OHS in Tank and Silo Cleaning

Silo cleaning safety: first ask "can it be done without going in?"

Material sticking to silo walls in flour, feed and grain processing plants, sediment at the bottom of tanks in tomato paste and fruit juice plants and residual product in chemical plants all require regular cleaning. The classic solution for this work is to lower a worker inside. Silo cleaning safety, however, starts from the opposite direction: is there a way to clean without going inside?

Tanks and silos are typical examples of confined spaces: entry and exit are limited, they are not designed for continuous work and the atmosphere or material inside can create a sudden hazard. Art. 4/1-d of Law No. 6331 on Occupational Health and Safety requires the necessary measures to be taken so that workers other than those given adequate information and instructions do not enter places where there is a life-threatening and special hazard; Art. 10 requires each silo and tank to be addressed separately in the risk assessment. For the general framework of confined space work, see our article on working in confined spaces; this article covers the risks and cleaning methods specific to tanks and silos.

Grain silo hazards

In grain and flour silos, most of the hazard comes from the material itself:

• Engulfment in flowing material: While discharge from the bottom is under way, a person inside the silo is drawn into the centre of the grain flowing like a funnel. Once the flow has started, they cannot get out on their own. Even a person buried up to the knees may not be able to free themselves without help because of the pressure of the grain.
• Bridging: Damp or spoiled grain forms a crust on the top surface; even if the material underneath has emptied, the top looks solid. A worker who steps on it falls into the void when the crust collapses and ends up under the grain.
• Collapse of a vertical wall of material: A tall wall of material stuck to the silo wall can collapse while it is being scraped from below.
• Dust: Fine organic dust can explode when it meets an ignition source in an enclosed space. Dust accumulated inside the silo and in the galleries above it can cause the first explosion to feed a second one. For the dust explosion mechanism, see our article on dust explosion risk.
• Gases: The gases used in fumigation to protect the stored product are toxic; spoiled or fermenting product also gives off carbon dioxide and oxygen is depleted.
• Mechanical: Floor augers, discharge gates and sweep augers can catch a person inside.

Tank cleaning: risks in liquid tanks

In liquid tanks, the hazard comes more from the atmosphere and residues than from the flow of material:

Two special risks are often overlooked. The first is cleaning chemicals reacting with each other or with tank residues: a chlorinated disinfectant meeting an acidic cleaner in the same tank can release toxic gas. The second is not waiting for surfaces to cool before entering a tank cleaned with steam or hot water. We explain the order of measurement and detector selection in our article on confined space gas testing.

Prioritise cleaning methods from outside

The first step in the hierarchy of controls is to eliminate exposure to the hazard. In tank and silo cleaning, this means cleaning from outside.

1. Prevention through design: A conical bottom at a suitable angle, aeration pads or vibration units that help the material flow, and surfaces that reduce sticking. When buying a new silo or tank, the cleaning method should be asked about at the design stage.
2. Cleaning in place (CIP): In liquid tanks, fixed or rotating spray heads wash the tank without anyone going inside. This system, already used in food plants for hygiene, is also the most important safety measure.
3. Mechanical cleaning through the hatch: In silos, rotating cleaning machines lowered through the top or side hatch and long-handled tools break up bridged material and wall build-up from outside.
4. Remote observation: A camera lowered through the hatch makes it possible to see the situation without going in; it often removes the need to "go in and have a look".
5. Vacuum removal: Sediment and sludge at the bottom of a tank can be drawn out from outside with a vacuum tanker.

If there is still work that cannot be done with these methods, entry is only made under the conditions in the next section.

If entering a silo is unavoidable: entry conditions

For entering a silo or working inside a tank, the following conditions are met together:

• Written entry permit: The permit includes the work, the duration, the people entering, the attendant and the measurement results; it is closed when the work is finished. For the general structure of the permit system, see our article on the permit-to-work system.
• Energy and material isolation: Feed and discharge augers, agitators and gates are locked out and tagged; pipelines are isolated with a blind flange or double valves with a bleed. For the method, see our article on LOTO lockout and tagout.
• Continuous gas monitoring: Measuring before entry is not enough; monitoring with a personal detector continues throughout the work.
• Ventilation: Mechanical ventilation is running; fans and lighting used in a dusty environment are selected to suit the zone in the explosive atmosphere document.
• Lifeline anchored above: The worker inside the silo is attached by a full body harness to a fixed point above or to a rescue tripod; the line is kept taut to prevent sinking into the material.
• Attendant and communication: At the entrance there is an attendant in constant communication with the inside; the attendant does not go in.
• Rescue plan: Rescue equipment is ready at the entrance and has been drilled. For how to plan the rescue, see our article on confined space rescue.

In dusty silos, hot work, spark-producing tools and unsuitable electrical equipment are not used. Which explosive atmosphere zone the silo and galleries are in should be written in the ATEX document; if there is no document, it should be prepared before planning the entry.

Risk assessment, training and health

Each silo and tank is addressed as a separate line in the risk assessment: what is stored, how it is cleaned, who goes in and which gases can form. The assessment is renewed when the cleaning method changes, when a new cleaning chemical starts to be used or when a near miss occurs.

Workers who will enter and attendants learn about confined spaces, gas testing and rescue in practice in training on job-specific risks. The fitness for work of these workers is assessed by the workplace physician, taking into account work in confined spaces and with a harness. If dust measurement is needed, we plan it with accredited laboratories and enter the results into the risk assessment.

Silo and tank cleaning is often contracted out to an external company. In this case, responsibility does not change hands; it is shared: the cleaning company is responsible for the training and equipment of its own workers; the facility informs this company in writing of the silo's contents, its fumigation history, energy isolation points and emergency arrangements. The entry permit is issued from the facility's permit system and locks are fitted according to the facility's procedure. For admitting contractor workers to the facility, see our article on contractor induction training and permits.

We prepare silo and tank cleaning instructions at flour, feed, tomato paste and fruit juice plants in Bursa and the surrounding provinces as part of our risk assessment and OHS specialist services. For other topics specific to food plants, see our food sector page, and to track gas detector calibration and drill dates, the OHS calendar tool.

Frequently Asked Questions

Can a silo be entered while discharge is under way?

No. The flowing material draws the person inside into the centre of the silo and they cannot get out on their own. Entry is only made with a written permit after discharge has been stopped and the discharge and feed equipment has been locked out.

Is a safety harness enough for silo cleaning?

A harness alone is not enough; a lifeline attached to a fixed point above or to a rescue tripod and kept taut is needed. In addition, nobody goes inside without gas testing, an attendant and a ready rescue plan.

When can a fumigated silo be entered?

It is not entered until the waiting and ventilation period specified in the instructions of the fumigation contractor has been completed and gas testing has been carried out. The measurement must be made with a detector suitable for the gas used.

Which gases are measured for tank cleaning?

At least oxygen is measured; depending on the tank's contents, flammable gas (LEL), carbon dioxide, hydrogen sulphide or carbon monoxide are added. Which gases are measured is determined in the risk assessment according to what is stored in the tank and which chemical will be used for cleaning.