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Flow of Fluids
Published in Siddhartha Mukherjee, Process Engineering and Plant Design, 2021
Improper suction conditions are the largest source of pump troubles. Careful attention should be given to net positive suction head (NPSH). NPSH is the net remaining pressure at the suction flange of the pump after all negative forces that restrict liquid from getting into the pump are subtracted from all the positives forces that assist liquid in getting into the pump. Two terms of NPSH are referred to as follows:NPSHa: Net positive suction head available in the system expressed as meters of liquid.NPSHr: Net positive suction head required by the pump expressed as meters of liquid.
Plants and Equipment
Published in Carl Bozzuto, Boiler Operator's Handbook, 2021
Sudden formation of vapor in a pump driven by a steam turbine will result in rapid over speeding of the pump and turbine. Occasionally, that happens so fast that the turbine over speed trip cannot respond before the turbine blades start flying out of the casing. When the bubbles start forming, they will collapse later when the pump increases the pressure in the liquid. In centrifugal and turbine pumps, the result is bubbles forming and then collapsing. The liquid rushing in, to fill the voids as bubbles collapse, hammer away on the parts of the pump. That is called “cavitation.” It is evident by a small to fair amount of noise that can be heard. It is also evident when the pump is dismantled. There will be heavy wear, consisting of lots of tiny indentations where the bubbles collapsed. To prevent pump damage, be sure to have adequate NPSH.
Balance of Plant
Published in S. Can Gülen, Gas Turbine Combined Cycle Power Plants, 2019
If the pressure of feedwater at the pump suction is below the vapor pressure, vapor bubbles form in the inlet flow. As the pressure increases above the vapor pressure while passing through the pump impeller, the bubbles collapse at the speed of sound of the water, sending shock waves throughout the water. This situation, known as “cavitation”, is damaging to the equipment in the long run. In order to prevent cavitation, net positive suction head (NPSH) available at the pump inlet should satisfy the vendor’s required value by a typical margin (usually, 1 m of head or 3.3 ft). NPSH is the total (absolute) suction head of water determined at the suction nozzle minus the vapor pressure of water. Note that required NPSH is proportional to the inlet flow; higher pressure head is required at the pump inlet for high flows than low flows. Ideally, most pump operation is expected to be within 70%–120% of best efficiency point (BEP) flow. At much lower flows, actual NPSH may fall below required NPSH leading to cavitation. Furthermore, operation at lower than the optimal flow range may require recirculation, which can lead to increased vibration and component damage and, thus, to an unplanned outage.
Modelling and performance prediction of a centrifugal cargo pump on a chemical tanker
Published in Journal of Marine Engineering & Technology, 2020
The head is inversely proportional to the fluid density. It can be seen that the efficiency increases while the density of liquids decreases. The kinematic viscosity and the vapour pressure of the fluid also affect the overall efficiency of the pump. The castor oil is the most viscous fluid among others, and its vapour pressure is 0. Its density is close to the reference liquid (water). As a result of the comparison between these liquids, it was determined that the pump efficiency and head increase when the fluid has higher viscosity and lower vapour pressure. The analogy between Chloroform and nitric acid indicated that the higher vapour pressure leads to the head drop. Furthermore, net positive suction head required was more elevated when the vapour pressure and density of fluid are higher. This leads to the opening of the difference between the net positive suction head acquired and the net positive suction head required, in other words, cavitation. If there is a significant increase in the fluid density relative to the reference value, it is observed that the pump head is significantly reduced.
Effect of inlet sweepback angle on the cavitation performance of an inducer
Published in Engineering Applications of Computational Fluid Mechanics, 2019
Xiaorui Cheng, Yibin Li, Shuyan Zhang
Cavitation is a common phenomenon in fluid machinery. Cavitation will occur when the partial pressure of the fluid is lower than the pressure of vaporization at the local temperature (Guan, 2011). The high-speed rotating blade will reduce the pressure of partial fluid. Meanwhile, it will cause performance drop, produce vibration and noise, and seriously damage the over-current components under cavitation conditions (Hong, Kim, & Kim, 2015). One of the most effective ways to prevent cavitation of a centrifugal pump is to install an inducer before an impeller. The inducer can increase the pressure of the fluid before entering the main impeller, and improve the available net positive suction head (NPSHa) (Torre, Pasini, & Cervone, 2011).
Development of a Sodium Fast Reactor Cartridge Loop Testing Capability for the Versatile Test Reactor
Published in Nuclear Science and Engineering, 2022
Mitchell T. Farmer, Matthew Weathered, Darius Lisowski, Nathan Bremer, Dennis Kilsdonk, Tim Stack, Caleb Tomlin, Chris Plucker, Ed Moreno, Ran Kong, Zhengting Quan, Adam Dix, Seungjin Kim, Mamoru Ishii, Mark Anderson, Andrew Napora
Sufficient net positive suction head (NPSH) needs to be provided on the test coolant side to prevent cavitation during pump operation. Preliminary analysis indicates that for a seven-pin test assembly, the pump head needed to supply sufficient flow to match the target coolant temperature rise across the test fuel while removing fission energy plus gamma heating within the cartridge is <1 bar. Thus, the NPSH can be ensured simply by the increase in plenum pressure as the gas heats up during reactor ascension to operating temperature. Additional details on the status of the pump design and testing follow.