When seated, the disc of a safety valve has an area of 0.75 square inches (1.9 sq cm). When the valve lifts the area is increased by 10%. If the valve lifts at 300 psig (2170 kPa), at approximately what pressure will the valve reseat?
• Force = Pressure × Area relationship for the disc when seated vs. lifted • How a 10% increase in area affects the pressure needed to get the same force • Difference between set pressure (lift) and blowdown (reseat) on a safety valve
• First, calculate the force on the valve disc at the lift pressure with the original area. What numerical value do you get? • When the area increases by 10%, what new pressure would produce that SAME force on the disc? Set up the equation before plugging in numbers. • Compare your calculated reseating pressure to the answer choices. Which one is closest to the pressure you computed?
• Compute the original disc area and the increased area correctly (0.75 in² and 10% larger). • Use consistent units for pressure and area (stick with psi and in²). • Double-check that you are holding force constant between the lift and reseat conditions: (F_{lift} = F_{reseat}).
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