The disc of a relief valve has an area of 0.85 square inches when seated, lifts at a set point pressure of 250 psi. When the valve lifts, the area of the disc exposed to pressure increases by 20%. At what pressure does the valve reseat?
ā¢ Force on the valve disc = Pressure Ć Area ā¢ The spring force that lifted the valve at the set pressure stays essentially the same when it reseats ā¢ When the valve lifts, the effective area increases by 20%, which changes the pressure needed to balance the same force
ā¢ First, compute the force on the disc at the set pressure using the original area; keep that number as the spring force you must balance on reseating ā¢ Then, find the new effective area when the valve is lifted, and figure out what pressure acting on this larger area would produce the same force ā¢ Compare that reseating pressure to the set pressure: should it be higher, lower, or the same once the area has increased?
ā¢ Be sure to convert the 20% increase in area correctly: new area = original area Ć 1.20 ā¢ Use the relationship Pressure = Force Ć· Area when solving for the reseating pressure ā¢ Check if your final pressure value is less than 250 psi, since a larger area needs less pressure to create the same force
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