, ensuring reliable and continuous level measurement despite the presence of extreme dust, moisture, corrosion, vibration, flooding or extreme temperatures, making them ideal for the solids and bulk industries.

Efficiencies and good performance throughout the supply chain is achievable through effective control and instrumentation equipment and can potentially make a real difference in a company's annual performance and should therefore be an integral part of the continuing progress of the solids and bulk handling industries.

Steven Howes is flow & level specialist at Siemens

Siemens Process Instrumentation

Century House
Bridgwater Road
Worcester
Worcestershire
WR4 9ZQ
UK

T: 01905 450500