A Sonic Sound Device that works like magic to increase production
Donald F Cameron – Managing Director
Primasonics international Limited
Global CemProcess Conference - 24-25 April 3017 – London
Would you say that I was peddling snake oil medicine if I told you that I could sell you a simple sonic sound device which would:-
Remove process bottlenecks
Optimise process performance
Maximise production output
Improve thermal efficiency
Well that is exactly what is happening every single day in cement plants in over 70 countries worldwide!
Translate sonic sound device into acoustic cleaners and the picture may become somewhat clearer. A few seconds ‘sounding’ at periodic intervals is all that is required for example to:-
• prevent material build up within silos, fans, filters, cyclones, ESPs, SCRs
etc.
• maximise material flow in silos, hoppers, ductwork etc.
• improve thermal conductivity in boilers, economisers, air heaters etc.
• eliminate unscheduled plant shutdowns for example in fans and filters.
• reduce opacity spiking from ESPs and filters
So what exactly are sonic sound waves created by acoustic cleaners and how do they work? Acoustic cleaners are employed wherever ash, powders or granular materials are processes, generated, stored or transported.
Sound can be best described as the passage of pressure fluctuations through a medium by means of a vibrating source. It is these pressure fluctuations and the resulting ‘compression’ and ‘rarefaction’ cycles that are used by acoustic cleaners to move powders by breaking particle bonds.
The ear does not actually hear sound, it is a pressure sensitive mechanism that detects very rapid pressure fluctuations and it is these very rapid pressure fluctuations which cause dry material to debond from both adjoining particles and from any structure.
In the case of an acoustic cleaner, the wave generator creates the ‘base’ tone and the variety of bell sections amplify and convert this into a particular fundamental frequency. Within the Primasonics range there are six selected frequencies ranging from 420 Hz down to 60 Hz. All that is required to ‘power’ the titanium diaphragm within the wave generator is normal plant compressed air at 5 to 6 bar.
So what do these high energy – low frequency sound waves actually do?
They produce very rapid pressure fluctuations (up to 840 times per second) which break the cohesion bonds between one particle and another and between each particle and the structure to which they are adhered to. Once these bonds have been broken the particles e.g. cement powder, fly ash etc. are removed either by gravity or in the gas stream.
Before we take a look at some typical acoustic cleaner installations let me answer one important question I am often asked
Will the acoustic cleaner cause any damage to the structure?
The answer is ‘NO’
The wave generator and exponential bell shape of a Primasonics acoustic cleaner has been very carefully calculated to ensure that the unit has a very high acoustic impedance, just as in electrical engineering coupling between components is only efficient if they have similar impedance.
The Primasonics Acoustic Cleaners are designed with this high impedance for two key reasons the first of these is that the sound produced from the acoustic cleaner does not couple with the structure, this prevents the structure from being driven to dangerous vibration levels and ensures that the sound is reflected inside the structure which then builds up a high diffuse sound pressure level to ensure that all parts of the application remain free from particulate build up. The second reason for the high acoustic impedance relates to the design frequency of the Primasonics Acoustic Cleaner, if the cleaner were allowed to couple to the structure the geometrical property of the structure would alter the frequency of the sound produced by the acoustic cleaner to a frequency nearer the natural resonance of the structure. For these two key reasons Primasonics working with the Acoustic Research Unit at the University of Liverpool have conducted many both laboratory and on site tests to ensure that coupling does not take place between the acoustic cleaners and structures. The figure above shows the sound pressure level shown as the two large peaks in the dark line caused by a PAS75 acoustic cleaner and the vibration levels of the structure shown by the lighter line. It can be seen that there is no corresponding increase in vibration at the horns fundamental frequency of 75 Hz or at harmonic frequencies. This particular test took place in a large steel flour silo where an accelerometer measured the vibration on the silo wall.
Let us look at some of the key areas within the cement plant that can benefit from the installation of acoustic cleaners.
Silos and Hopers
The three main problems that occur in silos and hoppers are bridging, ratholing and cross contamination.
Bridging - this is caused when the design of the hopper section has insufficient angle of repose, side walls with excessive frictional properties or an insufficiently sized outlet for the material being contained. Consequently these problems can become severe if a silo is used for materials different to that it was designed for either through process or plant layout changes over time. To combat bridging an Acoustic Cleaner is mounted just above the outlet, when this sounds single particles and clusters of particles move at different speeds therefore bridges are collapsed and material flow is restored. Once the Acoustic Cleaner has ceased to sound new bridges and arches will begin to form, to counter this the sounding may be controlled in one of two ways either simply on a timer device typically set for a few seconds every few minutes or alternatively it can be controlled automatically via a flow sensor downstream of the outlet or even from load cells.
Ratholing - or funnel flow occurs when an insufficient angle of repose is used in the silo for the contained material this usually occurs with material that displays a mechanical interlocking properties or very quickly bonds without compaction for example because of heat or by means of solvent evaporation. To defeat ratholing Acoustic Cleaners with a long wave length are placed on the top of the silo usually through an existing manhole or inspection hatch. If there is a large amount of hardened material that has built up over a long period the silo is then cleaned using traditional offline cleaning methods such as the Prima Whip system. This is because if an Acoustic Cleaner were to be sounded in a silo which these very thick hardened build ups and the adhesion forces between particles were greater than those between the particles and the walls of the silo there is a risk that large lumps of material may dislodge and block the outlet because of the power of the low frequency Acoustic Cleaners to continue their sound transmission through the bulk material until the solid and reflective surface of the silo wall is reached. If we consider a full silo that is prone to ratholing being slowly emptied we can imagine the central core of the material lowering small amount while an outer ring close to the side walls remains at the same height. Without acoustic cleaning this process would continue until eventually the central core had been discharged and the outer ring is left adhering to the side walls. In this case a decision has to be made whether to remove the silo from active process lines while offline cleaning takes place or refill the silo and risk lower control over both the quantity and quality of product as older adhered material will mix with the new material coming in. Using Acoustic Cleaners and once they sound the bonds holding the outer ring will collapse and this will form a flat surface along the top of the material in the silo. This pattern is repeated as the silo empties even if the silo is refilled before complete discharge has taken place. This now allows a first in first out mass flow pattern. Again the Acoustic Cleaner can be controlled by a simple timer or a PLC or central control system.
Cross Contamination - if we take the example of a pre packing hopper or weigh hopper sometimes not all the material is discharged before the next batch enters the hopper. This can lead to product contamination which can lead to quality control problems. The reasons for material failing to discharge from hoppers are similar to those already mentioned but in pre packing hoppers, weigh hoppers or similar batch filled vessels an additional problem when material hitting or contact with colder side walls and moisture precipitates from the bulk material onto the side wall. This moisture forms a meniscus between the side wall and the particles which holds the powder in place so that even after discharge a thin layer or powder remains. In this instance the Acoustic Cleaner is activated when the bin is being emptied the alternate compressive and rarefactional forces break the surface tension of the moisture and allow this material to flow out leaving the vessel completely empty.
Typical Acoustic cleaner installations in Silos & Hoppers
Bridging Ratholing Pre Packing Hopper
Further information & case studies can be found on this link - http://www.primasonics.com/industries/silos-hoppers
ID Fans - Acoustic Cleaners are being used for cleaning fans in a variety of industries. Advantages include:
· a uniform cleaning pattern
· cleaning inaccessible parts of the fan
· eliminating out of balance fan conditions
· elimination of unscheduled fan shut down
Acoustic cleaners can be used to prevent build up on ID fans which can cause the fans to run out of balance causing unplanned shutdowns or potential damage to the bearings. The acoustic cleaners are operated intermittently and controlled by a simple timer, via a PC or connected to vibration sensors as part of a Monitored Preventative Maintenance (MPM) system. Major savings can be achieved by eliminating unplanned shutdowns caused by fan imbalance.
Typical Acoustic cleaner installations in ID Fans
Further information & case studies can be found on this link - http://www.primasonics.com/industries/id-fans
Baghouse Filters - Acoustic Cleaners are being used on reverse air, pulse jet and shaker units. The Acoustic Cleaners are effective in reducing pressure drop across the collection surface, increasing bag life and preventing hopper pluggage. They can be very beneficial to Filter bag life and lead to significant reduction in compressed air usage. Acoustic Cleaners are generally mounted in the Hopper section below Filter bags where they can also help to clean frame works and keep the Hopper sections free from build-up.
By the sonic cleaning action of Acoustic Cleaners bags do not suffer the same shocks and weave deterioration caused by reverse pulsing and can last up to 1.8 times as long as bags subject to reverse pulsing.
Acoustic cleaning provides the ability to turn off or reduce the frequency of reverse pulsing helps save compressed air as significant volumes are expended within Filter systems. Acoustic Cleaners help to maintain clean and efficient filters, reducing differential pressure and avoiding any output particulate spiking.
Typical Acoustic Cleaner installations in Baghouse Filters
Further information & case studies can be found on this link - http://www.primasonics.com/industries/filters
Electrostatic Precipitators (ESPs) - Acoustic Cleaners are proving invaluable in ESP applications, bringing advantages such as:
· superior cleaning
· elimination of excess build up problems
· increased precipitator efficiency
Acoustic Cleaners can provide many benefits for the effective cleaning of turning vanes, distribution plates, collecting plates and electrode wires. Acoustic Cleaners have been employed to either replace or assist Mechanical Rapping Systems and provide superior cleaning through the precipitator.
In addition they prevent particulate build up in the under hoppers, which can result in opacity spiking. Other advantages include, cleaning of walls, roof and steelwork with no mechanical stress or damage to the vessel or equipment, the elimination of excess build-up problems, increased up-time and increased precipitator efficiency.
Typical Acoustic Cleaner installation in ESP
Further information & case studies can be found on this link - http://www.primasonics.com/industries/electrostatic-precipitators
Boilers, Superheaters, Economisers, Air Heaters – acoustic cleaners are now the first choice to prevent ash build up from occuring in all sections of the power generation plant. The main advantages over steam
There are quite a few with the advantages in each case pertaining to the Sonic Soot Blowers. The main advantages of Sonic Soot Blowers have over steam soot blowers are:-
Price/Operating & Maintenance Costs – The Sonic Soot Blower is cheaper to purchase and install. It is considerably more economical to operate and has virtually no maintenance costs.
Physical Damage/Corrosion & Erosion - Sonic Soot Blowers will not cause any physical damage or wear to tube bundles or boiler structures. They operate at frequencies very much higher than the resonance frequency of steel, ceramic lining, concrete etc. and are therefore guaranteed not to cause any vibrational damage to any vessel or structure or tube bundles. Also because they do not use any high pressure steam or water they will not cause any corrosion or erosion problems and are Eco Friendly.
Cleaning Efficiency – because Sonic Soot Blowers use powerful sonic sound waves, these sound waves travel at a speed of 344 metres per second and in a 360° radius. This means that the entire circumference of the tubes and internal structure can be easily reached instead of just the leading edge of the tubes as with a steam soot blower. A periodic "sonic sounding" is all that is required to achieve these goals! Typically 5 - 10 seconds every 6 - 12 minutes. Within the power generation industry Sonic Soot Blowers have successfully solved particulate build up problems in:-
Typical Acoustic Cleaner & Sonic Soot Blower installations