June 2015, Vol. 242, No. 6
Features
High-Speed Compressors Spawn Need for Better Guidelines
With the growing use of high-speed separable reciprocating compressors in natural gas storage and transmission applications, the Gas Machinery Research Council (GMRC) sees the need for better guidelines and practices to govern the specification, design and application of this class of compression equipment.
With the culmination of a more than two-year research project, a new GMRC Guideline for High-Speed Reciprocating Compressor Packages for Natural Gas Storage and Transmission Applications was recently released. This summer the GMRC also introduced a three-day training course that teaches application of the recommended practices and methods provided in the new guideline.
Gas Compression Evolution
Gas compression for natural gas pipelines has continually evolved since 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 horsepower (hp). These conservative, reliable and versatile engine compressors, most of which operated at speeds in the range of 250-30 rpm, were installed by the thousands as the interstate pipeline infrastructure was developed throughout the country.
In the 1950s, centrifugal compressors were applied on mainline pipelines. With the development of aircraft derivative gas turbines in the 1960s, gas turbine-driven centrifugal compressors gradually displaced large integrals on new pipelines and expansions. The manufacture of integral engine compressors ceased in the 1990s, as the U.S. pipeline infrastructure essentially built out, and ever larger gas turbine-driven centrifugals 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 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 offered at ratings as high as 8,000 hp.
Robust Systems
Compared to traditional integral reciprocating gas engine compressors, high-speed separable reciprocating compressors, driven by natural gas engines or electric motors, provide lower capital cost, shorter installation time and compactness.
For several decades, they have been the prevalent type of compressor equipment applied in upstream and midstream gas gathering and processing natural gas compression markets. Current state-of-the-art, high-speed reciprocating gas engine drivers provide excellent compliance with exhaust emissions regulations. Compared to centrifugal compressors, high-speed reciprocating compressors are much more flexible, maintaining high efficiency over a broad operating range.
However, as larger (?2,000 brake horsepower (bhp)), high-speed (?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.
Higher speed compressors naturally create a broader spectrum of gas pulsation frequencies that must be addressed in the system design. Further, the lighter frames and I-beam skid mounting, typical of high-speed compressor packages, tend to be more flexible and reactive than traditional heavier, slow-speed compressors that are block-mounted.
Pulsation dampening and piping system pressure losses can also be more of a concern because of the higher frequency pulsation that is generated by high-speed compressors. This has driven the development of better and more sophisticated time-based methods of pulsation and vibration modeling and analysis, as well as additional pulsation control “tools” and suggested practices for damping, de-tuning or canceling pulsations.
Another important consideration is that for centrifugal and traditional slower speed (<700 rpm) reciprocating compressors, the compressor manufacturer commonly takes full responsibility for system’s design and performance. in contrast, high-speed packages are typically not offered by directly. instead, a packager purchases compressor, driver other high-value content from individual manufacturers, then integrates assembles them on fabricated i-beam skid with pulsation bottles, scrubbers, gas piping, utility instrumentation, controls auxiliaries. vast majority of units that upstream applications, which smaller size, always highly engineered high efficiency is primary consideration. some design, analysis fabrication practices common reliable smaller, necessarily adequate storage pipelines applications larger, heavier, high-flow, low ratio flexible compressors. economics also significant factor. while presence multiple competent packagers supply chain makes competitive procurement environment, this an advantage when end user’s specification insufficient. engineering capability experience level vary. it important to recognize packager’s value-added limited after buying large components make up package cost. gross margin comparatively small percentage total they manufacture, together mark-up purchased components, must cover costs engineering, overhead, financing, marketing sales commissions, leaving profit satisfying commercial expectations its owners or shareholders. Better Specifications
There is 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.
Recognizing this gap, in 2011 the Gas Machinery Research Council (GMRC) initiated a research project to develop a specification or guideline to address the foregoing packaged high-speed compressor concerns. Cambridge, OH-based ACI Services, Inc. was selected as the contractor for the supporting investigations, data gathering, research and development of a new guideline for the industry.
The GMRC, a subsidiary of the Southern Gas Association (SGA), is a not-for-profit research corporation, founded in 1952. It provides member companies and industry with the benefits of an applied research and technology program directed toward improving reliability and cost effectiveness of the design, construction, and operation of mechanical and fluid systems. GMRC also serves the industry through technical training programs and conferences, including the annual Gas Machinery Conference and Exhibit.
The research and data collection phase for development of the new guideline involved interviews of over 100 people at 12 end-user companies, five compressor manufacturers, two engine manufacturers, an electric motor manufacturer, fur compressor packagers, fur engineering services company and a foundation engineering company.
In addition, 11 pipeline or storage field sites were inspected, covering 30 different high-speed engine and motor-driven compressors. From this information, a long list of problems, solutions, preferences and suggested practices was documented to serve as the foundation for development of the new guideline.
An extensive search was also conducted to evaluate what existing materials could be cited and referenced in the guideline. After over 30 months of effort, the new guideline was released in October 2013.
The guideline defines recommended practices, not mandatory requirements. It is intended to provide end users and operators with more reliable procedures and references for selecting, specifying, procuring, applying and operating high-speed units with more predictable and reliable results.
It also provides compressor packagers with more comprehensive and detailed guidance for designing and building high-speed compressor packages that meet customer and equipment manufacturer expectations. While portions of the document may be applicable to gathering and midstream applications, the principal use is targeted for higher horsepower (?2,000 bhp), high-speed (?700 rpm), highly flexible, low-ratio gas transmission compressor applications and versatile gas storage and withdrawal applications.
A significant part of the document serves as a tutorial on how to handle specific aspects of large compressor package specification, procurement and design.
Areas of detailed coverage within the 200-page guideline include scope and introduction, project and schedule management, compressor system selection and specification, compressor capacity control, drivers and couplings, skids and foundations, pulsation and vibration analysis and control, equipment accessibility and maintainability, instrumentation and control, inspection and testing, installation and commissioning, and operation and maintenance considerations. Accompanying the guideline are spreadsheet-based tools for project scheduling, vendor bid evaluation and preliminary pulsation bottle sizing.
Successful High-Speed Compressor
Successful large, high-speed compressor projects share several common characteristics. First, a specification should be used to define the scope, requirements and expectations. Next, projects must be well-managed. A capable project manager should be assigned by the end user: either an experienced employee or a qualified, experienced compression system consultant contracted directly by the end user. The end user should stay involved in the project, especially at key meetings (those that establish scope, kickoff the project or review analysis findings and recommendations).
Reasonable project time lines should be established for the completion of the design, including vibration control recommendations, such as one week turnaround for preliminary bottle sizes, three weeks for pulsation analysis and time for mechanical analysis, collaboration with packager, assessment of alternatives and preparation of drawings.
A range of required and potential operating conditions should be provided in addition to the identification of key design points. This should include all possible off-design operating ranges over which the equipment could potentially be operated. Ideally, this is part of the pre-order process, but at a minimum, it must be part of the design process. Compressor unloading and operating requirements should be defined, considering required operational flexibility and potential pulsation and vibration concerns, not just a limited number of design points.
Early decisions about pulsation control vessels, compressor unloading and control approaches, and the skid and foundation can significantly affect the success of the compression system. It is, therefore, a good practice to select the pulsation and vibration consultant for the project early in the design process, and more preferably, in the specification phase.
A complete and rigorous analysis of the pulsation control system and the equipment-supporting structures is a critical success factor. The needs for trade-offs and rapid design decisions is common, and the end user should stay closely involved with the design and analysis process, so that not only a safe and reliable, but an optimal, system design results.
Ideally, the pulsation and vibration consultant should work directly for the end user or its representative to ensure that design goals are met and decisions requiring time-critical efficiency and reliability trade-offs are made prudently and quickly.
The majority of high-speed compressor systems are built by third-party packagers rather than the compressor manufacturer. Packager qualifications and the experience of current design staff should be carefully evaluated, and one or more competent packagers should be pre-qualified.
The package or the packager should never be treated as a commodity for this class of equipment. Once a qualified packager is identified as able to meet the end user’s requirements, it is generally advisable for the end user to continue working with that packager for its large horsepower, high-speed compressor projects as long as expectations and requirements continue to be met.
Since the compressor package and the station facility need to be integrated for a successful installation, it is recommended that the engineering, procurement and construction (EPC) company also be brought into the project as early as possible.
The EPC and packager should provide the required information to enable the analyses to begin early in the design process. Preliminary pulsation analysis results can define bottle size and placement to avoid later problems. The final analysis can then be completed as off-skid information, including plant piping, becomes available later during the project – and the design can be adjusted, if necessary, to ensure that any off-skid issues are addressed.
Finally, the end user should assign an inspector for the compressor fabrication phase at the packager’s plant to ensure quality work and expeditiously resolve questions that arise during fabrication. An inspector should also closely observe the installation and connection of the equipment on site.
Recommended Practices
Many additional practices help ensure the compressor package has a low risk of vibration, can meet the required performance and can achieve the operating flexibility required by the owner. Although the guideline provides many detailed recommendations, a few key ones are worthy of emphasis.
In the equipment selection phase, a risk assessment is recommended. This determines the scope required for the package vibration analysis. For example, a fixed-speed machine will have fewer vibration problems and lower risk, than a machine that is required to operate over a wide speed range.
Selecting and applying compressors conservatively, compared to their horsepower per throw rating, reduces the driving forces from gas forces acting on the system. A well-designed foundation is critical. Especially for extremely large units, a concrete block is preferred. If a concrete block is not used, the skid design must be evaluated in more detail. Effective mounting and installation of the equipment and skid are also critical.
System pressure losses – including vessels, pulsation dampening components, piping, coolers and headers – which affect overall compressor system performance, should be included in the performance calculations. Checking the system performance at key conditions is an important step for avoiding unexpected flow shortfalls or excessive power requirements.
For large, more critical applications, the scope of work should include an analysis that follows API 618 Design Approach 3 (DA3), including a forced response of the compressor manifold.
The vibration consultant should address the thermal design and mechanical design concurrently while considering pipe support assumptions and competing thermal and vibration requirements.
A torsional vibration analysis is necessary on all large units that are not an exact mechanical and operational duplicate of a unit previously analyzed. The effects of manufacturing tolerances should be considered in the torsional analysis.
Shop tests should include bump tests, piping inspection (including small bore piping) and a review of auxiliary device mounting to avoid mechanical resonance and significant vibratory responses at running speed and harmonics of running speed.
During commissioning, a vibration check is recommended for main piping, vessels and small bore piping (on-skid and off-skid), as well as the rotating equipment. Torsional testing may also be required, depending on results of the torsional analysis.
Author: W. Norm Shade is senior consultant and president emeritus of ACI Services Inc. Headquartered in Cambridge, OH, ACI manufactures custom engineered reciprocating gas compressor products used throughout the world. Shade received BME and MSME degrees from Ohio State University, graduating summa cum laude in 1970. He is a registered professional engineer in Ohio, Oklahoma and Texas.
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