单项选择题 Which of the following operating features do displacement pumps bear if they are working under no-cavitation condition?（） (1) output is almost directly proportional to speed (2) they are self-priming (3) they will develop a discharged pressure equal to the resistance to be overcome, irrespective of speed

单项选择题 Which of the following pumps will build up a bursting pressure or stall their motors if the discharge valves are closed?（） (1) gear (2) centrifugal (3) screw

单项选择题 Attempts are made to eliminate axial thrust in centrifugal pump by （）.