Screens Etc.
Greetings Thelaz,
first things first, what are you screening?, when you refer to screen pegging are you inferring screen blinding/plugging?
It can be one of two things, either the material is to wet to screen
or you are feeding to much material across the screen deck plugging it.
If the screen is not throwing material fast enough in its elliptical orbit it will plug simply because of the lack of movement and material compacting on the screen deck.
In my opinion your client is putting too much material across the screen deck and further complicating the matter by removing the counterweight, add the counterweight back and reduce the delivery rate to the screen and you will see a marked difference.
It also sounds as if the down stream samples are over size, if this is the case you have a lot of fines in your material if is this the case it is because of screen plugging.
Allways remember gravity, poor screen incline, and lack of screen orbit are the enemy. ■
Poor Screening Efficiency Troubleshooting
Hello Thelaz: Thanks for the question. We need a bit more detail before attempting to troubleshoot this application.
Screening: TOP DK: 18mm over
2nd DK: 10mm over
3rd DK: 5mm
BOTTOM Deck exciter is operating at 15mm stroke at 750rpm
Some IMPORTANT questions to review:
1. You did NOT state what the actual problem is
2. Fines in the coarse? Inefficient? What is the spec he is trying to make off of each deck please.
3. What is the product please.
4. Is it dry or wet?
5. The opening sizes are stated but, are they clear openings, slotted openings, are they T304 Stainless steel or other wire, what is the size of the wire diameter? What is the slot direction - cross/slot side tension or long slot wirecloth?
6. How is this screener FED specifically...quite important aspect.
Width of belt feeding the screenbox. Troughing idler angles on the feed belt.
7. Sieve analysis of the feed material. This will identify the percent of near-size material to the cut you wish to make and will well define HOW HARD it is for the screen to do this job. LOTS of nearsize equals lots of inefficiency. What are they attempting to take out of the feed to make what product?
8. What is the SLOPE of this screen box. 20, 30, degrees?
9. What direction are we rotating the drives.....WITH FLOW of material or COUNTER-flow?
10. Are you PLUGGING? if so what percent?
11. Are we spreading the feed material full width of the screenbox WIDTH?
12. Are there holes in the SCREEN CLOTH?
13. iS THE screencloth, drum head TIGHT?
14. What is the size of this screen BOX width x length?
15. Total exciters.....TWO? or more? Is top deck worse or second or third for problems?
Welcome to vibrating screen world.........any answers would be very helpful. Thanks for the question.... ■
Changing Throw General Rule Of Thumb
Generally speaking and practically:
1. If we INCREASE the stroke SIZE, we decrease the SPEED
2. if we DECREASE the stroke SIZE, we increase the SPEED
3. THE SMALLER the material, the faster we want to go to be a bit more aggressive with it and to INCREASE the number of introductions to the opening in the screencloth or other media.
4. The bigger the material, the slower we will generally go ...as we are usually SCALPING oversize off a multiple deck unit.
As I was taught ....we generally do TWO THINGS when changing STROKE SIZE ........not one as most people do.
* The factory should always be consulted for the cheat side of things.....they may OKAY grabbing say 50 RPM with a little bigger motor pulley....but, the key is to NOT run too fast....and too high of an ACCELERATING force or G's to crack out a sideplate. ■
Re: Changing Throw Of Vibrating Screens
Thanks gents,
Great tips so far- i really didn't expect such useful information so soon. Now I feel deeply immersed itno the magical world of screening.
With response to your first email, George
1. The real problem is too much fines leaving the system. There are 3 decks of screens b/c there is so muich material to screen. Fine material is -5mm and is internally recycled as RM. All oversize is chuted to a common conveyor.
2- The spec off each deck is unimortant. What is most important is reducing fines leaving the system- i.e having as much fines go through the 3 decks as possible. We have identified that most of the fines going in the product is entrained in the oversize from the 5mm aperture deck, as would be expected, and have chosen this as the most logical position to optimize, although we have reduced aperture size on the middle deck to reduce loading on the bottom deck. This has shown a small improvment. The obvious thing to do is increase aperture size on the bottom deck, but I am saving this as a last resort as it will sacrifice yeild.
3- Prodcut- sintered iron ore
4- Dry (for the most part)
5- Openings are slots (elongated tear-shape )
6- Bottom deck is fed via a chute from the above screen. I blieve there is a crash plate onto which the matireal falls. Angle =?
7 What is the definitioin of near-size material?
8- Slope= approx 20 degrees
9- Cannot specify this. Screens are linear vibration. Angle of throw is approx 35 degrees to hrizontal,.
10- Plugging? I assume you man pegging. It is pretty bad, and only got worse with a reduced stroke
11- There is some shift of material on the screens, but not in any considerable abount.
12- waht do you mean here? The scren cloths are changed regularly. No holes are identified.
13- What is head tight??
14- Cannot specify this
15- there are several exciters. 4 on each drive. 2 drives per screen deck.
Thanks for your help George
I have ordered the Vibrating Screen Handbook from VSMA. Is this the "Holy Grail" I've been looking for? Any other useful papers out there? I'm a nut for more technical info.
Cheers ■
Magical World Of Screening "I Like That"
Replying to your responses to my questions: tks for that !
First: I somehow neglected to ask: HOW many tons per hour are being fed to this screener? What is the sieve analysis of the feed material?
1. Summarize: You are carrying over fines (Minus 5mm which is under a 1/4" opening) VERY TOUGH SCREENING
2. I did not understand, bottom deck fed via a chute and crash plate? My understanding is top to second to third..@ 20 degrees
3. Near size means particles in the feed mtl that are CLOSE to the size of the opening in the deck. They cause inefficiency because they are trying to pass but, cannot and interfere with particles that are -5mm and want to pass but have to compete with NEARSIZE. The more NEARSIZE the more fines passing over that should pass theoretically.
4. I am confused with #8 answer slope approx 20 deg
5. Drive operating direction is directional or counterflow established by motor lead hookup. Here you say angle of throw is approx 35 deg from horizontal....conflicts with #4 above
6. Plugging or pegging ...you state is PRETTY BAD....and worsened with reduced stroke. That makes perfect sense. ..when you reduce stroke. But, all you did was ONE THING....reduced stroke. When you reduce stroke you should SPEED the rpm up also....so we do 2 things.....we do not throw the fine material up high in the air......now .... a good thing when screening fines. but, we must also increase the number of times the small fines are INTRODUCED to the 5mm openings to attempt to pass before the stroke throws it back up in the air again.
7. NO holes in screencloth means damage, etc.
8. Drum head tight. means ensure the screencloth is properly stretched over the deck and NOT LOOSE. (tight as a drum head)
9. why can you not tell us the width and length?
10. The screencloth specification could be the CULPRIT...WHAT is the exact opening, wire diameter, metal (steel or stainless steel)
longs running lengthwise feed to discharge END TENSION or cross slot and hooked?
.....................
a) YOU ARE PEGGING.... this will cause carryover for sure.
b) You need higher accelerating force........ ■
Re: Changing Throw Of Vibrating Screens
thelaz
Increasing the stroke would be the way to clean the deck but as you mentioned all plugs are allready in the counter weights you can increase the speed to 770 rpm and that would effectivelly give you 5g acceleration. Another way is to look at different screening deck. To change the speed even 20 rpm I would suggest doing impact test to find the natural frequencies so not getting additional problems. That will help moving the material faster and possibly improve the stratification and therefore screening. You can also chack if you can get lead inserts for teh mechanisms and increase the stroke without changing the speed.
Regards
Ziggy
www.vibfem.com.au ■
Changing throw of Vibrating screens
I am doing some work for a client who is having difficulty with their vibrating screen operation.
The layout is three decks of screens with 40*18mm/40*10mm/40*5mm apertures. each deck feeds onto the one below it.
In assessing performance I have been taking belt-cuts off the conveyors during plant downtime, and have measured stroke and rpm of all screens.
The exciter on the bottom deck of screens (5mm aperture) is completely full of counterwieights (5 total on each exciter), and the stroke is 15mm @750 rpm (g factor of 4.71). After discussing with a supplier, it was adivsed that the amplitude may be too large, and we removed a counterweight to get the stroke down to 12.5mm (g factor 3.92).
After this change, the screens appear to be operating much less efficiently, and pegging is now becomign a problem. Has this been the right approach? Will screen efficiency only improve with an increase in stroke? Any help would be fantastic
Thanks ■