How to install a gravity feed metal detection system for powder and bulk product
July 3rd, 2009 | 
Author: The installation and testing of a gravity feed metal detection system used in many powder and bulk applications is critical to the assurance of overall system performance. The following guidelines will help ensure you have continued performance for running products such as spices, grain, sugar, flour, plastic pellets or other granular materials.
Design Considerations
Free fall of product.
Three key factors are used to determine whether a system will react in time to reject the metal contamination.
1 ) Distance between search head and reject valve
Maximizie the distance between the search head and reject system within the overall system height
2 ) Response time of electronics and air operated components
Make sure the metal detector manufacturer is using the fastest acting electronics, relay, and solenoids.
3 ) Free fall of product
Reduce the freefall of the product as it enters the system to reduce its initial velocity. If this is not possible use bafuls to redirect and slow the product down. Failing that you must increase the distance between the search head and reject device.
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Many metal detectors have a setting which allows for immediate response from the control system. This response should be around the 50 milli sec mark. The distance between the detector and the reject needs to be close with no restriction because the product must fall exactly the same way every time. Your possible contaminent could snag on a baffle and be let through later on. Use a trouser leg reject, this is an inverted Y connection in the ducting. Always reject more product than you need to.
Moving metal cannot be instaled within 1.5 to 2 times the aperture height so dont fit it too close.
Gravity is calculated at 9.81 m/sec but which travels faster….
loose feathers or dried peas?
Most new solenoids and relays will act fast enough to work efficiently just dont try to do too much..
Regards
Brian
Brian,
Agreed that all metal detectors have a ‘zero’ setting that is almost instantaneous in response time. Setting this is a must for gravity feed systems. Typically what happens is the system reacts to fast and the metal contaminate has not reached the reject device. The simple solution is to increase the ‘on’ duration for the reject to make sure you capture the tramp metal.
Reject devices.
One major problem with a pant leg reject device is that the reject stream and normal stream touch the same surface. Many products tend to stick to the inner walls of the reject device. This allows for a good possibility of tramp metal lodging in to cracks etc. and not being rejected.
I would always recommend that any reject device that touches raw product have 2 separate surfaces. One for normal flow the other for contaminate flow.