Hello,

The diagnosis of the problem is not so hard. The solution will be .

My diagnosis :

1. All screws - in tons per hour sizes - or grams per hour sizes - can only give volumetric consistency with speed if the flights are 100% full .

2. This depends on if a flow aid device as an agitator or wall flexing device is in place - or the flow regime at inlet is designed for mass flow . One element in your screw is existing - the tapered inlet. Angles ?

3. Without flow aids , or appropriate inlet section design - under gravity flow, lower the speed - higher is the fill level into the flights.

So it happens that at low speeds, your screw is filled more . Unfortunately - the motor is unable to push this fill level - so it

trips.

If the screw is mechanically able to carry the higher fill - may be the motor can be changed. Or use a higher ratio gearbox, a higher capacity frequency drive, run the motor at super base speed to deliver higher torque .

Unfortuantely this step will only solve your screw tripping - not the inconsistency leading to process instabilities you described. For that - only a consistent infeed will help .

To go further, one may use a rotary feeder - weighfeeder synchronous combi at the bin outlet to deliver gravimetrically accurate output - and then carrry it by a volumetric screw .

Good luck ■

"What are the symptoms of flooding? " >> Nothing but water is visable as far as the eye can see and all the animals are getting about in pairs. hahahha

On a serious note, it sounds like the material is fluidising and flooding the screw body, this is evidenced by the two points; that it only occurs when you have a higher percentage of fines and the hatch breaking off under pressure.

Although the material is fluidised enough to flood into the screw, it isnt fluid enough to flood out through 9m of spiral flights.

However what does suprise me is that a 22kW drive isnt able to handle the torque of that fully ladden screw. Especially since I would assume the drive torqe should should reduce when material is fluidised which should counteract to some extent the increased volume to be driven.

I have a suspicion that you may have some problem with gearing or VSD.

What is the gearing of the drive? You mentioned a VSD is used and that 30rpm is max screw speed and that problems occur below 20rpm.

VSDs are wonderfull things but they are often abused in what is asked of them. they really shouldnt be run at excessively low or high freq output for extended periods as this can casue the motor or VSD to overheat & trip.

Consider this senario; It is possible that the origianl gearing was wrongly selected, but the comissioning engineer was blessed with a VSD which comensated for the design error. So the VSD only supplies 30hz to give the desired max speed of 30rpm. But when system tries to produce 20 or 10 rpm the VSD is supplying only 20 or 10 hz respectively, at which point everyting overheats and trips out.

Why does the VSD/motor overheat at low speeds? well for many reasons and it can depend on the type of VSD and the cable length between VSD & Motor.

"Most induction motors can operate with modern VSDs through moderate speed ranges (around 30% to 100% speed). Sustained operation at low speeds and, in particular, high load at low speeds may require a special or larger drive and special measures to cool the motor.

AC induction motors operate hotter with a VSD because of harmonics, impurities in the electric power they provide to the motor and also the slower rotating speed of the motor's integral cooling fans. This is usually not a problem if speeds are continuously above 40% or where there are brief periods of slow-speed operation. However, prolonged operation at or below about 30% speed, especially when driving significant loads, can cause rapid and potentially damaging heat in some motors. "

source:

http://www.synergy.net.au/BusinessS...edDrives.html

Another good laymans page on VSDs (although it tends to taking in the context of fans and pumps)

http://www.seav.vic.gov.au/manufactu...&resourceid=31

Cheers ■

Well, sorry to take a differing approach but your feeder design may need some tweaking. The reason for both the stepped pitch and the tapered inlet is to control the problem you have described.

If the feeder is a "U" trough, you need to be sure there is an internal shroud for the first several pitches after the feed inlet. Secondly, be sure that the pitches do not widen beyond a short pitch beneath the inlet. The "short pitch" is what controls your feed rate.

If you would care to share an autocad drawing of the feeder design, I would be happy to try to help.

Ted Key

tedkey@carbonblack.us ■

1) you have not given the length of the inlet, although your description indicates it must have some length if you have tapered pitch and centre tube on the feeder. What are the dimensions and profiles of the bin (attach a sketch?)

2) for a feeder based on a U trough a shroud over the top of the screw is necessary for correct operation as a feeder.

3) IMO 22.5 degrees for a screw feeder is a bad idea, I'd have feeders horizontal.

4) fluidising valves do not promote mass flow in a bin. They are an addition to overcome a bin that does not mass flow. Adding air needs to be carefully done. Too much air can totally fluidise a bin resulting in uncontrollable flow.

5) overall sounds like your bin/feeder/material combination are incompatible. The addition of air has been provided to try and make the system work but it isn't .

6) maybe Lyn will pick up on this thread and impart some of his considerable knowledge on the subject of bins and screw feeders.

7) note also the comments of Young&Silly with regard to variable frequency drives, they are definitely NOT "Silly" :-) ■

So Anthony P what is the status of the screw problem?

Have you solved your problem or atleast conclusively determined the root problem?

Did you check out the gearing ratio & the VSD fequency output at the low running speed.

I have a tall bottle of Tiger beer wagered on this with one of my colleagues. This is VERY important, the beer is only worth about USD$6 but the bragging rights are priceless.

Cheers

Young&Silly ■

Your description is consistent with the lime aerating in the bin. In such a condition the lime can be thought of as having a viscosity less than water. It will flow through any small gap, certainly through the clearance between the scroll and casing of the screw feeder.

I might suggest that initially the lime flows to the bin outlet OK, but at some point arching or piping in the bin has occurred followed by a collapse, the collapse aerating the lime so it flushes through the screw feeder.

You will not solve your problem without modifying your bin/feeder arrangement to ensure consistent flow to the feeder.

What rate (te/hr) are you looking to extract? ■

Hi Anthony P,

Thanks, I think I have enough to justify my Tiger claim with my colleague.

It seems pretty conclusive that the problem (with the over feeding & resulting uncontrollable reaction) is due to over fluidisation. This couldn't cause the motor to trip as the torque req’d will drop substantially during fluidisation. Of course when fluidisation ends the screw will be 90% full and packed hard which might cause the tripping, especially in combination with the low rpm/freq causing the trip especially when combined with the heat coming from the out of control reaction nearby.

btw your freq data doesn’t seem to make sense.

You gave 75hz = 30rpmSCREW and 50hz=1480rpmMOTOR=39.7rpmSCREW (using gear ratio)

and

30hz=20rpmSCREW.

none of these figures seem to correlate.

When the Expert gave their report to switch to fluidisation system, their should have been some accompanying data on the fluidisation properties especially de-aeration time of the material; if this de-aeration time is short then you can use cutting the fluidisation air to control the flooding. However if the de-aeration time is long you cant control the flooding this way.

Has the expert been told of the problem their fluidisation approach has now caused? what was their recommendation?

It would appear the expert has only solved half the problem they were presented.

Problem was previously (i can assume) insufficient feeding with vibrator, as it didn’t provide your req’d feed rates.

Fluidisation as solved the insufficient feeding problem, but now provides uncontrollable feeding, so you still aren’t provided the feed rates you require. Out of the frying pan into the FIRE..... especially since now you've got an out of control reaction!!!!

IMHO the experts job is not done.... they have not provided the feed rates you required from your bin. In fact it would seem they have made the system worse/DANGEROUS with an uncontrollable reaction.

Of course I am assuming you followed the experts recommendations to a Tee and didn’t leave out some seemingly unimportant parts of their recommendations which may have been to actually control this flooding.

cheers ■

I trust your high profile expert is returning to fix your problem, or at least return some of his fee if you've still got problems.

I might comment further but without further details of your bin it's hard, you're tying one hand behind our backs.

Is it square, rectangular or round.

Is the converging section a cone or pyramid and what are the flank angles.

The outlet is 500mm, what's the barrel diameter/section ■

Oh dear, you pay for an "expert" opinion, then don't follow what he says!

I suppose the reason was that his mods would have been a bit expensive and time consuming to carry out and someone came up with a "cheap" quick fix. But you got stuck with the problems from the fix. ■

Actually I am also not surprised they didnt follow exactly what the expert recommended.

I would say this happens in about 90% of cases for a few reasons.

1) The experts usually/often provide the 'ideal' design, without regard to costs

2) Even if the expert has taken costs into consideration, they almost never consider the practicality of carrying out the proposed changes to a system that already exists and has minimal opportunity for extened down time.

3) Before the experts recommendations are implemented someone discovers that expert was not given the full envelope of data for the exisiting system. So their recommendations are built on flawed data/assumptions and thus wouldn't work flawlessly in reality.

4) Before the experts recommendations are implemented the system requirements change thus their recommendations are no longer entirely applicable/suitable.

5) Someone comes up with a better cheaper system.

I disagree with ' designer's ' comment

"someone came up with a "cheap" quick fix. But you got stuck with the problems from the fix."

Yes the soluntion of adding 8 fluidising valves may have been "quick" and also even "cheap" but that doesnt mean its a flawed solution.

The problem turned out to be one of faulty components, not faulty system design. Once the faulty components were isolated the system has run faultlessly.

Add a simple check on the operation of these valves to the PM schedule or even the operational checks and the problem disappears.

Of course to be fair in cost comparision, the system the expert was proposing seems to have no ongoing energy costs, while the current ssytem will consume compressed air for forever, so it may well not be cheap but actually have a higher life cost.

Cheers

Y&S ■