The first requirements for a good seal is to ensure that the shaft is running concentric and correctly aligned. Screws are usually fabricated, not precision machined components like car gearboxes, and it is usually necessary to straighten the end shafts to correct for weld distortion during the fitting of the flights and the end shafts in the auger tubes. Unless this is done carefully, minute eccentricity will cause a wobble that is fatal to the entrainment of fine particulates. Also, where there is an offset mounting of the bearing, it is also difficult to secure perfect alighment when the bearing height is packed up and bolted in adjustable slots to fix its location with respect to the end gland. No matter how carefully the selection of packing it is virtually impossible to check that the bearing is absolutely concentric when the bearing holding down bolts are finally tightened. It is therefore good practice to clamp the bearing first and have the gland on a separate fixing plate that can be fastened after the bearing is firmly located.

Gland seals are reasonably good for most applications, but the local shaft surface can be ceramic coated for severe duties or a wear sleeve fitted that may be hard coated and a bleed air supply fitted to diffuse a controlled volume of air through a lantern ring to avoid an adverse pressure differential. It is also possible to fit various designs of shield discs, shedders and internal protective devices inside the casing, occording to the particular nature of the product and screw type.

As a last resort, mechanical seals can be fitted, but these tend to be rather expensive for general use. For example, the price of a mechanical seal for a pharmaceutical application, ATEX rated, to suit a 60 diameter shaft recently came in at £ 7,000 each. ■

Try air purge seals...use small amount of compressed air...can use lantern ring in front of packing

Increase de dusting take off volumes from the screw as well...U will get dust coming out of any other small holes etc..

If the screw is not been pressurised from the impactor..then maybe U have dust accumulation at tail seal - is it inclined screw..suggest double flight to sweep the tail end clean.

Thanks

James ■

We have a range of mechanical seals that are used very successfully on screw conveyors. They are nowhere near £7000 per unit!!! About 1/10 of that.

They wre originally developed for use on our rotary valves and were so effective that our customers have asked us to fit them to screw conveyors.

We gave up fitting gland packing seals on any powder duty about 10 years ago as they are very old technology and high maintenance. Our seals are "fit and forget".

http://www.mid.uk.com/documents/misc/cartridge.php for more details. ■

You do not state whether the screw is open topped, encased for much of its length or how long it is. Have you measured the casing pressures?

Assuming the screw is substantially encased---

If you construct an air bypass duct along the top of the screw chamber this would release the fan pressure from the high downsteam resistance caused by the screw & material labyrinth. It would work as a corollary to the conventional air bypass conveyor so that plugs would form & fall into the mainstream. Duct cross section would probably need to be similar to a balloon flue but with baffles to seed out the airborne grit. In the old days screw conveyors, scooby-do's, were used to clean out large balloon flues.

Regardless of the quality of your shaft sealing the overpressure will get out of the line somewhere.

Once you have reduced the overpressure then you should go for one of the 700quid seals mentioned earlier. ■