A Reasonable Approach to Pump Design While Avoiding Resonance

For the majority of pump application, the growing use of variable speed operation has increased the likelihood of resonance conditions that can cause excessive vibration levels, which can negatively impact pump performance and reliability. Mechanical resonance is the tendency of a mechanical system to absorb more energy when the frequency of its oscillations (external excitation source) matches the system’s natural frequency of vibration more than it does at other frequencies. To avoid vibration issues, potential complications must be properly addressed and mitigated during the design phase.

Some of the factors that may cause excitation of a natural frequency include rotational balance, impeller exit pressure pulsations, and gear couplings misalignment. The effect of the resonance can be determined by evaluating the pumping machinery construction. All aspects of the installation such as the discharge head, mounting structure, piping and drive system will affect lateral, torsional and structural frequencies of the pumping system. It is advised that the analysis be conducted during the initial design phase to reduce the probability of reliability problems and the time and expense associated.

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Impeller Design Challenges on Integrally Geared Centrifugal Compressors

The integrally geared compressor, also known as a multi-shaft compressor, is a technology that has been around since the 1960s, but remains underdeveloped.  Usually seen in applications in the industrial gases industry, integrally geared compressors (IGCs) can range in size from small product machines to steam turbine driven high-horsepower, high-flow compressors for air separation plants.  These compressors modular construction principle, consisting of as many as eight different stages, allows for implementation in a large number of varied customer processes.  The main advantages of IGCs in the industrial gases industry is the compact design and smaller installation footprint, efficiency increases due to the use of multiple speeds for separate impellers, and overall lower operational and installation costs.

semi-open-impeller
Figure 1 – Semi-Open Impeller

One of the key design differences between the standard inline compressors and the IGCs is that the integrally geared compressor makes use of both closed AND semi-open impellers.  The reason for the use of open impellers in IGCs are the higher strengths due to better manufacturing techniques, speed of manufacture, and the inherent lower costs.  However, the main drawback to having an open impeller in your system is that in the event of impeller rub, the damage to the compressor would be significantly worse than with a closed impeller.

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The Balancing Act – Rotor Stability

When designing rotating equipment, it is extremely important to take into account the types of unbalance that can occur. Forgetting this step can result in vibrations that lead to damage of the rotating parts, increasing maintenance costs and lowering efficiency. Currently, if a rotating part already vibrates or makes any noises, maintenance engineers rely on OEMs (Original Equipment Manufacturer) or third parties services companies to conduct balancing services.

Types of Unbalances

ubalance
Figure 1: Static and Couple Forms of Unbalance

The three types of unbalances to consider are static, couple and dynamic. Static unbalance (Figure 1) occurs when a mass at a certain radius from the axis of rotation causes a shift in the inertia axis. Couple unbalance, usually found in cylindrical shapes, occurs when two equal masses positioned at 180 degrees from each other cause a shift in the inertia axis, leading to vibration effects on the bearings. Lastly and most common, dynamic unbalance occurs when you have a combination of both static and couple unbalance.

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The Future of Turbocharger Technology

­One of the main setbacks in scaling different turbochargers for diesel, petrol, and gas engines is the inherit variability that different turbochargers would exhibit at low or high RPMs. In order to understand this further, a common term used to describe a flow characteristic of these machines is the A/R ratio.  Technically, this ratio is defined as the inlet cross-sectional area divided by the radius from the turbo center to the centroid of that area (Figure 1).  This ratio is essentially a metric for the amount of air that is allowed into the turbine section of the turbocharger.

Ratio visualization
Figure 1 – A/R Ratio Visualization

For smaller turbochargers, lower A/R ratios allow the fast exhaust velocities to drive the turbine at lower speeds.  This results in a more responsive engine and overall higher boosts at lower RPMs.  However, once a vehicle starts to navigate at a higher RPM, smaller turbochargers experience a significant reduction in performance due to the high backpressure present in the system.  This occurs because of the low A/R ratio limits the flow capacity and does not allow a sufficient amount of air to feed into the turbine.  The same effect is present for larger turbochargers, only in reverse.  They will perform most efficiently at higher RPMs, but in turn exhibit a significant reduction in performance at lower RPMs.

In order to overcome this phenomenon, many engineers have developed more complex turbocharger systems over the years, which attempt to leverage the benefits of each type of turbo.  One of the first solutions to this dilemma was the twin turbo: simply comprised of two separate turbochargers operating in the system in parallel or in series.  The problem with this system is that it disproportionately increases the cost, complexity, and space necessary for implementation.

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Optimizing the Cooling Holes in Gas Turbine Blades

To increase the overall performance of the engine and reduce the specific fuel consumption, modern gas turbines operate at very high temperatures. However, the high temperature level of the cycle is limited by the melting point of the materials. Therefore, turbine blade cooling is necessary to reduce the blade metal temperature to increasing the thermal capability of the engine. Due to the contribution and development of turbine cooling systems, the turbine inlet temperature has doubled over the last 60 years.

thermal-effiency
Figure 1: Variations of Thermal Efficiency with TIT [1]
The cooling flow has a significant effect on the efficiency of the gas turbine. It has been found that the thermal efficiency of the cooled gas turbine is less than the uncooled gas turbine for the same input conditions (see figure 1). The reason for this is that the temperature at the inlet of turbine is decreased due to cooling and therefore, work produced by the turbine is slightly decreased. It is also known that the power consumption of the cool inlet air is of considerable concern since it decreases the net power output of  the gas turbine.

With this in mind, during  the design phase of gas turbine it is very important to optimize the cooling flow if you are considering both the performance and reliability. Cooled Gas turbine design is quite complicated and requires not only the right methodology, but also the most appropriate design tools, powerful enough to predict the results accurately from thermodynamics cycle to aerothermal design, ultimately generating the 3D blade.

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Performance Effects of Axial Turbines & Compressors Due to Roughness Variations

As turbomachinery technology continues to advance in efficiency as well as overall power, many engineers want an estimate on how long these manufactured machines will operate.  Specifically, in high-temperature and high-flow turbomachinery applications, one of the main sources of performance degradation can be attributed to increases in surface roughness.  Gas turbine and compressor blades in particular experience a substantial amount of surface degradation over their lifetime.

gas turbine blade
Figure 1 – Gas Turbine Blade and Annulus Surface Wear (Source PowerMag)

There are many mechanisms that contribute to surface degradation in airfoils and annulus surfaces.  Foreign particles adhering to the material surface (or fouling) is generally caused by any increase in contaminants such as oils, salts, carbon, and dirt in the airflow.  Corrosion occurs when there is a chemical reaction between the material surface and the environment that causes further imperfections on the machine surfaces.  Additional mechanical factors such as erosion and abrasion will play a part in a machine’s surface degradation as well.

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Rotor Dynamics – Importance of Fundamental Understanding & Software tools

Rotor and bearings are the most critical components of any rotating machinery. Rotor lifetime and reliability depend, first of all, on the level of rotor vibrations. In order to meet highest requirements of reliability each step of the rotor design should be based on accurate Rotor Dynamics prediction.

Rotor dynamics is the branch of engineering that studies the lateral and torsional vibrations of rotating shafts, with the objective of predicting the rotor excessive vibrations. Rotor Dynamics is different from structural vibrations analysis because of gyroscopic moments, cross-coupled forces, critical speeds, whirling effect, etc. These difference makers are all due to the rotation of the rotor assembly.

Rotor-Dynamics

Understanding of basic rotor dynamics phenomena and the various types of problems is absolutely mandatory when designing and developing rotor-bearing systems for various applications. Fundamental approach for Rotor Dynamics analysis generally is based on the following steps:

  1.  Predict critical speeds.
  2. Determine design modifications to change critical speeds.
  3. Predict natural frequencies of torsional vibration.
  4. Predict amplitudes of synchronous vibration caused by rotor unbalance.
  5. Predict threshold speeds and vibration frequencies for dynamic instability.
  6. Determine design modifications to avoid dynamic instabilities.
  7. Calculate balance correction masses and locations from measured vibration data.

Another factor that determines accuracy of Rotor Dynamics calculation is rotor system simplification and the adequate modelling for rotor parts such as Impeller/disks, Sleeves, Balance pistons, Seals, Thrust collars, Couplings, Addition of Stiffening Due to Shrink Fits and Irregular Sections etc. Continue reading “Rotor Dynamics – Importance of Fundamental Understanding & Software tools”

Product Development: Rescale existing turbomachine design vs full design process

When deciding on a new product line, manufacturers of turbomachines and their engineering teams must often decide whether to rescale a product that they already manufacture or to begin a full design process for a completely new machine. For example, a producer of 5 MW axial turbines wants to start manufacturing 10 MW turbines, does it make sense to create a brand new design from scratch or to simply scale up the 5 MW turbine they already produce to a similar 10 MW version? To answer this question, many considerations have to be taken into account, the general answer however is, that it is almost always a better idea to start a new design.

Improved Design Technology

Many manufacturers wrongly believe that by simply scaling their current product that they will save not only on design costs, but that they can leverage their existing manufacturing capabilities to stamp out a similar product. What is not factored in however is the progress of design technology and theory since their original machine was first conceptualized. The result from a simple scaling process will simply be a less optimized and efficient machine for any use as compared to a new configuration using the latest in design software. Increasing software sophistication and computing power are constantly pushing the boundaries of efficiency while minimizing operating costs. Simply put, your competitors will have designed a superior product compared to yours.

BladeProfiling-Turbomachinery-Design-Software

        AxSTREAM 3D Blade Design Software

Improved Materials

When was your current machine designed? Many older machines were created using materials that by today’s standards are simply not capable of operating at the extreme conditions  (mostly temperatures) required today to attain the energy efficiency requirements set up by ever increasing regulations. Depending on materials used, the optimal blading structure, bearings, etc. geometries would be significantly unique. If one were to simply scale up their current product, they would either be using old materials or have inefficiently designed machine components for a different material. In either case, their scaled machine will be inferior to a configuration that was conceptualized and optimized from scratch.

Scaling Factors

Another very significant aspect of machine resizing is that it is not a straight forward process; if you want to double your power generation in a turbine for example you are not going to be doubling the blade size or mean diameter, for example, even when considering the same boundary conditions (inlet pressure and temperature, as well as, outlet pressure, rotation speed, and so on). For each specific set of conditions, fluid, rotation speed, mass flow rate, etc. a unique flow occurs inside the different blades. Changing one parameter will lead to changes in the flow and therefore result in inefficiencies, as it is what happens in off-design conditions (the machine is not operating at its maximum performance). This is why flow similarity parameters become relevant.

Machine Purpose and Type

One of the obvious questions to ask is, what is the purpose of my new machine and how much larger (or smaller) will I need it to be? If the new machine is intended for use with a completely different fluid, a new design will be optimal as different fluids interact in unique ways with varied rotor and stator configurations.

The machine type that you are considering is also critical to the decision. Different turbomachines do not scale in similar fashion with increase in size. For instance, radial turbines are usually not as efficient as axial turbines when one starts to approach the 2 MW range. In this instance the ideal solution is for a complete redesign since a smaller scale version that the manufacturer may have had would not be configured to operate at higher power ranges efficiently.

 

Improving Turbine and Compressor Design Matching

Compressor-Turbine-MatchingOne of the most prominent steps of complete gas turbine design is turbine-compressor matching. There are three major components to a gas turbine: compressor, combustor, and turbine. Although all of the components are designed individually, each of the components needs to correspond within the same operating condition range since all are integrated into one cycle. Consequently, an optimal design of each component must fit the requirement of other component’s optimal parameters. Corresponding operating points for each component must be found at equilibrium with the engine, thus the overall performance of gas turbine can be reached within the defined range of parameters.

The idea behind component “matching” process is to find flow and work compatibility between corresponding components. Based on the mechanical constraints, gas generator speed and firing temperature of a gas turbine have limitations depending on: ambient temperature, accessory load and engine geometry. The match temperature chosen should be the ambient temperature which reach both upper limits at the same time.  Pressure ratio needed to allow a certain gas flow is also one of the most prominent parameters that has to be taken into consideration. Designers need to make sure that the gas flow through the power turbine from gas generator satisfy the pressure ratio needed for compressor power requirements. Gas generator can easily show an altered match temperature due to some conditions i.e: reduction in compressor efficiency (due to fouling, etc), change of thermodynamic properties of combustion product, gas fuel with lower or higher hearing value, etc. Match parameters of an engine could also be altered by changing the flow characteristics on the first turbine nozzle.Turbine-Compressor

Using characteristic map/curve as well as thermodynamic relationships of turbine and compressor, calculations can be performed to identify the permitted operating range. It must be taken into consideration that all calculated value must match the value from map data.

Trying to find the fastest solution for this step? SoftInWay’s turbine-compressor matching feature in AxSTREAM could help you cut engineering time and simplify the process. Combining performance maps of turbine and compressor, making it easier for the user to determine points of joints operations.

Take a look into AxSTREAM’s to learn more about this.

Reference:

https://www.grc.nasa.gov/www/k-12/airplane/ctmatch.html
http://cset.mnsu.edu/engagethermo/components_gasturbine.html
http://turbolab.tamu.edu/proc/turboproc/T29/t29pg247.pdf
http://scholarcommons.sc.edu/cgi/viewcontent.cgi?article=3255&context=etd
Turbine Compressor Matching Compatibility Mode Document

The Economic Optimization of Renewable Energy

Global warming has been a very popular topic these days. With up-trend of clean technology and realization that strict climate policy should be implemented, demand of renewable energy sky-rocketed as conservative plants popularity falls. Number of coal power plants have significantly dropped since its peak era, being known as the largest pollutant contributor as it produces nitrogen oxide and carbon dioxide, the technology is valued less due to its impact on nature. Renewable energy comes from many sources: hydropower, wind power, geothermal energy, bio energy and many more. The ability to replenish and having no limit in usage and applications make renewable energy implementations seems attractive. Aside from that, they also produce low emission, sounds like a win-win solution for everyone. Theoretically, with the usage of renewable energy, human-kind should be able to meet their energy need with minimal environmental damage. With growth rate ranging from 10% to 60% annually, renewable energy are getting cheaper through the technology improvements as well as market competition. In the end, the main goal is still to generate profit, though these days taking impact on nature into the equation is just as important. Since the technology is relatively new, capital cost still considerable higher compared to some cases with more traditional (–and naturally harmful) implementations. So the question is: how to maximize the economic potential of a renewable energy power generation plant?

The Economic Optimization of Renewable Energy

Living up to the maximum potential of any power generation plant starts in the design process. Few examples for solar power plant: designers should take into consideration type and quality of panels, it’s important to see the economic-efficiency tradeoff before jumping into investment; looking into the power conversion is also one of the most important steps, one should take into consideration that it would be worthless to produce more energy than the capacity that are able to be transferred and put to use, though too low energy generation would mean less gross income.

Another example, for a geothermal power plant, many studies have shown that boundary conditions on each components play a big role in determining the plant’s capacity and efficiency. High efficiency is definitely desired to optimize the potential of a power plant and minimized the energy loss. Though, should also be compared to the economic sacrifice; regardless of how good the technology is, if it doesn’t make any economic profit, it would not make sense for one to invest in such technology. Low capital cost but high operating expenses would hurt the economic feasibility in the long run, whereas high capital cost and low operating expense could still be risky since that would mean a higher lump sum of investment upfront, which might or may not breakeven nor profitable depending on the fluctuation of energy market.

Modern technology allows investors and the engineering team to make this prediction based on models developed by the experts. SoftInWay just recently launched our economic module, check out AxCYCLE to optimize your power plant!

Reference:

[1] Optimal design of geothermal power plants 

[2] Strategies in tower solar power plant optimization

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