What must be closed when performing a hydrostatic test on a boiler?

When performing a hydrostatic test on a boiler, it is essential to close the main steam stop valve. This valve is crucial because it isolates the steam system from the boiler during the test. By closing it, you ensure that no steam can escape, which is critical for maintaining the integrity of the test pressure being applied to the boiler. The hydrostatic test involves filling the boiler with water and applying a pressure that exceeds the normal operating conditions to check for leaks and to assess the strength of the boiler components.

Closing the main steam stop valve helps create a sealed environment, allowing operators to accurately assess the strength of the boiler structure under specified test conditions. It prevents any potential release of water or steam that could interfere with the test, ensuring safety and accuracy when determining the boiler's reliability.