Issue link: https://www.e-digitaleditions.com/i/1035980
Tablets & Capsules October 2018 41 until the leakage has ceased. In one unusual case where the refill device had been positioned some distance from the hopper because of limited headroom, an analysis found the post-refill weight disturbance to be caused by the protracted trailing off of flow resulting from the tran- sit length. To fix such a condition, check your refill device for proper operation and confirm positive shut-off. Venting of the feeder's hopper is another potential cause of post-refill weight disturbance. Proper venting permits the air displaced from the hopper by incoming material to escape and facilitates material de-aeration and settling. Venting can be an internal feeder issue or an external process issue, depending on whether the venting is passive or active. With passive venting, the displaced air exits the hopper of its own accord, impeded only by the size of the aperture and the resistance that any sock or filter presents. An improperly sized vent or a clogged filter can delay com- plete venting, temporarily pressurizing the hopper and inducing stress on flexible connections. At worst, this can pressurize the feeder's hopper enough to force material out through the discharge. These conditions can produce a perceived weight disturbance, a feedrate error, and/or an abnormal motor-speed trendline. The fix is simply to clean or replace the filter and if needed, increase the vent size. Alternatively, active venting and dust collection uses a vacuum to encourage the air to exit from the hopper, which involves pressure forces that can compromise weighing. If active venting is too aggressive, the low pres- sure in the feeder's hopper can induce stresses on flexible connections that directly register as weight disturbances and provide a path for the transmission of vibrations from the process environment to the feeder. In such a case, check the inlet, vent, and discharge connections for full flexibility during active venting and correct as necessary. New electronic, pressure-compensation devices are available to detect pressure fluctuations and correct for their effects. This control addition uses pressure transmit- ters on the feeder's hopper and discharge to detect pres- sure fluctuations and then filters them out from the weight signal to avoid a change in the mass flow. Scenario 5: Constant disturbances. The final scenario depicts weight measurement swamped with constant con- tamination, as shown in Figure 7. This most visually intimidating category of weight disturbance is also the most performance damaging. After you have eliminated possible internal causes—electronic noise, static, or bind- ing of scale flexures—you can see clearly that the process must have found some direct route, some point of least resistance, to manifest its contaminating influence on the feeder's weighing environment. Constant disturbances are commonly caused by poor mounting practices. For example, the company may have installed the feeder without adequately considering the transmission of shock or vibration through the feeder's base or other supports, or the installation may have resulted in stiff or stressed flexible inlet, venting, or dis- charge connections or electrical wiring and cabling. Material factors The material being fed is the only part of the external process invited to cross the LIW feeder's defensive line. Unfortunately, the flow properties of most pharmaceuti- cal excipients and active ingredients are not ideal. Familiar difficulties include bridging, arching, and other problems related to the material's flow through the feeder, such as caking, clumping, or buildup on the feed screw or tube or on the agitator, if used. You can best anticipate, address, and resolve these problems during feeder selection and testing, or at worst, in the pre-operation shakedown. However, actual pro- cess conditions can and do change, and the character of the material can vary as well. Such changes can cause old Figure 7 Constant disturbances Constant Scenario 5