Dynamical analysis of pumps

Tez (Yüksek Lisans) -- İstanbul Teknik Üniversitesi, Fen Bilimleri Enstitüsü, 2013
Thesis (M.Sc.) -- İstanbul Technical University, Institute of Science and Technology, 2013
Günümüzde artan nüfus ve sanayileşme sebebiyle ortaya çıkan tüketim artışları, üretim işlemlerinin hızlandırılmasını ve süreçlerin kısaltılmasını gerekli kılmaktadır. Ancak bu hızlı gelişmeler henüz tecrübe edilmemiş bazı problemleri de yanlarında getirmektedir. Bununla beraber, özellikle enerji, kimya, ilaç ve petrokimya gibi sektörler hassas ve yüksek güvenirlikli üretim süreçleri içerdiği için üretimde kullanılan ekipmanların hem bu hassas üretim şartlarını hem de tasarım ömür şartlarını yerine getirmesi gereklidir. Santrifüj pompalar belirtilen sektörlerdeki birçok üretim sürecinin ya ana ekipmanı ya da ana ekipmana destek ikincil kritik ekipman olarak görev yapmaktadır. Santrifüj pompa içerisinde akış genel olarak üç boyutlu, türbülanslı, sınır tabaka ayrılmaları, giriş/çıkış sirkülasyonları ve kavitasyon gibi sebeplerle çalkantılı bir görüntü sergilemektedir. Pompanın tasarımı birçok parametreye bağlı olup, çark ve gövde geometrisi karmaşık bir yapı içermektedir. Bu durum, daimi olmayan çalkantılı akışın hangi etkileşimler sonucunda oluştuğunun belirlenmesini zorlaştırmaktadır. Tecrübe ve benzeşim yaklaşımı ile hidrolik verim açısından iyi tasarlanmış bir pompanın, basınç çalkantıları, yatak ömürleri, titreşim ve gürültü seviyesi gibi diğer kriterler açısından da durumunu belirlemek genel anlamda pompa üretildikten sonra yapılan deneylerde ortaya çıkarılmaktadır. Bu bağlamda, tasarımda gerekli görülen iyileştirmeler ve düzeltmeler ancak iteratif prototip üretimleri, deney tekrarları ve yüksek maliyetlerle gerçekleştirilmektedir. Özellikle son yıllarda bilgisayar teknolojisinde ortaya çıkan gelişmeler, pompa tasarımında yapısal ve akış analizlerinin sayısal olarak yapılarak görselleştirilmesine olanak sağlamış, tasarımın iyileştirilmesinin ve optimizasyonunun bilgisayar üzerinde yapılarak model, kalıp ve deney maliyetlerinin indirgenmesine ve zaman kayıplarının azaltılmasına zemin hazırlamıştır. Yapılan bu çalışmada bir pompanın üç farklı açıdan (Modal, Akış ve Yapısal) sayısal analizleri ANSYS Workbench ticari yazılımı altında yapılmıştır. Yapılan modal analizlerde pompanın üç farklı mesnetleme durumuna karşılık gelen doğal frekanları belirlenmiştir. Belirlenen doğal frekansların pompanın dönme frekansı ve kanat geçiş frekansları ile karşılaştırılması yapılmış, kritik olabilecek mod şekilleri incelenmiştir. Akış analizleri Fluent ortamında URANS(Unsteady Reynolds-Averaged Navier-Stokes) algoritması kullanılarak çözümlenmiştir. Buradaki akış, çark gövde içerisinde tam bir tur atarken incelenmiş, özellikle dil bölgesi ve çıkış basıncındaki çalkatılar irdelenmiştir. Yapı-Akışkan etkileşimi açısından yapısal analizler akış analizleri ile eş zamanlı yapılarak, akış çözümleri ile elde edilen basınç dağılımları gövde üzerine uygulanmış ve gerilme-şekil değiştirme durumları incelenmiştir.
Increasing amounts of consumption, due to the explosion of population and industrialization, necessitated the production processes to be faster and with shorter lead times. However this rapid progress brings some design problems for which solutions based on experience does not exist. Since accurate and highly reliable production processes are mandatory for energy, chemical, pharmaceutical and petrochemical industries; the tools used during production in these industries must assure both accurate production conditions and design life criterions. Centrifugal pumps are being used as the main equipment or the second critical equipment at the stated industries above. Pumps are recorded to be the second most common machinery on earth after electric motors. Recently centrifugal pumps are being designed and produced in various sizes and constructional features. For instance, miniature sized Blood Pumps, giant axial flow pumps with impeller diameters exceeding 4 meters for Flood Control and boiler feed pumps in Power Plants of motor power above 65.000HP. This wide range of application brings challenging problems to cope with on different levels of criticality. It is obvious that the problem arising in the design phase has no direct link to the size of the pump but closely related to application itself. As a result, specific requirements based on application specific standards and additional end-user measures are quite common and define every single step through design, production, quality control, testing, site installation and even shipment of the pump. Pumps are composed of two main components in terms of hydraulic design: Casing and Impeller. Fluid accelerates inside the impeller and steps up kinetic energy and then diffuses to casing. The energy transfer mechanism inside casing and impeller is quite complex. The flow in the centrifugal pump is generally characterized as three dimensional (3D), turbulent and also fluctuating when the flow is dominated by boundary layer separations, inlet/outlet circulations and cavitation. The design of the pump depends on many parameters and the pump geometry consists of very complex structures. These aspects make it difficult to define under which interactive process the unsteady fluctuating flow develops. Vibration, being one of the challenging problems of today, is one of the most important parameters pump designer shall consider. The excitation mechanisms in rotating machinery, especially fluid bearing equipments, may be very complex. Flow separation, cavitation, unsteady flow, recirculation and pressure pulsations are some of the reason for that. But these are not considered as main excitations for vibration, the coupling phenomenon between fluid and structure is defined as the major source. The influence of pressure pulsations on vibration can be characterized by the following; wake flow at impeller outlet part load recirculation distance between impeller vanes and diffuser vanes or volute tongue geometry of impeller trailing edge combination of number of impeller and diffuser vanes fluid properties and gas content inlet conditions resonance and piping excitations Taking into consideration above influencing parameters, the forced vibration of pump is very complex. It is not easy to design a pump just by experience with fair knowledge of fluid-structure interaction. Extended analysis and prototype tests are needed to be conducted in most cases. The evaluation of a hydraulically efficient pump; that is designed by the designers’ experience and similarity laws; with pressure pulsation, bearing life, vibration and noise level criterions are generally inspected with experiments after the pump is manufactured. In this context, necessary modifications and improvements in the design are only possible to be accomplished by checking the experimental results, thus with high costs. Recent developments in computer science led to reduction in pattern/mould production, casting and experimental costs since the flow and structural behavior of the pump can now be numerically simulated and design improvements and optimization process are made using computers. Numerical methods have wide-spread applications on different subjects. FEM (Finite Element Method) and CFD (Computational Fluid Dynamics) methods became indispensible part of design processes and have given a chance for designers to make “Virtual Experiments” before production. In these methods, the computational domain discretized to small computational grids or elements where governing formulations are kept valid for each computational node. FEM is generally used for static and dynamic structural analysis and the calculated results corresponds to nodal points at the element. CFD has different algorithms on discretization, FD (Finite Differences) or FV (Finite Volume) on defining nodal or elemental properties. FEM can also be used in CFD analysis but it is more easy and common to use other methods. Some commercial sofwares use both techniques depending on problem type or users’ choice. Some bottlenecks arise using computational methods and affect the validity of results; accuracy and numerical error high resolution mesh small time steps (transient analysis) hardware requirements software and license requirements post processing huge sized result data To overcome stability and accuracy problems on solutions, optimization of mesh size and measures of quality shall be proposed in the beginning of the analysis. It is also important to consider the computational cost (time and hardware), when defining the optimum computational grid. In terms of accuracy, mesh independence to be analyzed and convergence criteria are to be stated. In many analyses, it takes more time to generate an optimum computational grid than the time on solution phase. Furthermore in some cases, accuracy may have a second level importance where immediate estimate results are needed with an expected level error. Weather forecast can be a good example for this. In this study numerical analysis of a pump is performed with the commercial software ANSYS Workbench considering modal, fluid dynamics and structural aspects. In modal analysis, natural frequencies of the pump are calculated under three different supporting conditions. The calculated frequencies are compared with shaft rotation and blade passing frequency, and critical mode shapes are analyzed. Flow simulations are performed with Fluent using URANS (Unsteady Reynolds-Averaged Navier-Stokes) algorithm. Flow is analyzed for the impeller making one complete rotation in the casing, and special attention is paid to the volute tongue region and outlet pressure pulsations. Structural analysis is simultaneously conducted with flow analysis. Pressure fields, calculated with CFD, are imposed to volute casing and stress-strain-deformation levels are analyzed. Validations of simulations are done by the tests conducted. The pump is tested under specified operating conditions and off-design conditions in order to calibrate CFD model, and the results are found consistent with the simulations. The vibrations at the bearings are measured on both driven and non-driven side, and levels are checked to corresponding standards. To investigate the influence of piping and installation, the tests are conducted with two different flow control configuration. Results show that poor installations and piping design is dominant than pump itself on the vibration levels.
Yüksek Lisans
M.Sc.