单项选择题 A shallow dish positioned at the normal water level is connected to the（）This enables the blowing down or removal of scum and impurities from the water surface.

单项选择题 The（）enables water to be blow down or emptied from the boilerIt may be used when partially or completely emptying the boiler.

单项选择题 The（） is fitted in pair one is the main valve, the other the auxiliary or standby.