Dredging Technology with Körting Ejectors
Ejectors on Assembly- and Drilling Platforms The English terms ejector, jet pump or eductor all describe the same ejector technology, as does the standard term Strahlpumpe in the German language area. They are transport systems in which the drive is provided by a drive medium. Water or compressed air are generally employed in ship's technology, depending on the planned application case. By means of the motive medium a secondary flow this can be liquids, gases or vapours, solids or a mixture thereof - can be sucked in and delivered to a higher pressure level together with the motive medium. With this operating method ejector technology covers practically the whole field of pumping technology, also known from the mechanical systems. So you can find ejectors just for transporting of liquids and equally as venting equipment, compressors, vacuum ejectors, dosing systems or as mixing equipment. Körting Hannover AG, located in Hannover since its founding in 1871, is known as the world s leading manufacturer of this type of ejector technology. Based on the developments made by Ernst Körting the company s system technology stands for a broad spectrum of applications, also in the field of marine engineering. This covers the very wellknown bilge ejectors and their outstanding characteristics with regard to reliability against dry-running, selfpriming operating method and long-term maintenancefree service life as well as the central evacuating equipment for non selfpriming mechanical feed pumps. For example, a further application case may be found in the generation of operating vacuum and the suction removal of lye solutions at thermal seawater evaporators. The above given application cases and system characteristics stand here for a multitude of employment possibilities on a ship without describing these in greater detail here.
Degassing Systems One recent development, also based on this technology, are the so-called degassing systems for use on board dredgers especially the so-called hopper dredgers. Should these ships dredge out sediments containing gas pockets - this happens quite often when increasing the depth of shipping lanes or reclaiming land - then these gas pockets can become dangerous in two different ways. First of all though, the technical aspect is the predominant point but in addition to this, those aspects dealing with the environmental point of view are also becoming more and more important. The technical aspect refers to the possible rupturing of the suction flow at a mechanical dredger pump. This is accompanied by the so-called water hammer which caused by the liberated forces, can lead to a total destruction of the mechanical dredger pump. Such damages could lead to considerable material expenses. In addition to this, there would be a certain period of outage time for necessary repairs which again would lead to further costs, e.g. contractual penalty payments to the customer. In less critical cases, gas contained in the dredger pump's suction flow can lead to a continued rupturing of the suction flow and inevitably to repeated start-up actions each time this happens. In extreme cases, the removal of dredged material with a high gas content just cannot be accomplished without degassing equipment. A solution to this problem may be found in freeing the extraction flow sucked through the dredger pump of all incorporated harmful gas pockets before it enters the dredger pump casing. This calls for an exceptionally robust type of vacuum system as the process takes place under relatively harsh operating conditions and the intake gas flow can be mixed with water and solids. Usually this duty is fulfilled by the exceedingly reliable and practically maintenance-free operating water jet vacuum ejectors. The previously mentioned environmental aspect for utilising degassing systems has resulted from the fact that a non-degassed excavated flow is loaded with a higher portion of secondary water. Generally speaking, due to its high pollutants contents this water may not be passed out via the overflow. So the dredger's cargo holds are filled with secondary water which reduces the dredger's performance by diminishing its loading capacity and thereby increasing the operating costs. A further undesirable subsidiary effect may be found in the high strains put on the irrigation fields by this secondary water. All efforts are being made to relieve the irrigation fields of avoidable ballast as only a restricted field area is available for dumping purposes. The degassing technology offers an appropriate possibility.
Advantages The loading diagram in Fig. 4 clearly shows which advantages may be gained by using a degassing equipment with the resulting cutback of secondary water flow. Degassing equipment was utilised from the 8th journey out onwards. On using the degassing equipment it could be seen that there was an immediate increase in the portion of solid matter and that the transport behaviour became visibly smoother. The measuring data was collected by the dredging contractors Johann Bunte Bauunternehmung GmbH during operation of their dredger F. Volker SR in Leer. These measuring data are of special interest as they show the very rare situation in the dredging operations were carried out in the the same area with the same dredger. So for evaluation constant conditions can be assumed. The only deviation consists of the changeover to a degassing system based on the water jet vacuum technology developed by Körting Hannover AG. After modification the same dredger achieved an up to 100 % improved loading result in comparison to an operating mode without a degassing system. Dredging characteristics 2200 2000 1800 1600 1400 m3 1200 1000 800 600 400 200 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Journeys solid material mixed liquid & solid material total load Figure 4
Design In the most simplified form such a degassing equipment consists of a single conventional- type water jet vacuum ejector. The constructional size of the required ejector for the degassing process will increase in relation to the large suction ratings of today's dredger systems. As water jet vacuum ejectors achieve their optimum performance when they are installed in a vertical position, space problems soon occur in the shipbuilding industry when constructional sizes reach more than 3 m in length. One solution for this problem can be found in the parallel connection of smaller ejectors. This type of installation also offers the advantage of being able to switch individual ejectors on and off and so adjust the degassing performance to the actual operating conditions. However, this installation form can also be disadvantageous because it is necessary to mount a greater number of ejectors. This can result in quite considerable installation expenses with regard to necessary pipeline construction. The installation of so-called multi-ejectors (bundled ejectors) can help to reduce these cost factors and simultaneously reduce the amount of installation space required for the degassing system. With this constructional form it is quite usual to have several individual ejectors - up to approx. 30 units - bundled together in a common casing. Each bundle of ejectors therefore has just one suction- and one motive connection. So the necessary expenditure for piping as required by conventional parallel- connected individual ejectors can be reduced to a considerable extent.
Suction power adjustment The last remaining question concerns the performance adjustment of such a water jet vacuum ejector unit to the differing gas quantities which usually occur in the dredger flow. Even this problem can be solved by connecting several of these bundled ejectors in parallel in the usual large installations. A reduction in performance can, however, only be achieved within the range of the executed number of bundles as each bundle of ejectors is to be considered as an individual ejector unit. As water jet vacuum ejectors have a performance curve which can be influenced by the supplied motive pressure this too offers a further possibility of adjusting the suction performance to the actual composition of the dredged material requiring degassing.
Figure 6 shows a bundle of ejectors in the process of being installed. The increased number of mixing tubes for the installed water jet vacuum ejectors can be clearly seen. In this actual case the whole system consists of 3 parallel connected bundled ejectors and was executed for use on board the hopper dredger Alexander von Humboldt. The necessary supplementary component parts such as hot well, piping, fittings etc. were planned in collaboration with Körting Hannover AG but were actually manufactured by the shipyard. With this type of construction there is the possibility of separating out solids entrained in the suction flow to prevent them from entering - and therefore jamming - the bundled ejectors. This is effected quite simply by the gravity separation method. Solids and entrained water removed from the flow are collected in a buffer tank and are pumped out when required by means of a special liquid jet solids ejector. Figure 6 M-type ejector