PRODUCTION • PROCESSING • HANDLING
connected through, all the control pilots (high and low) are activated and the gas pressure is adequate. If a pilot trips, the gas pressure in the control
line is released and the two rapid venting devices are switched over with the aid of the gas pressure present in the cylinder. Te gas pressure in the cylinder falls and the SSU is closed by spring pressure.
Te SSU closes if the pressure in the
measurement line increases to a level above the maximum set value of one or both of the high pilots or if the pressure in the work cylinder of the actuator falls to a value below the minimum set value of the low pilot. All the control pilots have a mechanical restart interlock that prevents the SSU opening again in an uncontrolled manner as soon as the pressure as measured has reached the set values of the pilots again. Te restart interlock must be unlocked manually on site. Te upstream dryer-filter unit treats the control gas, which is removed directly from the gas line. A pressure regulator limits the system pressure
and therefore the actuator’s torque. Te system pressure upstream and downstream of the unit is displayed on two pressure gauges. Two safety valves arranged in parallel are integrated in the control to safeguard the system against impermissibly high excess pressure.
Te test device permits the high pilots and their settings to be checked regularly. Te check valve in it ensures that the maximum permissible line pressure is confined to the high pilots. Te test valve and the pressure line to the high pilots can be depressurised with the spring-return manual valves.
USER-FRIENDLY MAINTENANCE For maintenance work the manual valve is closed and the gas pressure in the control is released by manually actuating the low pilot. When the pressure has fallen below the minimum set value of the low pilot, the control valve of the low pilot switches and the SSU is closed by spring pressure. When the maintenance work has been completed, the manual valve must be brought back into its open position. Te SSU moves to the position ‘On’ again as soon as the regulated system pressure has reached at least the low pilot set value. To check the system the SSU can be supplied with nitrogen via an N2 connection. l
Eike Kobbe is with Böhmer.
www.boehmer.de
www.engineerlive.com 31
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