Re: Densephase Conveying - Plugging
The problem is due to the control system that regulates the supply of conveying air. This "on-line" system must respond to the actual conveying pressure and increase the air flow to prevent line blockage. Installation of boosters or secondary air injectors is not necessary if the air control system design is correct.
If the coal is really wet, or overly moist, pneumatic conveying will always give problems whether you use boosters or a correctly designed air supply control system.
Regards.
A. T. Agarwal
Pneumatic Conveying Consultants ■
Plugging
please send us your e mail address so that we can suggest a simple solution ■
Clyde System Handling Coal
Dear Matthew
I think from your description of the material, the problem is that it is not a very suitable one to convey in dense phase! The problem is that it is not retentive of air (like say pulverised coal) nor is it highly air permeable (like coal graded to a narrow granular condition). Hence it needs help to keep the slugs from coalescing and causing a blockage.
However I can say from experience that the density stabilisers can be often be made to work for these sort of problems as a simple, practical sort of band-aid fix. Not the ideal solution, but workable in many instances. Clyde do have a pretty good idea what they are doing, I might say.
A more sophisticated approach using active boosters (and some of them do work but many are unnecessary!) would be more effective if well engineered but probably a hundred times more expensive!
We do a lot of line blocking and unblocking in our labs to explore the conveyability of material, and we find that just a small amount of air bled in through relatively closely-spaced air tappings will usually clear a blockage when it occurs.
It occurs to me that if the system operates well most of the time but blocks now and then, the solution is to run the system normally with the injection points turned off, then if a blockage is sensed by increasing pressure, turn the "density stabilisers" on to prevent/clear the event.
For local expertise on the matter get in touch with us on 020-8331-8646 and ask for either myself or Richard Farnish. We can give some general advice or even run some trials if you want. Or have a look at our web site www.bulksolids.com
Mike. ■
Regulator Burden
Matthew:
The sort of pressure burden I would expect for a properly sized regulator is about bar, by the way. So it looks like the reg is probably ok.
The function of the regulator is to keep a constant pressure upstream of the orifice plate so that there is a constant flow rate of air into the system.
Using an orifice plate, the pressure downstream of the orifice plate (expressed in absolute terms, i.e. gauge pressure plus 1 bar) must not be more than about 60% of the upstream pressure (again in absolute terms) - if it is then you'll lose air flow and inevitably block. Check that the pressure on the downstream side is not exceeding this limit - if it is then a change from the orifice plate to a convergent-divergent nozzle is required - give us a ring if you want more details.
Mike. ■
Re: Densephase Conveying - Plugging
I agree with Dr. Bradly - your material is suitable one to be
transported by a belt conveyor - this is a simple and a
"head-ache free" solution.
In any case, forget about "dense phase" and increase the
air consumption via additional piping connected to the
pipe outlet.
If you are going to suffer from your pneumatic system further,
I can advise you how to destroy blockages and to clean
pipeline automaically.
Michael Rivkin ■
Densephase Conveying - Plugging
I am trying to resolve a blockage issue on a Macawber/Clyde 12/12 densephase conveying system. Unfortunately I don't know too much about this technology; hopefully the expertise in this forum will be able to give me a few pointers.
We are transferring unwashed smalls coal. The colliery sizing specification states 20% min 12.5-25%; 60% max 3.15-12.5%; 25% max below 3.15mm, of which 15% max below 1mm, and 7% max below 0.5mm. Coal is typically stockpiled in the open for several weeks before being loaded into the silo above the pressure vessel - which will give variable moisture content, and probably increased fines.
The problem we suffer from is occassional plugging/blockages in the pipeline. Pipework is 150mm x approx 100m (all but 10m horizontal), with 3 long radius bends, and alignment flanges between pipe sections.
The manufacturer is advising that we need to install 'density stabilisers' every 4m along the conveying pipe. They are not prepared to offer any form of performance guarantee due to coal variability. The stabiliser is essentially an orifice tube and non-return valve and would be connected between a 0.5" hole in conveying pipe and a 2" airline running alongside the conveying pipe. The installation advice is that the 2" airline is connected to the discharge of the main blow valve on the pressure vessel (but before the 24mm orifice plate which limits air flow into the vessel), so that when the main blow valve operates to push coal out of the pressure vessel air is simultaneously put onto the stabiliser air main. The stabiliser non-return valve would close if the pressure in the conveying line exceeded the pressure in the 2" airline.
Currently (no stabilisers installed), prior to blow valve opening the air regulator is set to 4BarG, immediately after the blow valve opens the pressure drops to 3BarG then tails off to 2BarG as coal clears the pipe (typically 24s later). We have no gauge indicating the pressure in the vessel itself, but it will obviously be lower than the regulator discharge pressure due to the orifice plate.
I have 2 questions:
1. Has anyone any good or bad practical experience of 'density stabilisers' (also termed secondary air injectors or booster systems I think) for this type of, or similar, application, or words of advice?
I have read the 'Boosters? do they boost anything??' thread in this forum; extremely relevent and interesting, but I remain sceptical. Our supplier says they work because (a) they stop a long plug forming, and (b) they will break apart any plugs formed during a blockage. How?? The air flow through each stabiliser would be approx 1% of the main conveying air flow I'm told.
2. Is it likely that the air regulator is undersized (Clyde are questioning this)? The 1.5" regulator is dropping 1Bar immediately the blow valve is opened. Clyde have calculated the flow through the 24mm orifice to be 770scfm, which is on the limit of the regulator, and regulator curve does predict approx. 1BarG pressure drop. I would love it if someone could explain what effect the orifice plate and upline pressure have on coal conveying velocity, and any potential significance on blockages.
Any other advice will be greatfully received,
Many Thanks, Matthew.
Telford, UK. ■