Originally Posted by mamarciani

Hi everybody,

we have an arching problem in a bin with wedge hopper, for synthetic gypsum.

Based on our bulk material's data, the hopper's rectangular outlet has been dimensioned as Width x Lenght by some specialized consultants.

But eventually, due to extraction-fitting reasons we doubled the major outlet size L. So presently the outlet size it is W x 2L.

Presently we have arching.

My question is:

is there a leanness factor, a L/W lower limit rule to be respected (cannot find any on internet)...

...or any (L;W) size is good, provided that both L and W are greater than critical ratholing-arching size?

Thanks alot

Your arching problem may simply be due to the material compacting on itself

and not breaking away and falling if the slope of the hopper is too low in the

degree of ascent.

Your problem may simply be solved by installing slick sheet material which

allows the material to fall away and not bridge or arch when compacted during

periods of no drawdown of the synthetic gypsum.

The other often fatal mistake made is the hoppers are not discharged completely when

the production shift has ended to ELIMINATE THE PROBLEM TO BEGIN WITH!!!! ■

The flow benefit of a slot is not fully attained unlit the length is three times the width. However, to secure this benefit, it is essential that extraction takes place over the full length of the slot, otherwise the slot will have a shorter effective length and have a 'rough wall' condition on at least one side due to static powder. The fact that your slot is twice as long as specified is unlikely to work worse than one half its length due to the possible, and somewhat marginal, effect of having an dead region at one end. It seems more likely that the original dimension was inadequate.

The question of how to overcome this depends on the existing design. The suggestion to fit 'slick' sheet liners on the hopper wall may work if, and only if, the reduction in wall friction converts the flow regime in the hopper from funnel flow to mass flow, otherwise it is a waste of time and money. This also presumes that it is not already mass flow and that the replacement surface has a lower value of contact friction. There is no substitute for measuring wall friction and establishing the wall angle needed for mass flow that, for a V shape hopper, is roughly 45 degrees plus 1.2 x wall friction. (the accurate relationship is given on Jenike's charts that take account of the minor influence of the internal angle of friction of the bulk material). The key feature for arching is the 'critical span' that is determined from shear tests on the material.

Starting flow from a hopper that has been filled is more difficult flow than when flow has previously taken place so a useful technique in marginal cases is to remove a small amount of material at an early stage of filling. The virtue of emptying the hopper completely to avoid time consolidation has to be balanced against this re-start problem if it is not practical to carry out a minor extraction at an early stage of fill. It is also good practice to ensure that the in-feed stream does not imping on the outlet region, as this induces an additional degree of compaction.

Vibration MAY be useful for initiating flow, but is a crude brute force technique, as are air cannons, air lances, 'magic mushrooms' and mechanical agitators. The bets approach is to secure a correct gravity flow design. Provided dry air is used, the continuous injection of a limited volume of air at a height about half the outlet width above the outlet can help to maintain a degree of dilation that will ensure reliable flow, but this technique should be either based on specialist advice or subject to trial and effort, if you can afford the time and errors. Some advantage can be gained by the use of hopper inserts to reduce compacting pressures, but this is also the domain of specialists. see CHEMICAL ENGINEERING WWW.CHE.COM July 2010 P 32 - 37. Otherwise, if you care to send details of the installation to lyn@ajax.co.uk, I will see if I can advise. ■

Originally Posted by mamarciani

Thank you lzaharis. Most of your advices have been already followed (not the last one unfortunately!).

Just to be sure, I'd like anybody to answer to my former question, though probably a silly question:

Is there any leanness factor rule (I'd better call it "Slenderness Ratio", sorry for my english) that gives

an upper or lower limitation to the ratio of width/lenght of the rectangular hopper outlet?

I'd like to make sure that if I have a hopper WxL with no arching problems, after I enlarge that hopper to Wx2L I'll still have no arching problems.

Thanks

In a word No If the length formula is followed BUT!!!!!

The only real limitations is/are the type of ore or processed byproduct (and moisture content)

that must be removed from the hopper AND the surface area the hopper is going to occupy and

the desired feed rate.

Your english is fine and no problem. As Lyn bates has so kindly mentioned the length must

be three times the width of the feeder to solve the issue of bridging, arching,

and ratholing-which is something that killed 2 of my friends 30 years ago while they were

being supervised by my uncle............................................................................................... .....

The length of the hopper being 3 times the width lets say 6 meters long and 2 meters wide ABOVE THE ACTUAL SQUARE AREA OF FEEDER ITSELF wide gives you 12 square meters

above the feeders total USABLE surface area which allows the material to fall away FROM ITSELF and let gravity do it s work and prevent any material from bridging and compacting upon itself and the added weight of the material above it simply adds to the pressure on the material all ready bridging above the the feeder.

If you do not have a lot of room to do corrective surgery in a poorly designed bin and feeder the suggestions that Lyn has so kindly provided will solve the bridging issue,

BUT if the cardinal rule of emptying a troublesome storage bin at the end of an operating shift is not followed(as apparentently you are very familiar with you are

already fighting a losing battle if the problem is one the senior managers are not willing to solve- as it was in my case. ■

It is important to appreciate that flow through a slot outlet is very dependent on the design of the extraction device. This is usually a belt or screw, but often the slot is parallel, which limits the effective length that can be served with a live flow over the whole area. The overal combination of hopper and feeder has to be considered together, particularly if there is a flow problem, but in any case is needed to secure an effective outlet. The reason that there is not a formula available for a slot proportion on the net is that the outlet form very much depends on the feeder design as to width and length over which live flow can be extracted. Extremely extensive slot length can be used in some limited circumstances, but only with free flowing, inert products, where large 'dead' regions of storage can be allowed and the slot width is usually quite narrow. Adopting a large size of opening can impose very high loads on the feeder, so it is wise to know the flow charactistics of the bulk solid and employ a systematic design proceedure, a copy of which is available from lyn@ajax.co.uk, with a publication on feeder loads. ■