How to Unclog the Asphalt Silo Safely
BY Sam Bojarski
For the 10-year anniversary of AsphaltPro magazine, we will help asphalt companies teach new workers some back-to-basics techniques for best success in the field, at the plant and in the lab. Even veteran employees will be reminded of best practices with these refreshers throughout the year, but the goal is to help readers who are bringing in new employees who may or may not be well-versed in the industry yet. On-the-job training takes time and energy, and we’re here to help with the Asphalt Paving 101 online training course and these free articles every month.
The liquid asphalt cement (AC) binder only constitutes about five percent of the total material in asphalt mix. But all too often, this binder can harden during storage, clogging the asphalt silo and creating clusters of mix no softer than the roads we drive on.
“Normally when silos get clogged they start at the cone, and then everything builds on top of that,” said Dennis Braswell, division manager at Blasters Inc., Tampa.
When this occurs, asphalt plant managers need a way to remove the mix efficiently, while minimizing safety risks.
The type of work needed depends on how much buildup has accumulated in and around the cone. Many companies still put workers up on scaffolding to hammer out hardened mix, but high-pressure water blasting has become more common due to its efficiency and safety benefits.
To keep costs down or for the sake of practicality, asphalt companies often use their own workers when dealing with relatively small amounts of buildup. Plant managers need to take a few safety precautions into account before work can start.
“Anytime you do any work whatsoever on an asphalt plant, you’ve got to tag it out, lock out the power, especially the silo that you’re working on,” said Patrick Ahern, president of Ahern Industries Inc., San Antonio.
While turning off the power, remember to also shut off the air compressors near a silo’s gates, to ensure the gate doesn’t accidentally shut on a worker. This isn’t always intuitive, because the compressor may be set up on a separate electrical circuit from the power.
If a silo is on the end of a row, contractors can lay it down, making it easier for workers to clean from the inside. Even though this makes the cleaning process safer, crews might still need to build scaffolding up to where the hardened material has accumulated, just as they would when cleaning a vertical silo.
“That silo could be 10 feet or 12 feet in diameter, and inside of that silo you might need to get six or seven feet off the ground,” Ahern said.
When cleaning a silo in the vertical position—a more common scenario—contractors must build scaffolding from the ground to the cone opening at the bottom. Occupational Safety and Health Administration (OSHA) regulations apply, whether the silo is vertical or horizontal.
If workers are higher than four feet off the ground, they must have the appropriate fall protection. This includes harnesses, handrails and often safety nets.
Workers also need hard hats, safety glasses and steel-toed boots to protect them from falling material. Ideally, when working on a vertical silo, a worker won’t fully break the plane of the cone opening and will hammer away at the hardened asphalt from outside. But sometimes crews have to climb into the silo, and in these cases, they must wear an OSHA-approved dust mask that covers the entire face (See this month’s Safety Spotlight on silica dust regulations at the asphalt plant).
Before cleaning begins, workers must lock both gates in an open position with a chain or strap. This allows for a free flow of air if a worker has to get inside and ensures compliance with OSHA’s confined space regulations, which call for adequate ventilation.
When it comes to physically breaking off hardened asphalt, most contractors use the air hammer, or electric hammer. This handheld tool, which resembles a drill, removes small chunks of mix at a time. It’s slow work that could stretch a cleaning job to a week or longer.
“You’ll have your scaffolding off to the side, and you’ll be chipping away so it won’t fall on top of you,” Ahern said.
Anytime a hammer (or even a water blaster) is used, workers can potentially wear into the silo’s walls. Ahern recommends using an ultrasonic thickness gauge to measure the thickness of the steel prior to hammering. If the steel is thin, then the cleaning crew might have to take a less aggressive (and longer) approach.
High-pressure water blasting lessens the risk of doing damage to silo walls, since water won’t cut steel under normal conditions. But this technology offers the most value in the area of safety, according to Braswell. As a matter of fact, it removes the need to put a worker up on scaffolding.
“While it may be less expensive to put somebody up there who makes 10 to 15 dollars an hour, it’s going to cost hundreds of thousands of dollars if that guy breaks his neck, hurts his leg or kills himself, which has happened in the past,” Braswell said.
Instead of hammering away at asphalt mix from a scaffold, blasting companies wheel a water blasting system in on a trailer. The system contains a long arm that stretches up near the bottom of a silo’s discharge and blasts material out, using water pressures of up to 40,000 pounds per square inch. The arm pivots, allowing the nozzle at the end to spray a large area—usually the entire diameter of the cone opening—with water.
Blasting mix out from the bottom creates a void that then allows material near the top to begin falling down. After the bottom is cleared, the mix falls into a haul truck placed beneath the silo.
Plants often try to recycle most of the hardened mix that comes out during blasting, according to Braswell.
If there’s any asphalt mix stuck in a higher position on the silo walls, blasting contractors can either raise the arm of the water blaster, use a longer nozzle, or increase the water pressure, depending on the technology a company uses. This provides for a thorough cleaning job and doesn’t require putting a worker into a confined space high off the ground.
When a job is complete, the contractor will often perform a visual inspection with the customer, either from the top or bottom, where both parties check for residual material. All in all, the cleaning process takes about two days on average.
Although water blasting is relatively safe, plants and contractors should take certain precautions before and during the procedure. With water moving at extremely high pressures, it could be catastrophic if someone accidentally gets hit.
“We put caution tape up around the area so that no one comes in, because you’d never want a water blast hose to break,” Braswell said.
Workers perform whip checks where the hoses hook together, as another precautionary measure. The entire water blasting process is fairly standardized across geographical regions.
It’s helpful to know how long the mix has been stuck before blasting occurs. If hardened material has accumulated for several years or more, water blasting runs the risk of removing the tiles in a silo cone right along with the mix.
While water blasting has minor drawbacks that plants must consider, the benefits it offers in the areas of safety and efficiency make it an attractive option compared to other more traditional methods.
Sam Bojarski is a freelance writer whose work has appeared in national trade publications serving the maritime and human resources industries. He has also written extensively for the industrial services sector.