By Bridget McCrea
Reaching high into the air, concrete silos present unique safety challenges for employees who work in and around these massive structures. As a result, these buildings require special attention, particularly when it comes to overpressurization. Whether through neglect, poor equipment maintenance or excessive pressure input, overpressurization can lead to onsite hazards.
Falling, being struck by a falling object and inhaling cement dust are just some of the potential hazards associated with these structures. When not maintained and monitored properly, silos can literally “blow their tops,” leak powder or buckle – all of which can result in potential injuries.
Don’t Ignore the Problem
To keep workers safe and their silos operational, precast concrete manufacturers must have the right equipment, pressure sensors, filters, pressure relief valves and safety procedures in place.
An unmonitored tanker that leads to a single discharge can create a serious problem. That’s why silo safety prevention is paramount – not just when something goes wrong.
“Many companies ignore silo safety until it becomes a problem,” said Keith Knox, vice president at Deland, Fla.-based Knox Companies. “Like when the top of the silo flows off or the baghouse (dust collector) blows up.”
Proper safety control valves are a major deterrence, especially when a top-heavy silo filled with cement requires human intervention to repair or clean.
“In the past, companies would send someone up to the top of the silo with a harness or lanyard that, when hooked to a rail, becomes an OSHA violation,” Knox said. “The only true and proper way to work at the top of a silo is using a horizontal lifeline kit with D-rings on it, for hooking the lanyard to.”
Even a short fall of a few feet can result in injury.
“Precasters really need to have a game plan and a safety plan for operating in and around these structures,” Knox said. “If there’s an incident, know in advance how you’re going to rectify it and make sure your workers are safe or, if they’ve been injured, how you’re going to get them down off of that silo.”
An Ounce of Caution
According to OSHA’s website, the regulations governing proper silo usage are fairly basic and fall under the agency’s Safety and Health Regulations for Construction. For bulk cement storage, OSHA says any bulk storage bins, containers or silos shall be equipped with conical or tapered bottoms and either a mechanical or pneumatic means of starting the flow of material.1
“No employee shall be permitted to enter storage facilities unless the ejection system has been shut down, locked out, and tagged to indicate that the ejection system is not to be operated,” OSHA states.
Control the pressure during deliveries and have the right people on hand to address any concerns early on.
Silo protection systems (SPSs) keep tabs on pressure sensors, emergency relief valves and other key indicators during the delivery process. These integrated safety systems control and monitor the pressure and level in a structure. Should a problem arise, the SPS also will take the required steps to prevent accidents or silo damage.
The maximum height of a typical cylindrical silo is 275 feet, according to a resource from the state of Oregon. Fatal accidents typically resulted from lack of proper equipment, including a proper harness or lanyard. In one 2013 incident in Ohio, a worker attempting to unclog a fly-ash silo without being properly harnessed fell and became engulfed.
“Engulfment is one of the six major hazards present in silo-type storage facilities,” OSHA’s Bill Wilkerson said in a press release at the time. “This was a terrible, preventable tragedy that underscores the importance of complying with safety precautions. Employers are responsible for identifying hazards and ensuring workers follow proper procedures to prevent injury or death.”
Jim Mantz, a sales professional at Standley Batch Systems in Cape Girardeau, Mo., outlined some other silo safety concerns.
- Overpressurization. The worst-case scenarios of overpressurization are either a silo rupture or the filter unit being physically blown off the roof. “Both cause extensive disruption and loss of production, requiring costly repairs and equipment replacement,” Mantz said. Filters that often weigh more than 200 pounds each being disrupted cause additional damage when they land.
- Damaged or faulty sensors. Pressure sensors prevent silo overpressurization, detect pressures as low as 40 millibar (0.5 psi) and must be accurate and capable of operating in a low range. “In some cases, these sensors are ignored and/or not tested often enough,” Mantz said. “Alternately, they are tested by applying hand pressure to a sensitive rubber diaphragm, a force 50 times higher than the required set point.” Sensors fitted to silos must be fail-safe and suitable for the specific application.
- Emissions released into the environment. Overpressurization during the filling process can eject the powdered product into the atmosphere through the silo emergency pressure release valve (PRV) vent system. “Clouds of product blowing out during fills are indicative of silo protection failure,” Matz said. Emitted corrosive or hazardous products that lead to environmental damage may result in large fines, expensive cleanups and negative publicity.
- Falls. In 2020, fall protection was the most frequently cited standards violated, according to OSHA. Working at heights requires care and training as well as equipment that only can be tested in situ. In other words, someone has to climb the silo before every delivery to perform a functionality test. “Even with correct safety gear, working at height requires great caution,” Mantz said. “In silos, all practicable steps should be taken to reduce the need to work at height.
Knox advises precasters that he works with to check pressure relief valves and sensors regularly in order to ensure good operability. Something as simple as a device being clogged with cement dust, for example, will restrict its ability to do its job properly. Make sure the sensor area is ventilated properly and that dust isn’t “blowing out” out the dust collector.
“If it’s blowing dust,” Knox said, “the chances are good that there’s a clog somewhere.”
For workers who ascend into the silo, make sure they are using proper fall protection, have been fit-tested for air breathing apparatus and are trained for working at heights.
“Twenty years ago, someone may have just tied a piece of rope around their waist and headed into the silo to fix the problem,” Knox said. “Things have changed a lot since then. Now, companies that don’t have the right certifications and safety equipment in place have no business trying to get in there.” PI
Bridget McCrea is a freelance writer who covers the manufacturing industry and technology. She is a winner of the Florida Magazine Association’s Gold Award for best trade-technical feature statewide.
1 OSHA standard 1926.702 (a) Bulk Cement Storage.