As the demand for larger dredgers rises, there is a growing need for developing bigger dredge pumps that can reach up to 1,800 mm suction diameter.
The dredge pump is one of the main components of the dredger, and it is critical to achieving production. Dredge pumps also have the biggest power consumer during operation so efforts to reduce the pump's power needs through smarter design are critical.
Engineering Challenges of Dredge Pumps
Dredge pumps involve more complex engineering challenge than water pumps. They must carry water filled with mixture of several types of soils like mud, fine sand, coarse sand, gravel, stones, and other debris. The mixture of those materials with pumped water doesn't behave as homogenous fluid as the soil tends to settle down and slide on the pipe bed depending on the flow speed. Another challenge is that pump parts wear quickly. Clogged impellers and breaking vanes also pose risks to pump performance.
Dredge Yard's Pump Design
In 2011, Dredge Yard began developing a new range of dredge pumps ranging from 200mm to 1,400mm - the largest size in the dredging industry. These pumps are designed to counter wear problems inherent in dredge pumps. Impellers are designed in multiple categories of 3, 4 and 5 vanes. To resist wear, the pump parts are made from high chromium iron having a hardness of 60HRC. Unfortunately these materials don't have high elongation properties and are not impact resistant. For this reason, some dredge pumps are built with an outer casing.
The large pumps are connected in series, reaching a final pressure of 30 bar by the 3rd pump. The pumps often weight more than 100 tons for one complete assembly including wet parts, outer casing, plates, shaft, bearing assembly and pedestal.
Size, weight and component resistance, among other factors, create an imposing challenge for dredge pump designers. Demand has recently grown in the market for even bigger dredgers. Dredge pump size must increase from 1,400 mm to 1,600 mm - even 1,800 mm. Larger suction pipes and dredge pumps must be designed to fill the dredgers in less than 2 hours.
New design methods are needed to adapt to the market efficiently. Pumps must be tested and analyzed for their efficiency and performance. Foundries with casting capacity of 50 tons and heavy steel machinery 6 meters in diameter are required when casting and machining the pump case.
Performance analysis of a dredge pump
The size of these dredger pumps makes building a test and analysis laboratory extremely difficult if not impossible. The manufacturer must design and manufacture the pump as a prototype and deliver the prototype to the dredger without testing and performance checks. All performance will be tested in the dredger when it starts working. The design has to be flawless, performance oriented and reliable before its production. Engineers at Dredge Yard test dredge pump performance with computational fluid dynamics (CFD) instead of physical testing. CFD software calculates and simulates the pump performance measurements like flow rate, aeration, cavitation, pumping head, efficiency, power required, and net positive suction head (NPSH).
CFD codes, can provide steady-state simulation, also known as multi-frame reference (MRF) simulation or "frozen blade" simulation, which put the impeller in a reference frame that rotates at the pump speed, while solving for the rest of the pump in the stationary lab frame.
3-D Modeling
After achieving the hydraulic design, a basic 3-D pump model is made in several CAD programs. Once the basic design is completed Dredge Yard runs several trials on major FEA calculations on the model to ensure getting the right stiffness and strength of the pump. This model incorporates pressures output from the CFD analysis to determine the wall loadings.
The gap between the impeller and front plate must be tightly controlled in terms of geometric tolerance. If deformation widens the gap, efficiency will drop significantly. The 3-D modeling and FEA process is repeated as needed to ensure the highest safety margin and lowest weight. At 1,800 mm a dredge pump weights approximately 200 tons. Any weight savings from the FEA calculation could be significant.
Design work continues on the 1,800 mm dredge pump. Dredge Yard aims to finalize the development in 2016 and build the pumps for the dredging and mining market.
One of the examples of a geometric tolerance that needs to be tightly controlled is the gap between the impeller and the front plate. If there is a deformation in that area and the gap widens, the efficiency will drop significantly. Besides the impeller gap effect, the stiffening of the pump structure will assure a symmetrical tension on the pump case and plates. As the pump case is of a volute shape and pressure is not evenly spread, deformation could be asymmetric which is not desirable in pump structures.
The 3-D models take into account such performance measurements as flow rate, head and cavitation for many of the pump's components.
