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Milling & Grinding

How to Design an Explosion-Safe Milling System

| Editor: Dominik Stephan

Frewitt hammer mills were able to demonstrate conformity in all assessment procedures. This indicates that Frewitt provides a safe system with its inerted milling system.
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Frewitt hammer mills were able to demonstrate conformity in all assessment procedures. This indicates that Frewitt provides a safe system with its inerted milling system. (Picture: Frewitt)

The basics of explosion protection apply not only to refineries and paint manufacturers, mill operators are also concerned. Hammer mills users find themselves on the front line of explosion prevention, as the powders must be very finely milled with high RPM's.

Frewitt hammer mills, suitable for fine milling down to 30 µm, are, nevertheless, up to the task: As one manufacturer of drugs needed to replace his milling system for additional efficiency, he required a hammer mill which could meet all guidelines and could produce in a clean room with high throughput. The customer needed to mill a heat-sensitive product to a particle size of 100 µm, while ensuring high throughput.

Frewitt’s Hammer Witt offered the optimal solution for this application, as both the particle size distribution and throughput requirements could be achieved. In order to prevent the formation of an ignitable mixture during conveying, dosing, milling and filling, the process environment room had to be inerted.

Monitoring Oxygen Levels

In addition to other safety precautions, the monitoring of the highest allowable oxygen level is paramount. In this case, it was 3 %, and this level was programmed as an alarm threshold that would immediately halt the process to prevent a potential explosion. Only if the oxygen concentration returned to non-critical values could the milling resume.

A level sensor is placed in the feed funnel of the vacuum conveyor, monitoring product feed and regulates it in order to ensure a continuous process. In order to keep the milling chamber temperature at a constant, cool temperature the milling housing was equipped with a water-cooled system.

The temperature of the milling chamber, as well as the drive bearings, is constantly monitored. All these measures help to maintain the milling chamber at ATEX II G/D (Zone 0/20) and the area of the milling head at ATEX II 2G/D (Zone 1/21). The Profi-Clean 250 system, equipped with an H13 filter, prevents even the finest dust or powder leakage. The filter is continuously cleaned by an efficient, blow-back system.

Easy Handling and Bag Changing

Bag-changings is done with special attention. When the bag reaches the prescribed weight; a valve stops the product flow. The remaining nitrogen is aspirated from the bag. Thereafter, the ProFi-Bant bag holder is disengaged, the filled bag removed, and a replacement bag is installed. The system remains inerted during the bag exchange.

This unique inerting design is not only technically complete (fulfilling GMP standards, ATEX guidelines and all rules of the country-specific Institution of Quality), it also has considerable economic advantages. Continuous monitoring of oxygen contents reduces the use of nitrogen, a very expensive raw material, to a minimum.

ACHEMA: Hall 3.1/Stand F3

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