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How to Achieve Optimum Silo Level Measurement

How to Achieve Optimum Silo Level Measurement

Being able to accurately assess the level of liquids throughout industrial and chemical processes is key to achieving efficient automated control.

The measurement of liquids in industrial and chemical processes has evolved significantly in recent years. Increasingly sophisticated processing systems and the introduction of stringent environmental regulations require a high degree of accuracy with reliable performance. Manufacturers have responded by developing alternative measurement methods, using modern technologies.

Process engineers are faced with a multiplicity of options, from point level measurement sensors to continuous level transmitters which measure range using technologies ranging from differential pressure to ultrasonic and radar.

With so many options and such a range in investment costs, how do you choose?

There is no such thing as a perfect technology. Typically, several methods may be suited for an application. In selecting the most appropriate solution, advantages and limitations of each need to be weighed against operational parameters. In the chemical industry, measurement equipment must also deliver a consistent performance in harsh and often hazardous conditions.

To get the best performance on your silo level indication you first need to understand your product, its behaviour and the equipment it is handled by.

The product you are measuring

- Understand a bit about your product behaviour, when it’s delivered and in storage. Does it aerate during filling, how does it flow and handle?

- Its characteristics, is it adhesive, aggressive or abrasive?

- Is it stored at temperature, does it give off humidity is condensation high inside?

- What does the product surface behave like? (angle of repose)

- Finally, is there a lot of dust expected? Perhaps speak with process operatives or suppliers for some insights if you are not sure.

The silo

- How is the silo filled?

- What is the surface profile during filling?

- Does it give off a lost of dust?

The internal geometry/shape of the silo: diameter versus height i.e. a wide height to diameter may require multi point measurements with surface mapping, whereas tall and narrow is perfect for one sensor. 

A dimension drawing is extremely useful. What is the product out-take like? Is it a core flow, or mass flow (Google it!) discharge or if it's a flat bottom, is material emptied via screw feed or a moving floor, how does it affect the surface profile during emptying? This may affect how far down you can measure or how much material stays behind, even when nothing is coming out.

The measurement goal?

Accuracy? Measurement of volume or conversion to weight? – does the bulk density vary by product source, varying material or suffer from aeration during filling, how high up the vessel do you need to fill, how likely is overfill and what are the consequences for safety and environment?

Do you need an additional point level protection to your volume measurement? Varying product density can even fool weighing systems into getting the silos capacity wrong! Also who and how do you want to share the information? From an indicator to internet, there are many different possibilities here.

Ash Silo Level Measurement

Technology and sensor positioning

If you can get answers to most of the questions above it will help narrow down the technology or device. You can use something like a Technology Finder and a Configuration Tool to specify the device you need. But you don’t have to do this on your own. Better to get a reputable sensor manufacturer with multiple level technologies at their disposal.

Get them to review your information, suggest the device, location and an idea of accuracy to make sure it meets your goals. Options may vary with access into the silo to install or location of filling points.

Striking the right balance

Whilst fitness for purpose is essential, this must be balanced against cost (especially where there are multiple measurement points), in relation to potential operational gains.

Analogue transmitters continue to offer an economical and effective solution suitable for a wide range of applications. Now the latest models to come onstream are set to deliver increased freedom of operation.

Striking the right balance

Whilst fitness for purpose is essential, this must be balanced against cost (especially where there are multiple measurement points), in relation to potential operational gains.

Analogue transmitters continue to offer an economical and effective solution suitable for a wide range of applications. Now the latest models to come onstream are set to deliver increased freedom of operation.

Maintenance and cleaning are also important considerations where a build-up of liquids could occur, so ensure transmitters are designed for ease of installation and removal for routine cleaning.

In the face of increasingly sophisticated measurement solutions, analogue transmitters have a secured their place, ensuring higher product quality, improved safety and less waste at an economical price, now with the added benefit of programmability

Correct location of level measurement transducers during bulk solid material

During the transfer of bulk solid material to and from storage silos, the shape of the surface of free-flowing material changes. Correct location of level measurement transducers and probes mounted at the top of silos will lead to greater accuracy and reliability in product contents measurement. The following refers to large round silos, which often cannot easily be fitted with load cells.

Normally, transducers are installed on the top of the silo and measure distance down to the surface of the material. This measured distance can then be converted into the material volume. Technologies used for this purpose include radar, ultrasonic and TDR (Time Domain Reflectometry).

Figure 1 – basic geometry

Figure 1 – basic geometry for Silo Level Measurement

Fig. 1(a) shows a conical pile of material of height L and base radius of R. The volume (V1) of such a cone is given by V1 = 1/3 πR^2L

Fig. 1(b) shows a cylindrical shape having the same base radius R and a height of H. The volume (V2) of the cylin

der is given by V2 = πR^2H

If the cylinder is to have the same volume as the cone (V1 = V2), its height H will be 1/3 x L.

In Fig. 1(a), the point on the upper surface of the cone which is at height of H, or 1/3 L, above the base is at a radius of 2/3 R. Therefore if the height of a conical pile of material is measured at a radius of 2/3 R, the volume can be found by multiplying this height by the base area. This will give the correct volume irrespective of the angle of repose of the material.

Consider now the conical depression shown in Fig. 1(c). The volume V3 of material in this shape is given by V3 = 2/3 πR^2 L

Similarly the cylinder shown in Fig. 1(d) will have a height H of 2/3 L. The point on the upper surface of the depression of Fig. 1(c) which has this height, is at a radius of 2/3 R. So again, if the height of a conical depression is measured at a radius of 2/3 R the volume of the material can be found by multiplying this height by the base area. The correct volume is given independently of the angle of repose.

For a cone or a conical depression the correct volume can be found by measuring the height at any point that is 2/3rd of the radius of the silo from its centre. This is the ideal location at which to mount a level transducer, accurate when the surface is a perfect conical pile during filling or a perfect conical depression during emptying.

However, there will still be errors during the transition phase when emptying follows filling and vice versa. If the probe were centrally located this would lead to significant errors. However, by locating the transducer at a position 2/3rd of the silo radius the error during change over from filling to emptying to filling again is typically no greater than 3.0% of the maximum content.

Acoustic phased-array technology

Measuring the level and volume of solids in silos is complex and challenging. The surface of solids is uneven and constantly shifting, and the difference in level between its various peaks and troughs can be substantial. Especially in larger silos, a single-point level measurement is therefore of much less value than an understanding of maximum level, minimum level and total volume.

Traditional mechanical methods of solids level measurement have limited accuracy, reliability and repeatability of measurement. Also, they can expose workers to hazardous conditions, either through performing measurements manually or carrying out regularly-required maintenance. Consequently, many modern facilities are instead employing continuous automated measurement technology.

One of the most widely-applied automated technologies is acoustic phased-array antennas. 3D solids scanners, based on acoustic phased-array technology, have three antennas that generate a mix of audible or acoustic signals and receive multiple echo signals from a silo’s contents.

Digital analysis of these echoed signals produces multiple measurement points to achieve accurate continuous level and volume measurement. Matching the received data with known silo dimensions allows these scanners to calculate the volume of practically any kind of stored contents, including difficult-to-measure fly ash and materials with a low dielectric that would challenge other technologies.

In very large or irregularly-shaped silos, multiple 3D scanners can be used to provide the necessary level of control. By merging their individual measurements, users are provided with a combined wall-to-wall surface map.

The latest enhancements to these acoustic devices are analytical features which address additional market needs by supporting improved safety and inventory management through the continuous analysis of product flow and movement.

3D solids scanners for silo level measurement

For example, new devices with 3D Scanning features help to optimise filling and emptying processes by dividing a large silo into as many as 99 individually-monitored sections.

Average, minimum and maximum level readings are provided for each section, enabling better understanding of material flow and movement, and filling points can then be switched to ensure even distribution of material across the surface area. This eliminates the need for manual surveys of material distribution, thereby improving worker safety.

Centre of gravity (COG) is another important consideration in stored solids applications. When most of the material lies outside the silo’s COG, it produces stresses that can cause the structure to tilt or even collapse.

Laser level and volume measurement

Laser level measurement provides an easy way to get precise, reliable silo level measurement.

Why laser level measurement?

Because of the simplicity of its use, which translates into low cost of operations. Laser beams move through space with very little divergence, meaning they remain tightly focused even at long distances.

Since the laser beam doesn’t interact with the surrounding environment, there is no need to cancel false echoes: only the liquid or solid surface is detected. Commissioning is thus simpler. Also, changes in the environment do not require changes in sensor parameters.

For instance, material accumulation on the side of a vessel will change over time, requiring a remapping of false echoes. The same would happen if the sensor is moved. This will never be required with laser level measurement. Therefore it leads to more reliability and more up time during use.

Laser beams also bounce back from surfaces very differently from ultrasonic or radar waves, which can be advantageous in several applications. Plastics, polymers, and low density materials are easily detected by laser sensors, as opposed to radar sensors. Also, there is no limitation on the angle of incidence for measuring solids with lasers, which simplifies installation.

Being very narrow, the laser beam can also be used in tight spaces and difficult applications like measuring through pipes and valves, grids, and also in the presence of agitators and mixers, where the laser beam can be sent between the edge of the agitator and the vessel side to measure without interference.

Laser level measurement is used in many types of silo level measurement applications. For instance in agriculture it performs well on many types of grains such as corn or wheat. For wood products, the ability to measure in the presence of wall buildup greatly reduces maintenance cost, as the measurement is insensitive to buildup.

For plastic pellets, widely used for making plastic objects, laser level provides an easy solution since it can measure plastics which is difficult for radar-based sensors. Lasers are also used in several aggregate materials silo level applications.

In conclusion, lasers will change the way you see level measurement. It will simplify operations and allow non-contact measurement in applications where non-contact was not possible up to now. By using new technology applied to industrial level measurement, laser level makes measurement easy.


Wireless Sensor Control System

A wireless asset monitoring and control system, eliminates the need for cabling sensors, which can be prone to lightning strikes that will propagate along wires and destroy equipment attached to it, and also damage caused during periodic maintenance tasks.

Radio nodes wirelessly interface with level sensors installed in silos to extract and transmit data to a gateway that serves as the central processing hub. See Graphic 1. An Ethernet interface module connected to the gateway ties information into a local area network (LAN, wired or Wifi) for local access or a cellular modem so, for example, farmers can access inventory levels in real time on laptops and smartphones.

Graphic 1. The Connected Farm – Access Information regarding farming operations from anywhere for Silo Level Measurement

Graphic 1. The Connected Farm – Access Information regarding farming operations from anywhere

Unaffected by ground faults associated with cabling, a wireless telemetry system is less susceptible to damage from lightning strikes as only the hit sensor might be damaged and not the entire network.

The wireless remote sensing system also allows for the installation of the level sensors at any height and location on the silo. Operating on a mesh network, the sensor control system can be deployed over large areas to go around hills, buildings and other structures that may obstruct the radio transmissions of other networks.

Robust gateways can accommodate hundreds of transceiver inputs from the field sensor, enabling the network to cover a geographic range of a mega farm of 10,000 acres that roughly equals 15 square miles.

By automating silo monitoring activities, farmhands know when they need to switch to another silo or refill one low on inventory. As a result, the farmer increases production efficiency.

The Flow Characteristics of Silo Measurement

As previously mentioned, the measurement of silo level can present difficulties due to the surface profile varying considerably according to the filling and emptying pattern. These will depend on the design adopted for the silo and its discharge facility, with the flow pattern generating either Funnel Flow, Expanded Flow or Mass Flow.

The selection of the flow pattern is normally based on the properties of the bulk material stored; ‘Mass Flow’ is chosen to avoid indefinite residence time for products that deteriorate in flow prospects or quality with time, ‘Expanded Flow’ for poor flow materials that are not adversely affected by extended residence and ‘Funnel Flow’ for inert, relatively easy flow materials or for silos with some form of assisted discharge.

‘Funnel Flow’ extracts preferentially from the region over the outlet with a surface inclination of drained repose. Combined with a single point outlet, this creates a conical depression in the surface profile when the silo is discharging. In contrast, a single point entry point forms a rising cone during the filling process.

An ‘Expanded Flow’ pattern will develop a central plateau within this cone during discharge, the size depending on the diameter of the transition in the hopper section. Superimposed on this pattern, a single point fill will generate a growing pile that is characterised by the angle of poured repose of the product.

Discharging by means of a progressive extraction screw feeder will spread the flow channel and reduces the emptying difference to a 2D situation, but complicates the refill position.

A side mounted level probe on a circular silo in Funnel Flow therefore cannot indicate a true volumetric capacity within an accuracy of +πD3.Tanθ/8 and - πD3.Tanα/8, Where D = silo Diameter, θ = poured angle of repose and α = drained angle of repose.

Taking θ and α as roughly 300, on a silo 3 M diameter, this equates to a volume of about πD3/4√3 or 12 M3 for which filling and discharge condition would show no change on level. Fig.1.

Funnel Flow Silo With side mounted probe

It may be thought that repositioning the probe to an intermediate radii of the silo would compensate for this radical profile shape, and so it will, but only when it is placed near the silo inlet. Unfortunately, wherever the probe is placed on the side of the silo this discrepancy will not be eliminated, fig 2.

Funnel flow silo with probe near the inlet

 A single-point, surface level detector can be placed in a radial position where either a poured or drained repose slope will indicate similar contents, but the inner and outer volume variations can accumulate, rather than compensate for the difference, if the sequence of filling and emptying surface profiles overlap around this level. Fig.3.

Volume variations possible between filling and emptying

In summary, Sophisticated radar scanning can accommodate variations in surface profile and weighing of the silo offers accurate content value, but Mass Flow is essential to indicate accurate volume indication by level probe, and even then a low level indication is only true on locations sufficiently above the hip level to counter the velocity differential that inevitably develops in the converging flow channel.

Mass flow carries many other benefits in density consistency, flow reliability and countering segregation, at the expense of extra headroom and possible long term wear on walls near the outlet.

Selecting your measurement systems supplier

If they are a reputable manufacturer, and it's the first time you have used them, get them to stake their reputation on a test or trial system (particularly if you have multiple silos to do) or perhaps on a sale or return basis. If you are getting a silo manufactured, involve someone early enough to help you get the sensor type and positioning right at the design stage.


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