Regrettably this will probably be a matter of conjecture and opinion.

Question -

how many screw conveyors do you have that have had an internal dust explosion?

how many chain conveyors do you have that have had an internal dust explosion?

are there published statistics for confirmed dust explosions in screw or chain conveyors?

The real answer is to have knowledge of the above and the measured dust density in actual machines in operation so that an informed judgement can be made. Without this the true probability of an explosive dust concentration being present cannot be made and the danger is “if in doubt lets say its Zone 20” with all the knock on effects that will bring. ■

HI Ian,

I would say that zone 20 should apply. Explosions are un common in this type of conveying, however, ignitions elsewher in the process regularly tranmit to these areas. An ignitable cloud could be present on an ongoing basis or could be created at any time due to the presence of the combustible powder.

A detailed analysis could be undertaken if you have the details of the dust - this could vary the hazard classification.

Cheers,

Tony. ■

Always be careful with “over zoning” as the more arduous the Zone the more you will pay. You may also have difficulty in getting the equipment you want. My opinion would be that internally mechanical conveyors (screw conveyors, chain conveyors etc) should not be more than Zone 21.

Regarding explosions in other equipment, any equipment with a significant change of exploding (mills etc) should be provided with isolation from other handling and storage plant in the event of an explosion to avoid explosion propogation.

Zone 20 is significantly more arduous than Zone 21, you do not “relax to Zone 20”, you relax to Zone 22. ■

ATEX apart: in the field of coal sampling I have heard from alleged HSE specialists in a well known Anglo-French power consortium that a 35m high pipeline/chimney guiding coal dust down to a ground level collection point was less likely to explode than was the coal discharged from a boom stacker, no pun intended, in open air. As a simple European Engineer I found that difficult to comprehend.

Hazard assessment is one of those opportunist subjects populated by mathematicians who couldn't quite manage to work things out.

Presented with your situation Professor Northcott Parkinson would have formulated a law which simply stated "Insurance premiums will reflect the level of investment." ■

"..I reckon that void will contain an explosive dust.

So surely that means that the internals should be rated as Zone 20?"

The question posed by ATEX is not "is there an explosive dust ?", but "is there a potentially explosive atmosphere ?".

If there is a potentially explosive atmosphere then does the occurance of this atmosphere correspond to the requirements for Zone 20, 21, or 22?

As far as I have seen, consideration of the internals of machines is all based on supposition and conjecture. I have yet to see any evidence of measurements of dust concentrations inside enclosed materials handling equipment being used in considerations of ATEX Zoning. (Please feel free to contradict this statement if you have the evidence.)

With regard to leakage of material out of equipment through minimal gaps, the alternative approach is to provide a slight negative internal pressure so that air is drawn in, rather than dust leaking out. You may also wish to consider upgrading your housekeeping to remove any slight material leaks. ■

As I previously suggested.."supposition & conjecture" it most certainly is.

Also, if you ingest fresh air into a machine you might, will, create more voids.

Where is a 1 eyed man in all this APEX mumbo jumbo? ■

"I have been advised that, as I suspected, I ought to regard the inside as Zone 20."

Umm, can you by a Zone 20 screw conveyor? Doesn't it have to be third party approved by a Notified Body? Isn't that a contradiction to "we dont do anything special" ?

I'm sure that there are plenty of people who will advise Zone 20, after all it's safest for those giving the advice to go for the higest level and they don't have to source the kit and pay for it! All part of the consultants money-go-round.... ■

Hi guys,

Interesting discussion. Why not considering using a special type of compressed air driven, purely pneumatic Vacuum Conveyor. These have many advantages against conventional, mechanical conveying screws.

Multijector Vacuum Conveyors are free of internal ignition sources. The only risk comes from the marterial to be conveyed and -of course- the parameters of this materials should be known.

This Vacuum Conveyors can be rated internally as zone 20. Details are described in the manufacturer's ATEX-certificate from German TÜV.

Please see www.Volkmann.info ■

Vacuum conveyors are no more free from internal ignition sources that the pressurised jobs if the material is potentially explosive. Interparticle friction can increase temperatures locally. Digressing ..but...

I remember a learned colleague commissioning a pharmaceutical plant who was not getting a mass balance. It turned out that the most effective germicide in the recipe was vaporising in the pipe, escaping through the filters & landing over an adjacent residential area. The germicide was also known to produce sterility. The factory shut down very soon after the discovery. Like Thomas says the material properties must be known; but who is realistically going to forecast vaporised germicide? ■

"I presume when they say "we dont do anything special" the manufacturers mean that they dont change the design - merely get their normal design approved."

I'd want hard evidence from suppliers that they have had third party approval by a Notified Body before going too far down that route.

My view remains the same that any potentially explosive atmospheres in screw conveyors and chain conveyors do not satisfy the requirements for ATEX Zone 20. ■

In addition to the possibility of an exp atm you should consider what could be the result if there was an ignition source present e.g.hot surface from friction,failed bearing,static discharge etc etc.

Inside an enclosed space there would be a large pressure increase resulting in a failure of the enclosure.The extent of this failure could be as simple as a lifting of an access hatch or, perish the thought,an almighty bang throwing casing and components everywhere around the site.

If the conveyer is open then a pressure increase will be unlikely and any explosion will not be able to provide sufficient energy to be damaging,however a fire could result.

This is what you need to focus on,your judgement here is necessary as it could save injury to people in the vicinity.

The drive motor is usually outside of the casing and could well be irrelevant to the ignition.If in doubt look at the construction and ensure that it will fail quickly following a pressure increase or better, install relief doors or panels. p.s. I am not an explosion panel supplier... ■

Step 1. establish if ATEX applies

Step 2. establish the Zone (i.e. the extent of time the EXPLOSIVE, not dusty, atmosphere exists.

Step 3. establish possable causes of ignition

Step 4. establish what action is to be taken to mitigate Step 3 according to Step 2. ■

Screw conveyors cannot be considered in isolation for ATEX purposes unless served by a rotary valve on the inlet and the outlet, so usually have to be treated as part of a total flow route system.

With regards to possible dust concentration, it has to be assumed that optimum explosive conditions may pertain at some time and location during the unconfined, mechanically-agitated transport of a combustible fine solid. The assessed frequency and operating circumstances surrounding the situation have to be taken into account in the conduction of a risk assessment There are various potential hazards present in the normal working conditions of a screw conveyor, such as seal contact, flight tip speed and, possibly, intermediate bearings. Fault hazards must also be considered, as with the intrusion of foreign bodies, shaft breakage, over-speed and the like. This combination could quite reasonably point to the designation of a Zone 20 Category 1. Any reserve on this point should be tempered by the work that was organised by The British Materials Handling Board on bucket elevators.

However, the cost of compliance with the stringent requirements demanded of this specification focuses attention to reality, particularly for custom-built equipment, and sense can prevail. DSEAR recognises that there may be circumstances where the risk assessment finds that the selection criteria for equipment is inappropriate, or unnecessarily restrictive. A degree of flexibility has been introduced to Schedule 3 of the regulations - by allowing employers to use equipment that does not conform to the requirements in the DTI's regulations, where the employer's risk assessment finds otherwise. Section (3) of their document cites circumstances where equipment of the required category is simply not available, but a lower category can be used in combination with other protective measures to achieve the purpose of these regulations.

The way that this normally works is that a responsible screw conveyor manufacturer can supply machines rated for Zone 21, Category 2. If the user employs this equipment in a situation calling for Zone 20, he is obliged to add another level of protection according to the overall, installed flow system. This calls for a clear understanding, agreement and hopefully co-operation, between the supplier and the user and, when involved, third intermediate parties such as OEM's and main contractors.

ATEX is not a monster, but a serious attempt to provide a safe working environment. Its implementation does demand a degree of rigor and costs that were rarely incurred by equipment designed before its introduction. Being mandatory in Europe, the playing field is theoretically level for regional suppliers, but interpretation and methodology on safety techniques may vary at different steps of the evaluation. ■

"..it has to be assumed that optimum explosive conditions may pertain at some time and location.."

Quite reasonable.

"This combination could quite reasonably point to the designation of a Zone 20.."

Surely the appropriate Zone is determined only by the frequency/time that an EXPLOSIVE atmosphere exists?

"ATEX is not a monster.."

Well it shouldn't be.

"Being mandatory in Europe, the playing field is theoretically level for regional suppliers"

But in the real world? ■

True, on safety issues one should err on the side of caution and, in many cases, it is difficut to establish how frequently explosive conditions exist. This is where the real world comes in. In many, many years of making screw equipment, I have only one knowledge of a explosion, (caused by a trapped foreign body in a large, virtually empty, screw discharger). Experience would indicate that the incidence of an explosion in normal screw conveyors, excluding associated equipment, is very rare but prudence dictates that one should consider the implications and take proper steps if the risk is at all present. ■

Prudence is one thing, over Zoneing is something else. ■

It certainly happens, but this lies in the hands of the user, not the equipment manufacturer. ■