What are these valves?
Answer: Pressure relief valves
What do they do?
Answer: They limit the pressure in a system to their setting
What is the difference between the two symbols?
Answer: The symbol on the left is a pilot operated pressure relief and on the right is a direct operated pressure relief. The symbol on the right has one internal component plus a spring. The one on the left has a main valve then a smaller valve, and pressure galleries to direct pressure to the internal valves.
Why have two different types of valves?
Answer: One is better for maintaining a constant pressure, the other is fast acting and not suitable for constant flow. A direct acting relief valve is faster acting.
There are more questions to be answered such as does flow affect pressure and what do these valves look like internally. Also how do you set the pressure.
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