单项选择题
A.stuffing boxes eliminate the leakage problems usually associated with other gear pumps
B.it is not necessary to closely maintain design clearances with this pump
C.they are essentially self-priming and produce a high suction lift
D.these pumps are designed with extreme tooth angled
单项选择题 One of the reasons that causes oil flowed from sludge outlet of a self-cleaning purifier is （）
单项选择题 To ensure oil purifier working in normal condition,the interval between two sludge discharging should not exceed（） hours.
单项选择题 Some refrigeration systems have chemical moisture indicators installed in conjunction with the sight glass in the liquid lineIf excess moisture is present in the system, the indicator will （）