One of the biggest causes of pump failure is related to the rotor bearing system. Now, pumps are usually planned to run at higher speeds and loads to increase power. These operations require special attention to rotordynamic analysis at the planning stage. This includes analog bearings, seals, and other components of the pump.

There are many pump types available for industrial applications. These pumps can be classified according to planning, operating principle, special functions, characteristics of operating fluid, equipment (centrifugal, axial, screw, screw, volumetric), etc. For each type of pump, there are challenges in modeling and analyzing the rotor bearing system. On the other side of the coin, many rotor dynamics methods and principles are similar to each rotating machine.

This article will highlight two basic types of pumps: horizontal and vertical. Without further explanation of the planning, it will outline the common methods and differences in rotor dynamics, bearing and seal simulation. IMO pump ,

Vertical and horizontal pumps: planning differences

The main difference between vertical and horizontal pumps lies in the direction and shape of the shaft. Horizontal pumps have a horizontally placed shaft (above), located between bearings or cantilever orientations. The shaft in a vertical pump is positioned straight. The most common type of vertical pump is the vertical turbine pump (VTP) - Figure 2. Vertical pumps, such as VTP, typically have long, spaghetti shaped shafts that are coupled to the motor (above or below). Another planning feature of vertical pumps is the cylindrical shell that affects the dynamic characteristics of the pump. These planning specifications have an impact on how to deal with rotordynamic modeling and vertical pump profiling.

What makes the rotordynamics of vertical pumps different?

flexibility

The vertical pump has a long shaft to improve flexibility. These flexible shafts have closely spaced modes and closely spaced frequency ranges. In this case, resonance oscillation with increased amplitude may occur, especially when the pump operates at a wide range of speeds.

The shell structure of vertical pump (pipe) is also very flexible. In consideration of this, the flexibility of the shell and the drum assembly should be considered when calculating the stiffness characteristics of the intermediate radial shaft support. In addition, the casing structure of the vertical pump may experience high vibration due to its flexibility, so the frequency of the pipeline should also be analyzed. IMOUS pump

axial force

The cantilever vertical pump supported by the thrust bearing on the top of the machine bears the axial tension caused by gravity load. On the contrary, if the thrust bearing is placed at the bottom of the machine, the compression force acts along the shaft. The thrust of the impeller contributes more to the shaft tension and stiffness. All these forces will change the flexure stiffness, natural frequency and critical speed of the rotor, so it is very important to consider these factors through rotor dynamics analysis before the machine is put into operation.

Bearings and seals

Bearing is one of the most critical components in pump. The bearing supports the shaft and maintains the smooth rotation of the rotor to reduce the conflict of the moving parts of the pump. The bearing also brings stiffness and damping to the rotor bearing system. The bearings used for the pump can be divided into radial (transverse support shaft) and axial (applicable to axial load) bearings. The most common bearing types in pump operation are ball and roller bearings, hydrodynamic oil film (babbitt) journal bearings and pivot pad bearings (axial thrust load support).

In pump products, seals are equally important. As with bearings, pump seals are the source of stiffness, damping and additional "mass" coefficients of the rotor bearing system, which will change the dynamics of the entire system. Compared with the rigid support system, the pump with bearing and seal system has different natural frequencies. , IMOAB pump ,

Compared with horizontal pump, the bearing seal system modeling of vertical pump is different. One difference is that there may be a large number of radial bearings supporting the long shaft in vertical pumps. In many cases, many stages in the pump (such as spiral/spiral stage type) will add the number of bearings and seals - the total number of bearings and seals may reach dozens. Figure 4 shows how many elements need to be modeled to achieve accurate rotor dynamics. The combination of long shaft, added tolerance and misalignment with many radial bearings may lead to rapid and nonlinear changes in the bearing stiffness in the bearing, where the axis is close to the bearing wall.

The second difference, perhaps more important than the above, is that the radial bearing load in the vertical pump is lighter (there is no gravity in the radial direction), which makes the estimation of dynamic bearing coefficient more disorderly. The no-load cylindrical bearing is one of the reasons for the safety and stability of vertical pumps. Therefore, nonlinear analysis is critical to accurately evaluate the rotor behavior of vertical pumps with long shafts and unloaded bearings.

Finally, in most VTP submersible pumps, the bearings are in a pressurized environment and lubricated by the process fluid, usually with contaminants. In addition, the working fluid mixture may change composition, and the operating conditions (speed) of the pump are usually variable. Therefore, these radial bearings experience accelerated wear. Considering the random characteristics of application conditions, the prediction of their characteristics is very messy. The worst case modelling approach can be used to predict dynamics and reliability to prevent severe failures.

Whatever the pump type, what effects should be considered?

The analysis of some fields is similar. Some other important influences that should be considered in the rotordynamic analysis of vertical and horizontal pumps are:

Static and dynamic radial load generated in the impeller direction due to uneven distribution of the gap between the impeller and volute

Inertia and hydraulic unbalanced force that should be introduced in the impeller direction

Useful additional mass of impeller and shaft

Dry, wet and process fluid conditions, and "new" and "worn" clearances considered during bearing and seal profiling

Lomakin effect: force generated at the wear ring and throttle bushing in the centrifugal pump

Other general functions and technologies similar to most rotating machines described in API 684 standard 2

Although the modeling and analysis methods of horizontal pump and vertical pump are usually similar, the vertical pump has its own set of characteristics, which makes the rotor dynamics analysis and the simulation of bearings and seals more disorderly. The primary challenges found in vertical pumps relate to structural and operational standards, including:

the major axis IMO agent

Many stages

Its bearings and seals

No load radial bearing

Axial force caused by gravity

Because of these planning characteristics, the vibration problem and the structure/life number problem of the vertical turbine pump are more simple. This may be a headache for rotordynamic analysts dealing with such pumps. Fortunately, today's engineers can use digital tools to deal with these problems. Through advanced simulation software, dynamics standards and technical publications (such as references 1 and 2 below), these impacts can be modeled and analyzed to ensure safe and reliable operation.