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This post was last edited by 3983596_FPPZ on 2019-1-13 at 23:47. Rules for the bg2.png multiple-choice questions: 1. Those who get both questions wrong receive a reward of 2 wealth points as an incentive for active participation; 2 wealth points are given for getting one question right and the other wrong, while 4 wealth points are awarded for getting both questions correct ； 2. Rewards for answers are available once per person only ； 3. Each reply allows scoring for only one person ； 4. The answer and its explanation will be visible after responding ； 5. The reward is valid for 48 hours; no reward will be given if the deadline is passed. Again winning awards in studies – come by here every day! 1. The shafts of multi-stage centrifugal pumps are usually slender, and the keyways on these shafts that engage with the various impellers are often arranged alternately at 180-degree intervals. The main reason for this is to ensure the dynamic balance accuracy of the rotor system. ( ) 2. Under normal circumstances, the efficiency of positive displacement pumps is generally higher than that of centrifugal vane pumps. ( ) Answer: 1. Incorrect; the main reason is to avoid having the keyways on the same side, thereby further reducing the torsional strength of the slender shaft ； 2. Correct. Positive displacement pumps have a low flow rate (small volume flow, high pressure), which results in significantly less hydraulic loss (such as shock, vortices, and swirls) and mechanical loss (such as disk friction), hence they offer higher efficiency.