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In HVAC control systems, actuators that control dampers and valves may be fail-safe, for example, to prevent coils from freezing or rooms from overheating. Older pneumatic actuators were inherently fail-safe because if the air pressure against the internal diaphragm failed, the built-in spring would push the actuator to its home position – of ...
The safety objectives associated with Catastrophic Failure Conditions may be satisfied by demonstrating that: No single failure will result in a Catastrophic Failure Condition; Each Catastrophic Failure Condition is extremely improbable; and; Significant latent failures are addressed in accordance with § 25.1309(b)(4) and § 25.1309(b)(5). [10]
For example, while fail-safe electronic doors unlock during power failures, fail-secure ones will lock, keeping an area secure. Fail-Passive systems continue to operate in the event of a system failure. An example includes an aircraft autopilot. In the event of a failure, the aircraft would remain in a controllable state and allow the pilot to ...
An FMEDA can predict failure rates per defined failure modes. For Functional Safety applications the IEC 61508 failure modes (safe, dangerous, annunciation, and no effect) are used. These failure rate numbers can be converted into the alternative failure modes from the automotive functional safety standard, ISO 26262.
HVAC (Heating, Ventilation and Air Conditioning) equipment needs a control system to regulate the operation of a heating and/or air conditioning system. [1] Usually a sensing device is used to compare the actual state (e.g. temperature) with a target state. Then the control system draws a conclusion what action has to be taken (e.g. start the ...
A fault tree diagram. Fault tree analysis (FTA) is a type of failure analysis in which an undesired state of a system is examined. This analysis method is mainly used in safety engineering and reliability engineering to understand how systems can fail, to identify the best ways to reduce risk and to determine (or get a feeling for) event rates of a safety accident or a particular system level ...
No control approach can work unless the actuators produce this necessary force. So the common solution is to err on the side of safety by over-actuation: much more control action than strictly necessary is built into the system. For critical systems, the normal approach involves straightforward replication of the actuators.
A separate actuator on the piping releases pressure in the line if it crosses a threshold. This releases the pressure on the back of the seal, causing the valve to open. The essential parts of a PORV are a pilot valve (or control pilot), a main valve , a pilot tube, the dome, a disc or piston, and a seat.