How do I prevent Pegging or Blinding of Screen?

You may find this page interesting; maybe even useful.

http://www.goodquarry.com/article.aspx?id=158&navid=19

Hi, Prem!

First of all, welcome to CR4! You've brought your inquiry to a real opportunity for it to be read and thought about by a very diverse group of engineers, from almost every engineering discipline; so you can expect some diversity of opinions from a group of very helpful and knowledgeable people. And I hope that you will participate as well, from time to time, giving us the benefit of your thoughts in a variety of interesting topics and enjoying yourself in here as we all do.

Now, as for your question...

It sounds like you are considering a two-pronged approach to diminish the pegging effect that results in screen clogging: increased throughput pressure and a lubricant that will assist ease in throughput.

The screen is the bottleneck in the process through which your product (you didn't mention what the product is) must pass as part of its journey to end result. It's a wire screen in a vibratory mode and it catches product, i.e. "pegging" on the way through.

Increased pressure on a wire screen poses two problems: deformation (potentially resulting in increased pegging), and breakage due to lateral stress on the screen's (and its supporting structure's) individual wires.

Teflon or other lubricant coating of the screen wires poses another potential difficulty: flaking, peeling, or powdering of the teflon off the screen in minute amounts that will contaminate the end product. The forces against the teflon are pretty fierce, with the eccentric vibration drive and the throughput pressure.

Have you considered exchanging the wire screen for a solid screen with rounded edges on the holes? The structural integrity of such a screen might withstand a greater throughput pressure without deformation or breaking, while the rounding on the holes' edges would potentially prevent sticking of the product as it passes through the mesh, which can be sized pretty finely according to your needs.

An occasional sweep, wash, or blow across both upper and lower surfaces of a solid screen in mid-process might both lubricate and move potentially overlarge particulate out of the way without breakage as well, possibly into a hopper for resizing in some manner. Although if you keep to the wire mesh screen the sweep might damage the mesh material, an upper and lower blow or wash during the process might lessen the pegging difficulty; and if the product is absorbent and light enough, just a blow might do the trick.

It might be helpful to know what material you are putting through the screen as well and for what purpose, from the point of view of offering suggestions to vary the state/shape/moisture content/etc. of the material slightly to prevent pegging without changing its basic composition or useful nature.

I hope some of this will help you form ideas that might lessen the pegging problem.

Mark

Guruji,

The material that is flowing over the screen is Fertiliser. The material is has been granulated and have passed through the dryer. The material coming out from the process of granulation is not a perfect sphere.

Thank you for your prompts.

Kind regards,

Prem

Hi, Prem!

When you chat with the manufacturer about

• an increased vibrational rate, it might be useful to
• experiment with the solid screen (holes with rounded edges) as well as
• increased air pressure in the throughput, hoping the wire screen will cut the product on its way through to ensure less pegging if you are currently using pressure to assist the process; and even
• using a wire with a sharpened upper edge in the manufacture of the screen.

I'm sure you are aware of pressure-less vibratory screen systems that

• screen out larger particles by using a jerk motion and angling to jump them off the screen's lower side into a return track while the smaller (usefully-sized) particulate falls through into the packaging system, and
• those that use serial belt-like screens (ranging from smallest hole to largest hole) to access the correct size of particles for packaging while rerouting the oversize particulate that did not fall through any of the screens for resizing or use in some other manner. These are especially useful if several sizes of particulate can be used, as in gravel screening.

Good luck with your machine's manufacturer. I would like to know what answer you came up with, so when the problem's solved, be sure to post it here for us to read.

Mark

Mark,

• The installation I am working on does not use air pressure in the screening process.
• The screen is also pressure less vibratory screen. The product is sliding over the inclined screen surface. Oversize is finished product and the underside is recycled.
• The use of high pressure air for de pegging is prohibitted on site.

Since I wrote the first time, I have learnt that the issue of pegging has escalated in the last twelve months. I have investigated from the supplier of the screens that nothing has changed in the manufacture of the screen.

I am investigating now whether the frequent stoppage of the plant in the last twelve months is the cause of the problem. Fertiliser absorbs moisture and the pegged material upon startup would be hard to move if moisture content has risen.

I looked at Poly Ripple Screencloth (www.locker.com.au/downloads/technicaldatasheet/polyripple) as an alternative but had to drop this because of the size. The size I need is 2 mm, the smallest polyripple is 2.5 mm.

Prem

Hi, Prem!

Great link. On my own behalf, I've queried their Screenex branch in Au to see if they can do 2mil, or offer a setup preparatory sequence*, or provide a lubricated screen surface. I'll let you know everything they send me.

"Fertiliser absorbs moisture and the pegged material upon startup would be hard to move if moisture content has risen."

*Your concern as to whether the product is absorbing moisture between-times got me thinking on another tangent as well. You might also investigate as to how thoroughly they do a complete cleanup each time they shut down. If they don't do a meticulous screen cleaning, they would be perpetuating an additional clogging problem on startup.

If they do a thorough clean up each time, then perhaps a preparatory sequence on or before the screen is necessary on startup (such as a product re-heat or the lubrication you were discussing earlier).

If the extent of cleanup on shutdown is part of their problem, an additional service you might offer is the installation of a simple shutdown cleanup sequence for them.

Some thoughts on what a cleanup system might include: If they chose to employ a manual system, the installation of a set of heavy duty overlapping plastic isolation sheets that can close around the machine during cleanup, some blowers to expel the humidity, and a high pressure washing hose next to the machine to be used each time they shut down might be all they need. The plastic sheets and blowers will afford any nearby product isolation from the hosing exercise, and the water used can be drained away and completely evaporated to the plant's exterior before restart (with the assistance of the blowers) so as to not affect the humidity levels in the plant.

I think they must be cleaning up each time they shut down. If they aren't then I think the pegging problem has a funny side, like forgetting where your sunglasses are when they're on your head.

So if they haven't been cleaning up on shutdown, I would think designing a cleanup system for them should be made as simple as possible . e.g.

• As they shut down the screening process for temporary or longer-term plant shutdown, a cleanup switch is supplied with power.
• A flashing red light and signal horn alerts the shutdown person that a decision must be made to clean or to not clean and won't go off unless the decision is made (or a polite voice asks the operator whether cleanup is required. If the voice is ignored after n number of requests or x amount of time and the cleanup system is automated from beginning to end, the cleanup sequence begins automatically.)
• If the decision is made to clean the screen immediately upon shutdown, the shutdown person throws the cleanup switch.
• The plastic curtain closes off the machine.
• When a sensor says the curtain is closed, the machine-adjoining vents or windows to the outside open.
• When the curtain is closed and adjoining vents or windows are opened to the outside, the blowers come on.
• When the blowers come on, the high power washing hose or automated washing system is supplied with water.
• When the water comes on, the electric pump on the hose or system is supplied with electricity.

All in sequence, and all automatic, leaving nothing to a decision-maker except where to manually hose the screen clean. If the plant is unwilling to leave that task to a shop hand, an automated wash system might be:

"On sequence /wash /inspect /blow-dry /shut down (reverse of 'on') sequence."

Obviously, I have too much time on my hands.

Mark

Hi! Mark,

The screen is cleaned by the operator with a mechanical hand held scraper which is time consuming, hence the issue was raised by the operator via the idea generation system the plant has in place.

Since high pressure air is not allowed for cleaning, I will now put my thinking hat and consider hot air jets moving on the surface automatically after each stoppage as a sequence. Note: there is a curtain over the screen which would minimise safety issues.

Use of water is going to be very messy hence not pursued.

Prem

Hi, Prem!

Good idea re the hot air jets (I would think you are writing of higher pressure air, drying temperature or a degree or so higher, but not so hot as to bake the stuck particulate in place) as routine maintenance.

[The jets could be helped out by a simultaneous shaking of the screen. Perhaps hit the screen with air from both sides while it goes into empty super-speed mode for a couple of brief clearance shakes. To do this you might employ a sonic low frequency vibration transmitted into the screen or the entire machine along with the hot air jets. All the nuts and bolts are already supposed to be secured against vibrational loosening.]

If I understand your intention correctly, the idea is that after any such scouring process as you are proposing is used, those wires aren't just fairly clean; they're as clean as brand new without even a single bacterium left behind.

I agree with the basic thought. What you propose might not stop the pegging, but it'll sure as heck clear it every time.

I have another suggestion: If you can figure out a way to get past the no-high-pressure-air requirement safely, you might even build a quick cleaning sequence into the process on a regular interval during production. (Flow stops temporarily, screen undergoes clearing sequence, 15 or 20 seconds later, flow resumes for another x hours until experience has shown it needs another periodic clearing.) Then, probably not until a plant-stoppage clearing is undertaken, if any clogs or clinging bits still abide during a visual inspection, the operator can polish the wire at the sticking point with some degreasing cleaner on a soft cloth (to avoid scoring it), noting that point on a chart. A repetitive charting alerts the management to replace a wire.

{Do you think I could get a job running the machine? It would be easier than this...}

Mark