Rotor Dynamics Study of 4-Stage Compressor – from Theory to Application

Rotating machines have huge and important roles in our daily life although we may rarely think about them. Steam turbines at electrical power plants rotate the electrical generator shafts which produce electricity coming into our homes and offices. Driving to or from work, the reciprocating cycle in your vehicle’s internal combustion engine results in rotation of the transmission and the wheels of vehicles, while the electric car wheel operation is a result of induction motor rotation. If you get on an airplane, rotation of the turbo reactive gas turbine engine produces the effective thrust to sustain flight by moving, compressing and throwing the gas behind the plane. We can even find the useful effects of rotation in our kitchens when we are blending the food or washing our closes.

Although these rotating machines are different, the approaches to modelling their rotor dynamics are pretty much the same, since similar processes occur in rotating parts which differ in their vibrations from the non-rotating machines.

Do you remember the example of rotating washing machine? Have you ever seen it jumping on the floor trying to squeeze out your closet? We bet you have. This is the simplest example of the increased unbalance affecting the amplitudes of machine vibrations. Washing machines are designed to experience these noticeable vibrations during their operation without breaking. But the steam turbine or compressor rotors which have the tight clearances between the impellers and the casing can not boast of that leeway. In addition to that, the excessive vibrations significantly influence the machine’s useful life due to the increased fatigue.
This is why the rotor dynamics predictions are one of the most important parts of rotating machine analyses. And although they may seem easier than comprehensive stress-strain investigations of machine components, in some cases the rotor dynamics analysis can be trickiest part.

Usually, the rotor dynamics analyses are divided into lateral and torsional stages depending on the nature of rotor response to be used. They are discussed in different types of standards (API [1], ISO [2], etc.). Let’s consider the example of the lateral vibrations of a 4 stage compressor rotor with an operational speed of 8856 rpm.

Fig. 1 - 4 Stage Compressor Rotor
Fig. 1 – 4 Stage Compressor Rotor

This rotor rotates in the 4 pad tilting, pad oil film journal bearings. The characteristics of these bearings should be determined carefully to ensure that there will not be an excessive wear, heat generation or friction in them. Read More

Common Challenges in Rocket Engine Rotor/Bearing Systems

Rocket engines are the perfect creation of the human mind, incorporating our existing knowledge in aerodynamics, thermodynamics, solid and fluid mechanics, and rotor dynamics. Believe it or not, rocket engines designs contain turbopumps that move fuel and the oxidizer into a combustion chamber creating the perfect conditions for their burning and high-efficiency rocket motion. The word “turbopump” means that the pump is driven by the turbine installed on the same shaft or connected to it through a gearbox. This thrilling tandem results in a bunch of rotor dynamics effects inherent in pumps, turbines, high-speed rotors, cryogenic temperature materials, etc. And all these effects must be carefully taken into account during rotor dynamics studies.

A standard schematic of an internally geared turbopump consists of the liquid hydrogen (LH2, fuel) and liquid oxygen (LO2, oxidizer) rotors.

Fig. 1 - Internally geared turbopump model
Fig. 1 – Internally geared turbopump model

Although the rotor dynamics model is usually simpler than the CAD models, it looks quite complicated in the case of the turbopump. The rotors contain sections that are hollow and sections with some elements inside the hollow space. Read More

Rotor Dynamics Challenges in High-Speed Turbomachinery for HVAC Applications

In comparison to large steam and gas turbines, the rotating equipment found in heat ventilation and air conditioning (HVAC) applications is often seen as more simplistic in design. However, sometimes a simpler model of a rotating machine does not mean a simpler approach can be used to accurately investigate its rotor dynamics behavior. For example, a large number of effects should be taken into account for single-stage compressors used in HVAC applications. Three important ones include:

  1. High values of rotational speeds above the first critical speed;
  2. Rigid rolling element bearing used in the design and therefore a relatively flexible foundation which should be modeled properly;
  3. Aerodynamic cross-coupling adding additional destabilizing forces to the structure.

All these effects should be modeled properly when performing lateral rotor dynamics analyses of HVAC machines. And, in some cases, this simpler model can prove a much more challenging task than building the complex model of a steam turbine rotor.

Let’s consider a seemingly simple example of a high-speed single-shaft compressor for HVAC application (Figure 1). It consists of the compressor and motor rotors, the flexible coupling connecting them, the ball bearings connecting the rotors to the bearing housing joined with the compressor volute, and the structural support.

Fig. 1 - Single stage compressor model
Fig. 1 – Single-stage compressor model [1]
The compressor rotor is connected with the motor through a flexible coupling. Its lateral vibrations can be considered uncoupled from the motor rotor vibrations, and the lateral rotor dynamics model appears pretty straightforward (Figure 2).

Fig. 2 - Rotor dynamics model of the single stage compressor rotor
Fig. 2 – Rotor dynamics model of the single-stage compressor rotor

However, additional factors are discovered if you include the mechanical properties of the supporting structure when considering the lateral rotor dynamics calculations. These factors are very important to an accurate model. Read More

Pump Rotor Dynamics – from Residential Pools and Human Hearts to Heavy Duty Industry Applications

You rarely find a rotary machine with a wider range of applications than pumps. These machines acting in a single role can be installed both to supply the water to a garden pool and move the crude oil in pipelines.

And even more, the same simple pump can substitute the functions of the human heart by moving the blood through it.

Fig. 1 - Left ventricular assist device - a tiny pump moving the blood in the human body
Fig. 1 – Left ventricular assist device – a tiny pump moving the blood in the human body [1]
Although the heavy duty industry applications of pumps are less delicate at first sight, they can still generate similar effects of this unique nature which is inherent only to this type of machine and should be studied carefully when executing rotor dynamics calculations. Read More

Active Magnetic Bearings – When Magic Serves Engineers

From the beginning of the turbomachinery era, in the 19th century, engineers have been thinking about ways to reduce losses in rotating machines. Losses connected with fluid motion or producing the useful effects are related to the main purpose of machine operation,while losses in rotor bearings are just annoying and inevitable. Fluid film and rolling element bearings are effective solutions, but their operational principles cause increased friction – the best predictor of losses. But what if we could reduce the losses in rotating machines by avoiding the friction in required supports? What if a rotor could levitate and rotate in the air held by some magic forces? And furthermore, what if this magic could give us even bigger dividends, for example, enabling variable stiffness of rotor supports and safe passing through resonances? Luckily, engineers have already invented how to turn this magic into reality with active magnetic bearings.

The early patents of active magnetic bearings principles were recorded during the World War II, but the decisive breakthrough in production and applications of them were made during the the last three decades when the latest research about the active magnetic bearing operation and control made utilization feasible and economically viable [1].

The early patents of active magnetic bearings principles were recorded during the World War II, but the decisive breakthrough in production and applications of them were made during the the last three decades when the latest research about the active magnetic bearing operation and control made utilization feasible and economically viable [1].

Active Magnetic Bearing and its Components
Active Magnetic Bearing and its Components [2]
The main idea of an active magnetic bearing is based on the electromagnetic processes. Electrical current passing through densely wound copper coils creates magnetic fields which interact with a magnetized sleeve connected to the rotor.

Sounds pretty simple, right? But why on Earth did it take so much time to go from the general ideas to a real industrial application of active magnetic bearings? Read More