The disc of a relief valve has an area of 0.75 square inches (483.87 square mm) when seated, and lifts at a set point pressure of 250 psi (1723.50 kPa). When the valve lifts, the area of the disc exposed to pressure increases by 10%. At what pressure does the valve re-seat?
ā¢ Force = Pressure Ć Area relationship for the valve disc ā¢ How a change in effective disc area affects the pressure needed to create the same force ā¢ The idea that the spring force at lift and re-seat is essentially the same (ignoring hysteresis for this problem)
ā¢ First calculate the force on the disc at the lift (set) pressure using the original area. What is that force? ā¢ When the valve lifts and the effective area increases by 10%, what new area should you use in your calculation? ā¢ If the same force is now spread over a larger area, what happens to the pressure? How can you solve for the new (lower) pressure that will balance the same spring force?
ā¢ Be sure to convert the 10% area increase into a multiplication factor (not an addition of pressure). ā¢ Confirm you are keeping the force constant when comparing lift and re-seat conditions. ā¢ Double-check that your final pressure should be lower than the set pressure because the effective area increased.
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