4.5.5.2 – PhotoThe need to accurately and continuously weigh in a process environment presents specific challenges to a weigh meter / rotary valve feeder.

The need to accurately and continuously weigh in a process environment presents specific challenges to a weigh meter / rotary valve feeder. The particular types of demands result from the feeder’s operating principle These weighing challenges are introduced below for weigh meter / rotary valve feeders.

Range
As a dual-element feeder in which the weigh meter’s flow rate measurement is fed back to control rotary valve (or delumper) speed, the range of the meter’s weighing system must be sufficient to support the maximum planned resin throughput rate, which in many of today’s large capacity operations can be several hundreds of thousands of lb/hr. The operating range of the meter’s weigh system must also be sufficient to accommodate the dynamic force applied by the falling material.

Resolution
To precisely resolve and trace the quickly changing dynamic forces applied to the meter’s inclined and vertical surfaces, high measurement resolution is a necessity.

Repeatability
Although weigh-system repeatability is an important performance dimension for other principles of gravimetric feeding, its significance is diminished in weigh meter feeding due to measurement impracticabilities.

Linearity
While continuous, reasonably steady-state operation characterizes the desirable norm in polyolefin production, the common use of an upstream surge hopper underscores the possibility of variation in supply rate and/or the necessity to modify extruder rate. Under these circumstances, the linearity performance of the meter’s weigh system increases in importance.

Tare
Unlike loss-in-weight feeding where only weight differentials count, the weigh meter’s operating principle involves absolute measurements, increasing the importance of accurate taring. In polyolefin production, two main factors can contribute to tare shift: the adherence and accumulation of process material to the inclined and/or vertical weighing surfaces, and the effects of material-induced ambient temperature variation within the measurement region. Thus, periodic re-taring based on experience is recommended.

Dynamic Response
As a direct consequence of its operating principle, a highly responsive, deflection-free weigh system is required for precise and accurate measurement of throughput rate.

Shock and Vibration
Because weigh meter feeding relies on the measurement of the dynamic inertial forces applied by the falling material stream, the need to discriminate between those forces and the contaminating forces associated with the transmission of ambient (external) shock and vibration underscore the need for effective signal filtering as well as proper installation practices.

Temperature
At some times and under some conditions the elevated temperature of freshly precipitated resin may approach the upper limit of the weigh system’s design operating temperature envelope. Under such circumstances more frequent taring may be required. This is a consideration in weigh meter feeding due to the proximity of the weight sensor to the material. If material temperature is known to or is anticipated to exceed the upper operating limit of the meter’s weigh system, an alternative feeding solution is recommended.