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Mixing Safely Safety Interlock System - Food Processing Plant Mixing Tanks


Mixing Safely
Welcome to The Journal readers of our Mixing Safely article in the Feb. 2009 issue.
You can download a copy of the article by clicking the logo below (2.6 MB PDF):

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Safety Interlock System - Food Processing Plant Mixing Tanks

Our customer received a safety violation
from WorkSafe BC (WCB)

  • Operators were exposed to rotating parts when they manually added ingredients to batches of product in several mixing tanks.
  • CIP (Clean In Place) procedures include a manual step that requires the operator to reach into the tank from outside, exposing the operator to the agitator with a risk of unintended or unauthorized startup.
  • There was no interlocking system for the lids of the tanks to prevent agitator motion.
  • There was no guarding of the opening under the lid to allow addition of ingredients while differentiating for limited body access for the CIP step.
  • Full LOTO (Lock Out Tag Out) was not being practiced for either operation in the absence of a qualified safety system.

Typical Mixer

Agitator View

(click for a larger view)

Industrial mixer tank
Industrial mixer sharp edged scraper

Identified Hazards
Moving Parts

  • Agitator
  • Limited body access required to clean inside


  • None on the lid


  • None over the opening

Hazardous Gas

  • Flammable


  • None or Cat 1 only
  • Type 1 enclosure

Solution and Equipment Installed

The top of the mixer tank was modified to permit a hinged grate to be added under the existing lid. The lid can be opened at any time, so that the operator can manually add ingredients by pouring them through the grate while the agitator is running.

The grate protects the operator from accidentally coming into contact with rotating parts. The grate is locked closed by a dual trapped key chain interlock. Access into the mixer tank for CIP is permitted only after the operator makes the system safe.

A safety relay de-energizes the safety contactors stopping the motor. After a preset time delay to allow the agitator to run down, the safety relay closes its time delay contacts energizing the coil of a solenoid release trapped key, permitting removal of the A key. The operator removes the A key and takes it to the dual trapped key chain lock at the mixer tank. He inserts and rotates the A key, which untraps the B key.

Safety Grate installed under lid

Rotation and removal of the B key traps the A key preventing its return to re-energize the motor. It also releases the chain lock permitting the grating to be opened for access into the mixer tank. The operator puts the B key into his pocket while performing the CIP.

When CIP is complete the operator removes the B key from his pocket and reverses the above key exchange to return power to the system for normal operation.

Modified Mixer
(click for a larger view)

Retro Fitted

  • Safety Relay with run down time delay
  • Dual Channel Type 7 E-Stop button
  • Solenoid Release Trapped Key
  • Dual Trapped Key Chain Lock
  • Dual Safety Motor Contactors
  • Stainless grate under tank lid

Trapped Key System:
(click for a larger view)

Dual trapped key chain interlock

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