Analytical Tools for Determination of Damped Unbalanced Rotor Response

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Representative Rotor Response Plot (API Standards) & Analytical Simulation

Lateral rotor-dynamic behavior is the most critical aspect in determining the reliability and operability of rotating equipment in the oil and gas industry – be it a centrifugal pump , compressor, steam or gas turbine, motor or generator. One way to evaluate operating reliability is identifying lateral rotor response to unbalance, i.e. by analytically determining damped unbalanced rotor response. Torsional response is sought only for train units comprising three or more coupled machines (excluding any gears). Experience suggests that the effect of other equipment in the train is normally not included in the lateral damped unbalanced response. Hence brief summary of various characteristics and a technique for analytical predictions of lateral behavior deserves attention by all.

The purpose of damped unbalanced rotor response is to identify critical speeds, associated amplification factors-AF (as per API standards AF greater than or equal to 2.5 is considered critical) and ability of rotor dynamics system to meet the separation requirements (margin of operating speed away from critical speed/s). The first step is ‘undamped’ unbalance response analysis for identifying mode shapes and critical speed-support stiffness map.  ‘Damped’ unbalanced response analysis then follows for each critical speed within the speed range of 0 % to 125 % of trip speed. Unbalance or side load is placed at the locations that have been determined by the undamped analysis to affect a particular mode most adversely. The magnitude of the unbalances is four times the value of U as calculated by U = 6350 W/N. The aim then is to identify the frequency of each critical speed, frequency-phase and response amplitude data, deflected rotor shape due to unbalance and bode plots to compare absolute shaft motion with shaft motion relative to the bearing housing (support stiffness <  3.5 times the oil-film stiffness).

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Simultaneous Design for Turbocharger Compressors and Turbine Wheels

AxSTREAM Blade Profiling
Figure 1- AxSTREAM 3D Blade Profiler for Radial Designs

Increasing regulation for reducing emissions has forced the automotive industry to accept different technologies over the years in order to stay ahead of the market. In an industry that is so accustomed to internal combustion engines, new solutions such as electric motors and turbocharger systems have allowed experts in other industries to cultivate an influence in the automotive market. Specifically in the realm of turbomachinery, increased development has gone into designing turbochargers in order to minimize the effect and size of internal combustion engines. Design challenges are inherent in the fact that an engine is a positive displacement device whereas the turbocharger falls under aerodynamic turbomachinery. The two separate machine types have distinctly different flow characteristics, and the proper sizing of a turbocharger for its parent engine requires proper modeling of the engineering system as a whole.

In general, initial turbocharger sizing becomes a matter of obtaining the necessary boundary conditions required for a preliminary design. A thermodynamic cycle analysis of an ICE-Turbocharger system will allow the designer to obtain an initial idea of the bounds

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Steam Heat & Mass Balance Considerations in Refineries

Optimizing the heat and mass flow i.e. steam balance in a plant that has several levels of steam pressures is not a simple task due to the vast array of equipment such as turbines, heat exchanges, steam auxiliaries and accessories used. The steam balance of a refinery plant is further complicated because of use of steam for chemical processes and compression. Depending on processor licensor, technologies and many other traditional factors, it is not uncommon to see steam pressure levels defined in refineries as simply HP & LP or HP,MP & LP or as complex as VHP, HHP, HP, MP and LP.

The traditional approach to designing a steam system is to install steam generators able to generate steam at the maximum pressure and temperature with enough redundancy in capacity as required by the process. Modern steam generators tend to be inclined towards higher pressure steam rather than lower pressure steam – saturated high pressure steam has higher temperature meaning  less exchange surface in heat exchangers and reboilers, high density of high pressure steam requires less bore in the steam mains. Consequently, the usage of high pressure steam represents less capital expenditure. The resultant philosophy is to generate steam at the highest possible temperature and pressure, expand steam from a higher pressure to a lower pressure level through the most efficient means possible and use process at the lowest economically attractive pressure and temperature.

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Aircraft Engines: A Need for Increased Performance and Safety

Turbine engine of airplaneThe necessity for a robust aircraft engine design is strongly associated with not only flight performance, but also to passengers’ safety. The fatigue on the blade of CFM56 engine did not prove to be fatal in last August’s incident. None of the 99 passengers was hurt, but parts of the engine broke apart damaging the fuselage, wing and tail, and forcing the Boeing Co. 737-700 to an emergency landing. However, that was not the case in July 6, 1996, when the left power plant on a Boeing MD-88 broke apart while accelerating for take-off and the shrapnel was propelled into the fuselage killing a mother and a child seated in the Delta Air Lines Inc. aircraft [1]. A few years earlier, in January 8, 1989, a CFM56-3 blade failure proved to be fatal for 47 out of 118 passengers of the British Midlands Airways (BMA) Ltd Flight 92 departed from London Heathrow Airport en route to Belfast International Airport. Based on Federal Aviation Administration’s accident overview [2] post-accident investigation determined that the fan blade failed due to an aero-elastic vibratory instability caused by a coupled torsional-flexural transient non-synchronous oscillation which occurs under particular operating conditions. An animation describing this process is available at the following link: (Fan Blade Failure).

The last example [3] of this not so cheerful post took place on July 29, 2006, when a plane chartered for skydiving experienced jet engine failure and crashed. Tragically, there were no survivors. The failure was attributed to aftermarket replacement parts. The aircraft was originally equipped with Pratt & Whitney jet engines, specifically made with pack-aluminide coated turbine blades to prevent oxidation of the base metal. However, during the plane’s lifetime, the turbine blades were replaced with different blades that had a different coating and base metal. As a result of the replaced turbine blade not meeting specification, it corroded, cracked and caused engine failure.

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Rotating Equipment Specialist in the Oil and Gas Industry – A Turbomachinery Professional

Turbomachinery is a core subject in many engineering curriculums. However, many graduates joining the oil and gas industry are designated as rotating equipment engineers. Though turbomachinery and rotating equipment are used synonymously, all turbomachinery are rotating equipment but not vice versa. Turbinis in Latin implies spin or whirl, and a natural question that arises is – what are the factors that differentiate turbomachinery?  In a general sense the term, “rotating” covers  the majority equipment used in the industry be it in the upstream, mid-stream or the downstream segment. Yet top rotating equipment specialist in the industry are seen spending their prime time or often being associated with certain unique and specific types of critical rotating machines – the turbomachines.Oil and Gas

In a classical sense, turbomachines are devices in which energy is added into or taken out from a continuously flowing fluid by the dynamic action of one or more moving blade rows. By this definition propellers, wind turbines and unshrouded fans are also turbomachines but they require a separate treatment. The subject of fluid mechanics, aerodynamics, thermodynamics and material mechanics of turbomachinery when limited to machines enclosed by a closely fitting casing or shroud through which a measurable quantity of fluid passing in unit time makes the practical linkage to rotating equipment – those which absorb power to increase the fluid pressure or head (fans, compressors and pumps) and those that produce power by expanding fluid to a lower pressure or head (hydraulic, steam and gas turbines). Further classification into axial, radial and mixed type (based on flow contour), and impulse & reaction (based on principle of energy transfer) is common. It is the large range of service requirement that leads to different type of pump (or compressor) and turbine in service.

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Liquid Rocket Propulsion with SoftInWay

Preliminary Design of Fuel Turbine

Operation of most liquid-propellant rocket engines, first introduced by Robert Goddard in 1926- is simple. Initially, a fuel and an oxidizer are pumped into a combustion chamber, where they burn to create hot gases of high pressure and high speed. Next, the gases are further accelerated through a nozzle before leaving the engine. Nowadays, liquid propellant propulsion systems still form the back-bone of the majority of space rockets allowing humanity to expand its presence into space. However, one of the big problems in a liquid-propellant rocket engine is cooling the combustion chamber and nozzle, so the cryogenic liquids are first circulated around the super-heated parts to bring the temperature down.

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The Future of Nuclear Power Plants

With the blast of the French nuclear power plant a few weeks ago, safety of nuclear power plant designs has fallen under more scrutiny. Although according to sources the blast took place in the turbine hall and no nuclear leak was found, this event has brought more attention to improved design and operation standards.

Following the incident earlier this month Toshiba, a Japanese multinational company, announced the resignation of its chairman following a $6.3 billion loss in their nuclear sector –also withdrawing from the nuclear business. The two back to back events have highlighted the main two problems of nuclear power: high cost and environmental/safety concerns. Said to be a green technology, nuclear power raises concerns with potential nuclear meltdown and risk of safety from toxic waste, accompanying the fact that building a new plant cost around $5,000.00 per kilowatt of capacity with around 6 years of lead time. Each dollar invested on a nuclear power plant has about 2-10 less carbon savings and is 20-40 times slower compared to other alternatives. Yes, evidently nuclear power is found to be very reliable, enabling consistent baseload energy production at any time of day and night. Though, it has been questioned whether this reliability is worth the high cost of nuclear production, in fact all nuclear plants are still operating with 100% subsidized.

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Turbomachinery Rotor Dynamics – Latest Modelling & Simulation CAE for Design and Analysis, using SoftInWay’s Integrated Tool

The Rotor-Dynamic System of a typical turbomachine consists of rotors, bearings and support structures. The aim of the designer undertaking analysis is to understand the dynamics of the rotating component and its implication. Today the industry practices and specifications rely heavily on the accuracy of rotor-dynamic simulated predictions to progressively reduce empirical iterations and save valuable time (as repeated direct measurements are always not feasible). Be it a centrifugal pump or compressor, steam or gas turbine, motor or generator, the lateral rotor-dynamic behavior is the most critical aspect in determining the reliability and operability. Such analytical predictions are often tackled using computer models and accuracy in representing the physical system is of paramount importance.  Prior to analysis it is necessary to create a detailed model, and hence element such as cylindrical, conical , inner bore fillet/chamfer, groove/jut, disk / blade root and shroud, copy/mirror option, bearing element and position definition are built. Stations (rather than nodes) having six DOF (degrees of freedom) are used to model rotor-dynamic systems. Typically for lateral critical analysis each station has four DOF, two each translational and rotational (angular). Decoupled analysis followed by coupled lateral, torsional and axial vibration makes prediction realistic and comprehensive. The mathematical model has four essential components, i.e. rotating shafts with distributed mass and elasticity, disks, bearing and inevitable synchronous imbalance excitation. Components such as impellers, wheels, collars, balance rings, couplings – short axial length and large diameter either keyed or integral on shaft are best modelled as lumped mass. Bearings, dampers, seals, supports, and fluid-induced forces can be simulated with their respective characteristics. Bearing forces are linearized using dynamic stiffness and damping coefficients and together with foundation complete the bearing model. The governing equation of motion for MDOF system require  determination of roots (Eigenvalue) and Eigen Vectors. Lateral analyses – such as static deflection and bearing loads, critical speed analysis, critical speed map, unbalance response analysis, whirl speed and stability analysis, torsional modal and time transient analyses are then performed.

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Double Flash System Application in Geothermal Power

Geothermal power market has been showing sustainable growth globally, with many installations in developing countries. As people turn to renewable sources while demand for energy is experiencing rapid growth, geothermal is found to be a reliable energy source and current development is calculated to increase global capacity by over 25%. Geothermal power plants can usually be divided into several types of cycles, including binary, flash, double flash and more. Flash power plants are found to be the most common forms of geothermal power plant and specifically, we are going to talk about the double flash cycle.

A flash system produces high pressure dry steam to move the turbine, generating electricity after going through a flash separator. A double flash system uses two flashes separating systems in order to generate more steam from the geothermal liquid and increase cycle output. The cycle starts with high temperature fluid extracted from a geothermal source to a high pressure separator (HPS) for flashing. The HPS produces a saturated steam that enters the high pressure turbine and the remaining brine is directed into a secondary low pressure separator (LPS). Reducing the flashing pressure increases the mixture quality in the LPS, which results in higher steam production. Low pressure saturated steam is mixed with the steam flow exhausted from the high pressure turbine and the resulting steam flow is directed to the low pressure turbine and produces more electricity. Steam that is exhausted from the low pressure turbine will then be compressed and injected back to the ground. In a flash system, separator pressure has a significant effect on the amount of power generated from the system – and the flashing pressures also influence double flash cycle significantly. In order to optimize one design, the value of parameters versus cost of operations should be taken into account.

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Surge Conditions of Centrifugal and Axial Compressors

Centrifugal and axial compressors must operate within certain parameters dictated by both the constraints of the given application as well as a number of mechanical factors.  In general, integrated control systems allow compressors to navigate dynamically within their stable operating range.   Typical operating ranges for compressors are represented on a plot of volumetric flow rate versus compression ratio.  Compressors have a wide number of applications, from household vacuum cleaners to large 500 MW gas turbine units.  Compression ratios found in refrigeration applications are typically around 10:1, while in air conditioners they are usually between 3:1 and 4:1.  Of course, multiple compressors can be arranged in series to increase the ratio dramatically to upwards of 40:1 in gas turbine engines.  While compressors in different applications range dramatically in their pressure ratios, similar incidents require engineers to carefully evaluate what is the proper operating range for the particular compressor design.

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Figure 1- Typical Performance Map Limits – Compressor Ratio (Rc) vs. Volumetric Flow Rate (Qs)

For intensive applications of centrifugal and axial compressors, the phenomenon of surge resides as one of the limiting boundary conditions for the operation of the turbomachine. Essentially, surge is regarded as the phenomena when the energy contained in the gas being compressed exceeds the energy imparted by the rotating blades of the compressor. As a result of the energetic gas overcoming the backpressure, a rapid flow reversal will occur as the gas expands back through the compressor. Once this gas expands back through to the suction of the compressor, the operation of the compressor returns back to normal. However, if preventative measures are not taken by the appropriate controls system or any implemented mechanical interruptions, the compressor will return to a state of surge. This cyclic event is referred to as surge cycling and can result in serious damage to the rotor seals, rotor bearings, driver mechanisms, and overall cycle operation.

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