To relieve the chutework designer of the responsibility of selecting an appropriate valley angle for the material flowing down the chutes. ■

If your contract specified a "minimum" valley angle and your chutework valley angles are equal to or greater than the minimum then you have complied with the contract specification laid down. (What minimum has been specified?)

The problem arises when the physical layout of elements within the plant results in the minimum angles not being achievable!

Any deviation in minimum angles must be agreed and accepted by the buyer (and get it in writing!).

Based on prior experience just because a company is large, or a large engineering company has written a specification you should not be surprised if neither organisation recognised a valley angle if it fell on their heads! (sad but true).

Also read the specification closely as these things have a habit of containing the standard get out clause "... even if you do what we insist that you do, it's always your responsibility if it doesn't work as we accept no liability for the instructions we give ..." or something along those lines. ■

The 55 degree valley angle has appeared in just about every North American spec I have read in the last 30 years, regardless of whether the material was hard rock ore or wet clay.

The spec for your project probably has the same 100 pages of boiler plate that the consultant has issued, and charged fees for, on every job they have ever done. I agree with Designer, the guy who issued the spec may or may not actually recognize a conveyor if you ever got him out into the field.

If you issued drawings for approval, and if the chute angles were dimensioned and approved by the owner/consultant you may have some more ammunition.

We can only assume this is an issue because there is a problem. If so, putting a radius or chamfer in the offending valley works wonders and makes you look very clever. ■

The relevance is that the valley angle is the lowest inclination of slope in a hopper or chute, therefore a minimum angle may be specified based on experience or known wall friction values that this degree of slope is necessary to ensure product slip. Any such information should be accompanied by a specification of the contact surface. The fact that a specification is written by a consultant or large organisation is no indication of expertise in bulk technology, so prudence demands a suppliers evaluation of the slip and flow potential. If wall friction data is not given it should be requested or checked. No doubt the 100 page specification has little information about the flow properties of the bulk material to be handled, whereas this is a vital feature of the application.

Sharp corners are generally bad news for slip, so a small radius is very beneficial to resist the build-up of product from material sticking in corners. A valley angle implies convergence, which is usually limited in chutes but common in collecting hoppers. It is not satisfactory to base a valley angle on the self-clearing wall angle of a hopper because of the adverse effect of the corner and inflowing material from the adjacent walls. For similar reasons it is rare for a pyramid shaped hopper to be suitable for mass flow, as the valley angle must be at least as steep as a conical hopper for the same purpose. ■