Liquid thermal relief valves should be set at a pressure not higher than what percent?

Liquid thermal relief valves should be set at a pressure not higher than what percent?

• A. 100 %
• B. 110 %
• C. 120 %
• D. 130 %

Answer: (C)

When a liquid-filled vessel is heated, the liquid expands and pressure can rise. A liquid thermal relief valve is there to relieve that pressure before the vessel is overstressed, so the set pressure needs to be high enough to avoid nuisance openings during normal operation, but not so high that the vessel could reach or exceed its design limit before relief occurs. The standard practice is to set the thermal relief valve at a pressure not higher than 1.2 times the design (MAWP) of the vessel. This 120% margin gives a safe buffer: it allows normal thermal fluctuations to stay contained, while ensuring relief happens before the vessel reaches a dangerous overpressure. Setting it higher (like 130%) would risk allowing pressures beyond the vessel’s safe limit, while setting it lower (100% or 110%) would increase nuisance openings or unnecessary relief. So the best choice is 120%.

When a liquid-filled vessel is heated, the liquid expands and pressure can rise. A liquid thermal relief valve is there to relieve that pressure before the vessel is overstressed, so the set pressure needs to be high enough to avoid nuisance openings during normal operation, but not so high that the vessel could reach or exceed its design limit before relief occurs. The standard practice is to set the thermal relief valve at a pressure not higher than 1.2 times the design (MAWP) of the vessel. This 120% margin gives a safe buffer: it allows normal thermal fluctuations to stay contained, while ensuring relief happens before the vessel reaches a dangerous overpressure. Setting it higher (like 130%) would risk allowing pressures beyond the vessel’s safe limit, while setting it lower (100% or 110%) would increase nuisance openings or unnecessary relief. So the best choice is 120%.