Spring and summer in Central Texas (USA) bring unpredictable weather, and at times thunderstorms rage across the region, pounding the dry ground with so much water that parched riverbeds can suddenly become torrents in minutes, writes Don Papini, Martin Engineering Territory Manager. The warm seasons also bring humidity that averages more than 85%, which can make any bulk material containing clay or silica adhere to most surfaces.
This type of accumulation created problems for the Hunter Cement Plant in New Braunfels,Texas, which consistently fought against penetrating humidity and marble-sized raindrops dousing the material on its partially covered D-07 conveyor system. Terminating in a chute that acts as a major transfer point, the wet material would clog the shaft and result in backups on the belt, leading to spillage issues that halted production.
Martin Marietta took over plant operations in 2014, following a merger with Texas Industries (TXI), the largest cement producer in the state. Managerial inspections revealed that there were some improvements needed in the existing rotary feeder and chute system in order to reach continuous mill operation. Built in 1979, the facility had run on one kiln, producing 0.9mtpa (million tonnes) a year of cement. In 2012,TXI added a state-of- the-art second kiln, increasing the plant’s production to a total of 2.3mt annually.
According to a management source, “The frequent unscheduled downtime to clear the rotary feeder chute was a challenge. We approached Martin Engineering to explore the possibility of installing the air cannons that could help resolve bottleneck issues.”
DAMAGE AND RISK ASSESSMENT
Limestone extracted from Hunter’s quarry eight miles away is protected from the weather in an enclosed 40,000-tonne on-site storage dome at the Hunter plant. There, it is crushed to a size of 6-inch minus (150mm) and mixed with clay, then conveyed to a processing tower. Separate conveyors deliver iron ore and sand to the same tower, where they are loaded onto a weight belt feeder system for precise measuring. Two chutes — one with limestone and clay, the other with iron ore and sand — offload 350 to 400 tonnes (317.5 to 362.8 metric tonnes) per hour onto the D-07 conveyor, mixing them together on the 42in. (1,068mm) single belt running at 350FPM (1.8MPS).
The conveyor travels approximately 300 feet (91.4m) to an adjacent tower and offloads into a pyramid-shaped funnel, which feeds a chute with a flow control gate at the bottom that either diverts material to a conveyor leading to the raw mill or to a dump truck waiting on the ground below. In the Quadrapol mill, the mixture is ground to the kiln feed specifications of the grade of product to be produced.
Once the material is offloaded from D-07 into the funnel, it drops 12 feet (3.65m) down a square 3ft by 3ft (0.91m) chute, onto the conveyor below. Operators found that wet or damp material gradually formed into a rat hole and eventually created an obstruction, which led to the material back-up.
“Blockages could take days to form with normal humidity, but when it rained, the chute would require more attention,” said Jonathan Cole, Lead Service Technician for Martin Engineering assigned to the Hunter facility. “Once material backed up in the chute, the spillage and aggregate would also collect on the return
side of the belt. To address the problem, the raw mill would be
shut down for six to eight hours at a time so that workers could
clear the chute.”
Operators explained that two or three workers would gather
around the railing at the top of the chute and use air lances and
pneumatic jackhammers fitted with 8-foot long bits to clear the
AIR CANNON PLACEMENT FOR PROPER FLOW
The Martin Engineering team and Martin Marietta management
worked together to devise a solution using a series of Martin®
Hurricane air cannons strategically placed along the chute. With
more than 140 similar units already in place, aiding material flow
throughout the facility, Martin Marietta decision-makers agreed
with the conclusion.
“Once material stuck to the diverter gate and chute wall, it
quickly collected upon itself, so our first priority was to prevent
accumulation,” Cole said. “However, the higher the moisture
content, the faster material adheres and the harder it is to blast
off, so we also installed units that shot air at a 30° angle down
the centre of the chute to always keep material flowing.”
Martin technicians installed a total of five 70-litre air cannons
on two levels. The 94 lb (43kg) tanks are suspended from the
tower frame by steel cords and are fed by the plant’s existing
compressor line. The powerful cannons fire a shot of air at up to
120 PSI (8.27 BAR) from a pressurized tank through a fan jet
nozzle with a 12 inch (305mm) wide mouth, which spreads the
stream across the surface area of the wall. To clear obstructions
and assure proper material flow, the cannon has an effective area
of up to 22.6ft3 (641 L).
On the upper level, two of the cannons were placed along
either side of the chute to dislodge accumulation from the wall,
and a third unit was installed at the top to free any material that
could get trapped where the chute narrows. One floor below, a
cannon is directed at a downward angle to aid material flow,
whereas the lowest unit on the chute specifically addresses the
easily clogged diverter gate.
All fitted with a centrally located valve design, the tanks offer
the most direct air path with the maximum force output and low
air consumption. This efficiency reduces the burden on the
compressor system and shortens the lag time between
sequences. The central valve is a detachable piece that allows a
single worker to disconnect it without the need to remove the
tank, for faster and easier maintenance.
A ten-bank positive solenoid control panel — which can be
located as far as 200 feet (60m) away from the tanks to avoid
harsh conditions — is connected to the facility’s programmable
logic control (PLC) software. It allows operators to control and
monitor the proper firing time and sequence remotely from a
central location. To eliminate the risk of unintentional firing due
to drops in pressure, the valve requires a positive signal from the
solenoid in the form of an air pulse to trigger discharge.
According to operators, installation only took a couple days
and was scheduled with other projects that also required
downtime. After some testing, a workable firing sequence that
suited most weather conditions was implemented.
IMPROVING PRODUCTION AND SAFETY
A few months after the installation, operators report that the air
cannon solution has significantly decreased the plugging issues,
resulting in continuous mill operations with minimal unscheduled downtime.
Moreover, the area around the chute is reported to need only routine cleanup and maintenance accompanying normal scheduled inspections.
Proven to be able to withstand the demands of the, strong Texas storms, the cannons are discharged when needed, increasing and
decreasing in frequency with seasonal weather fluctuations.
“Due to our long and positive relationship, there was no
doubt that the Martin team was going to come through with a
good solution,” Martin Marietta management concluded. “The
equipment has worked out well and with the team’s consistently
attentive service, we are confident that this is a good solution.”
Along with hundreds of air cannons, Martin Engineering has also equipped the Hunter Cement plant with conveyor
accessories and bulk handling solutions. Founded in 1944 and
headquartered in Neponset, IL — with divisions on six
continents — the equipment designer and manufacturer is the
world leader in flow aids and conveyor products for a wide
variety of bulk material applications, including cement, coal,
rock/aggregate, biomass, grain, pharmaceuticals, food and other