2014 Real Knowledge Questions 55
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Which of the following descriptions regarding liquid transfer pumps are correct? (A) Two centrifugal pumps are operated in parallel; they have the same head, and their flow rate is twice that of a single pump. (B) Before starting a centrifugal pump, it is necessary to fill the pump with liquid and close the outlet valve. (C) The theoretical flow rate of a gear pump is related to the discharge pressure, and it increases as the pump’s speed increases. (D) Reciprocating pumps use bypass valves to adjust the flow rate slightly, and the outlet valve must be opened at startup. I only know that BD are definitely correct; how can AC be explained?
All that is known is that when centrifugal pumps are connected in series, the head generated is slightly less than twice the original value, and when they are connected in parallel, the flow rate is also slightly less than twice the original value
(A) Two centrifugal pumps are operated in parallel; they have the same head, and their flow rate is twice that of a single pump. This is consistent with the information provided by the examination board, so it should be correct. (B) It is essential to fill the pump with liquid before starting a centrifugal pump, and it is correct to close the outlet valve. (C) The theoretical flow rate of a gear pump is related to the discharge pressure, and it increases as the pump’s speed rises. Gear pumps belong to the category of positive-displacement pumps; their theoretical flow rate is determined by their structure and is not related to the discharge pressure, so this statement is incorrect. (D) Reciprocating pumps use bypass valves to adjust flow in small increments; the outlet valve must be opened at startup. This is also correct according to the textbooks, but in practice, the design assumes full-flow backflow. So the answer should be ABD
As a multiple-choice question, AB is too imprecise; neither can be determined with certainty.
The parallel flow rate is less than 2 times the flow rate of a single pump.
Correct answer: BD. A is incorrect – the flow rate is slightly less than twice the original value when connected in parallel, and the head is slightly less than twice the original value when connected in series; there are exact statements and explanations in \"Principles of Chemical Engineering (Part 1)\\" by Tianjin University; C should be independent of the discharge pressure!
I’d like to remind everyone of one thing: there are slightly more answers where two options should be chosen, while fewer options require choosing three. It’s impossible to choose four answers. Be sure to check your answers before submitting the paper; if you’re unsure about any option, definitely don’t select it!
A, if one wants to ensure that the flow rate is twice as high, there are many conditions required, such as keeping the outlet back pressure constant and ignoring the frictional losses along the pipes. The pump inlet pressure remains constant, with no cavitation. C is definitely correct. A gear pump is a positive-displacement pump, and its speed is proportional to its displacement, assuming no leakage.
For pumps A and 2 connected in parallel, \"if the head is the same as that of a single pump, the flow rate should be the sum of the flow rates of the two pumps\" or \"if the two pumps are of the same model, the head will be greater than that of the single pump, while the flow rate will be less than twice the flow rate of a single pump.\" Both of these statements can be correct. As for the term \"slight increase,\" there is some confusion here; using a bypass valve allows for significant adjustment of the flow rate, right?