Rotary Valves - Screw Conveyor Combination for Unloading a Hopper

Posted in: , on 27. Dec. 2014 - 09:50

Hi All,

For transferring a relatively free flowing powder ( less than 1 mm in size ) from a bin which includes a bin activator to a tank I am looking at two options :

1- Installing a Rotary Valve (VVVF Driven) as a feeder underneath the hopper and conveying the material into the tank by a screw conveyor ( Fixed speed).

2- Removing the Rotary valve from the option 1 and installing a screw feeder (VVVF driven)

I have seen both options some companies have done before , however not sure why some companies design option 1 which is more expensive. Any valid reason for that ?

Appreciate your thoughts



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Rotary and Pinch Valves

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Re: Rotary Valves - Screw Conveyor Combination For Unloading A …

Posted on 2. Dec. 2014 - 11:14


Option 1 is used when the material being conveyed is in an aerated or fluidized state. In your case, you are handling a fine powder which can get aerated depending upon the type of bin activator you have at the hopper bottom. If you have a mechanical activator, aeration should not be a problem and Option 2 with a screw conveyor should be acceptable.


Amrit Agarwal

Pneumatic Conveying Consulting


Screw Conveyor With Rotary Valve

Posted on 3. Dec. 2014 - 04:45

to discharge material from bafilter's hopper always use option 2 so rotary valve as airlock.

Re: Rotary Valves - Screw Conveyor Combination For Unloading A …

Posted on 3. Dec. 2014 - 07:09

It is just a little bit late in the day to raise this query as the system should have been considered with the bin design. For a 'fairly free flowing' powder a well designed screw feeder would have dispensed with the need for a bin activator. The problem now is that it is not good practice to restrict the output of a bin activator, so care must be taken to match the output with what a rotary valve or feeder can handle. If a conveyor is needed anyway, the transfer task may as well be done by a screw feeder that has a small buffer hopper capacity and probe to cut off the activator when the surge bin is full. This allow the discharge rate to be controlled, if required. Otherwise the feeder could run full chat to pass whatever comes out.

The question emphasises the importance of securing an integrated design for a hopper and feeder. Bin activators are well proven for stimulating flow, but only control the discharge rate as a phenomenological action that is dependent on the powder state. Rotary valves essentionally generate live flow channels of limited size and pass ambient gap back into the bin via the return pockets. Screw feeders can serve extended length slots and secure the many flow benefits and headroom savings of plane flow.


Posted on 7. Dec. 2014 - 03:05
Quote Originally Posted by mohandesView Post
to discharge material from bafilter's hopper always use option 2 so rotary valve as airlock.

This is only when there is not enough room for a standpipe of sufficient length. Catalyst regenerators in oil cracking are high temperature vessels where mechanical devices would last minutes. Such vessels rely on standpipes and so if standpipes are good enough for those applications they should be good enough for comparatively warm dust.

I just have a mission to eliminate as much moving machinery as is necessary: not because of the machines but because of the trouble they subsequently impose. Manufacturers of suction ship unloaders, 1MW variety, sometimes arrange rotary valves in a shuttling frame to minimise the inevitable shutdown time when these valves regularly fail. Such valves will, of course, only show their true colours when they are asked to do the job as described on the box. When unloading abrasive powders the fitting of a repaired unit in the vacant side of the shuttle frame becomes a matter of great urgency. Those clever folk between Germany and Italy who also make watches use standpipes instead.

John Gateley

Hopper Discharge Control

Posted on 7. Dec. 2014 - 07:59

These discussions highlight the need to understand the duty requirements fully to design a hopper and discharge system. There are many aspects to take into consideration for an optimum design and a systematic procedure should be followed. The designer should be aware of the advantages and limitations of the various methods of controlling the discharge, differing hopper geometry and the demands of the properties of the material to be handled. Examining the reasons why a rotary valve or activator should be fitted may throw some light on the enquirers dilemma but advice from others is handicapped by the limited information available.

Gilding The Lilly

Posted on 30. Dec. 2014 - 07:14

Firstly, please define 'relatively free flowing' and then consider why you need a bin activator.

That aside, why would you need a rotary valve at all? Both options include screw conveyors so why increase the cost and unreliability by adding a rotary valve?

There is no valid reason for adding cost, complexity and risk: except where there is plenty of cash and not enough knowledge: dangerous for some but most profitable for others.

When mixing thickener reagents there is no need for exact metering, as both options agree on this. If the screw ever gets flooded stop the bin activator and if it gets starved just restart it.

John Gateley