2 replies to this topic

[PaulC]

Hi all,

I am currently doing some preliminary design on solids storage and discharging systems. I am looking into the use of a flexible discharge line to transfer product from a silo to a road tanker.

The filling of the product is into the top of the tanker which remains horizontal and is not tipped. The use of a flexible line is intended to reduce the labour needed as a stationary discharge point requires reparking of the tanker several times. The obvious issue here is that the hose will only be able to reach a certain length horizontally before the angle from the horizontal is too shallow.

My query is how to calculate the angle that the hose will be able to be at without the material blocking up. Am I right in thinking it will be governed by the angle of friction? I have the angle of repose and the cohesivity from previous tests done by others but unfortunately not the angle of friction, is there a way that I will be able to calculate the angle of friction from solely the cohesivity?

Many thanks, Paul

[Steve Hall]

Paul,

Although important properties -- angle of internal friction, wall friction angle, and flow function (a measure of cohesive strength of the bulk material as a function of consolidation pressure) -- are used to design silo wall angles and the minimum size of the opening at the bottom of the silo, conveyance tubing design is not dependant on them.

Instead, conveyor piping including flexible lines are designed to meet criteria for velocity, bends (angles and proximity), smoothness, etc. The minimum velocity in a horizontal line is often established using the "saltation velocity," defined as the minimum sperficial gas velocity required to keep particles in suspension (for a dilute phase conveyor). This velocity is determined experimentally, although there are correlations in use for preliminary design work.

If your pneumatic conveyor was provided as a system by a single vendor, you are advised to ask that vendor for an assessment of your flexible line proposal.

[PaulC]

Thanks for your reply Steve and I am aware of the saltation velocity but here I was referring to a solely gravity conveyed system as pneumatic conveying will not be able to give the flowrate I require and provides too many problems with regards to venting.

So I would be looking at a silo with a discharge feeding into the flexible tube and then straight into a loading bellows (without any kind of conveying mechanism).