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

Axmap for turbocharger
Figure 2 – Simultaneous Turbine (color) and Compressor (dotted) Maps – Power vs. MFR (left) & Pressure Ratio vs. MFR (right)

necessary for the compressor and turbine design. Given the engine information, necessary inlet conditions of the compressor such as temperature and pressure, efficiencies required, and heat transfer of the system, the user can then obtain the boundary conditions for the turbocharger compressor and turbine wheels.

From this point, the process becomes an exercise in turbomachinery design and analysis. With SoftInWay’s turbomachinery design and analysis platform, a boundary condition realization of the system eventually manifests into a full 3D design of the turbine/compressor wheel. Once the engineer designs both the turbine and compressor wheels, they will be left with two discrete physical systems. However, these two designs must eventually coincide into a harmonious system that accurately represents the “turbocharger”. In order to facilitate this representation, the user can overlay the different compressor and turbine maps based on a number of varying parameters. Given the Power and Pressure Ratio curves for a number of varying shaft speeds and temperatures, an off-design performance of the turbocharger system can be analyzed via AxSTREAM’s matching module (Figure 2). Another simultaneous analysis of the turbine and compressor wheels must be made on the component that connects them, the rotor. Rotor design, rotor dynamics, and bearings analysis are crucial to a legitimate turbocharger design and will be a topic of a next week’s blog post. If you would like to learn more about turbocharger design and analysis methods, please follow this link

References:
http://www.automotive-iq.com/engine/articles/high-boost-and-two-stage-turbo-power-systems

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.

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.

As it can be observed, there are several reason why an engine can fail varying from inspection mistakes, manufacturing processes and design strategies. Nowadays, engine failures are far below the leading causes of accidents and death. Nevertheless, they are ranked fourth in the decade from 2006 through 2015 with 165 fatalities, according to Boeing statistics [4]. When it comes to blade fatigue regular inspections and maintenance play the most important role. However, the design process is equally important to ensure an efficient and powerful design. The design of the machine under specific flight conditions, taking into account aero-structure interaction, as well as vibration and Rotor dynamics analysis is essential to get a streamlined solution. AxSTREAM allows the user to investigate a variety of design points and further analyse the best solution that meets the constraints and operating conditions requirements. Moreover, AxSTREAM NET can now be used to estimate leakages and cooling or bleed air flow parameters for different fluid path sections while taking into account heat exchange of cooling flow with metal surfaces.

References:

[1] https://www.ntsb.gov/investigations/AccidentReports/Reports/AAR9801.pdf

[2] http://lessonslearned.faa.gov/ll_main.cfm?TabID=2&LLID=62&LLTypeID=2#null

[3] http://www.robsonforensic.com/articles/aircraft-engine-materials-expert

[4] http://www.dallasnews.com/business/airlines/2016/09/12/investigators-cracked-engine-blade-broke-southwest-airlines-flight-last-month

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 .

Revamping a Turbomachine Train

The demands of the plant construction and energy sector after a shorter response time for questions upon newly defined operating points of a turbomachine train are one of the biggest challenges in the service business. This becomes particularly obvious if the future points can only be realized by redesigning the flow-relevant components. Often, it is necessary to have more time to check the dynamic behavior of the train, than in the development of the appropriate revamp measures for the core machine itself.

In addition to the different utilization rates of the affected departments, the causes of the delays often lie in the lack of interface quality between the design/ calculation and train integration team. On top of that, a certain amount of time will be required by manufacturers of the critical components such as gearboxes or drives to perform a lateral check. This lateral check is not only mandatory, in case of a component modification such as changing the transmission ratio or upgrading the drive, but it is also necessary if the coupling between the train components must be changed to ensure torsional stability.

Blog post 1
          Figure 1 – Flow Chart 

The flow chart to the right shows the general process flow from revamping a turbomachine train. On the line to the left of the figure, the revamp’s main processes are shown. The process flow starts with the revamp of the core machine and ends with the fulfillment of the feasibility criterion in the torsional analysis. A calculation tool is usually available for each process step. It results from in-house development, university’s research or from a commercial manufacturer. Overall, in a process flow as per above, several tools which come from different manufacturers are used.

Normally, each tool outputs its result in a text file whose content conforms to the ASCII standard and is unstructured. The fact that the output file is unstructured, makes it clear, an update or a completion of the tool means an enormous effort for the company which is responsible for the train integration. It must examine the individual interfaces and adapt them as necessary. Furthermore, it must also carry out the immensely important work of the benchmarking of the overall result.

Implementing a text file with a hierarchical data structure as output is one of the simplest solution approaches to fix the interface problem. However, because most of the tools historically have been written in the obsolete programming language, writing the results in a hierarchical data structure is very difficult to achieve. Another approach is the application of an integrated development environment for turbomachinery. Because all tools come from a single manufacturer, the interface quality is now guaranteed. To apply the core competencies of the machine manufacturer, the environment should be able to integrate the specific characteristics of flow-relevant components such as loss and leakage models into the calculation.

Check out  SoftInWay’s integrated platform AxSTREAM for flow path design and redesign

Axial Compressor Challenges in Hyperloop Designs

Back when the California high-speed rail project was announced, Elon Musk (CEO of SpaceX and Tesla Inc. and perhaps the most admired tech leader of present day) was not only disappointed with this project, but also introduced an alternative to this system called the Hyperloop in 2012.  Since the abstract of this project was introduced, many engineers around the world have started to evaluate the feasibility of this “5th Mode of Transportation”.

Hyperloop Alpha Conceptual Design Sketch
Hyperloop Alpha Conceptual Design Sketch

The general idea for the Hyperloop consists of a passenger pod operating within a low-pressure environment suspended by air bearings.  At the realistic speeds estimated by NASA of 620 mph, the pod will be operating in the transonic region.  While Japan’s mag-lev bullet train has succeeded at achieving speeds of up to 374 mph, the scale and complexity of a ground transportation system rising above 600 mph bring to surface an unusual number of engineering challenges. As well, brand new designs such as the one proposed by Musk have a certain amount of risk involved due to this technology inherently having no previous run history on a large scale.

Of the many concerns with his original design, perhaps the largest resides on how to design and operate the axial compressor in front of these pods. The supposed function of the compressor is two-fold. The first function would be to overcome the Kantrowitz limit. Musk uses an analogy between the pod and tube and a syringe:

“Whenever you have a capsule or pod (I am using the words interchangeably) moving at high speed through a tube containing air, there is a minimum tube to pod area ratio below which you will choke the flow. What this means is that if the walls of the tube and the capsule are too close together, the capsule will behave like a syringe and eventually be forced to push the entire column of air in the system. Not good.”

Aero Booster
Figure 2 – Safran Aero Boosters Low-Pressure Compressor – Assembly View

An onboard compressor in front of the pod will allow the collected column of air traveling in front of the pod to flow through the system without compromising the increasing velocities of the pod. A second function of the compressor would be to supply air to the air bearings that support the weight of the capsule throughout the passage.

Traditionally, axial compressors are coupled with a complimentary turbine at the exhaust that provides mechanical power to the compressor. In the hyperloop, the proposed compressor arrangement will be driven by electric motors instead of turbines. This is a relatively new design that has only been tested on a handful of electric powered jet aircrafts for research purposes. Furthermore, Musk proposed a compression ratio of about 20:1, which would require several compression stages for an axial compressor arrangement and an intercooler system. The temperature increases resulting from this high order compression require a complex cooling method or a traditional steam pressure vessel for the proper dumping of hot air. A final challenge on the compressor end would be the fact that it will be operating at a very low pressure. Only a handful of companies like Safran Aero Boosters have the necessary experience with low-pressure compression.

In general, while this new proposed mode of transportation is very exciting and innovative from an engineering standpoint, the following challenges specific to the on-board compressor will require serious collaborations amongst the leaders in the compressor design industry:

  • Electric Motor Driven Compressor
  • High Compression Ratio – 20:1
  • Complex intercooler system
  • Low-Pressure Compression Environment

If you would like to learn more about SoftInWay’s integrated platform for axial compressors, please visit our axial compressor page

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.

Rotordynamics analysis
Rotordynamics Analysis

Because of the high pressure in the combustion chamber needed to accelerate the hot gas mixture, a feed system is essential to pressurise and to transport the propellant from the propellant tank(s) to the thrust chamber. In today’s rocket engines, propellant pressurization is accomplished by either (turbo)-pumps or by a high pressure gas that is released into the propellant tank(s), thereby forcing the propellants out of the tank(s). In space engineering, especially for high total impulse, short duration launcher missions, the choice is almost exclusively for pump-fed systems.

To design such systems, a highly sophisticated and complete tool is required. SoftInWay has developed AxSTREAM, the most integrated engineering platform in the market, for turbomachinery design, analysis and optimization. The long experience in the field along with the use of AxSTREAM allow SoftInWay to support its customers in the space industry. Below, you can catch a glimpse at AxSTREAM’s capabilities through a demonstration project of the RL10-A3-3 fed system. The RL10-A3-3 rocket engine is a regeneratively cooled, turbopump fed engine with a single chamber and a rated thrust of 15,000 lb at an altitude of 200,000 ft., and a nominal specific impulse of 444 sec. Propellants are liquid oxygen and liquid hydrogen injected at a nominal oxidizer-to-fuel ratio of 5:1 [1]. The design focused creating new rotating parts of the RL10-A3-3 feed system as presented in Figures 1 and 2, including full scope of rotordynamics analysis.

New Rotating Parts for RL10-A3-3 Feed System

Contact us for an AxSTREAM demonstration and attend one of our training courses to get a trial with AxSTREAM and become SoftInWay’s next success story.

References

[1]https://pslhistory.grc.nasa.gov/PSL_Assets/History/C%20Rockets/Design%20Report%20for%20RL-10-A-3-3.pdf

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.

Transatomic power, a company started by two MIT PhD candidates, came up with a new approach to safer and cheaper nuclear reactors. Utilizing molten salt reactors, which has not really been used commercially and so far is only existed in paper, the technology is promised to cut initial cost and increase safety. Today’s conventional nuclear reactor is cooled by water, due to the high operating temperature, failure to do so will open the risk of radiation leak as well as hydrogen explosion. The high boiling point of salt helps solve some of the problems associated with the technology. The new design also incorporates ways of producing faster neutrons, enabling the reactor to burn most waste materials, thus keep waste to minimum. The ability of this smaller unit to be made in a factory (and not onsite) as well as cost reduction on the safety side makes this attractive economically as well. That being said, this generation 4 nuclear reactor is still in design and development will take years and high capital cost.

References:

https://www.nytimes.com/2017/02/14/business/toshiba-chairman-nuclear-loss.html?_r=0

http://fortune.com/2017/02/16/toshiba-nuclear-power-plants/

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.Aashish blog 1 image

Indeed rotor-dynamic modelling with practical experience and engineering judgement improves accuracy.  Its ability to model complexities such as flexible supports, foundation, rotor seal interaction, and instabilities while making the CAE model comprehensive, user friendly, and fully integrated with other well proven and mature suites for flow path and  blade design makes SofInWay’s software platform unique.

Integrated Approach within AxSTREAM® Platform

Suited to meet the diverse needs of designers, analysts and users of turbomachinery, SoftInWay’s webinar in collaboration with Test Devices Inc scheduled on Mar 2, 2017, 10 AM EST helps you to understand HOW rather than why. It will  cover rotor and bearing types, principles of an integrated approach to rotor-dynamics system design and simulation, purpose and procedures for rotor-dynamic and structural analysis. The software demonstration will include modelling features, import/export options, lateral and torsional capabilities, bearing analyses and modelling capabilities, and case studies. It will also briefly highlight fundamentals such as characteristic influence of shaft rotational on natural frequencies in comparison to classical natural frequencies and modes in structures, gyroscopic effects, rigid vs flexible rotors, free and forced vibration as applicable to turbomachine rotors, impact of bearing characteristics and concept of cross coupling, modes, Campbell diagram, stead and transient response, instabilities, condition monitoring, testing, evaluation and acceptance criteria (log dec and margins) and much more. Testing methods covered by Tech Devices Inc. highlight testing procedures and methods for design validation and building confidence that the design exceeds expectations.

 

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.

Dan Post 10
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.

Because of surge and other phenomena such as stall, engineers must embed proper control systems that effectively handle different off-design conditions seen in particular compressor arrangements. Depending on the application, certain compressors will rarely operate away from their design point, and such control systems are not necessary. However, in advanced applications such as large gas turbine unit compressors, controls systems allow the compressor to navigate within a range between the choke, stall, minimum speed, and maximum speed limits. The chart seen in Figure 1 describes the operating range of a compressor using a Rc—Qs map. In many cases, an antisurge valve (ASV) working in conjunction with an antisurge PI controller will action open or closed based on varying transient conditions seen on the compressor. For design purposes, it is vital to understand compressor limits in order to properly develop or outsource a compressor based on the performance metrics needed for the application.