September 2016, Vol. 243, No. 9
Features
Guidelines Aim for Better High-Speed Compressor Systems
With the growing use of high-speed separable reciprocating compressors in natural gas storage and transmission applications, the Gas Machinery Research Council (GMRC) identified the need for better guidelines and practices to govern the specification, design and application of this class of compression equipment.
A 2½-year research project yielded the GMRC Guideline for High-Speed Reciprocating Compressor Packages for Natural Gas Storage and Transmission Applications that was released in October 2013. GMRC, a subsidiary of the Southern Gas Association (SGA), is a not-for-profit research corporation.
As the guideline developed, a need for better reciprocating compressor torsional vibration analysis guidelines was also identified. This resulted in the release of the GMRC Guideline and Recommended Practice for Control of Torsional Vibrations in Direct-Driven Separable Reciprocating Compressors in April 2015.
The torsional guideline derived from a 2013-15 GMRC project, led by Cambridge, OH-based ACI Services, Inc., and supported by the extensive collaboration and contributions of 39 industry experts from 26 companies, including motor, engine, compressor, coupling and variable frequency drive (VFD) manufacturers, torsional analysis and testing companies, end users and engineering consultants.
Gas Compression Evolution
Gas compression for natural gas pipelines has continually evolved since first emerging in the late 1800s. Early compressors were huge, slow-speed horizontal integral reciprocating gas engine compressors. In the 1930s, manufacturers introduced angle-integral reciprocating gas engine compressors that were comparatively easier to transport and install.
Over the next four decades, larger and larger integral units were introduced, with the largest exceeding 10,000 hp. These conservative, reliable and versatile engine compressors, most of which operated at speeds in the range of 250-330 rpm, were installed by the thousands as the interstate pipeline infrastructure was developed throughout the country.
In the 1950s, centrifugal compressors began to be applied on mainline systems. With the development of aircraft derivative gas turbines in the 1960s and beyond, gas turbine-driven centrifugal compressors gradually displaced large integrals on new pipelines and expansions.
The manufacture of new integral engine compressors ended in the 1990s, as the U.S. pipeline infrastructure was essentially built out, and gas turbine-driven centrifugal compressors filled most expansion needs. Yet, because of their inherent flexibility and high efficiency, the need for reciprocating compressors to support gas storage and transmission applications has never disappeared.
Beginning in the late 1950s, high-speed gas engines and matching separable reciprocating compressors, packaged as ready to install skidded systems, steadily took over upstream gas applications. As technology improved, these units grew in rated power and speed. By the late 1990s, high-speed separable engines and matching compressors were being offered at ratings as high as 8,000 hp.
Better Guidelines
As larger – at least 2,000 break horsepower (bhp) – high-speed (at least 700 rpm) reciprocating compressor packages were applied, especially in low ratio, high-flow, highly flexible pipeline transmission applications, it was too often discovered that the technology required for analyzing, designing and fabricating high-speed packaged systems lagged behind the fundamentally reliable and efficient engine and compressor equipment itself. Various concerns emerged with packaged compressor system efficiency, vibration, pulsation, and ancillary components and sub-systems.
Prior to the GMRC’s effort to address these issues, there was no comprehensive specification document for the purposes of procuring, designing and applying large, high-speed compressor packages. For example, the API 618 pulsation and vibration standard only applies to low-speed compressors, leaving confusion in the marketplace about what standard should be applied to high-speed units. In the absence of a standard, many units are fabricated without a proper level of pulsation and vibration analysis.
In particular, the range of potential operating conditions is typically not adequately explored. The former API 11P and the current ISO 13631 standards, intended primarily for field gas compressors, provide no in-depth guidance in many of the areas of concern. The GMRC Guideline for High-Speed Reciprocating Compressor Packages for Natural Gas Storage and Transmission Applications filled the gap by addressing these concerns. Since its release in 2013, it has increasingly served as an excellent tool for purchasers, engineering companies, packagers and operators of this class of gas compression equipment.
Silent Killer
Torsional vibration is the relative dynamic motion about the axis of rotation of different components of a machine or machinery system. This results in associated dynamic twisting of shafts, shaft sections and couplings between the components. All trains of rotating equipment have a number of torsional natural frequencies. These can be excited by the time variation of torques acting on the individual components – compressors, engines or motors – of the equipment train.
When the excitation frequency coincides with, or is in close proximity to, a torsional natural frequency, resonance results. Unless very well damped, this “interference” with a natural frequency can lead to high torsional vibration, high cyclic torsional stress and fatigue failure. High torsional vibration is typically not detectable without special measurement instrumentation, so it is like a “silent killer” that, with no prior warning, can lead to a sudden failure of some part of the machinery train.
Despite the development and evolution of sophisticated design and analysis tools, torsional vibration-related problems continue to be a recurring issue for reciprocating compressor installations. The majority of problems have occurred with motor-driven compressors.
However, engine-driven systems are not immune from experiencing torsional vibration issues. Reported problems have included failures of crankshafts, couplings, motor shafts, welded joints on motor spiders, auxiliary drive assemblies and cooler fan shafts.
These issues have often required the need to add torsional dampers, add or change flywheel inertias, change or avoid operating speed ranges, or change coupling, or even shaft, designs to improve the tolerance of system components to cyclic torque and the resulting torsional stress.
Better Torsional Guidelines
The new GMRC Guideline and Recommended Practice for Control of Torsional Vibrations in Direct-Driven Separable Reciprocating Compressors, defines recommended practices for helping to ensure the integrity of separable reciprocating compressor applications with respect to torsional vibration.
Its intent is to introduce some uniformity to the processes of torsional vibration analysis and testing by focusing on the practice of torsional vibration analysis and associated assessment criteria. Tutorial discussions of items related to torsional vibration inclusive of drive-train design, configuration, fabrication and assembly are also provided. It also provides information for those who are responsible for a compressor installation as to what to expect from a torsional vibration analysis, as well as the significance of the different elements of the analysis.
Direct-driven separable reciprocating compressor installations, which are the focus of the new guideline, are normally assembled and delivered by a packager that takes responsibility for designing and assembling a system that includes a compressor and a driver, usually connected by a coupling. In some cases, the packager may be the compressor manufacturer. The compressor may have multiple stages or services.
The driver may be a reciprocating engine or an electric motor. On occasion, the drive line may include a gear box and, in addition to the aforementioned drivers, it may be powered by a high-speed driver, such as a gas turbine or steam turbine. While much of the content of the guideline is relevant to the analysis of these less common gear box drive configurations, such equipment has analysis and operational requirements that are not addressed specifically in the torsional guideline.
Following introductory and scope explanations, the torsional guideline sets forth the general requirements for analysis, most directly oriented toward the entity or individuals with responsibility for initiating the torsional analysis and interpreting its results.
The next and largest section provides detailed requirements for torsional analysis, directed at the entity of individuals responsible for executing the analysis. A section on torsional validation testing covers the experimental verification of torsional behavior in the as-built equipment, and a section on installation, operation and maintenance considerations provides some recommended practices for maintaining the integrity of torsional tuned machinery systems.
A series of detailed appendices provides checklists of data required for a torsional analysis, recommended computational procedures that have been identified as best practices, miscellaneous information that provides more detailed technical discussion and insight for various subject areas referenced in the body of the guideline, a long list of references, and examples of detailed, comprehensive torsional analysis reports for engine-driven and electric, motor-driven separable reciprocating compressors.
Analysis Needed
Regardless of whether the equipment is new or existing, it is recommended that a torsional analysis be performed for every non-duplicate reciprocating compressor package. Subtle differences in configuration or operation from other successful installations can change a system’s dynamic characteristics or the excitation levels and frequencies, and may lead to intolerable dynamic torques and resulting stresses.
For example, a reciprocating compressor package with a proven history of “trouble free” operation, when all of its compressor throws are double-acting, may experience “troublesome” levels of torsional vibration when one, or more, of its throws are partially deactivated (single-acting).
Often, the pressures of schedule, cost constraints, or other considerations may lead to a decision to bypass the torsional analysis. For those contemplating such a decision, the torsional guideline provides a basis for assessing the risk of neglecting a torsional analysis, both for reciprocating compressor installations with electric motor drivers and for those driven by reciprocating engines.
Basic factors included in the risk assessment tables take into account the equipment power, speed range (if any) and the number of compressor throws. Additional factors include the type of driver, the usage and type of compressor cylinder capacity control devices and, of course, the criticality of the installation. Each risk factor is given a score, and the scores for all risk factors are then added together.
Guidelines are then provided for threshold scores that are indicative of an elevated probability of encountering a torsional vibration problem. A score at or above these thresholds indicates that a torsional vibration analysis should be specified and included in the design process.
Although the torsional risk assessment is not perfectly objective and is intended primarily to determine when a torsional vibration analysis is required, it can be also be used for guidance in the selection of equipment configurations and operational schemes to reduce the risk of torsional problems.
End users, in consultation with engineering consultants, compressor manufacturers, packagers, torsional analysts and other resources, may want to reconsider proposed solutions that involve lower risk factors. For example, if variable speed is specified, is it a critical requirement or would a constant speed machine be good enough? Or, if infinite-step unloaders and single-acting load steps are specified, are they really necessary, or could an optimal selection of volume pockets be good enough?
A high score is an indication that an acceptable torsional solution that meets all objectives may not be obtainable, that very careful engineering of a torsional solution may be needed, or that increased operational risk will result. It may also necessitate “blocked-out” speed ranges, reduced ranges of operation or elimination of some load steps.
Best Practice
The torsional guideline also provides guidance on the responsibility for instigating an appropriate torsional analysis. When a compressor package is purchased, it is with the expectation that it will operate reliably at the stated conditions. Therefore, it is logical, and often a contractual requirement, that the compressor packager, systems integrator or the organization taking responsibility for procurement and assembly of the equipment components will provide a torsionally sound design.
Thus, the torsional guideline recommends that this organization should have responsibility for ensuring that an appropriate torsional analysis is undertaken and completed. This organization has the most ready access to the necessary lines of communication to ensure the analysis is performed and to expedite the needed transfers of data between parties.
Nevertheless, the end user must operate the equipment safely and reliably once it is commissioned and turned over. Therefore, it is important that the end user impose due diligence on the process, commensurate with the criticality of the installation. The torsional guideline provides further responsibilities and typical project steps with regard to a successful torsional system specification, design, fabrication, installation and operation.
GMRC emphasizes its guidelines are not specifications or mandatory requirements. The new torsional guideline is available at http://www.gmrc.org/resources.
Field Gas Compressors
In addition to updating the 2013 guideline, a new GMRC research program is focusing on the requirements of reciprocating compressor packages used in field gas, or upstream, applications such as gas gathering, gas lift and gas processing.
The upstream compression sector applies higher-speed (up to 1,800 rpm) reciprocating compressor packages that are generally under 2,000 horsepower. Field gas applications have a number of characteristics that are not covered in the 2013 guideline. These include the need to handle wet and dirty gas; wider variations in gas analysis including sour, CO2 and N2 content; on-skid fuel gas conditioning; high-pressure ratios requiring multiple stages with intercooling; engine accessory end drives for auxiliary equipment; portability; outdoor packages; and integrated enclosures for cold weather packages.
Economics is a major factor driving the behavior of the supply chain. While the presence of a number of competent packagers in the industry makes for a competitive procurement environment, this is not necessarily an advantage when the procurement specification is insufficient.
With a continuing and accelerating decline in average engineering experience throughout the industry, many best practices are being lost. In addition, more packagers, with varying levels of experience and capability, serve the upstream reciprocating compressor market that demands many more units. Since the advent of the shale boom, more aggregate new horsepower has been placed in the upstream than in the natural gas pipeline and storage industry each year. Purchasing decisions in much of the upstream sector are usually driven by price and lead time.
Packagers complain they often have better solutions to offer, but the lack of adequate industry standards, inexperience of end users and the competitive nature of this sector, make it difficult to include and charge for improvements that aren’t specified by end users.
Similar to the situation that existed with large compressor packages prior to release of the 2013 guideline, existing industry specifications are of limited value for the purposes of procuring, designing and applying high-speed upstream compressor packages.
API is working on a new field gas compressor specification. Nonetheless, the development and approval cycle typically takes many years of effort. Elements of the existing GMRC guidelines are being considered as references for new API and Energy Frontier Research Center (EFRC) specifications. However, the GMRC decided a new program is needed for developing a comprehensive guideline that covers all the unique elements of field gas compressor packages.
Launched early this year, the program is beginning with an extensive survey of end users, packagers, contract compression providers, equipment manufacturers, engineering companies and published literature to determine important field gas – from wellhead through first level of gas processing – packaged compressor issues and best practices.
Another of the surveys will identify problems that lack adequate practices. Then use literature searches, observation and the collective experience of the extended project team, which includes representatives from 32 companies, develop guidelines. GMRC is targeting completion of a formal guideline for field gas compressors in the next 18 months.
Author: W. Norm Shade is senior consultant and president-emeritus of ACI Service, Inc., the manufacturer of custom-engineered reciprocating gas compressor products, headquartered in Cambridge, OH. Shade, who received his BME and MSME degrees from The Ohio State University, has over 40 years in various engineering and management roles.
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