Dead stock in silos, hoppers and storage vessels can pose major problems for cement plants, manufacturing facilities and terminals, hindering and restricting the capacity of silos and in many cases reducing production. But, dead stock also presents other serious issues for plants:
- Danger upon access to silos – unpredictability of falling cement.
- Expensive material written off due to material not flowing.
- Expense of cleaning when the situation arises for maintenance purposes.
- Removal expenses when changing product in silos for quality purposes.
- Depending on the silo design, dead stock could cause false readings of material levels.
Dead stock can be caused by a variety of factors, but the most common are water entering into silos, and faulty air slides/air pads, the latter of which is not always obvious immediately, therefore allowing the material to settle. With the increasing weight and subsequent pressure, gradually hardened masses of product result.
Figure 1. Angled and standard sockets installed on discharge chute.
Increasingly, Cardox has been called upon to reduce dead stock from silos and bunkers alike, using various different methods – from standard Cardox sockets being fitted on the silo discharge areas and walls of silos, to deploying abseiling crews to place Cardox tubes in hardened cement coating on the silo walls.
The recently introduced hydraulic whipping system, ‘SILOWHIP’, has also earned some positive plaudits within the cement and raw feed silo industries. Due to the unpredictable nature of how cement and clinker can build up within silos, a flexible approach is required when applying blockage clearing systems. There are different sizes of Cardox Tube that can be deployed to afford varying pressures and volumes to reduce dead stock, aerate the product and clear build ups.
Figure 2. Acute angled sockets installed on discharge gates.
Reducing dead stock
In November 2019 Cardox International was invited to a cement plant in the UK where two silos had dead stock issues.
Case study 1
The first silo was a blending silo, with a capacity of 2500 t and over 70% dead stock. The silo was discharging, albeit at a very slow rate, which indicated that material was perhaps trickling out through a small ‘rat hole’. With the very limited access to one central discharge chute, operators at the plant had tried using long air lances which had to negotiate the concrete silo base of 1.5 m, before a void of approximately 2 m, after which the lances encountered resistance, i.e. the ‘bridged’ product.
The operators had intimated on the company’s ‘silo questionnaire’ that they had encountered lumps that could not pass through the rotary valve of the discharge chute.
The air lances had little, if any, effect and so Cardox was tasked with providing a solution to not only access the bridged product, but also to pro-vide a safe means of discharge, as the rotary valves were to be removed and replaced with a fabricated chute, meaning that the manual valve would be critical in controlling the discharge. Sockets were installed on the discharge gate (Figure 1), affording two angles to fit the Cardox tubes.
The silo tube, fitted with a spike at the front end and extension pipes at the back end, was inserted 3.5 m up through the discharge chute, past the void and pushed up high into the bridged material. It was estimated that the tube, fitted with a multi-holed discharge head was approximately 75 cm up into the product. The Cardox tube was then activated, releasing a cold mass of CO2 which broke the ‘bridge’ and crust of the material and aerated the product so that it regained its fluidity. The aeration system was then switched on and the product began discharging immediately. The discharge continued for several hours.
Figure 3. After activation.
Case study 2
The second silo at the plant was a cement silo that had two discharge gates which had not run for over five years.
The silo itself was over 100 years old and the discharge chutes appeared to be blocked approximately two metres above the discharge gate, with approximately 500 t of dead stock cement above.
In order to insert tubes up into the ‘lumpy’ cement, acute angled sockets were installed onto the base of the discharge chutes (Figure 2). An extended drill rod was used to create a hole in the dead stock product and a Cardox tube with multi-hole discharge head was inserted high up into the product. Two tubes were activated, blasting the lumpy product down and aerating the cement.
Figure 4. Deadstock removed from bunker wall by the mole system.
Flexibility is key
Both the aforementioned examples involved access from the discharge are-as of the silo but sometimes access at the discharge area is not feasible, so versatility is required in applying systems to reduce dead stock.
One such silo in a Holcim plant in Europe had 360° material build up around the walls of the silo, effectively choking it and creating thousands of tons of dead stock.
Cardox silo tubes fitted with spikes were dropped into the dead stock cement from the top of the silo, with the weight of the tubes immersing them some 2 – 3 m deep in the product (Figure 3).
On activation, the tubes blasted the material down into the ‘rat hole’ and thus cleared the dead stock down 3 m. The tubes were then retrieved by an electric winch, with the steel wire affixed to the tube by an eye bolt connection. With the development of elongated discharge heads with multi-hole nozzles, extension poles, spikes and eyebolt retrieval fittings, the system has been adapted and modified for use away from the traditional applications on the pre-heater tower.
Figure 5. Whip head lowered into silo as operatives receive training in applying the SiloWhip.
The mole system
The mole tube operates by 5 bar compressed air supply, and pneumatically hammers and self-propels the tube into product, displacing the material and driving the tube to a designated position or depth where the tube can then be activated.
This affords the advantage of placing tubes where normally rope access teams maybe required; this reduces the requirement of extra permits and the overall advantages and safety issues of no man entry into silos. The example in Figure 4 is a very common problem for plants worldwide, therefore the specialised silo tube, which carries the largest capacity of liquid CO2 on the market, would be beneficial in these scenarios. The piston of the mole is oilless, which means there is no contamination of the product and production of dust is lessened.
Another addition to the Cardox silo cleaning kit is the Hydraulic SiloWhip, which is applied from the roof of silos – the whip sweeps down material and build up clinging to the silo walls, and again, this prevents the need for man entry into the silo (Figure 5). The boom arm can be moved around to place the whip head in the relevant position to sweep the material off the wall, the head can be lowered and raised to depths of 50 m, and a variety of sweep attachments or ‘knuckles’ can be provided, such as bronze anti-sparking attachments and plastic nylon attachments.
About the author
John Hodson is the Managing Director of Cardox International and has over 40 years of experience in the cement industry.
Read the article online at: https://www.worldcement.com/special-reports/09032020/dead-set-against-dead-stock/