单项选择题
A.main stern stop valve
B.auxiliary steam stop valve
C.scum valve
D.blow down valve
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.
单项选择题 The（） is a smaller valve fitted in the auxiliary steam supply line, and is usually of the non-return type.