Many instrument manufactures claim they have just such an instrument. Unfortunately it has been my experience this is simply not the case. Often these claims will promote instruments that will report only a symptom of a plug condition.
There are two general categories of sensors often touted as plug chute detectors; field disturbance and point level.
Field disturbance instruments detect motion – material flowing is motion and no material flowing is no motion. Sensor 'A' depicts microwave solids flow field disturbance detector. Unfortunately field disturbance sensors cannot differentiate between a plugged no-flow condition and an empty no-flow condition. A logic function must be employed to compare the material flow state against the operating state of the process. If no-flow is detected and the process is operating with sufficient material feed/supply then a problem can be concluded. If no-flow is detected and the process is shut down or material feed/supply is exhausted it can be concluded there is no problem. Not exactly straight-forward.
Point level sensors detect the absence or presence of a material at the exact point at which they are installed. Unfortunately, these devices may not reliably differentiate between a saturated dynamic condition (e.g. flow stream) and a highly saturated static condition (e.g. line plugged). And don’t even mention the physical punishment imparted on a sensing element!
So how does one detect a flow blockage in a timely fashion?
Like many other situations it comes down to choosing the best of the options available. My experiences have proven that clever installation of a point level sensor can prove the most fool-proof method.
The big secret: install the point level sensor in an area that the material will occupy only in a plug condition. It is often necessary to make some modification to the process so as to isolate the point level sensor from the material flow stream. Sensors 'B' depict rotary paddle type and RF capacitance type point level sensors in just such an application. With this method it is very important to ensure the point level sensor is in an area that will clear of material when the plug condition passes.
With all the details presented up front it will be much less stressful for an engineer to properly instrument a process to achieve the desired results.
Andy Bowman
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