Turbomachines feature several types of joint connections. Bolt connections, flange connections, and bolt joints are the most widely used; however, flange and bolt connections are major points for leakage and operational problems.

Many turbomachinery components, such as casings and piping, need to be dismantled or disconnected. Bolt connections, which make disconnection possible, may seem simple, but they can be complex in their mechanical behavior, performance, and operation. Typically, the behavior of a bolt connection is highly nonlinear and complicated, so bolt connections should only be used to disassemble or adjust for operation or maintenance. In addition, bolt connections are expensive compared to welding and other types of connections, and they are relatively heavy and need special care and provisions, such as inspection, maintenance, etc.

CRITICALITY OF DIFFERENT BOLT CONNECTIONS

Bolt connections only account for a small amount of the overall budget of a turbomachine. However, they are responsible for many leaks, operational problems, shutdowns, and failures. Bolt-connection requirements often determine the size and design of attached items, modules, and subsections. Specific nonstandard bolt connections may require extra thought in connection sizing, configuration, and installation.

Many components are involved in a bolt connection. Bolt details (size, patterns, etc.), the application/details of nuts/ washers, connecting plates/components, and the main connected assemblies all have a significant effect on the performance and reliability of the bolt connection. Any one of the involved parts can fail or cause failure in the bolt connection—bolt failure, cracks, and/or damages at bolt holes, failures due to bending stresses, buckling of parts, such as the compression flange, and others.

Despite widespread applications, bolting connections are surprisingly unfamiliar to engineers, operators, and experts. Some sources of difficulties are:

Local yielding in different parts. This involves localized loads and highly localized stresses.

High nonlinearities. For example, material nonlinearity due to stress-strain relationships and the involvement of complex contact and contact stresses.

As components yield and plastic zones extend, the behavior of the bolt connection gets more complicated and becomes highly nonlinear. After some yielding, there are bolt load (plastic) redistributions that usually lead to an increased moment capacity of the bolt connection and reduced stiffness of the connection and connected components. However, the redistribution of plastic/inelastic bolt loads/forces is a complex phenomenon. In general, many theories and methods, even modern ones, cannot predict the behavior and performance of bolt connections accurately. Some underestimate the forces and stresses, and others overestimate the forces/ stresses.

Bolts should be tensioned using proper methods before a turbomachine assumes loading. This is a necessary step for any bolting joint to ensure that the load-bearing and load-transmitting plies are brought into effective contact. The final tensioning of the bolt connection should proceed from the stiffest part of the connection toward the free edges. Previously tensioned bolts could have been loosened by adjusting adjacent bolts, so re-tightening and -tensioning may be required.

Bolt connections should be sufficiently strong, rigid, and stiff to prevent any leakage or operational mishaps under adverse scenarios. On the other hand, connections in general, and bolt connections in particular, should be lightweight, compact, and cost-effective. Given that bolt-connection components are expensive when compared to the connecting module or section, selecting the appropriate size and bolt optimizes installation and cost.

If bolts, relative to the members or parts they connect, are not strong enough, they will break. If the reverse is true, the member or part might weaken due to fatigue. If connecting plates or sections are not strong enough, they may fail in the bolt hole. Although the entire bolt connection should be strong and ideally designed for infinite life, it is easier to periodically replace a few bolts rather than break the entire bolted connection, which can be troublesome and expensive.

Key areas of concern for bolt connections are:

losses in bolt tension after installation (changes in bolt tension over time);

the ability to tighten the bolts using standard installation techniques; and

washer requirements for normal bolt holes and oversized (slotted) bolt holes.

For typical high-strength bolts in normal circumstances, the total loss of the preload due to relaxation should be below 25% of the initial preload, and, in many cases, it should be below 20%—so on average, 10-18%. Preload is usually unaffected by small temperature changes. For bolts tightened to their yield point, bolt relaxations between 6 - 12% can occur after 5 - 10 years of operation. A large portion of the loss in bolt load occurs within days or weeks after tightening.

Washers and their details play a vital role in this regard. The loss of preload is about 6 - 8% for a typical bolt connection with two washers and about 10 - 14% for a bolt connection with one washer (10 - 15 years of operation).

In general, bolt connections on turbomachines and rotating equipment under vibration or dynamic/shaking forces need focused care. Nuts that are subject to vibration should also be secured.

Some experts suggest not using slotted (bolt) holes under fatigue-loading conditions. However, others support the use of slotted holes in certain conditions. The risk of movement under dynamic/ cyclic load should be considered and properly addressed.

Bolt connections and their counterparts are often the weakest link in a turbomachine or connected piping. As such, the load capacities of each component and connection should be evaluated, including connectors such as bolts, nuts, and washers, and connection components such as plates or flanges.

There should be sufficient distance between bolt holes and between each bolt hole and the edge of the connecting plate/ part to avoid failure. Similarly, to tighten bolts, ensure there is access to each one and sufficient distances between bolts and the surrounding obstacles.

To prevent bolts from tearing, center-to-center spacing should be at least 2.5 times the bolt-hole diameter and edge distance, ranging from 1.5 to 1.75 times the diameter, depending on how the edge is cut. To be safe, it should be 1.75 times, or even two times, the diameter.

Amin Almasi is a Chartered Professional Engineer in Australia and the U.K. (M.Sc. and B.Sc. in mechanical engineering). He is a senior consultant specializing in rotating equipment, condition monitoring and reliability.

Articles

Bolt connections can be the weakest link of a turbomachine, so proper care, sizing, tension, and placement are key to avoiding leaks and other operational problems.

Turbo Tips: How to Manage Bolt Connections & Bolt Joints in Turbomachines

Approximately 76% of the carbon dioxide that is emitted in the United States by stationary sources comes from electricity-generating power plants. For many other major industrial countries, the percentage is even higher. Clearly, this is a significant motivating factor to decarbonize the power-generation industry using alternative energy sources, such as nuclear energy, hydrogen-fueled thermal power plants, as well as carbon capture with existing plants.

Blue hydrogen—produced from fossil fuel with carbon capture—and green hydrogen—produced from alternative energy sources—are often prominently displayed in the technology mix needed for the future of a decarbonized energy economy. However, green hydrogen for power generation is a rare animal, a unicorn, or some other mythological beast. Everyone believes in it, but nobody seems to have actually seen one credibly. I’ve been desperately seeking this unicorn for the last 30 years, and I still haven’t found it. There are images and talk about large power-generation projects for green hydrogen all over the news, but when you truly chase them, they seem to disappear or fade into obscurity.

Obviously, we may be exaggerating a bit to make a point, but green hydrogen is one of those topics that is hyped in the press but very little is actually produced. Over 99% of the world’s hydrogen that is currently produced comes from fossil sources and does not include carbon capture. Therefore, it generates significant amounts of carbon dioxide in the production process and is far from being green.

So, what is green hydrogen? As mentioned above, it is hydrogen produced by electrolyzing water using electricity solely from alternative energy sources such as wind, solar, and hydropower. Electrolyzers use an electrochemical reaction to split water molecules into their molecular components: hydrogen and oxygen. There are various types of electrolyzers commercially available to produce hydrogen. They usually operate at relatively low pressures (mostly below 20 bars), near ambient temperatures, and at efficiencies between 40% - 80%. Higher-pressure electrolyzers are being explored, but they seem to mostly operate at lower efficiencies. Regardless, by using electrolysis, green hydrogen is produced without any greenhouse gas emissions. Turbomachines, such as compressors, are then required to transport and store this hydrogen at elevated pressures so it can be practically used in industrial and power-generation applications.

Electrolysis requires large amounts of electricity and thus only makes sense if electricity is very cheap or freely available. Since the conversion efficiency of electrolysis is less than one on an electric energy in (electric kW-hr) versus hydrogen chemical heat energy out (in Joules or BTU) basis, it is self-evident that this concept only works for energy-storage applications where electricity is available from intermittent, non-dispatchable power plants, such as wind turbines or photovoltaic solar panel farms. This is also called “curtailed” energy since it is being produced at times or in locations where there is no demand for it and it does not have an immediate or practical use. Hydrogen from green electricity is effectively nothing more than an energy-storage technology option and possibly an energy transportation alternative if one considers hydrogen transport by pipeline, truck, rail, or ship.

It is important to emphasize that green hydrogen is only truly green if it comes from intermittently curtailed electricity. It must come from zero-carbon sources AND from electricity that otherwise has no uptake. That means wind turbines or photovoltaic plants producing electricity that are unable to sell it into the market due to a lack of utilization requirements or transmission capability. Otherwise, the production of green hydrogen will only create an opportunity for non-green (mostly fossil) power plants to sell into the electric market.

Also, nuclear- and hydropower-produced hydrogen make no sense given that all nuclear and hydropower plants are baseload plants, which means that all their electricity already has an assigned market—which, by definition, is what baseload power is. Again, taking any of this electricity to make hydrogen will only create opportunities for non-zero-carbon power plants to fill the gap.

Similarly, green hydrogen produced from dedicated zero-carbon power plants, which are built and exist for the sole purpose of exploiting government subsidies for hydrogen, is basically taking taxpayer money to produce electricity and wasting most of it with an inefficient conversion process. If this hydrogen is purposed for electric usage, at least 55% (and usually more than 60%) of the originally produced electricity is lost because of the low roundtrip efficiency of this process.

Blue hydrogen, which is at times falsely claimed as carbon neutral, is not necessarily green. Blue hydrogen is produced from natural gas using processes such as steam reforming or gasification, where natural gas and steam react to form hydrogen but also carbon dioxide. To make hydrogen “blue,” the carbon dioxide must be captured and sequestered. For example, if blue hydrogen is produced from natural gas using steam reforming, about 10 kgs of carbon dioxide are produced for every kilogram of hydrogen when including all process parasitic losses. For power-generation purposes, blue hydrogen also competes with methods to remove carbon dioxide from the exhaust stream of fossil power plants. In either case, it is expensive and energy-intensive to capture, transport, and sequester this carbon dioxide. Some of the proposed storage methods have limited assurance that they will not leak or escape over the long term.

Green hydrogen without subsidies, even if electricity is free and continuously supplied, is usually three to four times more expensive than blue hydrogen— including all costs of separation and sequestration. This is primarily driven by the electrolyzer’s first cost and overhaul/ maintenance costs. Unfortunately, there does not seem to be a near- or mid-term foreseeable technology path to drastically reduce electrolyzer costs. Thus, blue hydrogen will likely dominate over green hydrogen production for basic economic reasons unless it is replaced by highly subsidized green hydrogen or other government subsidies for green energy producers.

DOES GREEN HYDROGEN REALLY EXIST, AND IF IT DOES, SHOULD IT?

If excess power is produced by zero-carbon power plants, the emphasis should be on developing the electrical grid and distribution network to make sure this electricity finds a home rather than wasting most of it by converting it to hydrogen and then back to electricity. That is the key: Green hydrogen only makes sense if electricity needs to be stored or chemical hydrogen is actually needed at the end of the supply chain, e.g., for fertilizer or petrochemical use. Most power plants, especially wind farms, can shut their equipment down when they cannot find an off-taker for their electricity to keep mechanical wear, degradation, and operating costs to a minimum. Running a plant when there is no market for the product makes little economic sense. It only makes sense if the produced and stored hydrogen has a reasonable monetary value equivalent to at least the cost of producing it. Currently, in many cases, the only reason these power plants keep producing electricity is so that they can collect a production credit, which is basically just another artificial government subsidy.

As a quick aside, the often-advertised conversion of natural gas to blue hydrogen does not directly help to reduce methane emissions since in almost all cases the blue hydrogen is produced at the back end of the oil and gas value stream. Therefore, all methane-leakage losses happen during production and transport and are fully incurred by the time of the natural-gas-to-hydrogen conversion. For a power plant, blue hydrogen is local pre-combustion carbon separation and simply an alternative to exhaust gas (post-combustion) carbon capture. Also, hydrogen is hard to compress, store, and transport and has significant safety and leakage challenges associated with it (but that is a good topic for another Myth Busters article). Thus, the transport of blue hydrogen over longer distances makes little sense, as even the combined transportation energy of natural gas to the production site and the transport of the resulting carbon dioxide to a sequestration site requires less energy than the transport of hydrogen.

Consequently, an industry transition to green hydrogen is neither realistic nor necessarily desirable. If excess power is produced by zero-carbon power plants, the emphasis should be on developing the distribution network to make sure this electricity finds practical usage rather than wasting most of it by converting it to hydrogen and then later back to electricity.

We see the development of the decarbonized energy economy as a gradual evolution with the first steps focusing on zero-carbon electric sources—wind, solar, hydro, and nuclear—in combination with carbon capture in fossil-fueled power and blue hydrogen production plants. The onset of the production of appreciable quantities of green hydrogen is a much longer-term but necessary development that will require significant investment in the development of new technologies and infrastructure. In the near term, there is an obvious need for additional natural gas supply as a reliable and secure decarbonization transition fuel which, from a turbomachinery perspective, means more conventional production, pipeline, and LNG machinery. In the mid-term, we will likely see more pre- and post-combustion carbon capture from existing fossil-fueled power plants. This will require new compression machinery for the efficient transport and geological storage of carbon dioxide. The need to compress large quantities of green hydrogen for pipeline transport and storage is less likely to be an immediate requirement. It will probably remain a mythological beast for a little bit longer. The hope is that in the not-too-distant future, we will finally chase this unicorn down.

Klaus Brun is the Director of R&D at Elliott Group. He is also the past Chair of the Board of Directors of the ASME International Gas Turbine Institute and the IGTI Oil & Gas applications committee.

Rainer Kurz is a recent retiree as Manager of Gas Compressor Engineering at Solar Turbines Inc. in San Diego, CA. He is an ASME Fellow and has published over 200 articles and papers in the turbomachinery field.

For more on hydrogen, check our expert roundtable. Click here to read 

Green hydrogen is receiving a lot of hype in the news, but a closer look reveals how impractical its future is for power generation.

Myth Busters: Green Hydrogen for Power Generation: Is It a Unicorn?

Ansaldo Energia, a power engineering firm, specializes in the design, production, and maintenance of plants and power generation systems, including gas turbines, steam turbines, and generators. Having roots dating back to 1853 with the founding of Giovanni Ansaldo & Co., Ansaldo continues to leverage its 170-year history in thermoelectric and hydroelectric to a variety of power producers and industrial clients. Its headquarters in Genoa, Italy employs more than 3,000 employees across numerous subsidiaries such as Ansaldo Green Tech, Ansaldo Nucleare, and Ansaldo Nuclear Ltd.

Steam turbines are the company’s original offering, and it continues to update its designs for modern power requirements. The product range covers a wide berth of applications: power storage, geothermal, non-reheat, reheat, and nuclear. In terms of steam turbine power outputs, these models can range from 15 MW to 1,000 MW. Within the category of steam turbines, which includes the MT10, MT20, MT15, RT30, and RT70, Ansaldo provides several geothermal models based on an impulse or reaction design.

“Geothermal impulse steam turbines developed by Ansaldo can cover a wide range of applications, from 20 MW up to 60+ MW,” said Gian Marco Crespi, Head of Steam Turbine Engineering. “In geothermal applications, specific issues, mainly coming from the geothermal steam and the corrosive environmental conditions, must be addressed. Impulse technology, characterized by a limited number of stages and a compact design, has been demonstrated as a winning solution in extreme geothermal conditions.”

Geothermal steam turbines are subject to harsh conditions during operation, yet not only do they excel in such conditions, but maintenance can also be easily performed by replacing singular parts as soon as the damage occurs. Ansaldo’s impulse design has a limited number of stages and a compact design to perform optimally under the duress of geothermal steam.

Non-reheat steam turbines are manufactured as single- or two-cylinder designs, while reheat steam turbines are usually designed with two or three cylinders. Crespi offered some distinctions between these models to further clarify the purpose of cylinders in steam turbine operation. “The number of cylinders is determined according to the unit size and steam conditions,” he said. “The overall arrangement of the turbine is largely project-specific. A large condensing steam flow rate would generally require a split in one high-pressure (HP) or intermediate-pressure (IP) section and one or more low-pressure (LP) sections, while a reduced steam flow rate would allow the unit to be contained within a single cylinder.”

The split HP and IP sections of reheat steam turbines allow a degree of flexibility in the manufacturing process, as these sections can either be established in two separate cylinders or joined in a combined module. When choosing a reheat or non-reheat turbine, a few aspects should be taken into consideration: the overall plant architecture, costs, efficiency requirements, and simplicity.

Ansaldo assembles the majority of its equipment in its workshop at the Campi production plant in Genova, but large LP sections are often shipped unassembled due to several limiting factors. Shipping LP sections unassembled allows for ease of installation and transport.

“Usually, HP and IP sections are workshop pre-assembled, shipped, and installed onsite as pre-assembled modules,” Crespi said. “This is the most efficient solution for shipping and reducing site-assembly costs; however, LP sections usually ship disassembled due to their weight and large dimensions. Moreover, LP sections are usually mechanically connected to the condenser, which is installed onsite and is integral to the LP exhaust casing.”

Despite the steam turbine being classified as a “mature” component—one that retains a consistent design with relatively few technological innovations over time—Ansaldo continues to apply and advance its steam turbine portfolio for various power-generation industries, including combined-cycle and nuclear plants. Enhancements in extreme condition operation and the integration of renewable energy resources will secure plant and high efficiency during grid fluctuations. In addition, the company believes that steam turbines’ share of the power-generation market will increase as the technology is used to support micro-distributed generation, “as well as being the core of large and high-efficiency heavy-duty generation plants,” Crespi said. “They will be even more integrated within the industrial and urban framework, such as district heating and co-generation, and will be the basis for some energy storage technologies.”

Not only will steam turbines benefit from an increase in market share, but advanced control system functionality—such as cybersecurity, autonomous operation, and self-predicted maintenance—may help steam turbines bridge the gap into a digitally enhanced landscape. The integration of steam technology in various power-generation sectors—such as concentrated solar, geothermal, hydrogen, and new nuclear plant solutions—acts as a driver for Ansaldo’s steam turbine innovation.

Current policies require the integration of decarbonization and sustainability strategies, and Ansaldo is committed to these standards. “Besides the original steam turbine mission, which is to make energy transformations more efficient, decarbonization will see the steam turbine as a main player,” Crespi forecasted. “Ansaldo, as a steam turbine OEM, is strongly focused on the decarbonization process and active in designing, manufacturing, and installing a new portfolio of more flexible, efficient, and sustainable steam turbines.”

Ansaldo Energia focuses on decarbonization, efficiency, and sustainability in
the design and manufacture of its steam turbine portfolio.

Vendor Spotlight: Ansaldo Energia

Centrifugal compressor operations involving high molecular weight hydrocarbons represent some unique challenges. These operations include vapor recovery compressors gathering low pressure and storage tank gases in upstream operations, condensate and liquid stabilizer overhead flows in gas treating and LNG plants, and wet gas compressors in refinery applications. The gas compositions of these compressor streams contain higher fractions of heavier components from ethane to pentane as compared to typical natural gas. These higher molecular weight flows may be subject to partial condensation of the heavier components under some operating scenarios resulting in a potential reduction in mass and volumetric flow rates and a decrease in the molecular weight of the gas stream. While this may be anticipated during normal operation, it may be problematic during transient operation such as start-up and periods of partial recycle during continuous operation. It is also possible to experience significant gas composition variations during normal operations that can impact stable compressor operation. Additionally, these compression services are often viewed as less critical during design phases, so they are not evaluated as thoroughly as possible and are frequently supplied with constant speed drivers.

The basic configuration of a typical compressor system is shown in 

. The inlet to a compressor section should include an isolation (block) valve and a suction scrubber that removes any entrained liquids from entering the compressor. Flow from the compressor section discharge will generally have a large portion of the heat of compression removed with some type of aftercooler with a check valve and discharge isolation valve downstream. The check valve is intended to prevent reverse flow during process upsets while the isolation valve is often held in the closed position during compressor start-up. A recycle line is provided downstream of the aftercooler to upstream of the suction scrubber to accommodate any required recycle flows during normal operation or upsets such as surge avoidance. The included valve supplies the necessary pressure drop from existing compressor discharge to suction pressure conditions. A single valve for both anti-surge and recycle services is often provided, but this may also be accommodated by separate anti-surge and recycle valves since their response times and position accuracy requirements can be different.

Starting centrifugal compressors is typically accomplished with the anti-surge/recycle valves fully or partially open to avoid going into surge and control the starting power required. These valves may also be needed to recycle a portion or all the flow for extended periods of time when the inlet flow supplied is less than the design minimum (when flow rate falls below the minimum to avoid surge) or when discharge process conditions will not accept flow from the compressor. The flow across the recycle valve(s) is described as a Joule-Thomson (isenthalpic) throttling process with a normal accompanying temperature drop that is related to the pressure drop. In some situations, the temperature drop is sufficient to partially condense some of the gas mixture components and result in a gas-liquid two-phase flow on the compressor suction side of the valve.

It is also possible that the temperature drop provided by the aftercooler results in some partial condensation of gas components at the higher pressure existing at compressor discharge conditions but reduced temperature. It is possible to estimate the potential of a gas mixture to produce some condensate liquids by evaluating the vapor pressure of the different gas components. A plot of vapor pressures for a number of hydrocarbon components is given in 

These values were derived with the National Institute of Standards and Technology’s (NIST) REFPROP computer application.

At pressures above the curve for a given component and temperature, liquids may be present. While providing an approximate estimation of when liquids may form, the vapor pressures are only valid for ideal gas conditions due to molecular interactions between the various gas components and other real gas effects in a gas mixture. Nevertheless, reference to such a vapor pressure figure can indicate the potential for gas component condensation and suggest the need for a more accurate process simulation. Generally, the potential for partial condensation increases as the operating pressure range of the compressor increases.

To further illustrate this issue and compare alternative scenarios and system configurations, two representative compressor applications are presented. Both applications are based on a common gas composition of 20% methane, 25% ethane, 30% propane, 10% isobutane, and 15% normal butane. Relevant compressor operating parameters for the two examples are outlined in 

Three different operating scenarios are examined for each compressor example. The first scenario is the basic compressor configuration as defined in 

, which is a common compressor layout with the cooled recycle line located downstream of the aftercooler. If any portion of the flow is condensed in the aftercooler, it is assumed that it will flow with the gas phase through the recycle valve and back to the compressor suction. All resulting liquid phase is then assumed to be separated in the compressor suction scrubber before the gas flow is re-introduced into the compressor. A second case is illustrated in 

This case represents a configuration where any liquid condensed in the aftercooler will be separated from the recycle flow. Any liquid that is condensed will result in both a reduction in gas molecular weight and gas mass flow rate that enters the recycle line. If there is additional condensation due to the throttling of the gas in the recycle valve, it is assumed that it will be separated in the compressor suction, further reducing the gas mass flow rate and molecular weight.

In the third example, there is no cooling of the recycle flow upstream of the recycle valve, but cooling is provided downstream of the valve and upstream of the compressor suction scrubber. In this case, compressor discharge gas is throttled across the recycle valve first, and any remaining heat of compression is removed in the downstream heat exchanger prior to re-introduction into the compressor suction at the assumed suction temperature. A major drawback of this configuration is the need for an additional heat exchanger, shown in 

Selected results from the low-pressure simulations are shown in 

. These are shown for a range of compressor aftercooler discharge temperatures and assumed common with the recycle valve inlet conditions. The calculated results include the vapor flow molecular weight, the recycle flow to inlet flow mass flow rate ratio, and the comparable recycle flow to inlet flow volumetric flow rate. Changes in molecular weight and compressor inlet volumetric flow rate are particularly important because they correspond to impacts in the operating point of a centrifugal compressor.

It is evident from the simulation results that minimal compressor-operational impacts are associated with wide variations in aftercooler discharge temperatures in the low-pressure example. Some impacts are related to the recycle volumetric flow rates that tend to decrease as the aftercooler discharge temperatures drop. The cause of this volumetric flow reduction is due to the throttling effect of the gas flow across the recycle valve, creating lower recycle valve discharge temperatures and the associated increase in gas density.

A more pronounced sensitivity to compressor-operating parameters with aftercooler discharge temperature is demonstrated in the medium-pressure example. All the selected operating parameters are impacted with more substantial reduction in all parameters with decreasing aftercooler temperature. An exception is illustrated in Case 3, where the aftercooler location is changed to downstream of the recycle valve. Reductions in both gas molecular weight and mass flow ratio are more significant for Case 2—the case that includes any condensate removal downstream of the aftercooler. This relative trend is switched in the volumetric flow reduction and is due to a combination of both gas molecular weight and recycle valve downstream temperature impacts on gas density.

These reductions in gas molecular weight and flows will negatively impact compressor operation with the potential to force the machine offline. The compressor operating point will tend to move toward the surge control line and possibly unable to generate the required pressure ratio. While variable operating speed and the addition of some inlet make-up gas may reduce these effects, it may not be possible to avoid these issues. Of course, constant speed compressor operation or an isolated compressor loop will experience the full influence of these changes. Additionally, variations in gas molecular weight during normal operation may produce similar operating difficulties.

 presents a more graphical illustration of these parameter behaviors on the medium-pressure example.

Aftercooler discharge temperature is obviously a critical control variable in the prevention of partial condensation and its impact on compressor operation which becomes more influential as compressor-operating pressure range increases relative to component vapor pressures. Methods to control this temperature can be employed both during design and operating phases to minimize or eliminate the resulting operating issues. The most direct solution involves controlling the amount of cooling provided either through the cooling media or the process gas itself. Variation of cooling media flow is dependent upon the type of cooler provided. In the case of water or other liquid media cooling in a shell and tube exchanger, control of liquid media flow is achieved by modulation or bypass with a control valve positioned by a recycle gas temperature controller. Alternatively, fin fan air exchangers may be controlled by either dampers, fan speed, or operation control. Process gas temperature control is limited to cooler bypass control.

One such concept for process gas control is shown in 

where recycle flows upstream and downstream of the aftercooler are mixed to maintain a temperature setpoint downstream of the recycle throttling valve. Although effective in longer term operation, it does have limitations in startup and other transient operations. Temperature control loops typically suffer from relatively slow response times with significant amounts of dead time when initiated. This is particularly the case when the temperature sensing element is located in a stagnant section of piping as is the case downstream of the recycle valve before it is brought into application. In such cases, setting a preliminary stroke position on the temperature mixing valve based upon some initial conditions may be necessary to prevent condensation when the recycle valve is first opened.

While it is most desirable to address these issues during the design phase of a compressor application, in a number of cases this problem is not realized until the machine has been placed in operation.

Summarizing, aftercooler discharge temperature is both a significant indicator of partial condensing potential and a critical variable to control and prevent condensation. A number of actions can be completed to address the issue and avoid the negative operational impact of partial condensation:

Perform process simulations for the expected range of aftercooler temperatures and gas compositions to determine critical temperatures where condensation is initiated and the extent of the impact on compressor operation.

 If in the design phase, select one or more strategies to control aftercooler discharge temperatures that could include partial bypass of process gas (such as in 

), partial bypass or other temperature control method for cooling media, or movement of the aftercooler downstream from recycle take-off and the addition of another recycle/upstream cooler.

 If in operation, the solution becomes more of a challenge. Potential actions to avoid the associated problems could include manual throttling of coolant with an isolation valve or installation of a new control valve, addition or operation of dampers with air exchangers, and manual reduced operation of cooling fans (cycling fans or shutting off some fans).

 Field verification to ensure that incorporated modifications result in anticipated avoidance of partial condensation and its impact on compressor operation or surge avoidance are recommended.

Mark R. Sandberg, P.E. is the Principal of Sandberg Turbomachinery Consulting. He has more than 40 years of varied experience involving turbomachinery. Additional information is available at sandbergturbo.com.

To read about compression hydrogen, check out 

Hydrogen Compression - Prospects & Challenges.

Aftercooler discharge temperature was found to be a significant indicator of partial condensing potential and a critical variable to control and prevent condensation.

Challenges Associated with Heavy Hydrocarbon Compressor Operations

Accurately capturing the characteristics of fluid flow through a compressor or another turbomachine is critical to understanding its performance under different operating conditions. Many analysis tools are used by engineers to predict the behavior of gases and liquids to find the best design for their needs, including analytical methods, empirical correlations, physical experiment, and 

. Most engineers are familiar with and use analytical/empirical analyses and experimental setups when evaluating potential designs. Increasingly, more organizations are moving to accelerate their design processes by also incorporating CFD simulations.

Engineers use CFD in their design process by starting with mechanical computer-aided design (MCAD) models as a foundation. On these models, they construct 2D meshes that adhere to and resolve the model geometry to the level of fidelity required for the numerical simulation. A mesh is then constructed in the 3D volumetric space in which the fluid flow analyses need to be conducted. This 3D space can be internal flow, external flow, or a combination of both depending on the configuration. The topology used to define both the surface and volume mesh regions can vary depending on the type of fluid flow phenomena being studied. Different topologies can provide flow alignment or control over mesh resolution, which can be beneficial for increased accuracy in the simulation. These meshes are fed into CFD solver programs, which solve complex sets of partial differential equations to compute velocity, temperature, pressure, entropy, etc., for the entire flow field as defined by the mesh. With the use of high-speed supercomputers, which are often required for complex configurations, these solutions can be obtained quickly and with a high level of accuracy for challenging physical phenomena. Initial computational results are normally validated against experimental data. The final data sets from these simulations are later compared against existing experimental data from full configuration testing and against any analytical/ empirical calculations that were made.

Used in this way, CFD accelerates existing design processes as it enables the exploration of design spaces in ways and to an extent that would be cost-prohibitive, if not technically infeasible, using experiment alone.

Q: How does CFD software impact the turbomachinery industry? 

 CFD is an efficient way to use resources when doing fluid analysis because an operator doesn’t have to rely on the eight-hour workday to limit how much the analysis can progress. With CFD software, a simulation can run overnight. You press the button, go home, and come back the next morning with the job done. With scalable tools such as CFD software, the investment costs for enhancing workflow are the number of machines running.

 For turbomachinery design workflows, there are a wide range of computing platforms available that give companies flexibility to approach these challenges in a way that best suits the scalability they need for their design processes.

Q: How can CFD be used in the context of real-world design scenarios?

 Because of the scalability factor of CFD, a team can quickly review different designs and verify that its conclusions are correct. Everyone has access to a central processing unit, graphics processing unit, and the cloud, which means CFD is quite accessible to designers.

 CFD data can complement the work that designers are doing from an experimental point of view. For instance, it can be challenging to capture factors such as cavitation, fatigue, and other stressors that can occur. In an experimental rig, data will only come from where sensors are placed. With CFD, when you run a simulation, you’re getting the entire flow field. That allows you to rapidly identify where you might have some inefficiencies or where a failure could take place that you may not capture in an experimental setup. Here, engineers will get a more immediate idea of what the final design should look like.

Q: How can CFD help gas turbines curtail emissions and meet regulatory requirements?

In the turbomachinery industry, designers must be able to explore large design spaces and revolutionary new ideas to meet emissions standards. For conventional designs, with natural gas, for example, the objective is to make the system as fuel-efficient as possible. However, alternative methods are also being explored to find solutions. We have seen designers looking at alternative fuels, and as soon as you start going down that pathway, that presents an even larger design set that needs to be evaluated. Rapid turnaround times and artificial intelligence/machine learning (AI/ML) are tools that can assist in this process. One solution is how we use 3D designs to provide a streamlined turbomachinery design experience using ML methods.

Q: When it comes to working fluids in turbomachinery, how can CFD help address the major issues our industry is facing?

 Sustainability and decarbonization are and will remain two of the most abundantly discussed topics of interest in the future of energy. As it relates to a gas turbine power plant, improving operational efficiency is frequently underscored as the way forward in these ambitions. CFD simulations can be used to explore and optimize operational efficiency for multiple components in these systems and are an ideal way to navigate these challenges going forward.

TurboTime Podcast with Cadence Design Systems. 

Jean-Charles Bonaccorsi and Carolyn Woeber, Joint Application Engineering
Directors at Cadence Design Systems, discuss the role of computational fluid
dynamics in the design lifecycle, turbomachinery emissions, decarbonization,
and more.

A Q&A: How Computational Fluid Dynamics Benefit Turbomachinery 

Digital twins have been talked about for several years, but the complexity of turbine and compressor operations has limited their real-world applications. But that is changing. Most of the large OEMs report implementations or at least large-scale trials. And a variety of software, IT, and industrial vendors have leaped into the arena to add their digitization expertise to digital-twin initiatives. This includes Ansys, Nvidia, Emerson, Vital Technology Services (VTS), Expert Microsystems, Siemens, and AnyLogic.

, but the basic definition is relatively simple.

"A digital twin is a set of computer-generated models that map a physical object onto a virtual space with the most up-to-date data," said Anastasiya Malinovskaya, Content Manager at simulation and digital twin software provider AnyLogic.

A digital twin, then, is a digital representation of an intended or actual real-world physical product, system, or process. It can be seen as a digital counterpart of equipment or an entire facility that can be used for simulation, integration, testing, monitoring, and maintenance that accurately reflects the equipment or facility's ongoing operations. They can be used to simulate planned changes to see how the turbine will perform prior to the physical work being done. They can also be used to stay on top of performance and maintenance operations in real time. A multitude of sensors provide data on all aspects of operations, so the twin matches the equipment it mirrors.

Rainer Kurz, Manager of Gas Compression Engineering at Solar Turbines, divided digital twins into two categories: physics-based models that focus on thermodynamic and aerodynamic parameters; and digital twins that attempt to monitor vibration, temperatures, operating range, and other operational parameters. The former category is used by OEMs to simulate new designs. The latter category is directly related to operational decisions and thus demands that those running turbines trust the conclusions of the digital twin.

"The idea is to increase the reliability and the availability of machinery," Kurz said. "Physics-based models are a lot easier to implement than big-data models."

Probably the best example of a digital twin in the wild is at a power plant on the Ford campus in Dearborn, Michigan. A decade-long modernization project includes office complexes, research and development facilities, and the Dearborn Central Energy Plant (CEP). Operated by DTE Energy, the CEP generates electricity and distributes chilled water, hot water, and steam to nearby buildings. The CEP contains a combined-heat-and-power (CHP) plant and a chilled/hot water plant for power generation and steam production. Digital twin and other technologies make it possible for one person per shift to manage both maintenance and operations.

"A single person looks after the entire 34-MW 87,000-square-foot facility 76% of the time," said Kevin Siess, Regional Operations Manager at DTE Energy. "Our plant operators are also our maintenance staff who can monitor the plant and its control systems while they are doing their rounds."

Digital-twin technology is courtesy of the VTS HardHAT system. As well as a digital twin, it provides 3D modeling, inventory, and computerized maintenance monitoring systems (CMMS). It is supported and integrated with SureSense asset performance management (APM) software by Expert Microsystems. In addition, Solar Turbine's Turbotronic control system is used for sequencing, control, and protection of the gas turbine package and monitoring of associated auxiliary systems. A Rockwell Automation historian and Allen Bradly PlantPAx distributed control system (DCS) are used to control the entire CEP.

Collectively, that adds up to about 12,000 datapoints that are sent to the historian. All of it is available in VTS. Trip alerts are automatically generated on key equipment running in the CEP, such as two 14.5-MW Solar Titan 130 gas turbines, a 5-MW condensing steam turbine from Siemens Energy, heat recovery steam generators (HRSGs) from Rentech Boiler Systems, and feedwater pumps.

Digital-twin technology aids operators in monitoring all aspects of running and maintaining two HRSGs. For example, the high-fired waterwall, O-type HRSGs from Rentech are designed to provide up to 225 kilo-pounds per hour (KPPH) of 200 psig saturated steam.

"Pattern management and 3D modeling enable operators to keep a close eye on hot water and high-pressure steam metrics," said Siess.

Operators can view a digital twin of the HRSGs and drill down into systems and components as needed. The CMMS pushes out upcoming maintenance actions. Parts are flagged that need maintained or replaced based on usage. A recent incident involved power loss to the HRSG. Alerts were generated immediately. Investigation detected a power supply drop off to the programmable logic controller (PLC) that tripped the HRSG. It was resolved within 90 minutes. 

The Rentech HRSGs include supplemental firing. By raising the gas turbine exhaust temperature via supplemental firing, steam production can be greatly increased. Operators control supplemental firing from the booth and monitor temperature, pressure, and steam output numbers.

"This kind of automation is increasingly needed as the power generation labor pool is rapidly shrinking," noted Siess.

After a recent scheduled outage, the SureSense system generated an alert about the oil temperature exiting the shaft bearings rising from 219

F below the alarm level, the software flagged the condition as abnormal. Solar Turbines has scheduled a seal replacement during the next scheduled outage unless the problem worsens. 

"A sudden shift in oil temperature can lead to more varnish potential and make the unit and ancillary cooling equipment work harder," said Siess. "It is vital that we catch issues at an early stage to prevent a major failure."

Another example of the value of the digital twin concerned a problematic gas compressor. Drilling into data within the HardHat system, the operators discovered a slide gate out of calibration that caused unnecessary gas recirculation that had increased parasitic load and system wear. It was repaired before serious problems arose. 

Siess reported only two trips in 2022, leading to a total of four hours of unplanned downtime for the year. Scheduled maintenance takes place in spring and fall. Each gas turbine and HRSG is taken down for two to three days. While one unit is being serviced, the other continues to operate to ensure the flow of steam to Ford is never interrupted.

Features built into the Rentech HRSG provide further assurance of steam flow. A fresh air firing capability utilizes a forced draft fan so the HRSG can be operated when the gas turbine is offline. The fan draws in ambient air and the duct burner provides heat to generate 80 KPPH of steam. A louvre-type diverter valve installed between the HRSG and the turbine facilitates steam/electricity switching. If there is no current steam demand, flue gas from the turbine can be diverted up a bypass stack instead of through the HRSG, generating electricity and not steam.

Siess added that automation enables DTE to untether the operator from the chair to go on maintenance rounds for 45 minutes or more without anyone in the control room. Data on plant operation is available via mobile device, which brings the control room into the field. Alerts are issued if anything needs attention.

"The right technology gives people enough confidence in automation so they can walk away," said Siess. "Our operations staff is now comfortably performing 80% of all maintenance activities."

Few have gone as far as DTE in trusting their digital twins to run plant operations in real time, but there are other examples of digital twins being used for various turbomachinery-related functions:

Aluminum Bahrain (Alba) boasts the world's largest single-site aluminum smelter. Powering these smelters requires 800 MW to heat the metal to nearly 1,000

C. During product evaluation for this new facility, GE's HA-turbine lacked the operating hours to be approved for the project. Alba voiced concern about how the turbines might impact the Bahrain grid if they suffered an unscheduled outage.

GE Digital applied its Predix software platform to run a digital twin of the HA using data from the first HA running at a power plant in Bouchain, France, as well as numerous simulations to show how the turbines would react to the different situations. This demonstrated that if the turbines shut down, the effect on Bahrain's national grid should be minimal. Alba moved ahead with an order for three 9HA turbines as well as three GE steam turbines and three HRSGs.

"The digital twin helped us show that the HA should continue to operate as planned, even in the most challenging conditions," said Khalid Salem, a manager at GE Digital. 

Siemens Energy is active in digital-twin technology in applications like transformers and gearing. There was also a pilot project running a few years ago using the AnyLogic Agent-based Turbine Operations & Maintenance (ATOM) model. This digital-twin technology was implemented to address operational disparities between Siemens aeroderivative units and a unit it acquired from Rolls-Royce (now known as the SGT-A65). The ATOM digital-twin approach proved more effective than the Excel-based forecasting tools previously used to monitor the SGT-A65. ATOM simulated turbine conditions, temperature, engine characteristics, failure modes, and maintenance operations across the fleet; identified bottlenecks; and supplied the simulations needed to determine better investment decisions related to upgrades and turbine replacements. 

Emerson and Mitsubishi Power are working together on digital-twin technologies to optimize performance and reliability and enable predictive and AI-driven maintenance strategies. They are collaborating on solutions using Mitsubishi Power's Tomoni digital platform and Emerson's Ovation automation includes an embedded digital twin. The collaboration is aimed at enhancing the performance and reliability of power plants operating with Mitsubishi Power gas and steam turbines. 

"The simulation solution will seamlessly receive data and operate in parallel with the plant's integrated control systems and other enterprise platforms to support commissioning and training," said Marco Sanchez, the former Vice President of Intelligent Solutions at Mitsubishi Power.

Beyond the big turbine and control OEMs, Ansys helps organizations across many verticals to build their own digital twins. Ansys Twin Builder software combines physics-based data with analytics. The company reports that one utility used its model to predict turbine stresses, saving more than $100,000 in a year by avoiding unplanned downtime. 

Siemens, too, (as distinct from Siemens Energy) offers digital-twin software that has been implemented in aerospace, drones, fusion energy, electronics, and semi-conductors. The closest to a turbomachinery application is a project for SANY Heavy Energy. This real-time digital twin is said to have increased energy efficiency and lowered the levelized cost of electricity by 10%, improved wind turbine efficiency and wind farm design by 50%, and reduced maintenance costs. 

Solar Turbines also uses digital models extensively, both in the development and improvement of existing products, as well as in its Insight condition monitoring system. 

"The former involves detail, components and full engine simulations, supported and augmented by test data, while the latter provides a framework to interpret and act upon operational data," said Kurz.

Elliott Group currently uses advanced multi-physics based digital twins in its controls and fleet-monitoring systems. These digital twins for compressor performance, steam turbines, and vibrations rely on accurate modeling of the function of the turbomachine rather than big data and AI.

"They have been proven to be reliable and accurate," said Klaus Brun, Director of R&D at Elliott Group. "Nonetheless, for applications where significant test data is available either from factory or field operations, such as for bearings and seals, AI is being currently explored for utilization of subsystem digital twins."

Digital twins, then, have many applications. Brun believes AI and statistics-based digital twins add more value with gas and steam turbines that have hundreds and sometimes thousands of similar turbines operating around the globe. Those managing large fleets and OEMs monitoring their own units in the field can gather massive amounts of operational parameters and use the data to improve performance, lower emissions, spot degradation, and avoid unplanned downtime.

But data-based digital twins don't work well in every situation. The compressor market, in particular, is so geared to one-of-a-kind manufacturing that digital twins have limited applicability. 

"Even if you have 1,000 compressors out there, each one of them is going to have different impellers and bearings inside and be tailored to a very specific application," said Brun. "As each one behaves differently, you can't correlate data from one machine to another no matter how big your data set is."

Author: Drew Robb, former editor of Turbomachinery International, is a freelance writer specializing in engineering and technology. Email Drew at drew@robbeditorial.com.

Check out the TurboTime podcast on applications of digital twins and pairing them with AI. 

Digital twins can improve maintenance and plant efficiency.

Digital Twins in the Real World

January/February 2024 features a compilation of the latest industry news and new products in turbomachinery, advice on how to manage bolt connections in turbomachines, and a Q&A with Cadence Design Systems on the benefits of computational fluid dynamics (CFD). This issue’s cover story explores digital-twin technology and some of its turbomachinery applications in the real world, including examples and technologies from DTE Energy, Ansys, AnyLogic, and more. Mark Sandberg discusses the challenges with heavy hydrocarbons in compressor operations, and our vendor spotlight features Ansaldo Energia. Lastly, the Myth Busters contribute once again with an in-depth analysis of the feasibility of green hydrogen for power generation.

How to Manage Bolt Connections & Bolt Joints in Turbomachines

, offers a series of considerations, cautions, and tips for the proper selection of bolt components to avoid operational issues. 

How Computational Fluid Dynamics Benefit Turbomachinery

, with insights from Jean-Charles Bonaccorsi and Carolyn Woeber on the role of CFD in the design lifecycle, emissions, and decarbonization. Our vendor spotlight with Ansaldo Energia dives into the company's portfolio of steam turbine models, as well as the future of a technology that’s been deemed a “mature” component. Mark Sandberg of Sandberg Turbomachinery Consulting reveals case analytics and recommended actions to address aftercooler discharge temperature to avoid the negative impact of partial condensation.

And Myth Busters, Klaus Brun and Rainer Kurz, break down blue hydrogen—produced from fossil fuel with carbon capture—and green hydrogen—produced from alternative energy sources—and the reality of producing it, using it in power generation, and where/how it fits into a sustainable future. As the Myth Busters say: “[Blue and green hydrogen] are often prominently displayed in the technology mix needed for the future of a decarbonized energy economy. However, green hydrogen for power generation is a rare animal, a unicorn, or some other mythological beast. Everyone believes in it, but nobody seems to have actually seen one credibly.”

The January/February 2024 issue features an exploration of digital twins, hydrocarbon challenges for compressors, insights on Ansaldo steam turbines, and more.