Electronic Pressure Compensation for Feeder Hopper and/or Discharge
The Electronic Pressure Compensation (EPC) can be used to automatically detect changes in pressure within a feeder and adjust the weight signal accordingly. When it comes to handling air pressure issues in loss-in-weight feeders, especially for pressure issues in the feeder’s material hopper, EPC offers distinct advantages over traditional mechanical pressure compensation systems such as:
• simple, cost-effective design
With EPC the K-Tron Control Module (KCM) uses the data from a pressure sensor mounted in the hopper lid and/or discharge tube to adjust for changes to the weight signal caused by variations in pressure. Very small pressure changes inside the feeder material hopper cause weighing errors because the feeder hopper and discharge tube are not completely isolated vessels.
With the EPC compensation system, the KCM uses a sensor to measure the pressure and dynamically compensates the weight to eliminate the weighing error, using the entered value for the hopper lid or discharge opening(s). The EPC measures Gauge Pressure (relative to atmosphere). It will measure and correct for pressure fluctuations inside of a hopper in the range of + 50 mBar. If the pressure is outside of this range an alarm will be generated. EPC can be installed on the feeder hopper or the discharge or both. It is also possible to use EPC for the hopper while utilizing traditional mechanical pressure compensation on the discharge. However, EPC and mechanical pressure compensation cannot be used simultaneously on the same discharge.
Why Pressure Compensation?
Pressure compensation is necessary for gravimetric feeders which are part of a closed system, where changes in pressure within the feeder hopper or discharge tube can cause an incorrect weight signal. In a sealed pressure vessel the pressure forces balance perfectly in all directions and the measured weight is unaffected. However, the feeder hopper has an inlet in the lid for refills. Any positive air pressure in the hopper acts across the refill inlet area and pushes up on the fixed refill valve above. The flexible seal created by the bellows allows an equal force to push down on the hopper and results in an increase in the weight signal. The loss-in-weight controller would normally react to the added weight by erroneously increasing the feeder output creating a mass flow error.
Traditional mechanical pressure compensation on a hopper is quite complex and must be installed correctly – any deviations will adversely affect the result. In comparison, EPC is easy to install, self-optimizing and significantly less expensive. Pressure changes at the feeder discharge, when sealed and connected to the downstream process with a bellows, also create a weight error which can be corrected with a separate pressure sensor connected to the same EPC junction box.
For wiring details, technical data and pressure sensor options, download the EPC Specifications sheet here.