Why non-return valves are provided in the steam extraction lines?

Why non-return valves are provided in the steam extraction lines?
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Answer :

Non-return valves are provided because when the turbine trips there will be an instant drop in pressure inside the turbine. But there will be steam in feed water heaters, which is at high pressure. These will rush in to the turbine and overspeed will be there in turbine. So non-return valves are provided in steam extraction lines to prevent over speeding of turbine. 
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