FIVES Cryomec Inducer -...
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Improving NPSH Félicien Gebel and Stéphane Boeglin, Fives Cryomec, Switzerland, present a new inducer design that can help to improve net positive suction head in cryogenic pumps. When dealing with cryogenic fluids, such as nitrogen, oxygen, argon or LNG, which have boiling points at less than -150°C, net positive suction head (NPSH) improvement is an important topic. Keeping the NPSH under control is a key asset to avoid cavitation in cryogenic pumps. These fluids are oſten prone to evaporation, which can lead to a premature wear. Cavitation can also have other immediate negative consequences on pumps, such as vibration, gases development and instability. Fives conducted a study about NPSH improvement in partnership with the Luzern Technical University, based in Switzerland. Fives and its Cryogenics | Energy teams provided the Luzern Technical University with an inducer from the Cryomec cryogenic pumps range to evaluate. Over six months, the university conducted a study on the original design of the inducer provided in order to suggest new development paths. Due to this research, the overall performance and the NPSH in particular have been significantly improved. This allowed the pump to tolerate a lower suction pressure, which is particularly appreciated in road tankers applications, for instance. CFD STUDY AND OPTIMISATION The study showed clear enhancements to implement in the new design of the inducer. To understand the results of the study in more detail, it is important to look at how inducers work. Their main purpose is to pre-compress the flow to provide the impeller with a good suction capacity and a higher pressure than the suction flow. Inducers turn a laminar flow to a ‘helicoidal’ flow with a circumferential velocity. This is the first rotary element in the pump. The geometry of the inducer can have a huge impact on the velocity and pressure of the fluid. Therefore, computational fluid dynamics (CFD) was essential to estimate and understand how the geometry could improve the global performances of the inducer and therefore of the cryogenic pump. Fives’ initial inducer design is represented in Figure 1. Figures 2 and 3 perfectly illustrate velocity and pressure. Figure 2 represents a blade-to-blade view from the inducer before optimisation. It shows how the flow is aspirated and the fluid behaviour along the blade. This figure reveals the fluid impact at inlet blade side, the velocity is locally too high and the flow is not as smooth as it should be. Figure 3 represents the pressure before optimisation. It reveals the same as Figure 2, the high velocity leads to local pressure drop at the inlet blade side. Both figures confirm Fives’ belief that blades have to be redesigned to allow for smoother flow. CFD can handle a lot of different parameters, but results can be tricky to analyse to select the best approach. It appears that the edge profile and related angles (inlet and outlet) are key parameters to optimise inducer performance. Fives consequently focused its efforts on the blades, redesigning them in order to get the best results possible as far as reducing the NPSH. TEST BENCH TRIALS CFD soſtware is a relevant tool to evaluate potential performances, but Fives wanted to have the theoretical analysis confirmed. Organising a trial in actual conditions on its own test Figure 1 - Inducer initial design. Cryogenics | Energy
Transcript of FIVES Cryomec Inducer -...
Improving NPSH
Félicien Gebel and Stéphane Boeglin, Fives Cryomec, Switzerland, present a new inducer design that can help to improve net positive suction head in cryogenic pumps.When dealing with cryogenic fluids, such as nitrogen, oxygen, argon or LNG, which have boiling points at less than -150°C, net positive suction head (NPSH) improvement is an important topic. Keeping the NPSH under control is a key asset to avoid cavitation in cryogenic pumps. These fluids are often prone to evaporation, which can lead to a premature wear. Cavitation can also have other immediate negative consequences on pumps, such as vibration, gases development and instability.
Fives conducted a study about NPSH improvement in partnership with the Luzern Technical University, based in Switzerland. Fives and its Cryogenics | Energy teams provided the Luzern Technical University with an inducer from the Cryomec cryogenic pumps range to evaluate. Over six months, the university conducted a study on the original design of the inducer provided in order to suggest new development paths. Due to this research, the overall performance and the NPSH in particular have been significantly improved. This allowed the pump to tolerate a lower suction pressure, which is particularly appreciated in road tankers applications, for instance.
CFD STUDY AND OPTIMISATIONThe study showed clear enhancements to implement in the new design of the inducer. To understand the results of the study in more detail, it is important to look at how inducers work. Their main purpose is to pre-compress the flow to provide the impeller with a good suction capacity and a higher pressure than the suction flow. Inducers turn a laminar flow to a ‘helicoidal’ flow with a circumferential velocity. This is the first rotary element in the pump.
The geometry of the inducer can have a huge impact on the velocity and pressure of the fluid. Therefore, computational fluid dynamics (CFD) was essential to estimate and understand how the geometry could improve the global performances of the inducer and therefore of the cryogenic pump.
Fives’ initial inducer design is represented in Figure 1. Figures 2 and 3 perfectly illustrate velocity and pressure. Figure 2 represents a blade-to-blade view from the inducer before optimisation. It shows how the flow is aspirated and the fluid behaviour along the blade. This figure reveals the fluid impact
at inlet blade side, the velocity is locally too high and the flow is not as smooth as it should be. Figure 3 represents the pressure before optimisation. It reveals the same as Figure 2, the high velocity leads to local pressure drop at the inlet blade side. Both figures confirm Fives’ belief that blades have to be redesigned to allow for smoother flow.
CFD can handle a lot of different parameters, but results can be tricky to analyse to select the best approach. It appears that the edge profile and related angles (inlet and outlet) are key parameters to optimise inducer performance.
Fives consequently focused its efforts on the blades, redesigning them in order to get the best results possible as far as reducing the NPSH.
TEST BENCH TRIALSCFD software is a relevant tool to evaluate potential performances, but Fives wanted to have the theoretical analysis confirmed. Organising a trial in actual conditions on its own test
Figure 1 - Inducer initial design.
Cryogenics | Energy
Figure 2 - Blade-to-blade view with circumferential velocity.
THE INFLUENCE OF DESIGN PARAMETERSFigure 4 represents both initial and optimised NPSH curves (height in metres as a function of flow in litres per minute). It clearly shows that the optimised design is much better at every measured point, with up to a 30% improvement at its best efficiency point.
Another significant advantage that cannot be seen on this chart is the greater stability given to the pump. The pump even benefits from bette behaviour when working on reduced flow.
Thanks to the successful study realised with the partnership of the Luzern Technical University, Fives has now developed a new inducer design. The common involvement in the NPSH improvement led to significant enhancements, which Fives will be able to implement in several of its cryogenic pumps. The sharing of the university’s analysis, combined with Fives’ test bench facilities led to proven and exploitable results on which both parties can rely on and paved the way for potential future developments.
bench seemed essential before launching the new cryogenic pump to the market. The Cryomec pump and inducer have, therefore, been equipped with sensors and probes able to determine more accurately the values to be looked at. To compare performances of the new and old inducer as best as possible, Fives used the exact same cryogenic pump (a typical truck offloading pump) and only changed the inducer part itself without replacing anything else.
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CONTACT USFives Cryomec AGBinningerstrasse, 85a - CH-4123 Allschwil - SwitzerlandTel.: +41 61 487 33 00 - Fax.: +41 61 487 33 99Email: [email protected] – Website: www.fivesgroup.com
Figure 3 - Blade-to-blade view with pressure.
CONCLUSIONThis article shows how important it is to conduct such studies on various topics in order to stay at the forefront of new technologies. Partnerships with Universities combined with test bench trials are interesting processes to develop such studies.
The advantages provided by this new inducer design are key themes in the cryogenic market. How to avoid premature wear, gases development, instability and vibrations are daily matters of concern. Fives is hoping for further improvement on many other topics, like it recently did for NPSH.
