Which device prevents excessive vacuum in a pressure vessel?

• A. Check Valve
• B. Vacuum Relief Valve
• C. Air Release Valve
• D. Pressure Relief Valve

Answer: (B)

When a vessel risks pulling a vacuum, negative pressure inside can cause structural collapse or damage as the outside atmospheric pressure is greater than the inside. A vacuum relief valve is designed to prevent that by opening when interior pressure drops to a set negative level, allowing air to enter the vessel and equalize the pressure. This keeps the system from developing excessive vacuum and protects both the vessel and connected piping. It’s specifically for vacuum protection, unlike a pressure relief valve (overpressure protection), a check valve (backflow control), or an air release valve (vents trapped air but doesn’t guard against vacuum buildup).