5-10. High-speed  impellers  must  be balanced  to  avoid  vibration.  What is the purpose of a close radial clearance  between  the  outer  hub and the pump casing? 1. To decrease friction 2. To decrease axial thrust 3. To  minimize  leakage  from  the suction  side 4. To  minimize  leakage  from  the discharge  side 5-11. What is the function of mechanical seals  and  stuffing  boxes? 1. To improve pump operation 2. To seal between the shaft and the  casing 3. To  clean  bilges 4. To  prevent  liquid  from  being pumped 5-12. What type of pump is considered to be  nonpositive  displacement? 1. Sliding  vane 2. Rotary 3. Centrifugal 4. Jet 5-13. A pump that does not develop enough  discharge  pressure  could have  which  of  the  following problems? 1. Clogged  impeller  passages 2. A bent shaft 3. Excessive  suction  lift 4. Insufficient  pump  speed 5-14. Which  of  the  following  statements about a centrifugal pump is true? 1. It  is  essentially  self-priming 2. It loses no energy 3. It  is  a  positive-displacement pump 4. It requires a relief valve 5-15. What type of pump has no moving parts? 1. Screw 2. Jet 3. Gear 4. Sliding  vane 5-16. What  device  uses  feedback  to 5-17. 5-18. 5-19. 5-20. provide  automatic  control  of speed,  pressure,  or  temperature? 1. Regulating  valve 2. Flange  coupling 3. Proportional-flow   filter 4. Governor What is the purpose of a valve in a closed system? 1. To  sample  fluids 2. To  control  fluids 3. To  increase  fluid  pressure 4. To  decrease  fluid  pressure Brass and bronze valves are never used  in  systems  that  exceed  what maximum  temperature? 1. 450°F 2. 550°F 3. 650°F 4. 750°F There  are  many  different  types  of valves that can be used to control fluid  flow. What are the two basic  groups  of  valves? 1. Globe  and  check 2. Check and gate 3. Stop  and  check 4. Gate and globe Due to valve design, gate valves are not used for throttling purposes for which of the following  reasons? 1. They make it difficult to control fluid flow and can damage  valves 2. They make it difficult to control fluid flow and are too lightweight 3. They can damage valves and are too  lightweight 4. They make it difficult to control fluid flow and are excessively  expensive 26