Improving Flow in a 4000 t Coal Bunker using Insert Technology

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Ajax Equipment Limited

Bunker Insert System

The results of powder tests and model insert trials identified a critical aspect of the insert system design – the ability of adjacent outlets 2.5 metres apart to provide draw over a sufficient area, such that the minimum rat hole size that could form would be so large that any coal remaining would be substantially less than in the original bunker, and be more prone to collapse when the central flow channel emptied.

Of course modifying the flow pattern in a bunker of this size brings with it other concerns associated with the loads acting on the structure. Tata was very keen to avoid the possibility of repeating the failure of the Grange Coke Oven Service Bunker at Port Talbot in 1961. There a 3000-tonne concrete service bunker failed within two years of being built, due to stress cracking of the reinforced concrete walls. Whilst the bunker had been designed for mass flow, it was unable to withstand the pressures generated under the dynamic conditions occurring during mass flow.

Gallery

Tata calculated the loads on the Scunthorpe bunker walls due to mass flow. The results of this analysis confirmed that extreme care had to be taken not to generate mass flow in the bunker, due to the vulnerability of the hopper walls to the increased loads that total mass flow – if it occurred – would bring. Of course there was still the requirement to provide the maximum useable capacity of storage, but that would have to be satisfied by a combination of mass flow in the vicinity of the outlets, and avoidance of mass flow further up the bunker.

At Scunthorpe, a three-stage design with an insert was devised by Ajax. The first mass flow sections were internal hoppers which comprised of plane flow converging to the existing outlets. These hopper sections actually diverged slightly in the opposite plane to give the best chance of squeezing through the outlet – a technique called Sigma Two relief. The form of these sections was developed so that two adjacent outlet sections actually joined together to provide the one slot.

The second mass flow stage expanded the flow channel to a reliable flow width and connected a full row of four outlets together. To destabilise any large rat hole or cliff which might form, a third stage was added which would not mass flow but instead provide self-clearing of the remaining hopper contents. Due to the offset outlet construction, a set of inserts was fitted to reduce compacting pressures in the outlet region and encourage flow from the shallow side of the hopper (Fig. 3).

Installation and Performance of Inserts

The first row of inserts made such a huge difference to the flow of coal from the bunker; that a financial case was put together to line a further three rows of outlets. There are now two rows of inserts in the 3000-tonne section and two rows in the 1000-tonne section (Fig .4).

Prior to fitting the inserts poking for coal was required at least once per shift from November to March. This is no longer necessary. Moreover, there have been no reportable injuries sustained from poking activities from the service bunker since the inserts were installed in 2009.

When poking for coal there was a tendency for the chargecars to be under filled. This resulted in the ovens not being filled to the required standard, lower coke yield and subsequent refractory damage to oven chambers. Since the inserts have been installed, better filling of the chargecars has occurred resulting in improved oven filling and a higher coke yield. The live capacity of the 1000-tonne section is now approaching 80 percent.

In conclusion, powder testing for flow, insert system development using a model and careful consideration of the bunker's structural integrity, has enabled Tata Steel R, D & T and Ajax Equipment to deliver a successful solution to overcoming earlier flow problems at the Scunthorpe coal bunker.

* Dr. Eddie McGee, Technical Director of Ajax Equipment Ltd., Tel. +44 (0)1204/3867-23, E-Mail: sales@ajax.co.uk

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