Control & Instrumentation
require to control say 20 Te/hour of product and the refill rate should be 200 Te/hour, this becomes on most installations an impossibility. Even direct discharge from a silo may not keep up with this rate.
The alternative is to run a continuous flowmeter device whether screw or belt with a volumetric pre-feeder usually a screw. The two are run in closed loop control to maintain the set rate with the added advantage that the pre-feeder and hopper, because it is volumetric, does not require the same high rate of refill as the gravimetric control.
The pre feeder hopper is fitted with high and low level probes to control refills in the conventional sense.
Whilst this solution is invariably more expensive due to the number of machines employed, it provides a stable and sure method for large feed rates.
Batching Loss-in-Weight
Solution – Fit close tolerance discharge tubes and/or increase the number of pitches of the screw by fitting double start blading or half pitch.
• Cannot gain sufficient height to feed into the client’s process from the infeed position. Solution – Incline the screw. Be careful that combined with this is invariably an increase in feed centres so the motor power rating must be increased to compensate. With volumetric feeders just provide a frame to support the feeder, but in the case of gravimetric feeding ensure the feeder is suspended from a horizontal weigh frame to improve the stability and accuracy of weighing.
6. Loss-in-Weight or Gravimetric Feeding First thing to remember is that if you cannot handle any powder with reasonable accuracy volumetrically putting a weighed system on it will not solve the problem and could make matters worse.
Continuous Loss-in-Weight
Refill rates are critical and must ideally be 10 times the output rate of the feeder. When a feeder is re-filled it goes into a volumetric mode on the last known signal when in loss-in- weight. Until the weigh platform senses the high level of material in the hopper it will stay in volumetric mode. After the refill it will, within 3 seconds, re-learn the loss-in- weight signal and react accordingly.
Obviously the smaller the refill rate with the screw still discharging product, the longer the volumetric stage on refill and the benefits of loss-in-weight are reduced.
Any size of feeder up to 150 litre hopper can be mounted to a weigh platform. Larger hoppers will require to be suspended from a weigh frame to provide stability of the system. As with all weighing systems the main problems are caused by site vibration particularly if it is of a low frequency high amplitude characteristics such as locally sited ball mills or en masse vibrating tray conveyors with heavy reaction bases. The fitting of anti-vibration mounts can help under the feeder/weigh frame in limiting the effect.
On particular large rate loss-in-weight feeders, where you
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Of course, if you just want to control levels to 40m (and options of
Pulsar Process Measurement Ltd.,
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info@pulsar-pm.com www.pulsar-pm.com
August 2010 • Solids & Bulk Handling 39
Uses the same controller as continuous to measure the loss of material from the feeder but is set to switch off when a pre-determined target weight is reached on a fast/trickle speed setup.
Whilst it is ideal to batch in one operation, if it is impossible to get a large enough hopper in place, multiple runs can be made to make up the batch but refill rates are not so critical as a continuous loss-in-weight system due to the cyclic nature of batching operations.
For more information contact Rospen on tel: 01453 825 212 or visit:
www.rospen.com
MORE OPTIONS IN LEVEL CONTROL
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