Over what pressure is it required to have two sight glasses?

In the context of pipefitting and pressure systems, the requirement for additional sight glasses in a system is typically dictated by safety standards that address the visibility of liquid levels under pressure. Having two sight glasses instead of one at higher pressures provides an added layer of safety, enabling operators to monitor and verify liquid levels more effectively, which is critical in preventing over-pressurization and ensuring safe system operation.

The specifications indicate that when the pressure of the system exceeds 400 psi, two sight glasses must be installed. This threshold is based on industry standards and regulations that aim to enhance safety protocols. By using two sight glasses, operators can reduce the risk of misreading a single gauge or experiencing a failure that could lead to catastrophic results.

Therefore, the correct answer indicates that at pressures above this specific level, the added precaution of having two sight glasses is not just a recommendation but a requirement to maintain safety and operational integrity in high-pressure systems.