Control valves used in pneumatic and hydraulic fluid power systems often take the form of a spool mechanism, and the fluid power schematic symbols for these spool valves are quite unlike that of valve symbols in P&IDs and loop diagrams:
![Solenoid Valve Spool Valve Symbol](https://instrumentationtools.com/wp-content/uploads/2022/02/Solenoid-Valve-Spool-Valve-Symbol.png)
Explain what these spool valve symbols represent, and how they are to be interpreted. Also, comment on the construction of a spool valve.
Answer:
The symbols show each valve’s “normal” (unactuated) position. To understand what happens when the valve is actuated, you must visualize the boxes sliding over into alignment with the inlet/outlet pipes, like this:
![Valve Symbol in Normal Position](https://instrumentationtools.com/wp-content/uploads/2022/02/Valve-Symbol-in-Normal-Position.png)
The same holds for the reversing valve, except that it has three positions:
![Explain What these Spool Valve Represent](https://instrumentationtools.com/wp-content/uploads/2022/02/Explain-What-these-Spool-Valve-Represent.png)
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Credits: Tony R. Kuphaldt