The relief valve vent line maximum length is critical because it affects the valve’s ability to relieve pressure safely and efficiently. A vent line that is too long can lead to excessive backpressure, which can prevent the valve from opening fully or cause it to reseat prematurely. The maximum length of the vent line can be calculated using the equation provided in the API 520/521 standards, which takes into account the valve’s set pressure, the vent line’s diameter, and the density of the fluid being relieved.