If a spool piece has both a 150th and 300th flanges, which rating do you use to hydro test pressure?

In a situation where a spool piece is equipped with flanges rated at different pressure classes, the hydrostatic test pressure should be based on the lower pressure rating. In this case, if the spool piece has both 150-pound and 300-pound flanges, the test pressure must adhere to the 150-pound flange's rating.

Hydrostatic testing is performed to ensure the integrity and safety of piping and equipment under pressure. Using the lower of the two ratings ensures that the system can safely withstand the test without exceeding the capabilities of the weaker component. In this instance, while the 300-pound flange can handle higher pressures, the 150-pound flange is the limiting factor. Therefore, the hydro test pressure should be aligned with the 150-pound flange, thus selecting that rating for the testing process.

The other options represent pressure ratings that are higher or not applicable in this context, as they either surpass the maximum capability of the 150-pound flange or are not relevant to the given choices.