Down the Rathole

Discharge Aids

Down the Rathole

A Hopper Flow Solution for a Concrete Plant
A cement and concrete manufacturer faced a problem with their storage bins for the raw material, which accumulated in the hopper forming a rathole seriously affecting production. To keep the whole of the material flowing they installed an air cannon system with some interesting specifications.
(ed. WoMaMarcel - 09/10/2015)
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Planning a Flow Solution

A team comprised of Dunne, Dave Harasym, UK Sales Manager of Martin Engineering and the CRH Roadstone plant operations manager, Hugh McGee, collaborated on a plan that would use air cannons to address the build-up issue, a proven technology in cement manufacturing processes around the world. Complicating the plan was the fact that one side of the hopper abutted the retaining wall.

“Generally, proper flow from a square bin requires four cannons, one on each side firing simultaneously,” said Harasym. “We were already losing one side to a retaining wall, so figuring out how to do it with three cannons was a challenge.”
The planners came up with an innovative solution that set two 100-liter Martin XHV air cannons at the back of the bin. These work in tandem with a single, larger 150-liter XHV air cannon fitted to a split blowpipe manifold, which is connected to fan jet nozzles on the front sloping side of the bin. All three units are fitted with a 4 in (102 mm) quick exhaust valve (QEV) connected to the on-site compressor system 35 ft (10.7 m) away. Plimley also supplied and installed a new airline to feed the air cannons.

“This is the first application of the split manifold system on heavy, sticky material that I know of,” said Harasym. “Generally, this technology is used on light material such as fly ash.”

Pressurized air from the compressor is fed through the QEV and the piston, into the tank. When the solenoid is activated, the pressure stored in the tank forces the piston back and exits evenly from a single pipe and through the split discharge, with a strong blast of air into the bin. The XHV features a rugged 5/8 in (16 mm) stroke piston with a high-temperature polymer seal for long life. Working from one side of the tank, the complete valve assembly can be removed in one easy step and replaced within minutes, eliminating the need to ever remove the tank from the vessel for service.

Each unit is activated with a new type of 110 V, negative-firing solenoid, which provides the power, efficiency and easy maintenance of an advanced internal valve with the single-line plumbing of a traditional valve design. “Quite simply, we didn’t require the peak force that positive-firing air cannons would have delivered in this application,” Harasym pointed out. “The negative firing units we chose worked out to be the most cost effective in this price-sensitive environment.”

Keeping the Flow

After CRH Roadstone cleared the hopper with a mechanical digger one last time, installation was performed by two technicians and two engineers in just over 2 days to minimize disruption to plant operations. The pipework and manifolds were site measured, drawn, fabricated and fitted by Plimley Trading to assure maximized efficiency. According to Dunne, installation was a little involved, as all work was carried out from the basket of a cherry picker due to the height of the hopper and the placement required.


Installation was performed by a man team in just over two days.

Installers rigged a mechanism by which the discharge volume from the bin determines the firing sequence for the system. “On the block side outlet, the flow of material is controlled by a swinging door that drops the material into a weigh hopper,” McGee explained. “When the door opens, the air cannon discharges the compressed air into the bin. This happens every time the door opens and keeps the material flowing.”

On the concrete side, there is no door on the bottom, so discharge is controlled by a swinging flap placed over the conveyor that responds to material on the belt. If dormant, the flap operates a switch, which triggers the cannon. This assures that material is continually flowing to the belt with little monitoring or labor.

Results

Since installation in February of 2014, CRH Roadstone has not reported a single instance of ratholing in their hopper, and no need for a mechanical digger to clear an obstruction. This has accommodated the increase in volume the company has been experiencing, accompanying the economic rebound in Ireland.

This experience has strengthened the relationship between CRH Roadstone and Plimley, and fortified a belief in Martin Engineering products. McGee added, “Overall the installation of the air cannons has been very successful.”

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