Running bins and silos safely

Bins and silos look passive. They have no exposed moving parts, they sit in a corner of the yard or above a machine, and they work unattended for months. They are also the setting for some of the most serious incidents in bulk handling: people engulfed when hung-up material collapses, silos that split or buckle during filling or discharge, dust explosions in bins and their filters, and falls from silo roofs. Every chapter of this book has touched on a cause. This last chapter brings them together into a plan for running Plant C’s stores safely and reviews what the plant changed.

It is not a safety manual. The obligations that apply to a real plant are set by Australian work health and safety law, codes of practice such as Safe Work Australia’s model code for confined spaces, standards for explosive dust atmospheres and silo loads, and the equipment makers’ instructions. This chapter shows how the physics of the earlier chapters connects to those obligations, so that the reasons behind the rules are clear.

The hazards and their physics

Figure 1Bin and silo hazards and the chapters that explain them

Hazard	What causes it	Where this book explains it
Engulfment	collapse of an arch, rathole or hang-up onto or around a person; sinking into flowing material	Chapters 6 to 8
Confined space	oxygen depletion, dust, toxic gas from some stored materials, limited exit	this chapter
Structural failure	discharge and eccentric pressures, overfilling, blocked vents, corrosion, buckling	Chapters 5 and 10
Dust explosion	combustible dust cloud plus an ignition source, inside the bin, filter or around the bag station	this chapter
Stored energy	air cannons, pneumatic filling pressure, springs and counterweights	Chapter 8
Entanglement	screw feeders, rotary valves and agitators at the outlet	this chapter
Falls	silo roofs, ladders and platforms	this chapter

Engulfment

A person on or in a bulk solid can be engulfed in two ways. If the material is flowing, a person standing on the surface over a funnel-flow channel is drawn down with it, and the friction and pressure of the surrounding material make escape impossible within seconds. If the material is hung up, in an arch or rathole, it can collapse onto or around a person below or beside it. Chapters 6 to 8 explained why a funnel-flow bin with a cohesive powder can stand with an apparently solid surface over an empty cavity. That is why hung-up material is cleared from outside, and why the outlet feeders and any flow aids are isolated and locked out before anyone works on or in a bin.

Confined spaces

The inside of a bin or silo is typically a confined space under work health and safety law: it is not designed for occupancy, it may have a harmful atmosphere and it has restricted entry and exit. Safe Work Australia’s model code of practice on confined spaces sets out the risk assessment, permits, isolation, atmospheric testing, ventilation, standby person and rescue arrangements that apply. Plant C’s procedure for S1 and B2 is simple: nobody enters either while it contains material, and any entry for inspection or repair is a permitted confined space entry, with the silo emptied and cleaned, every feeder and inlet isolated and locked, and the atmosphere tested.

Dust explosions

Many bulk solids are combustible when dispersed as a fine dust in air: polymers, flour, sugar, wood, coal, many chemicals and some metals. Polypropylene dust and the fines and streamers from Chapter 10 are combustible. Calcium carbonate is not, but the filler bags may share a building with combustible dusts. A dust explosion needs a dust cloud within its explosible concentration range, oxygen and an ignition source such as a hot surface, a spark, static discharge or a smouldering deposit. The inside of a silo during pneumatic filling, the vent filter and the bag tipping station are the obvious places for a cloud.

AS/NZS 60079.10.2 sets out how areas where combustible dust can form an explosive atmosphere are classified into zones, which then governs the electrical and mechanical equipment allowed in them. Explosion venting, suppression or containment of vessels, isolation of connected ducts, earthing and bonding against static, and housekeeping that stops dust layers building up are the usual controls, designed by competent people for the material’s measured explosion properties.

Filling under pressure

S1 is filled by a tanker’s blower, which pushes pellets and air into the silo at a pressure of the order of a bar in the conveying line. The silo itself is not a pressure vessel: it is designed for the small pressures of its bulk contents and a little air, and it relies on its vent filter to release the conveying air as fast as it arrives. A blinded filter, a closed vent, a silo overfilled into its roof or a tanker driver who blows the line clear at full pressure at the end of a delivery can raise the pressure inside enough to lift a roof or split a seam. Silos filled pneumatically therefore usually have a pressure relief device on the roof as well as the filter, a high-level switch that stops the delivery, and a procedure for the driver agreed with the site. S1’s delivery procedure requires the driver to connect, confirm the high-level switch and vent filter status with the operator, and stop the blower on the high-level alarm.

Feeders and rotating parts

The screw feeder under B2 and the rotary valve under S1 are among the most dangerous items in the plant, because they are designed to grab and move material and will do the same to a hand. A blocked screw or valve is a strong temptation to reach in. Both are guarded so that the moving parts cannot be reached during operation, and both are isolated and locked out, with stored energy such as air cannons and pneumatic gates also isolated, before any clearing, inspection or maintenance. The safety of machinery standards in the AS/NZS 4024 series cover guarding and safety-related controls; the plant’s procedure follows them.

Falls and roof access

S1’s roof is 12 m above the ground, reached by a caged ladder to a guarded platform around the roof edge. Inspections of the vent filter and level sensors are done from the platform. Nobody walks on the silo roof itself, which is not designed for foot traffic and may hide a rusted or thin panel. Ladders, platforms and anchor points are inspected with the silo.

Planning for rescue

An engulfment or confined space emergency is not a time to improvise. Rescuers who enter a bin to reach someone who has been engulfed are themselves at risk of being engulfed, and in many incidents the would-be rescuers became victims. The plant’s emergency plan for its stores is therefore written before it is needed: who is called, what equipment is on site, how flow is stopped and material removed from outside, and the rule that nobody enters to attempt a rescue without the arrangements the confined space code requires.

Monitoring

Good monitoring catches most problems before they become incidents. Plant C’s stores now have the following, after the changes described in earlier chapters.

• Mass, not just level. B2 sits on load cells (Chapter 2). S1’s level radar is corrected for the heap and crater (Chapter 3), and the tanker acceptance check uses the more cautious reading.
• An independent high-level switch on each store, separate from the level measurement, to stop filling before the material reaches the roof or the vent. Overfilling a pneumatically filled silo drives pellets into the vent filter and can pressurise the silo.
• Vent filter pressure on S1, with an alarm if the pressure difference across the filter rises during a delivery, a sign that the filter is blinding and the silo may pressurise.
• Discharge rate. B2’s load cells show the actual feed rate; a rate that falls while the screw is running means a hang-up (Chapter 8).
• Feeder motor current on B2’s screw, rising when the filler has compacted.

Inspection

The plant inspects both stores on a schedule, with the items that matter most for bulk solids added to the usual structural and mechanical checks.

Figure 2Plant C's monthly and annual store inspection

• Avoid: Shell and hopper from outside: dents, bulges, out-of-roundnessmonthly; dents are ledges for build-up, bulges may be buckling
• Avoid: Welds at stiffeners, vibrator pads and supportsmonthly; fatigue cracks start at weld toes, Chapter 8
• Avoid: Vent filter condition and pressure differencemonthly and at each delivery alarm
• Avoid: Slide gate, rotary valve and screw feeder guards and interlocksmonthly
• Avoid: Earthing and bonding continuityannually
• Avoid: Wall surface inside: corrosion, wear, build-up, liner jointsannually at entry; wall friction changes with surface, Chapter 4
• Ask: Wall friction tilt test on an offcut stored beside each storeannually, compared with the original result

Change control

Most bin and silo problems at Plant C started with a change: a used bin bought with a line, a new pellet grade, a different steel finish, a replacement blower, a vibrator added to fix a symptom. Each changed one of the inputs this book has used, and nobody re-checked the outputs. The plant now treats any change of stored material, supplier, wall surface, outlet, feeder, flow aid or filling method as a change requiring review. The review asks four questions: does the material still flow from this outlet after its longest rest (Chapter 7); is the flow pattern still what we rely on (Chapter 6); are the loads on the structure still within its design (Chapter 5); and has the dust or explosion risk changed? For a new material, a flow function and wall friction test usually answer the first two for a small fraction of the cost of finding out the hard way.

The error: a bag station duct that was too big

This is an illustration. The new wedge hopper for B2 came with a new bag tipping station, a small enclosure in which the operator cuts open each bulk bag’s discharge spout, and a dust extraction system serving the enclosure and the screw feeder’s discharge point. The installer’s drawing showed a 250 mm duct from the two hoods to the plant’s existing dust collector, 18 m away, with each hood drawing 900 m³/h.

The engineer checked the drawing with the extraction duct check tool, which calculates the total airflow from the hoods, the duct velocity against the least transport velocity for the dust, the friction loss in the duct and fittings at the full flow, and the fan pressure needed with the hood entry loss, against the fan pressure available. With 1,800 m³/h in a 250 mm duct, a fittings loss coefficient of 2.5, a hood entry loss of 250 Pa and the dust collector supplier’s recommended transport velocity of 18 m/s for this dust, the tool gave a duct velocity of only 10.2 m/s and failed the transport velocity check: “use a smaller duct so the dust does not settle”. A duct running at that velocity would gradually fill with settled filler along its horizontal runs, losing airflow at the hoods and, in a plant that also handles polypropylene dust, accumulating a combustible deposit.

Figure 3B2 bag station extraction duct by diameter

Duct diameter	Duct velocity	Fan pressure needed	Tool verdict
250 mm, as drawn	10.2 m/s	493 Pa	duct velocity too low
200 mm	15.9 m/s	902 Pa	duct velocity too low
180 mm	19.6 m/s	1,296 Pa	extraction adequate

At 180 mm the velocity is 19.6 m/s, above the transport velocity, and the fan pressure needed, 1,296 Pa, is within the 1,500 Pa the collector’s fan provides at this flow. The duct was changed before installation. The tool’s own reminder applies after commissioning: the capture velocity at each hood must be measured, because a duct that carries the air does not guarantee that the hood catches the dust.

The error is a common one: duct sized for low pressure loss, as if for clean air, rather than for transport velocity. A dust extraction duct has to do two jobs, carry air and carry dust, and the second sets its size.

The final review

With all the changes in place, the plant’s engineer reviewed both stores against the checks in this book. This is an illustration.

Figure 4Plant C's stores before and after the review

Check	S1, pellets	B2, filler, before	B2, filler, after
Flow pattern	funnel, 3° outside mass flow	funnel with ratholes	* mass flow, wedge at 22° against 24°
Outlet against critical size	200 mm, far above interlocking size	150 mm round, arches	200 mm slot against 181 mm after 24 h
Capacity basis	poured density, heap corrected	tapped density on the display	load cells
Flow aids	none	vibrator on a switch	vibrator interlocked, now rarely needed
Segregation	managed: deduster, monthly cycle	not significant	not significant
Dust control	vent filter monitored	bag station extraction drawn at 250 mm	180 mm duct, capture to be measured

How this applies to a small Australian business

The physics of bulk solids explains the safety rules around bins and silos. Hung-up material can collapse, so it is cleared from outside. Bins are confined spaces with engulfment hazards, so entry is planned, permitted and rare. Dust explodes, so it is kept out of the air, away from ignition sources and out of ducts where it settles. Silos are loaded hardest during discharge and filling, so vents, high-level switches and inspections matter. A small business may not have a silo engineer, but it can make sure every bin has a reason to flow, a way to measure what is in it, a vent that works and a rule that nobody goes in while it holds material.

Exercise: a second hood

A third hood of 900 m³/h is proposed for the screw feeder’s other end, joining the same duct. Keeping the 180 mm duct, estimate the new duct velocity and say whether the fan pressure is likely to be enough. The fan provides about 1,400 Pa at the higher flow.

Worked answer. The airflow rises to 2,700 m³/h, or 0.750 m³/s. The duct area is π × 0.18² ÷ 4 = 0.02545 m², so the velocity is 0.750 ÷ 0.02545 = 29.5 m/s. The friction and fittings losses rise with roughly the square of the velocity: (29.5 ÷ 19.6)² = 2.26 times the 1,046 Pa duct loss found at 1,800 m³/h, about 2,370 Pa, plus the 250 Pa hood loss, about 2,600 Pa; the extraction duct check tool, run with three hoods, gives 2,331 Pa of duct loss and 2,581 Pa in total. That is far above the 1,400 Pa available, so the fan cannot serve three hoods through a 180 mm duct. A larger duct for the combined flow would be needed: at 230 mm the tool gives 18.1 m/s, just above the transport velocity, and 1,031 Pa, within the fan’s 1,400 Pa. Branch ducts from each hood must also be sized for transport velocity and balanced so that each hood gets its share of the air.