What prevents a relief valve from lifting at normal operating pressure?

The correct answer is spring tension. In a relief valve, spring tension is a crucial element that determines the pressure at which the valve will open. The spring is calibrated to hold the valve closed until the pressure in the system reaches a predetermined level. This level is designed to be the safe operating pressure limit. When the pressure exceeds this limit, the force exerted by the fluid overcomes the spring tension, causing the valve to lift and allow fluid to escape, thus preventing system overpressure.

Gravity plays a minor role but is not the primary reason the valve remains closed under normal operating conditions. While pressure settings are important in determining at what pressure the valve opens, it is the spring tension that actively holds the valve closed until that set pressure is reached. Calibration is essential for ensuring proper operation, but it does not prevent lifting; rather, it ensures the settings are accurate. Thus, spring tension is the most critical factor in keeping the relief valve closed under normal operating conditions.