Which factor is crucial before performing a hydrostatic test on a boiler?

Before performing a hydrostatic test on a boiler, it is essential that the boiler is empty. This requirement is crucial because a hydrostatic test is intended to check the integrity of a boiler's structure—primarily its ability to withstand the pressure it will encounter during normal operation. By ensuring the boiler is empty, you eliminate the risk of water or steam causing a sudden failure if an issue is identified during the test.

During a hydrostatic test, water is pumped into the boiler to a pressure that is significantly higher than the normal operating pressure. If the boiler were filled with water or under pressure at this stage, it could lead to dangerous scenarios, including blowouts or other types of failures that could result in injury or damage. Keeping the boiler empty ensures that the test can be conducted safely, allowing for a thorough examination of any potential leaks or weaknesses in the system.

Other options, such as having the boiler at operating temperature or under pressure, would be counterproductive for a hydrostatic test. The purpose of the test is to assess the boiler's condition without the complicating factors of heat and pressure, which could mask issues or create hazardous situations during the test process.