Trip mechanisms testing

SPECIAL CAUTIONARY NOTE AU circuit breaker manufacturers recommend that their circuit breakers be exercised annuaUy via the trip test button. Exercising the circuit breaker actuates the trip mechanism. The trip mechanism of the circuit breaker is separate from the on-off handle mechanism, and annual operation of the trip mechanism ensures that the breaker wiU trip when called on to operate. This is also a test indication if the breaker faUs to trip, it has malfunctioned and should be replaced. 

Plant engineer responsible for mechanical maintenance such as testing of alarms and trips, as well as installation of new instruments. 

An instrument mechanic was asked to test the trip on A furnace. He put the controller on manual and then went behind the panel. His next step was to take the cover off the back of the controller, disconnect one of the leads, applj a gradually increasing potential from a potentiometer, and note the reading at which the trip would operate if it was on auto control. 

This is the more normal test situation. For processes where prooftesting the final trip or control element cannot be made available (such as testing a furnace shutdown system), the maintenance mechanics perform a prooftest as described above (complete loop test) with the exception of testing the final trip or control element. All components in the loop including the final element are visually inspected for physical condition. Maintenance takes test credit for the portion of the loop prooftested. 

Class 1 safety instrumentation loops include alarms and trips on storage tanks containing flammable or toxic liquids, devices to control high temperature and high pressure on exothermic-reaction vessels, and control mechanisms for low-flow, high-temperature fluids on fired heaters. Other Class 1 instruments include alarms that warn of flame failure on fired heaters, and vapor detectors for emergency valve isolation and sprinkler-system activation. All of these alarms, shutdown valves, and other critical instruments are regularly proof-tested to a well-defined schedule. 

Failures which can prevent rod motion by a CRD mechanism are of concern because they might preclude a reactor trip of the associated rod. To detect such failures, redundant load cells and position sensors are installed. The load cells detect rod binding or breaking, and the position sensors measure rod position. The position sensors can be used to test all aspects of rod motion such as rod speed and rod mobility. When either the load cells or position sensors indicate a malfunction, this information is conveyed to the... 

Finally, the effective operation of the test button incorporated within the RCD should be tested to prove the integrity of the mechanical elements in the tripping device. This test should be repeated every 3 months. 

Instrument alarms and trips should be tested by simulation of trip conditions as far as possible during system commissioning. It is recommended in the case of trips that the function and mechanism in each case be verified by testing the system in its entirety. For example, in the case of a high-pressure rectifier trip, the system pressure should be raised to the trip value and all actions through to the circuit breaker should be observed. 

Centrifugal pumps are subject to mechanical damage when they lose suction pressure or when the discharge flow is blocked. Furnace tubes may overheat and fail when liquid flow through the tubes is greatly reduced. Low-flow trips protect process equipment against these failures. This trip may be used to shut down a pump or block off fuel to a furnace. To test an orifice-type low-flow trip, proceed as follows ... 

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