🔍 Key Concepts

• Blow down of a safety valve: the difference between the pressure at which the valve opens and the lower pressure at which it reseats • How a pop-type safety valve uses steam flow and area changes (huddling chamber, enlarged area) to change the opening force once the valve has lifted • The difference between a valve being held open by an external control (pilot line) versus opening behavior caused by its own internal geometry

💭 Think About

• First, recall in your own words what "blow down" means for a boiler safety valve. Is it about how it opens, how far it opens, or the pressure at which it recloses? • Ask which option describes a change in forces on the disc after lifting that would allow the valve to stay open until pressure falls to a lower reseating value. • Consider which statements describe a control method (external pilot or changing set pressure) versus a built‑in mechanical design feature that produces a distinct opening and reseating pressure.

✅ Before You Answer

• Verify which choice clearly involves steam acting on an increased effective area after lift, enhancing opening force and thus affecting reseating pressure. • Check that the option you pick actually produces a different closing pressure from the original opening pressure (that’s what provides blow down). • Eliminate any choice that only talks about the valve opening "gradually" or being held open externally, without explaining a built‑in blow down mechanism in the valve design.