Product buildup in Surge Hopper

[Wizard74]

Hi,

Plastic pellets in my company sometimes build up in surge hoppers even though the rotary valves below the surge hoppers are running near maximum speed. (The rotary valves are not under-designed for this application.)

One possible root cause is that the clearance between the rotary valve housing and the rotary wheel has increased. That can be solved easily through replacement of the rotary valve.

The other possible root cause is that the pellets have static on them, which can be observed on a rotary-type classifier screen.

1) Is there any possible root causes to the product buildup in the hopper, other than those 2 mentioned above?
1) Is there any way to remove static on product pellets?
2) What process can cause static on plastic pellets?

Many Thanks

 

[Wizard74]

One additional point is that the pellets are transported via dense phase conveying and when the product builds up in the surge hopper, increasing the airflow helps to clear the build up.

Thanks

 

[krecian]

Wizard74,

Check-up whether "bridge" formation is taking place at the conical bottom preventing the plastics flow to the bottom and to the RV itself.

KRECian

 

[Wizard74]

KRECian,

yes, that has been checked, no signs of bridging.
As an additional step to prevent product buildup, we have been operating the hopper at empty level, i.e. what comes in goes out immediately.

The product buildup is more frequent for non-surface-lubricated products than for surface-lubricated products.

Thanks

 

[smile1]

Sounds like static. The coating acts as an antistat. Solutions are changing the convey lines, humidifying the convey air, and adding an antistat.

 

[spettitt]

You mentioned that the rotary valves are running near maximum speed. Are they able to properly vent air? They could be acting as a fan and holding the pellets above the entry to the valve. Try slowing the valves down and see if product flow increases, if it does, then the valves need to be larger and designed to run slower.

 

[Wizard74]

Dear smile1,

Thanks. But the funny thing is that we have another line with exactly the same line-up but it does not show such problems at all. Strike any note? Let me know pls. Thanks.

Dear spettitt,

There is a air-leak pipe leading from the side of the RV housing to the top of the hopper. I have observed pellets being carried over before through this pipe. Could be one of the contributing factors.

Have tried varying RV speed and airflow but no significant improvement. Have even tried knocking on the conical bottom to ease product pellet but to no avail.

Many thanks to all, please keep your ideas flowing despite my product not doing so as much as I wish.

 

[Wizard74]

One thought came to mind though, why can't static in this case be removed through the usual earthings on the hopper/equipment?

 

[smile1]

Material handling is my company's focus, so I'll take another stab at this. Product flow problems can be static, where the particular product, the transfer line, or the holding bin are allowing static to accumulate in the product. This is generally the problem with light (30 pcf) plastic resin and pellets. Typically the problem arises in winter or if the plant is air conditioned and the humididty drops. If you are certain that the problem is not static, the next most common cause is hopper design. We design mass flow hoppers, designed specifically to deliver first in first out results. This keeps product from rat holing in the silo/bin, and stops bridging. Another area to examine is the hopper lining. Is it rough, dimpled from hammering, or uneven? Even slight imperfections can stop material flow. Rotary valves have a property known as blowby, due to clearances. Have the clearances checked against the rotary valve that works. Is the convey line pressure the same? Higher pressures increase blowby, and keep material out of the rotary valve. Is there a blowthrough adaptor in use, or is the rotary valve dropping into a pipe?

You may want to consider an eductor if all else fails.

 

[saxon]

wizard74, It sounds like a bridging and rat-holing problem aggravated by static charges. Have you thought of installing vibrators on the cone and body or possibly installing air blast nozzles to break up the agglomerations? Proper grounding of the equipment and system may help in the dissipation of static charges.

saxon

 

[Wizard74]

** smile1, thanks for the very insightful comments. I don't think I can conclude that static is not the problem because it is visible on the classifier screen, neither can I conclude it's the main cause. But it must be one of the contributing factors. Regrettably, I haven't have the chance to do an inspection of the hopper internal linings.

RV clearance is 0.23mm ~ 0.3mm versus 0.15mm (manufacturer's spec). Conveying line pressure is higher but due to this product buildup problem, airflow has been increased so the conveying line pressure has now dropped to a value which I deem pretty low. Rotary valve is dropping pellets into a pipe.

*** saxon, very good suggestions. I will keep them in my pocket after establishing the exact root cause. Thanks.

 

[azertyuiop]

I personally would decrease the dense phase air pressure to the same level as the installation that works well or go even lower , with the product that gives problems , just to see if RV blowby is the root cause. It seems to play a major role given that you are able to transport product through the RV airleak connection to the top of your vessel. If you have such overcapacity in supply air , it must go somewhere , and take the easiest way.

You probably have a combination of static accumulation leading to bridgeforming because of RV blowby , having read all the above mentionned comments.

I had similar problems in the past with similar products. I solved it by putting a rotating pin system inbetween the RV and the vessel , The rotating pin system being powered by the RV thru a chain connection. This kept the product constantly moving , just above the RV inlet , and we didn't have to worry anymore about air pressures leak or new different product also being transported in the line.

That component was subsequently installed every time we feared some problems with airleaks or static issues.

See on

http://www.spiromatic.com

for a picture of this system.