When you hire a carpenter, he repairs everything with a hammer. So, what happens when you ask an aeronautical student to solve some issues in a dredge pump? He models the pump as a badly behaving airplane. And with success, Alex de Rooij joined our company as a graduation student and recently graduated on the topic of ‘Numerical Study on NACA Profiles Sensitivity in Dredge Pump Impellers’.
The normal procedure for designing pumps is relatively straight forward. Set the performance specifications and try to hit that mark with the simulated behaviour from an iteratively improved design. This is well documented and I’ve been writing about this process before.1
One of the design parameters is the NPSHr. This is basically the amount of absolute pre-pressure the pump requires to operate.2 The system and operating conditions will result in a certain available pre-pressure: NPSHa. When the NPSHa drops below the NPSHr, the pump will experience cavitation at the low pressure side of the blade. The flow of the medium will be disturbed and the performance of the pump will abruptly crash. There is some warning. Operating near the NPSHr, there will be an incipient cavitation where the vapour bubbles start to form, but do not cause any issue. The imploding vapour bubbles may be detected audibly for the trained listener.3 Next will be a stage on the NPSHr, where the bubbles get larger and they loudly implode. At this stage, the pump sounds like it is tumbling nuts and bolts inside. These imploding bubbles will definitely cause damage to the impeller. At last, working below the NPSHr, the bubbles will become so large, they will block the passage between the blades. The result is an immediate drop in delivered head.
The flow of the medium through the impeller can be simulated in a CFD program. Specifically for impellers, you will need to model a rotating frame of reference. And that is the usual representation of the results. However, with mathematics being one of the most powerful tools invented by humanity, we can have an alternative view on these results. We can cut the impeller along a radial and stretch open the meridional passage and the blades to a row of foils. And that is exactly where our young aspiring engineer comes in. In literature, the blades in the impeller are modelled having a constant thickness. But, Alex has been investigating what the influence will be when we model the blades as foils. Selecting a proper profile makes the blade less sensitive to stalling.
Alex, thank you very much for your work here at Damen. We’ve learned the influence of certain profiles on the performance and geometry of the pumps. You have the right mindset to pass your time at the TU Delft and graduate successfully over there also. And whenever you have some days of the month left after you spend your allowance, know that we can give you a warm reception at our office.
This week is the triannual WODCON conference1. This edition will be held in lovely Copenhagen, in itself already a reason to be there. But there is more, the program is packed with interesting items. Starting with the opening ceremony with the Danish minister of transport and the Corporate Executives Discussion on sustainability. The Fehmarnbelt Tunnel Link will be a hot topic all through the program. My personal suggestion would be to attend Session 4: ‘Hydraulic Transport’ on Tuesday 17. Two of my highly regarded colleagues will be presenting their work on very interesting topics.
Rick Bekkers has been involved in the development of our new ProDredge edition. It is a slurry transport estimation tool. Where Ewout van Duursen has been working tirelessly on programming the tool, Rick has been checking the models and the results against known literature. He will show you the new reports generated and the insights they will give you for evaluating a proposed dredging system.
Suman Sapkota will be presenting his favourite topic: ‘Erosion estimation on the impeller blades of centrifugal dredge pumps’. He has been working on this topic ever since his own graduation project.2 Over the years he spend pondering over this issue and gradually grew in experience with the problem. Based on his ideas, he managed to start a cooperation with the Delft University of Technology and have a master student working on the practical details of his plans. Previously, I’ve written about the graduation of Wim Kleermaker in another article.3
Suman has been concentrating on gathering the various models for wear under different conditions during his own graduation. The results from his simulations could only be checked against my own experience with badly worn impellers and disasters. Immediately introducing observer bias and survivor bias. So, for this project Suman really wanted to include experiments to have at least one observation to match the simulations.
And what a struggle it was, to get the experiments done. Wim really gained more experience in fixing troubles than in fact finding. In the end he managed. After operating for a couple of weeks under heavy load conditions for the dredge pump, the impeller showed a noticeable wear pattern. And they came up with an innovative way of measuring the erosion pattern and match the test results with the simulation results.
The paper Suman is going to present on the WODCON, condenses the results from Wim together with Suman’s previous work as a graduation student himself. Don’t get deterred by the in depth analysis. If you fancy, you can follow the comprehensive literature references and understand his model. Or, just enjoy the ride and inspect the interesting pictures with the conclusions at the end. If you are really interested in his work, just approach him in the break hours, he is very willing to share his experience with you.
If you can’t find him, or Rick. Drop by at our booth #27 in the lobby.4 There will always be someone to get you into contact.
Yesterday, Wim Kleermaker graduated at the TU Delft on a research project he conducted on our slurry test circuit at Damen Dredging Equipment. Specifically, he was investigating the wear behaviour in our dredge pumps. The noteworthy aspect of this project, was that Wim was supervised by our colleague Suman Sapkota. As long time readers in the audience might remember Suman was my own pupil some years ago1.
Wear is a very common process in the dredging industry and one of the main cost factors in a project2. It is beneficial to know the amount of wear to expect in a certain condition and be able to predict the budget to reserve for this nuisance. This is only possible when we as a manufacturer will be able to predict the wear rate and pattern can provide the information to the operator for his estimates. We do have historical data that will allow us to provide a ball park figure, but a more analytical approach might assist us in particular unusual cases. Furthermore, it will also provide us insight in the impact of certain design decisions for the wear performance of a certain pump design. For Wim’s graduation, he had to approach this academically: come up with a simulation model and verify this with measurements.
The measurements were done in our slurry pump test circuit. This circuit has been highlighted a couple of posts back3. For Wim’s experiments, he used an impeller under a certain operating condition and mixture properties. Before and after a representative period, the condition of the impeller was measured and the difference is a measure of the wear experienced.
Wear (or scientifically: erosion) is related to the impact of the particles on the material surface. In order to know the kinetic energy of the particles, the flow field has to be known. As the flow fleild cannot be measured directly at the test circuit, we have to resort to Computation Fluid Dynamics. We already know of Suman’s graduation, to look for patterns in the flow lines, but Wim has extended the procedure to also quantitively estimate the related erosion.
Although there is only a limited amount of data available, comparing the results of the CFD estimation and the measured erosion are looking promising. This is certainly a workflow that will provide us the unique tools for engineering better pumps and assisting customers in their specific projects.
Although Wim will not join our ranks in the dredging community and pursue a different career in another interesting industry, we are sure he will be constructive and dedicated colleague at Marin.