Key points
In this article Ingemar Serneby, Emerson Process Management, discusses the difficulties of measuring the level of solids in vessels and silos, and reviews some of the available technologies considering their suitability for different applications.
Introduction
Selecting a suitable technology to measure the level of solid materials in vessels and silos can be a frustrating experience for many users.
The material being measured often behaves in a manner that makes level measurement difficult and the results unreliable. Many applications need to convert level measurements into mass, and accurate results rely on a consistent density of the material being measured, and a complete understanding of the dimensions and characteristics of the storage vessel.
The traditional approach to level measurement in the industry has been to use mechanical devices known as yo-yo’s. These are based on a weight attached to a wire being lowered onto the surface of the solids at scheduled times.
The device calculates the level of the media based on the length of wire required before the weight makes contact with the surface. However, as the spool mechanism is in contact with the media, these systems require regular maintenance – exposing operators to the hazardous conditions on tall silos.
Where continuous measurements are required, there are a number of options that cover a wide range of applications in the process industries. These include Guided Wave Radar (GWR), non-contacting radar, ultrasonic, acoustic, laser-based technologies and load cells.
Why are solids so difficult to measure?
Surface inclination
During the fill and emptying cycles, the surface of solid materials is rarely flat or horizontal, with a tendency to peak under the inlet and ‘rat hole’ above the outlet. The angle of repose, or surface inclination, will change as the vessel fills and empties, and this angle affects the recommended due to signal loss.
Dust
One of the most common problems for solids level measurement technologies is the effect of dust generated during the fill cycle. Many materials produce large amounts of dust in the silo and this can be problematic for ultrasonic and laser devices.
The presence of dust affects the signal strength and subsequently the reliability of the measurement data. As a result, ultrasonic and laser devices are unsuitable for some of these applications.
While radar signals are unaffected by dust, the antenna on non-contacting radar systems needs to be protected from dust build up, for example by using a PTFE bag which has non-stick properties, preventing dust from adhering to it.
Air purges are also often used to clean dust off the sensor surfaces. Acoustic based systems use low frequency pulse signals that are unaffected by dust, filling noise, humidity, or temperature. In addition, they can be made self-cleaning by using the vibrating membranes to drive out any dust particles that coat the horns.
Types of media
There are many different types of media and their physical characteristics vary widely. For example, particles can vary in size from fine micron size to large particles, rocks, pebbles or granules. They can be smooth or have sharp edges. Parts that are in contact with abrasive media – such as GWR probes will require regular maintenance.
Lightweight aerated materials have sound absorbing characteristics that affect the performance of ultrasonic and acoustic level devices. Some materials are hygroscopic – readily absorbing or trapping moisture and this can cause caking or clumps inside a vessel.
In particular, coarser materials are more likely to clump, bridge, leave voids and pile up. Products like grain have variable characteristics dependent on the specific crop and moisture content. This makes it difficult to determine a consistent crop density that will provide accurate volume results..
Dielectric Constant
The dielectric value of many solids is fairly low. For radar technologies, the dielectric constant is a key indicator of the amount of signal that will be reflected back to the gauge. If the dielectric constant of the measured material is very low (<2), this may require the use of a GWR device that uses signal processing techniques to overcome the effects of poor signal strength.
Bulk Density
The weight of the material per volume is important when monitoring bulk solids inventories so it is important that the bulk density (usually in Kg/m3) is representative of the material. Continuous level sensors will convert a distancemeasurement from the sensor to the material surface, to volume and weight/mass. Any errors in the bulk density will be translated into errors in volume and weight/mass.
Pull Forces
This is a particular problem for GWR where heavier materials can create a pull force that can break GWR cable probes or transfer loads to the silo roof, causing it to collapse. While this is more likely to be an issue in vessels taller than 50 ft. (15 m), care must be taken to guard against this possibility.
A further consideration is that the weight of the material may push flexible GWR probes towards other structures in the tank, creating false targets.
Electrostatic discharges and Electromagnetic Interference
In some applications, for example the level measurement of plastic pellets, electro-static charges can build up and eventually discharge. It is therefore important to provide a good earth ground to protect the sensitive electronics inmeasurement devices.
In addition, the device must be designed to handle the electrostatic charge and divert the excess energy away from the electronics. This design can help with electromagnetic interference from nearby equipment, which can be a problem with non-metallic silos.
Overview of the Technologies Available for Solids Level Measurement
Guided Wave Radar (GWR) for Solids Level Measurement
Guided Wave Radar (GWR) has many applications in the process and manufacturing industries providing anaccurate and reliable measurement of level in tanks and vessels. GWR is a top down, direct measurement, as it measures distance to the surface.
Since GWR is not dependent on reflecting off a flat surface, it works well with many powders and grains including plastics, fly-ash, cement, sand, sugar and cereals. Radar provides a fast response to changes in level, making it suitable for closed loop process control and safety applications.
GWR is based on a low energy pulse of microwaves being sent down the probe, when the pulse reaches the media surface, a reflection is sent back to the transmitter. The transmitter measures the time taken for the pulse to reach the media surface and be reflected back.
The on-board microprocessor accurately calculates the distance to the media surface using ‘time-of-flight’ principles. Because there are no moving parts, maintenance costs are reduced and the problems of false readings, which can result in potentially hazardous situations, are avoided.
In order to improve the maximum measuring range in low dielectric materials, technologies are available that allow for measurements when the surface pulse is weak.
For example, Emerson’s Direct Switch Technology provides an echo signal that is two to five times stronger than other GWR transmitters. The improved signal to noise ratio means enhanced ability to handle difficult measuring conditions like long measuring distances and lower dielectrics.
The company also developed Probe End Projection (PEP) which enables reliable measurements to be made with low dielectric materials when the surface pulse is too weak to be detected.
This method is based on the fact that microwaves propagate slower through product than through air. By monitoring the position of the probe end echo, an algorithm can calculate the surface position in case of an unavailable surface echo. The PEP function will only be activated as a backup if the surface echo is too weak.
Non-contacting radar for Solids Level Measurement
Non-contacting radar devices can be considered as an alternative to GWR for applications where no-contact with the media is required, for example where aggressive and abrasive media can cause damage or wear to the probe.
Like GWR, non-contacting radar provides a top-down, direct measurement as it measures distance to the surface. Non-contacting radar sends a signal through the vapour space that bounces off the surface and returns to the gauge.
Non contacting radar needs a clear view of the surface and the best results are obtained with relatively flat, rather than slanted surfaces. However, the specific challenges of sloping surfaces, low dielectric properties and high filling rates can be overcome by the use of specially developed software and algorithms that can manage some of the unique characteristics of the return echoes.
Ultrasonics for Solids Level Measurement
While ultrasonic level devices can be used for the measurement ofsolids they do have a number of issues that can affect the way the sound wave reflects off the surface and/or the generation of “false” echoes that can make detecting the true level signal very difficult.
These include their inability to make measurements in applications with dusty conditions, vapour space changes, changing angle of repose, large particle sizes, internal vessel obstructions, and coating or formation of clumps on the internal vessel surfaces.
While modern designs and technical innovations have improved the performance of ultrasonic level devices, in many applications, newer technologies such as radar and acoustic will generally provide better results across a wide range of applications.
Acoustic Solids Level Measurement
Acoustic based devices measure level by transmitting low frequency pulses that reflect off the surface of the contents of the silo, bin or container.
Because of the nature of the acoustic signal, which transmits over a wide area, it can be challenging to obtain the correct echo from the surface. Acoustic systems such as the Rosemount 5708 3D Solids Scanners, use three horn antennas that detect not only the distance to the surface, but also the direction of the echo to the object reflecting the signal.
A Digital Signal Processor then samples and analyses the received signals to provide very accurate measurements of the overall surface of the stored contents, and generates a 3D visualisation of actual allocation of product within the container for display on remote computer screens. Using the silo dimensions calculating the volume is therefore easy.
3D Solids Scanners provide continuous volume measurements that are based on the representation of the material’s surface. They are ideally suited to measuring solids in silos, large open bins, bulk solid storage rooms, stockpiles and warehouses.
They can measure practically any kind of material including grain, lime, cement, plastic powders, difficult-to-measure fly ash and materials with a low dielectric. Self-cleaning designs require minimal maintenance even when used in the dustiest environments.
Laser based technologies for Solids Level Measurement
Laser transmitters for level measurement employ the speed of light, using a very narrow beam focused on a small area. They are extremely accurate and can operate over large distances.
This makes them particularly suitable for applications that require level information at a precise location, for example at the emptying point of a silo. This provides operators with information about the ability of a storage vessel to deliver material when demanded.
Laser based systems can be used in high pressure, high temperature applications using specialised sight windows that isolate the transmitter from the process.
However, an accumulation of dirt, dust or other coating on the laser level transmitter seriously weakens the strength of the laser signal affecting their performance. Regular maintenance is important to prevent accumulations and ensure the correct functioning of laser level measurement devices.
Specialist systems for Solids Level Measurement
Nuclear systems work by directing a fan of radiation across the vessel. As the level rises and falls the amount of radiation received at the detector also rises and falls. Since the measurement system is not exposed to the material, nuclear systems are ideal for corrosive, abrasive, high temperature or high pressure process conditions.
However, these devices require special licenses, training and tracking, which add to the operating costs. In addition, serious consideration must be given future disposal and the associated cost implications.
Load cells and strain gauge weight systems are usually chosen for applications where certified mass measurements are required, for example in the food, pharmaceutical and aggregate industries.
They are used to support accounting applications or where all other technologies have failed. Load cells tend to be built into a system when it is designed and installed, although they can be retrofitted. Systems need to be engineered to individual requirements and are often difficult to calibrate.
Summary
With such a wide choice of technologies available, the characteristics of each application need to be carefully considered to ensure an accurate and cost effective solution to each solids level application.
For example, what are the dimensions of the storage vessel? Will dust in the vessel be a problem? Does the media being measured have a low reflective characteristic? With the surface of solids being typically uneven, will a single measurement point provide accurate results?
As a general guide, for applications in smaller vessels, GWR radar devices provide an accurate and reliable method of providing continuous level measurement of solids.
With their small connection size, GWR systems are easy to mount and can adapt to a wide range of vessel types and shapes. They have a fast response time for rapid filling applications and the measurements are independent of moisture, and material fluctuations such as density and temperature.
However, the selection of the correct probe type is important and special consideration needs to be given to installations in tall vessels or where the media being measured is aggressive or abrasive.
Care needs to be taken with low dielectric solids or where there is long distance to the surface, or where there is a source of electromagnetic interference.
These applications require products with technologies such as Probe End Projection which is incorporated in the Rosemount 5300 GWR transmitter. Probe End Protection enables reliable measurements to be made under low signal conditions.
For applications where aggressive or abrasive media can cause damage or wear to the GWR probe, non-contacting radar should be considered as an alternative. However, signal reflections are low in the measurement of solids which makes correct installation and the selection of antenna very important.
The latest generation products such as the Rosemount 5402 non-contacting radar use advanced software and algorithms to overcome the specific challenges of solids measurement which include inclining or sloping surfaces, low dielectric properties and high filling rates.
For larger vessels and silos up to 70 metres in height, acoustic based systems provide highly accurate measurements of stored contents.
They provide continuous volume measurements that are representative of the material’s surface and can measure practically any kind of material including grains, mineral ores and cement materials with a very low dielectric.
Devices like the 5708 Series 3D Scanners map the uneven surface typically found in solids applications and can provide the minimum and maximum level, the total volume and a 3D visualisation of the surface. The self-cleaning design of such products require low maintenance even when used in the dustiest environments.
For applications in large areas such as warehouses, several acoustic devices can be combined to provide an accurate and reliable measurement system for inventory and production process control in many industrial applications.
Care in selecting the most appropriate technology will minimise maintenance requirements and provide the longest operational life. Most suppliers offer a range of technologies and their technical and application engineers have experience of popular applications and are able to guide users through the selection process.
