What must a hydro test tree have built into it to be able to zero out a test gauge during a hydro test?

A hydro test tree must have a double block and bleed feature built into it to facilitate the zeroing out of a test gauge during a hydro test. This configuration allows for the isolation of the test gauge from the pressure of the fluid being tested, ensuring that the gauge can be calibrated to zero before the test begins.

The double block aspect involves two valves that provide a secure barrier to prevent any pressure from affecting the gauge. The bleed function allows for venting any residual pressure in the lines, ensuring that the gauge reads accurately at zero when it is disconnected from any influencing pressures. This setup is crucial for safety and accuracy during hydrostatic testing, as it ensures that test results are reliable and that pressure does not inadvertently affect the measurements taken.

The other options do not specifically enable the zeroing of a gauge. A pressure release valve is primarily used to relieve excess pressure, a flow control valve regulates the flow of fluid, and a safety pressure relief valve protects against overpressure scenarios but does not directly relate to the accurate calibration of a test gauge for hydro testing purposes.