Powder Storage Silos: Disasters Waiting to Happen?

How sure are you that your silo protection system will work next time you fill your silo? The latest silo protection technology provides much more than a safety system to prevent over-filling and over-pressurization.

Drawing parallels from its European experience in providing silo protection systems to prevent over-pressurization in powder storage silos, UK-based Hycontrol believes the issues encountered in Europe are similar in the U.S. market. If this is the case, according to their CEO Nigel Allen, many powder silos across the U.S. could be disasters waiting to happen, putting lives at risk and posing serious threats to the environment.
    
Level measurement specialists Hycontrol has been designing silo protection systems for over 25 years and has extensive experience of the potential problems that exist on sites that store and handle powders, particularly in the quarrying, cement, bitumen, food, plastics, and waste water industry sectors. The company’s integrated SPS silo protection system allows all key components, including the PRV, pressure sensor, and high-level alarm probe to be tested before each and every fill.
    
“Our engineers visit a wide range of sites and their findings are worrying to say the least. The photos they’ve taken speak for themselves,” says Allen. “Companies just don’t seem to understand the consequences of poorly maintained protection systems. It’s quite frightening that operators accept pressure blow-outs via the pressure relief valve (PRV), thinking that ‘It’s OK – if the PRV is venting then it is doing its job’. This couldn’t be further from the truth. PRVs are there as a last resort. If the silo protection system is working correctly and is fitted with an automatic shut-off feature to prevent over-filling, the PRV should never be used. If a PRV blows then there’s an inherent problem with the system or the filling protocol and corrective action must be taken.   
    
“I would advise anyone with a powder silo on their site to inspect their PRV. Material in and around a PRV is a tell-tale sign that there’s something wrong – you could be heading for a catastrophic blow-out,” continues Allen. “The material vented from a silo will almost certainly solidify over time and this will, at best, prevent the PRV from working correctly and, at worst, completely block it up. Unfortunately, many maintenance engineers just don’t realize the potential dangers, and think that simply cleaning off the material on and around the PRV is good enough. But if the PRV doesn’t lift next time an ‘event’ occurs, the over-pressure could easily rupture the silo or blow the filter housing off the top. For a silo located in a designated hazardous area (FM/ATEX), the over-pressure encountered during filling could be sufficient to simulate an explosion and open the protective blast panels, resulting in costly loss of product and silo contents being left open to the elements.“
    
With regard to filter housings, Hycontrol engineers have witnessed another worrying practice at a number of sites where companies fit chains to prevent the housing being blown off the top of the silo, seeming accepting over-pressurization is inevitable.

What Causes Over-Pressurization Problems?
Silo protection systems are designed to prevent the damaging and potentially dangerous consequences of over-filling or over-pressurization of silos when powdered material is being transferred pneumatically into them from bulk delivery trucks. Unfortunately, perched out on the top of silos, such protection systems are all too often ‘out of sight - out of mind’ - until a major problem occurs.
    
Problems during the filling process usually arise through an inherent problem with extant silo safety equipment or with the air filtration venting system on top of the silo. Problems can also occur through delivery driver/operator error. Such issues may relate to not knowing the available space in a silo or delivery drivers trying to offload too quickly at too high a pressure. The key point is that bulk delivery trucks are pressure-tested vessels, typically capable of withstanding up to 2 bar (29 psi) pressure. By comparison, storage silos are designed to withstand the weight of material stored in them and can rupture at pressures as low as 1-2 psi above atmospheric pressure. The potential consequences of over-filling or over-pressurization include:
•    serious or fatal injury to workers and the public
•    catastrophic silo damage
•    loss of material and production - plus
•    harmful environmental pollution
•    damage to company reputation

Ground Level Testing
The key issue with many pre-existing silo safety systems is the lack of adequate ground level testing capabilities, meaning that operators don’t know if they will work when needed. Working at height restrictions limit silo top inspections and maintenance, especially in adverse weather conditions. However, the main problem with sending an engineer to the top of a silo is: can they actually do anything when they are up there – after all, how do you physically test a relief valve or pressure transmitter without removing them?
    
Even if the protection system does do its intended job and prevents a major incident, companies rarely investigate the root cause of the problem so that remedial work can be carried out to prevent the situation re-occurring. Important ‘near miss’ events such as PRV lifts, high-level events, and high-pressure events, are routinely not recorded and often conveniently dismissed. Hycontrol has clear evidence that in practice there are more ‘near misses’ than realized and that the situation is a ticking time bomb.

Filter Housings
Filter housings at the top of the silos are designed to vent the silo during filling, while preventing dust escaping into the atmosphere. Normally these are fitted with some form of self-cleaning system to keep filters clear. These are typically mechanical shakers or reverse jet systems. Although filter manufacturers give recommended check routines and filter replacement schedules, in practice it would appear these guidelines are regularly ignored. Faulty operation can be caused by a range of issues, including blockages and the fitting of unsuitable or wrongly-sized filters. Most powders form hard compounds when mixed with water from the atmosphere, further exacerbating the problems at the top of the silo.

Effective Silo Protection
Surprisingly, in most countries there are no comprehensive guidelines to prevent over-pressurization and over-filling of powder silos. In the UK, the Mineral Products Association publishes comprehensive guidelines for silo protection systems in quarries and cement works, but there are little or no such recommendations for powder silos used in a broader range of industries including food and beverage, chemical, water treatment, and plastics. However the primary principles are the same for protecting any pneumatically filled silos.
    
However, even with guidelines in place, the benchmark for the effectiveness of any silo safety protection system can only relate to the last time all the components were fully tested.

Optimum Solution
The only effective solution is to take an integrated approach to silo protection design whereby the PRV, pressure sensor, and high-level alarm can be tested at ground level, prior to each fill. Only when all these safety devices have passed the checks should the safety interlock allow the silo inlet valve to open and the delivery to commence.
    
As an additional benefit, an effective protection system can serve as a powerful predictive maintenance diagnostic tool by recording critical near-miss events that occur during the filling process. This information allows managers to carry out effective predictive maintenance by means of a logical step-by-step root cause analysis (RCA) process to understand why the problems are arising. For example, high pressure and PRV lift events may be due to filter problems, prompting questions such as:
•    Are the filters the correct size?
•    Is the filter cleaning regime fully operational?
•    Have the filter bags/cartridges been changed as per manufacturers’ recommendations?

In parallel the logs will also indicate if the tanker drivers are routinely over-pressurizing during the fill process.

Conclusion
In summary, the optimized silo protection system should incorporate:
•    Pressure sensor, hi-alarm level sensor, and PRV testing (essential)
•    Simple, one-button press to test all components
•    Silo filling auto shut-off control
•    Pneumatic cleaning of pressure sensor
•    Recording of the number of events on incidents of over-pressure (time/date stamp)
•    Recording of the number of events of PRV lift and opening (time/date stamp)
•    Recording of the number of events of high-level probe activation (time/date stamp)
•    Filter On/Off output option to check filter status
•    Filter air supply monitoring alarm option
    
There is strong empirical evidence that many powder storage silos around the world are ‘disasters waiting to happen.’ The practical reality is that silos can split or rupture at pressures as low as 1 or 2 psi above atmospheric pressure. Malfunctioning filter housings can be ejected at similar pressures.
    
“The truth is that merely relying on a cursory visual inspection of silo protection equipment is woefully inadequate,” says Allen. “Therefore, it is imperative that any installed safety system must be capable of providing reliable protection that can be easily verified by testing critical components before each and every delivery – without having to climb to the top of the silo. This approach will provide total silo safety, protecting assets, the environment and, most importantly, site personnel and the public.”
    For more information, contact Hycontrol Ltd at +44 (0) 1527 406800 or [email protected], or visit www.hycontrol.com.