How to specify a bulk material conveying system

Most conveying problems are specified in, not built in. A belt that spills, a screw that jams on the first wet week, an elevator that was never going to reach the design rate — these usually trace back to a duty that was written down as a single number and a material that was described as "like sand". This guide walks through what a specification has to fix, in the order that makes each decision easier than the last.

What does a conveying specification actually have to fix?

A useful specification answers seven questions. What is the material, and how does it behave? How much of it, how often, and with what peaks? Where does it come from and where must it go? Which type of equipment carries it? What happens at every point where it changes direction or equipment? How is the line driven, controlled and protected? And what must the supplier be told to answer with a real proposal rather than a guess?

• Material — the physical properties that decide geometry, speed and wear.
• Duty — design capacity, operating hours, peak factor, availability.
• Route — start and end points, levels, obstructions, inclines, existing structure.
• Type — belt, pipe, screw, airslide, bucket elevator, or a combination.
• Transfers — chutes, skirting, sealing, access, dust handling.
• Drives, controls, safety — power, starting, interlocks, sensors, guarding.
• The enquiry — what to send, and what a proposal should come back with.

Step 1 — Which material properties decide the design?

Everything downstream is sized from a handful of properties, and most of them are cheap to measure compared with the cost of guessing. The dedicated note on bulk density, angle of repose and flowability explains each one; the short version is in the table below. Where a material changes between seasons, suppliers or process upsets, specify the range, not the average — a belt sized on dry density will be undersized on a wet day, and a hopper drawn for the free-flowing sample will bridge on the fines.

Step 2 — How do you define the duty?

"200 t/h" is not a duty; it is one number that will be interpreted five different ways. A duty statement separates the nominal rate from the design rate, says how many hours a day the line runs, and gives the peak the equipment must survive — a truck dumping into a hopper, a silo emptying at full feeder speed, the start-up surge after a stoppage.

Nominal capacity

The rate the process needs on average, in tonnes per hour of the material as handled (wet, not dry).

Design capacity

The rate the equipment is sized for. A margin above nominal — commonly quoted in the 15–25 % range — covers surges and belt loading that is never perfectly uniform. The margin is a decision, so write it down.

Operating hours

Hours per day and days per year. They drive wear allowances, maintenance windows and the choice between one robust line and two lighter ones.

Peak and surge

The highest short-term rate, and how long it lasts. Feeders and chutes are sized for this, not for the average.

Availability

What the line must achieve, and what redundancy the plant already has. This decides whether you need a spare drive on the shelf or a spare conveyor.

Step 3 — What does the route impose?

The route is where a good specification saves the most money, because it is where the site pushes back. A conveyor that could run straight from A to B on paper rarely can on site: there is a road to cross, a building to enter, a level change that turns a belt into a belt plus an elevator, and existing steel that may or may not take the load.

• Start and end points with levels, including the discharge height into whatever comes next.
• Obstructions and crossings — roads, rail, pipe racks, existing conveyors, property lines.
• Inclines — the maximum angle a belt can hold the material is set by the material, not the belt; above it the answer is a cleated belt, a pipe conveyor or an elevator.
• Existing structure — what can be reused and what must be checked; see new conveyor on existing steel.
• Access — for installation, for belt replacement, for cleaning under the return strand, for fire brigade lanes.
• Environment — wind, rain, freezing, dust regulations, noise limits at the fence.

Step 4 — Which conveyor type fits?

With the material, duty and route fixed, the type of equipment usually picks itself; when it does not, the choice comes down to containment, incline and footprint. The comparison note — belt, pipe, screw, airslide or bucket elevator — goes through each type in detail. The short table below is the first pass.

Step 5 — Why do transfer points deserve their own design?

Every place the material leaves one piece of equipment and lands on another is a small project of its own. It is where dust is made, where spillage starts, where belts are damaged and where blockages happen. The pillar note on transfer point design covers the mechanics; for the specification the rule is to list every transfer, give it a number, and require a drawing of each one — trajectory, chute, skirting, sealing, access — before the equipment is ordered. A transfer that exists only as a circle on a flow sheet will be built as a box with a hole in it.

Step 6 — What about drives, controls and safety?

• Drive sizing against the fully loaded, cold start — not the running load. State the starting method (direct-on-line, soft starter, variable speed) and why.
• Speed control where the rate must follow the process, and where a slower belt simply lasts longer.
• Protection — belt misalignment switches, pull-cord emergency stops, zero-speed and plugged-chute detection, bearing temperature on elevators. The note on safety switches and flow aids explains what each one does.
• Interlocks — downstream equipment starts first, upstream stops first; a stopped conveyor must stop what feeds it.
• Guarding and access to the current machinery directive and the plant's own rules; nip points, walkways, belt-change access.
• Weighing and sampling if the process needs a rate or a mass balance — a belt scale is much easier to include now than to retrofit.

Step 7 — What goes into the enquiry?

A supplier can only answer the question that was asked. The pre-enquiry checklist lists the twelve items an engineer needs; in the context of a full system specification, the sequence looks like this.

1. Write the material data sheet

   One page per material: the properties from Step 1, with ranges and the sample they were measured on.

2. Write the duty statement

   Nominal, design, peak, hours, availability — and the density used to convert tonnes to cubic metres.

3. Mark up the route

   A plan and a section with levels, crossings, structures to keep and structures to check. Photographs help more than most people expect.

4. List every transfer

   Numbered, with what feeds it and what it feeds, the drop height and the direction change.

5. State the constraints

   Site standards, electrical supply, control system, dust and noise limits, shutdown windows, delivery routes.

6. Ask for what you want back

   A general arrangement, a load list for the structure, a power list, a spares proposal, and the assumptions the supplier made where your data ran out.

The mistakes that cost the most

• Sizing on dry density and running wet material.
• Copying a belt width from a similar plant without checking lump size and surcharge angle.
• Specifying an incline the material cannot hold, then discovering it at commissioning.
• Leaving transfer points as circles on the flow sheet.
• Choosing the conveyor type before the route survey.
• Ordering equipment before the structural check on the steel it will sit on.
• Forgetting maintenance access — the belt has to come out one day.

When a project is still open on several of these points, a short feasibility study is usually cheaper than the first change order. Our design and engineering work starts exactly here: with the material, the duty and the site, before any equipment is chosen.