Which percent of boiler design pressure must the boiler be hydrostatically tested to if it has an unsatisfactory strength inspection at the five-year period?

The correct answer is 150% because the regulations for boiler safety and maintenance require that any boiler that has shown unsatisfactory results during a strength inspection must undergo a hydrostatic test at a pressure significantly higher than its designed operating pressure. This is to ensure that the boiler can safely withstand operational stresses and to identify any weaknesses in the structure that may not have been apparent during a visual inspection.

Performing the hydrostatic test at 150% of the design pressure provides a substantial margin to confirm the integrity of the boiler. This level of testing helps to ensure that it can handle unexpected pressures and operational conditions without failing, thereby prioritizing safety for both the equipment and personnel operating it.

Hydrostatic testing at lower percentages, such as 100%, 125%, or 175%, does not align with the typical safety standards or practices where a definitive assessment of structural integrity is needed after an unsatisfactory inspection. Thus, the requirement for 150% is a standard safety measure in boiler maintenance procedures.