High-speed reciprocating compressors, driven by natural gas engines or electric motors, provide the advantages of lower capital cost, shorter installation time and compactness. They have become the most common equipment type applied in the upstream and much of the midstream natural gas compression markets. However, as larger horsepower (≥2000 HP), high-speed (≥700 rpm) reciprocating compressor packages have been applied, especially in low ratio, high-flow, highly flexible pipeline transmission applications, concerns with regard to efficiency, vibration, pulsation, controls, ancillary components and systems, pipeline throughput and diagnostics have increased.
Higher speed compressors naturally create a broader spectrum of pulsation frequencies that must be addressed, and 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 generated by high-speed compressors. This has driven the need for better and more sophisticated methods of pulsation and vibration modeling and analysis as well as additional pulsation control “tools” and suggested practices for damping, de-tuning, and/or cancelling pulsations.
Another important consideration is that for centrifugal and traditional slower speed (<700 rpm) reciprocating compressors, the compressor OEM often takes full responsibility for the systems integration and its performance. In contrast, high-speed compressor packages are typically not offered by the compressor OEM directly. Instead, for high-speed compressor systems, a packager purchases the compressor, the driver and other high-value content from individual manufacturers and integrates/assembles them on a fabricated I-beam skid with pulsation bottles, scrubbers, gas piping, utility piping, instrumentation, controls, auxiliaries, etc. The vast majority of the compressor units that are designed and manufactured by packagers are for upstream applications, which are smaller size units that are typically not highly engineered and for which high efficiency is not a primary consideration. Some of the practices that are common and reliable for smaller high-speed upstream compressor packages are not necessarily adequate for applications with larger, heavier, high-flow, low ratio, or highly flexible reciprocating compressors.
Economics is a significant factor that drives the behavior of the supply chain. While the presence of multiple competent packagers in the industry makes for a competitive procurement environment, this is not necessarily an advantage when the procurement specification is insufficient. The engineering capability of individual packagers varies, with much of their experience developed around smaller, high ratio upstream applications. It is also important to recognize that the packager’s value added content is typically in the range of 20 to 25%, after buying the compressor, driver and other standard components which make up the majority of the package cost. The packager’s gross margin on the relatively small percentage of the total package cost that they add, together with a limited mark-up on the relatively large purchased equipment content, must cover the costs of the packager’s engineering, overhead, financing, marketing and sales commissions, while leaving an adequate profit margin for the packager’s risk and for satisfying the commercial expectations of its owners or shareholders.
Currently, there is no comprehensive specification document for the purposes of procuring, designing and applying large, high-speed compressor packages. For example, the API 61815 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 amount of analysis. In particular, the range of potential operating conditions is typically not adequately explored during the selection, procurement or manufacturing processes. The former API 11P6 (no longer in print) and the current ISO 1363157, intended primarily for field gas compressors, provide no in-depth guidance in many of the areas of concern. Neither fully provides the more comprehensive direction intended in this Design Guideline.
In 2011, GMRC identified the need for a specification or guideline to address the foregoing concerns. ACI Services, Inc. was selected as the contractor for the supporting investigations and the development of this GMRC Design Guideline for High-Speed Reciprocating Compressor Packages for Natural Gas Transmission & Storage Applications.
The research and data collection phase for development of this Guideline involved the interviews of more than 100 people at 12 end user companies, 5 compressor OEMs, 2 engine OEMs, 1 electric motor OEM, 4 packagers, 4 engineering services company and 1 foundation engineering company. In addition, 11 pipeline or storage field sites were inspected, covering a total of 30 different high-speed compressors. From this information a long list of problems, solutions, preferences and suggested practices have been documented and serve as the foundation for development of this Guideline. An extensive literature search was also conducted to evaluate what existing material could be cited and referenced in the Guideline.
This Guideline is not a specification and therefore defines only recommended practices, not mandatory requirements. It is intended to provide the end user and operator with more reliable procedures and references for selecting, specifying, procuring, applying and operating high-speed units with more predictable and reliable results, and it provides packagers with a more comprehensive and detailed guideline for designing and building high-speed compressor packagers that meet customer and equipment OEM expectations. While portions of this document may be applicable to gathering and midstream applications, the principal use is targeted for higher horsepower (≥2000 HP), high-speed (≥700 rpm), highly flexible, low ratio gas transmission compressor applications and versatile gas storage and withdrawal applications. A significant part of this document provides a tutorial on how to handle specific aspects of large compressor package specification, procurement and design. For guidance on field gas compressor packages, that are generally under 2000 hp, refer to GMRC High-Speed Compressor Package Guideline for Field-Gas Application52.
A specification is a communication document. There are lots of unspoken expectations. If the specification can bring them to light, it will be effective. This Guideline is not intended to be an all-inclusive specification. There are many features that are project, end user and packager specific, and some end users and packagers have their own preferred designs and standards. It is beyond the scope of this Guideline to reconcile every last detail of a potential specification. Unlike API, ISO and ASME, the GMRC does not issue standards and official specifications. The information contained herein provides a set of guidelines and recommended practices, and it is also intended to serve as a tutorial for the design, installation and operation of high-speed reciprocating compressor packages for natural gas transmission and storage applications. End users are encouraged to use the information in this Guideline to develop their own detailed specifications.