Demystifying S1-S2 Optimization in Turbomachinery

  1. Historically turbomachinery development began with empirical rules postulated by early pioneers. With the need for jet engine for aircraft propulsion, dimensionless analysis became popular, followed by the 1 D mean line design and 2D meridional methods. Today 2D meridional methods with 3D blade to blade CFD/FEA methods are a necessity as efficiency and reliability requirements are further pushed.

 

  1. One key aspect of 2D meridional design is S1-S2 optimization, which is a time consuming, laborious task and hence subject to human errors. S1-S2 optimization is a task of reviewing, adjusting and optimizing the flow path in the Tangential (S1 or blade-to-blade or pitchwise) and the Meridional (S2 or span wise) planes. The main purpose is to:
  • Fit the flow path to specific meridional dimensional constraints
  • Adjust blade-to-blade parameters while taking into account structural constraints.

 

  1. The need for preserving the turbine performance (that has been selected) and then allowing alteration of flow path dimensions requires use of inverse solvers. The parameters that affect the blade-to-blade calculations are the number of blades and the chord. These can be optimized in the S1 optimization, while meeting structural requirements. For optimizing the spanwise distribution, the S2 optimization is chosen to modify/ fine tune the Mollier diagram (Heat drop or Enthalpy) to meet the desired performance. In the S1 optimization adjusting important pitch-wise parameters such as the chord, pitch and number of blades to preserve optimal “solidity”/”relative pitch” by  considering structural constraints. The S1 optimization can be performed either for the full turbine (cylinder) or for particular components (rotors/stators).

 

  1. An effective analysis requires six different criteria for optimization which is basically grouped into two categories. The groups are based on the chord that gives the maximum efficiency or minimum length that is required to meet the structural requirements. For both of these groups the optimization can be performed to:
  • Obtain the optimum chord, with a constant relative pitch
  • Get the optimum chord and optimum relative pitch
  • Obtain the optimum relative pitch while maintaining the chord constant

 

  1. If the max efficiency chord is selected then the chord optimization is done with first preference to the aerodynamic criteria and meeting the structural requirement for the given material. If the min length chord is selected, then the chord optimization is done with the objective to obtain the smallest value which meets the structural requirement and MSF (Margin of Safety) value nearer to “zero”.

 

  1. The AxSTREAM® software suite provides an  automated process for S1-S2 optimization enabling improved designs and reducing engineering hours.

 

The Optimization Challenge in the Development of Turbomachinery

Optimization (or parametric studies) of a twin spool bypass turbofan engine with mixed exhaust and a cooled turbine can be considered one of the most complex problem formulations. For engine selection, determining the thrust specific fuel consumption and specific thrust is necessary against variables such as design limitations (Inlet temp, etc.), design choices (fan pressure ration, etc.) and operating conditions (speed & altitude). The task involves cycle level studies following machine, module, stage and component level optimization. This calls for an integrated environment (IE) and it is desirable to have such an IE operating on a “single” platform.

Historically IE was developed for the design of axial turbines (mainly steam). Later, it was expanded for gas turbines (especially blade cooling calculations) and axial compressors via plug-in modules. The new challenge designers face today is developing mixed flow machinery. An effective system for modern turbomachinery design needs to do the following: AxSTREAM_ION

  1. Involve a set of design modules necessary for design procedures under one operating platform (an umbrella, per se) that performs initial sizing and optimization, 1D formulations, and build 3D geometric blade models that are available for final refinement by means of 3D aerodynamic CFD and stress analysis
  2. Have the ability to automate and optimize calculations using embedded models
  3. Improve flexibility in carrying out interactive design scenarios including rollback to previous version(s), version support, project integrity, ensure expandability, scalability, and maintainability
  4. Provide users with convenient mechanisms to input, edit, display and export data to other systems.

The architecture presented below gives the designer an opportunity to design axial, radial and mixed flow turbomachinery in an integrated environment on a single platform. The objective is to review a large number of variants and design parameters to realize optimum results. From a software engineering perspective, the majority of modules are required to be compatible with every type of turbomachinery, and specific modules must be able to run simultaneously (axial and radial turbine, axial and radial compressor). Consequently, a solution using common modules (project data access, graphical display of information, multi-choice calculation and optimization, import/export, etc.) and specific machine modules operating in tandem emerges. It embodies cycle level analysis and further down to blade (impeller) 3D profiling, stress analysis, and 3D Flow analysis.

Such an IE platform shortens the design development process significantly, thereby decreasing engineering costs and improving productivity

To learn more about our new integrated software tool for automated design, register for our upcoming free webinar http://www.softinway.com/education/webinars/automated-design-of-an-industrial-steam-turbine/

Rerates, Upgrades, and Modifications to Steam Turbine

Steam Turbine DesignSteam turbines are designed to have long, useful lives of 20 to 50 years. Often, many parts of steam turbine are custom designed for each particular application, however, standardized components are also used. It is therefore inherently possible to effectively redesign a steam turbine several times during its useful life while keeping the basic structure (foot print, bearing span , casing etc) of these turbines unchanged! Indeed this is also true for many turbomachines. These redesigns are usually referred to as rerates and upgrades, depending on the reasons for doing them. The need for changes to hardware in an existing turbine may be required for (a) efficiency upgrades, (b) reliability upgrade (including life extension), (c) rerating due to a change in process (Process HMDB, use in combined cycle etc), and (d) modification for a use different from that of its original design. Typical changes include hardware components such as buckets/blades, control system,  thrust bearing , journal bearing , brush and laby seals, nozzle/diaphragm , casing modification,  exhaust end condensing bucket valves, tip seals and coatings.

Performance and Efficiency Upgrade The basic power and/or speed requirements of a steam turbine may change after commissioning for various reasons. The most common reason is an increase (or decrease) in the power required by the driven machine due to a plant expansion or de-bottlenecking. Other reasons include a search for increased efficiency, a change in the plant steam balance, or a change in steam pressure or temperature. Because steam turbines are periodically refurbished, an opportunity exists to update the design for the current operating environment. Turbine OEM’s , services companies and end users often face a challenge of undertaking engineering work within the very tight  time frame available for maintenance.  The AxSTREAM® software suite provides users with an automated capability of rerate, upgrade and modifications for performance and efficiency objectives. A summary of such features highlighting the capabilities is presented below:

Image for Retrofitting of Steam Turbines

 

Thermo-Physical Properties of Fluids for Simulation of Turbomachinery

Computer simulation and use of CAE/CAD are well-established tools used to understand the critical aspects of energetics (various losses), kinematics (velocities, mach no. etc.) and thermodynamics (pressures, temperatures, enthalpy etc) in thermodynamic cycles and turbomachinery. Computational models are now enabling the design and manufacture of machines that are more economical, have higher efficiency and are more reliable. Accuracy of complex processes that are simulated depends on thermos-physical properties of the working fluid used as input data. The importance of such properties was recognized when it became evident that a steam turbine cycle can have efficiency variance by a few percentage points depending on the chosen set of fluid properties.

Today the thermo-physical properties data is represented in the form of a set of combined theoretical and empirical predictive algorithms that rest on evaluated data. These techniques have been tested and incorporated into interactive computer programs that generate a large variety of properties based upon the specified composition and the appropriate state variables. Equations of state, correlations, or empirical models are used to calculate thermos-physical properties of fluids or mixtures. Examples of this include Helmholtz energy based equations, cubic equation of state, BWR pressure explicit equations, corresponding states models, transport models, vapor pressure correlations, spline interpolations, estimation models or calculation methods for vapor-liquid equilibrium or solubility, and surface tension correlations. Further fitting techniques, and group contribution methods are incorporated. The following broad level properties are often used in simulation tools:

  1. Thermodynamic properties including equation of state, phase equilibria, p-V-T behavior, heat capacity, enthalpy, thermal expansion, sound speed, and critical phenomena.
  2. Transport properties including thermal and electrical conductivity, viscosity, mass diffusion, thermal diffusion, non-Newtonian behavior, and thermal, thermoacoustic, and other diffusion waves.
  3. Optical and thermal radiative properties including dielectric constant, refractive index, emissivity, reflectivity, and absorptivity.
  4. Interfacial properties including solid-solid interfaces, surface tension, interfacial profiles, interfacial transport, and wetting.



Databases are now available for hydrocarbon mixtures, including natural gas, as well as a number of pure and mixed fluids of industrial importance. IAPWS, NIST and Coolprop are a few examples of such resources that provide valuable tools for turbomachinery and refrigeration engineers, and chemical and equipment manufacturers. One example is the IAPWS-IF97 that divides water and steam properties into five distinct regions.

Another example is properties of R134a expressed as 32 term, modified Benedict-Webb-Rubin (MBWR) equation of state, the accuracy of equation of state is estimated to be ± 0.2 % in density, ± 1 % in constant volume heat capacity and ± 0.6 % in sound velocity. The thermos-physical property databases provide core information for process modeling and development. The completeness, correctness, currency and reliability of the data as well as the integrity and management of the database itself are important factors in the ultimate reliability of the modeled process.

Turbomachinery Software for Education

Turbomachinery design has significantly evolved over the last two decades, as supporting education and training methods and techniques remains a challenge. Diversity of technologies covered in the varying courses and extensive use of software by industry designers makes the task of delivering the course curriculum that meets expectations of industry and students difficult. Many educational institutes and business use generic CAE tools for the purpose of learning turbomachinery through student projects. While generic tools have proven their value in research and design, the comprehensiveness of these tools to tackle real life turbomachinery situations is far from desired. The inexperience of fresh graduates from universities and colleges in their inability to perceive a 4D machinAxSTREAM EDUe (3D plus time), traditionally taught using a 2D blackboard, is evident. A student is not only required to have a very good understanding of underlying fundamentals, but is also required to address multitude of design, analysis and optimization problems within the limited time available for education. Coupling of theoretical and computer aided design knowledge to augment the capability of students to contribute to the industrial endeavor is necessary. Such a solution provides students with implicit understanding of the level of detail required by final designs, such as mean line design to the specification of a blade profile varying from hub to tip of a blade, and further complexities of iteration due to an aerodynamically correct blade profile being unsuitable because of stress levels or excitation frequencies and much more. AxSTREAM® EDU introduces multiple dimensions of design required by turbomachinery very early in the instruction process which, by using,  the students are able to develop insights that traditionally are difficult to attain in the same time frame. The use of AxSTREAM® EDU as a design software has been proven to multiply the skills of the students, enabling broad 3-D design considerations and visualization seldom possible otherwise.

AxSTREAM® EDU provides the user with the ability to design many different types of turbomachinery from scratch, such as axial turbines and compressors, radial compressors and turbines, axial fans, integrally geared compressors, mixed flow turbines and compressors and more. The moot question is how important is preliminary design? The efficiency gain possible to achieve in the preliminary design is of the order of 5-10 %, as compared to 0.5 % using 3D optimization (blade profiling, stress and CFD). One has an option of spending several weeks running  full 3D CFD calculations in generic software to try to optimize 0.5% of design, or spending much less time and resources using AxSTREAM® to figure out the best flow path design, followed by use integrated stress, CFD and rotor dynamic solvers!

Expander Configurations and Torsional Analysis

Lateral rotor-dynamic behavior is often discussed as one the critical aspects in determining the reliability and operability of rotating equipment. However, as multiple equipment are coupled together to form trains for centrifugal pumps, fans/blowers, compressors, steam or gas turbines and motors or generators, torsional behavior requires a thorough analysis. As per industry standards, torsional response is sought only for train units comprising of three or more coupled machines (excluding any gears).Blog 6

The configurations of the expanders used in the oil and gas industry makes it not only ideal but mandatory to perform train torsional analysis.  Expander trains are commonly used in CCU and FCU units and in the production of nitric acid. Serving the purpose of energy recovery, various arrangement for power recovery train are illustrated to the left:

As part of torsional analysis, the drive-train critical speeds (rotor lateral, system torsional, blading modes, and the like) need to be established to ensure they will not excite any critical speed of the machinery and the entire train is suitable for the rated speed and starting-speed hold-point requirements of the train. Finding frequency margins (torsional natural frequencies and torsional excitations) and if necessary undertaking stress analysis is mandated to demonstrate that resonances do not have  an adverse effect.RD

Such analysis requires modelling complexities of flexible supports, foundation, rotor seal interaction, instabilities etc. of the entire train and their interaction. SoftInWay’s CAE tool AxSTREAM® RotorDynamics is comprehensive, user friendly, and fully integrated with modules for flowpath and blade design making it unique to undertake train torsional analysis. Further information about the software is available by following the link

Performance Simulation and Optimization of CCPP with Turbine Inlet Air Cooling

It is well established that the performance of combustion air turbines (gas turbines) is sensitive to ambient air temperature. As the ambient air temperature increases beyond standard design point  (ISASLS), the power output and exhaust gas flow rate reduces while the heat rate and exhaust gas temperature increases. While the trends are similar for heavy duty and aeroderivative gas turbines, due to the inherent nature of design the results are steeper for aeroderivatives.  Various types of turbine inlet cooling technologies such as evaporative cooling, refrigerated inlet cooling and thermal energy storage systems have been practiced with varying degree of success, each having its potential advantages and limitations.  Simplicity and cost advantage gained in evaporative cooling is offset by limitation of cooling along web bulb depression line (and is a function of site relative humidity). Refrigerated inlet cooling (direct and indirect) offer advantage of higher cooling and lesser sensitivity to site conditions, and result in greater power output with an impact on relative cost and complexity. Selection of optimum technology of turbine air inlet cooling is hence a tradeoff between competing factors.

Combined Cycle
              Combined Cycle Power Plant

The complexity of combined cycles, without any turbine inlet air cooling, poses significant challenge in design of steam system and HRSG due to competing factors such as pinch point, heat and mass flows optimization etc. Knowledge of fluid viz properties of standard air (psychrometrics), standard gas for Joule Brayton cycle, steam for bottoming Rankine cycle and refrigerant for cooling system( for refrigerated inlet air cooling) as applied to complete cycle makes the process complete as well as complex. AxCYCLE™ is one such unique tool to simulate such combined cycle processes with multi fluid-multi phase flows including refrigeration. The standard HVAC features can easily be used for inlet air cooling refrigeration and integrated into the CCPP. Once a digital representation of the complex process is replicated and successfully ‘converged’ at design point, the challenge of optimization emerges. To facilitate optimization various tools namely AxCYCLE™ Map, Quest, Plan and Case are embedded integrally. As a first cut, users based on their experience apply AxCYCLE™ Map and vary one or two parameters to see the effect of operational parameters on cycle performance. AxCYCLE™ Quest opens the gates by allowing users to vary unlimited parameters, according to quasi-random Sobol sequences. mutli-Parameter optimization tasks are possible using AxCYCLE™ Plan – it uses design of experiments concepts. Once optimized the AxCYCLE™ Case tools allows off design simulation tasks. Exhibit below represents complexity of a combined cycle plant represented conveniently:

To learn more please check out the following demos:

Cost Estimation & Economic Analysis http://learn.softinway.com/Webinar/Watch/51

Vapor Compression Refrigeration System http://learn.softinway.com/Webinar/Watch/86

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).

Blog 4 image

To verify the analytical model, subject to various practical requirements, an unbalanced rotor response test could be performed as part of the mechanical running test. The actual response of the rotor on the test stand to the same arrangement of unbalance as used in the analysis is the criterion for determining the validity of the damped unbalanced response analysis. Sample summary results generated using SoftInWay’s integrated tool for rotor dynamics AxSTREAM®, as shown below indicate a very good agreement between test results and analytical predictions, for both amplitude and frequency.

For further understanding the analytical procedure, testing to validate damped unbalanced rotor response and implications please view http://learn.softinway.com/Webinar/Watch/90 or contact us to learn more.

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.

Concepts drawn from CHP namely topping and bottoming cycle have been implemented in refineries. In the topping cycle, fuel is used in a prime mover such as a gas turbine or reciprocating engine that generates electricity or mechanical power. The hot exhaust is then used to provide process heat, hot water, or space heating/cooling for the site. In a bottoming cycle, which is also referred to as Waste Heat to Power (WHP), fuel is first used to provide thermal input to a furnace or other high temperature industrial processes. A portion of the rejected heat is then recovered and used for power production, typically in a waste heat boiler/steam turbine system. To be effective, a bottoming cycle must have a source of waste heat that is of sufficiently high temperature (around 300 Deg C) for the system to be both thermodynamically and economically feasible.

A refinery with such steam pressure requirements at the design phase, in its life cycle however may operate under two distinct scenarios- excess or deficit steam. In the excess situation, equipment is shut or steam is vented off, and in deficit situation steam balance is made up by letting down from an immediate higher level.Blog 3

An optimal way to design and operate the steam balance system involves “what if” analysis and requires inputs from multiple sources such as process engineers, OEMs, equipment vendors, piping designers etc.  With the current trend in crude prices and need to maintain gross refining margins, an all possible mean need to be adopted for ensuring reduced costs. Though estimates vary, some figures indicate up to 130 Gigawatts (GW) of untapped potential at existing industrial and commercial facilities. To identify area of improvement in steam balance and optimize, a simplistic approach is an analysis using pressure and temperatures. A more detailed analysis requires estimation of enthalpy, entropy, exergy and anergy. Such an approach would consist of:

  1. Reconcile heat and mass balance for the steam system and achievement of LHS=RHS account (missing steam).
  2. Estimation of the performance of the existing cycle equipment from an efficiency and economic perspective.
  3. Ascertain inefficiencies in the steam system and opportunities for improvement and optimization by accurate simulation, modelling and scenario comparison.
  4. Assessment of the technical and economic feasibility of installing a rerate/upgrade/replacement


AxCYCLE™ is one such unique tool for design, analysis and optimization of thermodynamic systems (simulation, heat and mass balance calculation of heat producing and electric energy cycles) typically used for any heat and electric energy cycles including aircraft propulsion. Using AxCYCLE™ Economics – an extension to AxCYCLE™ it is possible to estimate capital/running investment calculation (CAPEX and OPEX) , fuel type selection, comparing different scenarios and return on investment.

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.

From the oil and gas industry perspective, standards namely API governs the specifications of design, material and systems requirements of rotating equipment. In line with such standards, equipment under the purview of APIs SOME (Sub Committee on Mechanical Equipment) are considered to quantify rotating machine. Falling under the turbomachinery group are centrifugal pumps (API 610), general and special purpose steam turbines (API 611 & 612), gas turbines (API 616) , axial and centrifugal compressors and expander- compressors (API 617), special purpose fans (API 673) and integrally geared compressors (API 672).

It is understandable that universities and academic institutions will continue their focus on turbomachinery as a fundamental subject of the curriculum, with increased emphasis on training and use of CAE tools. Basics remaining the same, the special nature of application of turbomachinery in the industry makes job of a rotating equipment engineer a challenge. At a conceptual level four important facets of rotating equipment (turbomachines) that an engineer in the oil and gas industry need to comprehend are as follows: Kinematic, energetics and thermodynamics of performance– fluid-aero thermal design and analysis; integrity of rotor, bearings, seals, casing and structure – mechanical design and analysis; the complete hardware – metallurgy, material mechanics and manufacturing and finally the associated systems. As the oil and gas industry entrusts its specialist rotating equipment engineers with a demanding level of reliability and availability of turbomachines, new ways are required to enhance competence. The practical experience of rotating equipment engineers coupled with exposure to design principles and use of CAE and simulation tools is one such way to help them add more value to their business.  AxSTREAM® is an advanced  software suite that covers many aspects of various critical rotating equipment for the oil and gas industry on a single platform.

For an overview of AxSTREAM for design, analysis, retrofitting, and so on, visit our past webinars recording http://learn.softinway.com/Webinar .