I recently saw a question in the maintenance question bank
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Among the following fluid machinery, the one in which there is no direct relationship between flow rate and discharge pressure is (C). A. Centrifugal pump B. Piston compressor C. Reciprocating pump D. Roots blower. I don’t quite understand this question; BCD are all positive-displacement pumps that force the medium to flow regardless of pressure, until either the safety valve activates or the equipment or pipes get damaged. Only A is different. Shouldn’t that mean A is the correct answer? I understand that the examiner might mean that option C refers to a vacuum pump in the retest, so the discharge pressure is zero?
My understanding is that for B and D, the medium is gas, which is compressible. When the outlet pressure increases, the actual displacement decreases. The medium in C is a liquid, and the displacement is essentially unaffected by the outlet pressure.
This post was last edited by zhaisanming1 on 2022-8-6 09:02. Only C reciprocating pumps have such a characteristic: the flow rate is independent of pressure. For centrifugal pumps, if you increase the pressure, the flow rate decreases. Piston pumps and Roots blowers both require a specific outlet pressure.
Piston compressors and Roots blowers are used to transport gases, which can be compressed; therefore, the volume flow rate is related to pressure. Reciprocating pumps, on the other hand, are used to transport liquids, which are not easy to compress, and their flow rate is independent of pressure