Re: Conveying System Issues
Dear Scott,
Experts and different opinions are not unusual.
May be my opinion is also different.
A filter pressure drop of 60 mbar (#600 mmWC) is high, but not excessive high.
This has to be evaluated against the total pressure drop of the system.
At a total vacuum of 450 mbar, the filter consumes approx 13%.
The feeding rotary valve only influences the solids loading ratio and thereby the resulting vacuum.
The rotary valve underneath the filter receiver vessel influences the airflow through the pipe.
The airflow through the pipe is the pump displacement mines the rotary lock air leakage and minus the pulse air of the self cleaning filter..
In case the pressure drop over a filter increases with vacuum, then the self cleaning functions not properly. A clean filter experiences less pressure drop with increasing vacuum.
(The air gets thinner as a result of the expansion)
May be this should be checked.
The pneumatic conveying behavior that you describe sounds like a plug conveying system.
This plug behavior should increase with higher vacuum as the mass flow of air decreases with higher vacuum.
This could be designed as such.
It is not clear what your problem really is.
Is it capacity or the irregular conveying itself?
The system can be made smoother in operation by increasing the air velocity, but that has consequences for the whole installation.
The feeding of the system should be based on the overall pressure drop of the system, whereby the rpm of the feeding rotary lock is changed as a function of the overall pressure drop, in order to keep the overall pressure drop constant at a set value.
To investigate the cause of the behavior of the system, it is necessary to know the product characteristics s.a. particle size, material density, suspension velocity and the exact pipe geometry and filter data and vacuum pump data.
Based on the actual behavior and calculations a more explicit answer can be formulated.
best regards
teus ■
Teus
Re: Conveying System Issues
Originally posted by sinclasc
Various companies have looked at the problem but all seem to have different opinions on the solution.
Welcome to the world of bulk materials handling. ■
Re: Conveying System Issues
Hi Scott,
Sorry for potentially sounding ignorant, but is this problem new, or did this problem exist from day one? ■
Re: Conveying System Issues
Hi Ralf,
Well it's best to split the problem out - some of it's new, some old:
1. Conveying to the furthest silo is the slowest; this is an old problem
2. Material slugging badly to closest silo; this is new. A new exhauster was installed just before I started to give a higher vacuum. The 2 plant system now runs a combination of old blower (for one plant) and new blower for the other.
3. Constantly blocked filters: not sure as we don't have historic data, but believed to be much worse since the new exhauster.
4. Material "slugging" up the vertical pipework. Again potentially caused by above (but note vertical pipework is 4" nominal bore, a difference from the initial pipework diameter of 4" OD). This is believed to be a new problem.
Originally posted by RalfWeiser
Hi Scott,
Sorry for potentially sounding ignorant, but is this problem new, or did this problem exist from day one?
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Re: Conveying System Issues
Hi Scott,
1.The capacity to the furthest silo should be less, because the SLR must be lower on a longer pipeline while the air volume and the vacuum stays the same.
2.A higher vacuum can cause a system to become irregular as the mass flow of air is decreasing with increasing vacuum. The material cannot be kept in suspension anymore. Is the air volume of the new exhauster the same as the replaced one?
Is the slugging frequency in any way related to the number of pockets per time of the feeding rotary lock?
3.If the new exhauster is bigger than the previous one, then the filter load is increased and could explain the filter blocking problem. Still check the cleaning pulses.
4.When the material is slugging, the first parameter to check is the air velocity.
If you can supply more actual data, a more to the point reply can be given
best regards
teus ■
Teus
Re: Conveying System Issues
Hi Teus,
I will try and obtain some airflow specifics tommorow.
In the mean time filter pulse cleaning is something we've looked at but are unsure where to set. The filters aren't cleaning properly. We clean with varying timings (not neccessarily right); the worst is a 6 bar pulse every 6 seconds with a 1 second duration. The air pressure is replenished well within the 6 seconds.
As this cleaning is ineffective, we continue the cleaning for a while "off-line" - after the vacuum has finished line clearing. I beleive the timing for this is 60 seconds or so, but still doesn't manage to clear all the powder from the filters.
Filters are anti static PTFE coated cartridges.
Originally posted by Teus Tuinenburg
Hi Scott,
1.The capacity to the furthest silo should be less, because the SLR must be lower on a longer pipeline while the air volume and the vacuum stays the same.
2.A higher vacuum can cause a system to become irregular as the mass flow of air is decreasing with increasing vacuum. The material cannot be kept in suspension anymore. Is the air volume of the new exhauster the same as the replaced one?
Is the slugging frequency in any way related to the number of pockets per time of the feeding rotary lock?
3.If the new exhauster is bigger than the previous one, then the filter load is increased and could explain the filter blocking problem. Still check the cleaning pulses.
4.When the material is slugging, the first parameter to check is the air velocity.
If you can supply more actual data, a more to the point reply can be given
best regards
teus
■
Conveying System Issues
Scott,
Here is another opinion (sorry Teus). It sounds to me as if the filters may be your problem. 600 mm d.p. is very high, especially for cartridges. They should run at 50 mm max. (Maybe the filter velocity has been selected too high, i.e, the dust collector is too small).
Cartridge filters are difficult to clean with 'difficult' dusts. 6 bar pressure is O.K but I would do off-line cleaning for much longer, 30 min. to 1 hour, if poss. and see if that helps. If the d.p. does not recover after that, the filters are blinded. In any case, your filters will wear out prematurely with this treatment.
I think you have the wrong kind of filter. A conventional bagfilter (with felted bags) and conservative filter velocity would work much better and maybe fix your problem.
What is the terminal airflow and filter area?
Regards,
Michael Reid. ■
Re: Conveying System Issues
Hi Scott, Michael,
Michael: it is not so much another opinion than an additional opinion that fits in the discussion.
A filter pulse of 1 second, every 6 seconds at 6 bar will blow the accumulated dust layer back to dust again.
And this dust is sucked in again by the convey air flow.
The cleaning principle of a filter is that the formed caking is released from the filter surface, but stays in agglomerations of dust.
These agglomerations have a higher suspension velocity than the dust particles and fall down between the filter elements against the upward flow of the convey air.
Gentle pulsing works therefore better than violent pulsing.
The upward can velocity between the filter elements must be low enough to make this possible, otherwise the dust stays between the filters.
In the mean time as the filter is pulsed, the convey air flow in the pipe is decreased for a while.
If the pulse air flow is relatively high compared to the convey air flow, it can cause momentarily sedimentation in the pipe, leading to slugs.
We wait for the specifics today
all for now
teus ■
Teus
Conveying System Issues
Scott, Tues,
There is such a thing as "over-cleaning". This mainly applies to fabric filters where the filter cake actually does the filtering. Removing every last layer of dust can lead to premature blinding of the fabric. It is why new filters must be seasoned gradually with dust before the pulsing system is activated.
The pulse duration is normally set between 0.1 and 0.3 sec. One sec. ON time is too long and 6 sec. OFF time is too short.
If the design filtering velocity is too high, cartridge filters cannot release the dust which builds up in the valley of the pleats. This is sometimes the cause of high d.p. Alternatively, the high filtering velocity collapses the peaks of the pleats, effectively reduces the filtering area and increases the d.p.
The system statistics will help further discussion.
Michael Reid. ■
Re: Conveying System Issues
Dear Scott,
The first calculations from your data show:
-air velocity ranges from approx. 19.7 m/sec at the inlet to approx. 32.9 m/sec at the outlet. (outlet rotary lock losses disregarded). This seems to be very high for PVC powder.
-Filter load is approx 1.167 m/min, which seems to be OK.
For further calculations it is necessary to know:
-particle size distribution
-material density
-outlet rotary lock data, s.a. lock volume and rpm
-present capacity and related vacuum and filter pressure drop
Do the pulse pauses you mention count for 1 filter cartridge or are you pulsing all the filter cartridges at the same time?
take care
teus ■
Teus
Conveying System Issues
Hi all,
I am having problems conveying powder from a plant to internal silos.
The basic process is that the powder drops from a surge hopper (through a rotary valve) into a vacuum pipe enters a vertical section, enters a receiving vessel on top of the silos, and falls through a rotary valve into the silo itself.
The problem is that the material transfers slowly up the vertical section of pipework. The differential pressure across the filters on top of the silo is 60mbar (and this with clean filters, too). Industry "experts" have different opinions on whether our filters are quite capable of the airflow or not.
The closest silo to the plant conveys the worst - perhaps because of a higher vacuum blocking the filters more? The further silo (of 4) conveys poorly too - perhaps because of the distance.
Any ideas? ■