tailieunhanh - Root Cause Failure Analysis Episode 4

Tham khảo tài liệu 'root cause failure analysis episode 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 82 Root Cause Failure Analysis Flow Figure 7-3 Pumps in series must be properly matched. One of the most common problems with pumps in parallel is suction starvation. This is caused by improper inlet piping which permits more flow and pressure to reach one or more pumps but supplies insufficient quantities to the remaining pumps. In most cases the condition results from poor piping or manifold design and may be expensive to correct. Always remember that when evaluating flow and pressure in pumping systems they always will take the path of least resistance. For example given a choice of flowing through a 6-in. pipe or a 2-in. pipe most of the flow will go to the 6-in. pipe. Why Simply because there is less resistance. In parallel pump applications there are two ways to balance the flow and pressure to the suction inlet of each pump. The first way is to design the piping so that the friction loss and flow path to each pump is equal. Although theoretically possible this is extremely difficult to accomplish. The second method is to install a balancing valve in each suction line. By throttling or partially closing these valves the system can be tuned to ensure proper flow and pressure to each pump. Entrained Air or Gas Most pumps are designed to handle single-phase liquids within a limited range of specific gravities or viscosities. Entrainment of gases such Pumps 83 Figure 7-4 Pumps in parallel may share suction supply. as air or steam has an adverse effect on both the pump s efficiency and its useful operating life. This is one form of cavitation which is a common failure mode of centrifugal pumps. The typical causes of cavitation are leaks in suction piping and valves or a change of phase induced by liquid temperature or suction pressure deviations. As an example a one-pound suction pressure change in a boiler-feed application may permit the deaerator-supplied water to flash into steam. The introduction of a two-phase mixture of hot water and steam into the pump .