单项选择题
In order to properly remove air from the casing of a centrifugal pump when starting, the pump should have a （）.
A.positive head B.negative suction head C.mechanical seal D.vent valve attached to the casing at the top of the volute
单项选择题 The valve best suited for throttling gas or liquid flow in a pipeline is the（）.
单项选择题 Which of the listed statements is a characteristic of the liquid charged power element used with thermostatic expansion valves?（）
单项选择题 Differential pressures can be measured with the use of a （）.
