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Drop through metal detector / rejector

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Posted 23 May 2013 - 06:06 PM

Hi everyone,
I'm involved in a project to install a drop-through metal detector c/w contaminant rejector diverter in our plant and I am wondering if anyone can share with me what equipment they have seen in use to feed such a detector.  We're looking at a Thermo Scientific APEX model.  All of Thermo's literature indicates that the product must be dropped from zero velocity through the detector from a point no more than about 19" above the opening of the detector.  The purpose of this is so that all material is moving through the detector at a speed that is well known and accurately repeatable.  This allows the reject diverter to be opened just in time to catch the contaminant but no longer than necessary to minimize the amount of good product that is rejected with the contaminant.  This "zero velocity drop" is a concept that appears to be common among all drop-through metal detector manufacturers.


The problem that I'm facing is that a "zero velocity drop" seems to be something that works in theory but no one, including the manufacturer, can offer concrete advice on how to attain this.  Certain other pieces of equipment (rotary valves, scalperators, screeners, etc.) drop the product nicely but none meet the letter of the law concerning the manufacturer's recommendation.  Thermo has provided no assistance in this aspect of the design; they seem to be afraid of recommending anything other than this theoretical ideal.


The first problem is that the drop height is often much larger than the recommended 19".  The concerns are that the product may be falling at a speed great enough that the diverter cannot open in time to catch the contaminant and that the dynamic impact of a faster falling product may damage the diverter or apply so much force to it that it's unable to close at all.


The second problem is that it's virtually impossible to match up the detector's inlet dimensions with a piece of equipment that has an outlet exactly the same size.  This may be possible with a detector with a circular opening but we're looking at one with a long, narrow opening for better sensitivity.  This requires some sort of funneling transition piece that results in some product sliding down the side of the funnel and therefore passing through the detector at a slower speed than the product falling through the centre of the detector.


I realize that we're going to have to compromise on our design but I'd like to draw on the experience of others who have similar systems installed in their plants.  How have you fed your drop-through metal detector and how much leeway is there in varying from the manufacturer's recommendation?





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Posted 28 May 2013 - 08:40 AM

Hi Ganning,


Have used 'drop through' systems on powders. Isn't it a case of design of installation and a valve would suffice within 19" of the detector?






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Posted 28 May 2013 - 05:39 PM

With drop throughs in our facility, we are running them from various lengths of poly tubing feeding them from screw conveyors and open dump hoppers.  My company has done it this way for years and I'm honestly not sure if it meets the manufacturer's requirements or not (although it makes sense).  What I can share is that no auditor thus far has questioned our setup, and has not asked us if we are following the manufacturer's recommendations.


It may be that our policy protects us sufficiently that auditors don't question installation.  We run MD checks every two hours, and require a total of (9) successful rejections in order to pass the test (3 fe, 3 non-fe, 3 stainless).  Any failure requires immediate shutdown of equipment, maintenance and management contact, repair, then rerunning of all material ran after previous successful test.  Our metal detector has a great history of being able to reject material dropped down through the long connection sock and successfully reject the test pieces at velocity.

Edited by john123, 28 May 2013 - 05:42 PM.

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