A engineer needs to write a program for an Allen-Bradley CompactLogix PLC to control a pressure relieving solenoid valve in a gas processing system.
A pair of high-pressure control switches signals the PLC when to open the solenoid valve: one telling the PLC to open the valve after a 3-second time delay and the other (called the “high-high” switch, with a higher trip setting) telling the PLC to open the valve immediately.
A pushbutton switch serves as a manual override to open the solenoid valve immediately when pressed.
In all cases, the solenoid vent valve will remain open (energized) until pressure falls below the setting of a low-pressure gas switch.
The High pressure contact instruction should be drawn as normally-closed rather than normally-open as shown in the engineer’s first draft of the PLC program.
This particular switch signals a high-pressure condition by opening its contacts, since the real-world switch is normally-closed (NC).
We want the 3-second timer to begin timing when this switch senses a high pressure, and so we need the contact instruction to color when the real-world high-pressure switch trips (opens).
A PLC contact instruction that colors when it senses a “0” bit condition is a normally-closed (NC) instruction.
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