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.
CEDA has always been an advocate of sharing and disseminating knowledge, with an attention to personal contacts1. Of course, there are the high profile Dredging Days2 and the WODCON3 level events, but once in a while there are also smaller events. As part of the smaller program, Young CEDA4 regularly organizes visits at projects and companies to bring together the new generation of our dredging community. This time, a group of young professionals visited our company Damen Dredging Equipment here in Nijkerk5 to get familiar where we are and what we do. Rick Bekkers, Jorgen Groeneveld, Suman Sapkota and Reinier de Vries where hosts of the event and organised, with assistance of the ladies of our reception and back-office, a nice program with presentations a yard tour and of course drinks and the proverbial ‘bitterballen’.
After Rick and Reinier gave a presentation on the premises and the products, there was a yard tour and a demonstration of our dredging simulator by Wouter Beekman. And I am always happy, that the visit to our Damen Dredging Experience was a great success again. We do have our own little museum to experience the difficult issues in the dredging process hands-on6. Literally! Specially for this occasion, the exhibits have been expanded with new informative screens, made by Judith Korver. In a few slides there is an explanation on what the visitor experiences by operating the exhibit.
Part of the experience is the staircase leading up to the third floor with the exhibits. In the staircase, there is a collection of wall covering pictures on the history of Damen Dredging Equipment. Personally, I think learning from history is essential to get inspired7. Back in the old days, people had to be very innovative to come up with solutions within their limited technological level. And Mr. de Groot, the founder of our local Damen company was very innovative8. He was the inventor of the double walled dredge pump9 and the soil pumping station10.
A lesser known fact by now may be, that originally Mr. C de Groot was a contractor. He was a true descendant from a Giessendam dredging family and tried his luck at the IJsselmeer werken of the 1930’s. As a basecamp, he leased a land lot next to the sea locks of Nijkerk. Later on, he had more fun on building his own equipment and eventually started a manufacturing facility more inland along the approach canal. You can still recognize the original footprint in the current urban landscape.
At retirement, Mr. de Groot had no successors and had to sell the company. After an intermediate period, De Groot Nijkerk was acquired by Damen Shipyards8 in 1988. Eventually in 2004 our company was rebranded to Damen Dredging Equipment and we proudly fly the blue colours of the Damen family.
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.