The Concept of Intrinsic Safety & Purge Systems on Automation

Flexible automation offers a fast and accurate means of performing repetitious tasks in the manufacturing sector, often under extreme conditions. In some extreme manufacturing environments, such as those experienced in the atmosphere of a paint spray booth, purpose built robots are required. These robots are designed to be explosion proof, with built in purge systems for safe operation in a combustible environment.

In order for a robot to operate safely in a combustible environment, a purged air cavity is required to negate the effects of a possible spark from the robots’ servomotors. The purge circuit, which is crucial to the successful operation of the robot, is not that simple. Before power can be applied to the servomotors of a robot designed for use in a paint environment, the controller activates a purge cycle. This consists of flooding an enclosed, sealed cavity (there is a check valve allowing air evacuation from the cavity) with 55 psi (dynamic flow) of clean, dry, compressed air. There is a flow switch mounted within the cavity that makes contact when this level of airflow has been detected. The controller then requires the contact to be held for 5 minutes, thus ensuring that any organic/combustible vapors have been evacuated from the cavity, creating an artificial atmosphere of fresh air, around each of the servomotors.

Following the completion of the purge cycle, the system switches to maintenance mode (maintenance of purge). This requires a low-pressure switch, which monitors ambient air, external to the robot, versus internal air. A differential pressure of 75 to 125 mm H2O or greater is required to maintain positive pressure, thus ensuring that impure air is kept out of the cavity. In the event that the maintenance purge is lost, all power is removed from the servomotors, thus avoiding the potential for explosion.

The timing of the purge cycle (in conjunction with the square area of the cavity), as well as the design of the maintenance air requirement is dictated by the safety organizations that provide their stamp of approval for classifying a robot as safe to use in a hazardous environment. In the U.S.A., this agency is Factory Mutual, or FM, in Europe, ATEX and in Japan, TIIS. Any company that plans to install an electric robot into a National Fire Protection Association (NFPA) Class 1, Division 1 (combustible) environment should be very strict in requiring these approvals on the products, or risk the liability of any associated damages from an accident.

In addition to the robot purge circuit, many additional safeguards are built into a robotic system to guarantee that make-up and exhaust air are present, that fire/spark detection equipment is active, fire suppression systems are engaged and interlocks are provided to remove the sources of combustion (paint supply, air supply and electricity), in the event that a fire is detected.