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Miscellaneous old school stuff
Let's say you work in a factory that manufactures a hot liquid that needs to be cooled down during an operation. If the product is allowed to continue flowing at a hot temperature with no cooling, a meltdown will occur in the machinery.
This problem is solved with the use of a heat exchanger. A cooler fluid flows in a pipe around the pipe containing the hotter liquid, and this cools it off. A valve controls the amount of coolant that passes through. If this is a traditional valve, when the valve is closed, no coolant can pass through, and the farther open it's pushed using power, the more coolant passes through.
Sounds simple enough. But what happens when the plant loses power? No power is available to push the valve open, so the hot liquid runs through the heat exchanger without any coolant to drop its temperature. This causes a meltdown.
Instead, a reverse-acting valve is used to control coolant flow. The default, closed, is used for complete coolant flow. Closing the valve causes less coolant to flow through. This way, in the event of a power outage, the valve will allow the full amount of coolant to flow through, rather than no coolant flow as in the previous scenario. This is a safety control.