Final answer:
Safe turbine operation requires appropriate lubrication, running at critical speed, and maintaining high discharge steam pressure at the setpoint. So, the correct options are B, C and D.
Step-by-step explanation:
Among the options listed for safe turbine operation, essential factors include the following. Appropriate lubrication ensures that the moving parts of the turbine operate smoothly and reduces wear and tear, preventing overheating and mechanical failures. Running at critical speed is the speed at which the turbine is designed to operate most efficiently and safely. Operating at or near critical speed reduces vibrations and stresses on the turbine.
High discharge steam pressure running at setpoint maintains the correct discharge pressure ensuring that the steam flow and turbine operation are controlled and stable. The options of low turbine inlet pressure and loss of casing insulation are not considered essential for safe operation. Low inlet pressure could impair turbine performance, and loss of casing insulation could lead to heat losses and potential safety hazards.