The graph above shows the opening characteristics of a valve set at typical values using R-22 to drive the pressure element. The characteristics of the valve can be designated by temperature or pressure.
Using the graph above, the pressure at which the valve begins to open or the "crack" pressure is 180 PSI (94°F). At pressures below the crack pressure the valve remains closed.
The cracking pressure is adjustable typically from 70-210 PSI by changing the force of the spring with an adjustment screw. Adjusting the force shifts the entire curve to the right (increasing spring tension) or to the left (decreasing spring tension).
For this example, the full open position of the valve, in Figure 5, is nominally 220 PSI (108°F). Full open represents the position of the valve at "full rated capacity" or rated C. The valve can usually open more than "full open" but this is not desirable as the span becomes unacceptably large.
Shown above, the increased pressure required to bring the valve to full open or "span" is 40 PSI. This value is calculated by subtracting the cracking pressure from the full open pressure. A typical span for a pilot operated valve is 25 PSI, for a direct acting valve it is 40 PSI.
When the actuating pressure is between 180 PSI and 220 PSI (108°F) the valve is open in proportion to pressure, to modulate the flow of water required for a given condition.
The span allows for operation of the system within a desirable range as system conditions vary. The refrigerant head pressure will vary as conditions change, but only within a narrow range in a manner desirable from a control and energy efficiency standpoint.