The schematic symbol for a direct acting pressure relief valve has two ports, port one and port two.
These ports are often labelled P and T for ‘pressure’ and ‘tank’ respectively (see diagram of the schematic symbol).
- Pressure: port 1
- Tank: port 2
The reason this schematic example has port 1 and port 2 is because that's often how a cartridge valve is described.
However, ‘pressure’ and ‘tank’ are interchangeable terms.
In the schematic diagram there is a sense of pressure at port 1 (indicated by dot in schematic diagram), and then via the pilot line the pressure is brought up to the control envelope.
On the other side of the control envelope is a mechanical spring variable (zigzag lines in schematic diagram).
In this schematic symbol the arrow denotes variability.
Therefore, in this case, the spring tension is variable and adjustable.
The dotted line (top left-hand side of schematic diagram) from the spring does not indicate a pilot line in this example. It denotes a spring chamber being drained to the tank.
The drain isn’t commonly featured in much schematic symbology and catalogues (dotted line at top left-hand side of schematic diagram) even though it is usually found there and assists with comprehending the function of the valve.
Essentially, any pressure at port 2 is additive to the setting of the spring.
When the valve operates, it opens a path to the tank.
The path is however closed off in this schematic diagram because it is offset in relation to the centre line.
The valve moves to a new position and opens a path to tank when mechanical spring tension is overcome (see second schematic diagram).
The valve must continue to overcome the mechanical tension to keep the path open.
Consequently, the pressure is higher at port 1.
The valve is adjusted by having a pressure gauge at port 1. It is set to open a path to tank at a certain pressure.
Therefore, the second schematic, never visible on pages, catalogues, or PDF files, enables the reader to imagine how it operates.
The schematic shows a mechanical spring tension being imposed by a hydraulic force, and whichever one wins results in the valve either pressing against the seat and holding it closed or the hydraulics overcoming the spring tension and opening it.
Therefore, it’s a mechanical device (spring) that becomes a pressure regulating device in a hydraulic system.
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