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When specifying a centrifugal compressor, the decisions made during the engineering phase can influence far more than equipment selection. Compressor specifications affect reliability, efficiency, project costs, installation requirements, controls architecture, lead times, and long-term operational performance.
For process engineers, EPC contractors, procurement teams, and plant operators, developing a comprehensive specification is one of the most important steps in ensuring a successful project outcome. A well-defined specification helps align equipment capabilities with operating requirements, while an incomplete specification can introduce unnecessary risk, costly redesigns, and schedule delays.
This guide examines why compressor specifications matter, the industry standards that govern centrifugal compressor applications, and the process data required for proper compressor selection. It also explores key considerations involving drivers, controls and instrumentation, the impact of late-stage specification changes, and the value of early collaboration in reducing project risk and improving long-term performance.
Unlike commodity rotating equipment, integrally geared centrifugal compressors are engineered around a defined operating envelope: every major subsystem, from aerodynamic stage configuration and bull gear geometry to impeller profiles, intercooler sizing, lube oil capacity, and control logic, is set by the process conditions and site requirements in the specification.
The specification becomes the project's roadmap for the compressor, shaping performance and efficiency, reliability and maintenance needs, controls architecture and package complexity, installation and commissioning, schedule and installed cost, and energy consumption and lifecycle cost. Changes made after equipment selection often require engineering rework, added procurement, and modifications to supporting systems, so an accurate specification developed upfront typically reduces project risk and lifecycle cost.
Successful compressor selection begins with complete and accurate process information. A centrifugal compressor specification should clearly define:
| Category | Required Information | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Process Conditions | Required flow range, inlet pressure, inlet temperature, gas composition or molecular weight, required discharge pressure and any other off design or performance requirements | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Site & Ambient Conditions | Ambient temperature, cooling water temperature, elevation / ambient pressure, relative humidity (RH) | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Driver & Electrical | Driver type and power requirements, electrical characteristics, hazardous area classification | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Controls & Standards | Controls and automation requirements, instrumentation , auxiliary system, applicable industry standards, future operating and expansion requirements | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Because centrifugal compressors are engineered around specific operating conditions, even small changes in process requirements can significantly affect performance, efficiency, and equipment configuration. Additionally, it is not uncommon for compressors to be sized for unrealistic ambient conditions, leading to inefficiencies and oversized machines.
Several industry standards help establish consistent engineering practices for centrifugal compressor applications.
The American Petroleum Institute (API) was established in 1919. As the primary trade association representing the oil and natural gas industry in the United States, API standards are used globally by the petroleum industry. Through its standardization activities, API promotes sound engineering and operating practices, along with the broad availability of standards for safe and reliable equipment and materials. API publishes a wide range of specifications for oil, gas, and petrochemical applications. These specifications are created and revised by committees comprised of users, specifiers, and manufacturers, reflecting a collaborative approach that aligns operating needs with engineering and manufacturing expertise.
API 617 covers centrifugal compressors used in critical process gas applications, including refinery, petrochemical, LNG, and industrial gas facilities. It generally applies to axial and centrifugal compressors with impellers in a "between-bearing" configuration rather than overhung directly on gearbox pinion ends, though a newer Chapter 3 was added to cover integrally geared machines in process-related services.
Because the control and instrumentation requirements in API 617 are complex and costly, it is not the appropriate standard to reference when specifying integrally geared air compressors for plant, instrument, or process air service. For those applications, API 672 is generally the more relevant standard.
API 672 applies to packaged, integrally geared centrifugal air compressors used in petroleum, chemical, and gas industry services for plant, instrument, and process air. It covers the compressor, gearbox, bearings, lubrication system, controls, intercoolers, baseplate, testing, and documentation. First published in 1979, the standard is now in its fifth edition (2019).
The standard classifies packages as Basic or Special Duty. Basic Duty suits facilities with adequate spare air capacity or non-critical service, while Special Duty adds instrumentation, redundancy, inspection, testing, and documentation for unspared, single-train installations. Special Duty is an API 672 classification and should not be confused with “special purpose” service under API 617.
Specifying Special Duty for a properly spared system can unnecessarily increase cost and lead time without improving system reliability. Common requirements include dual oil coolers, 2-out-of-3 (2oo3) voting on critical instrumentation, redundant controls, and additional lubrication system requirements.
Purchasers should also avoid unrealistic ambient conditions. API 672 Section 6.1.2.2, Note 1 states that a dew point above 33°C (92°F) seldom occurs and anything above that generally should not be used when defining operating conditions.
JIP 33 was initiated by the International Association of Oil & Gas Producers (IOGP) in 2016 to standardize and streamline procurement activities across projects and stakeholders. Historically, operators asked suppliers to meet a different, often hundreds-of-pages-long set of requirements per order, with specifications differing even across projects from the same company, an inefficient approach for all parties.
JIP 33 builds on existing industry standards to provide a full set of requirements for purchasing equipment and packages, organized into four documents: a Supplementary Technical Specification, Information Requirements (IRS), Quality Requirements (QRS), and a Procurement Data Sheet. By adopting JIP 33 and raising the minimum standards of the project with a standardized set of requirements, organizations in sectors such as oil, gas, and energy can reduce inefficiencies, improve project predictability, and achieve better cost control. For centrifugal air compressors, the relevant overlay is IOGP S-612, which builds on API 672. FS-Elliott has experience supporting JIP33 applications and can help project teams interpret and apply these requirements.
As industry standards continue to evolve, FS-Elliott works closely with customers to interpret and apply requirements from all of these industry standards. Our experienced engineering team regularly reviews project specifications, identifies potential conflicts between standards and site requirements, and helps customers develop solutions that align with both operational objectives and project schedules without overpaying for unnecessary requirements.
Understanding which standards apply to a project is one of the first and most important decisions during specification development.
The most critical section of any compressor specification is the process data sheet. Compressor aerodynamic performance is directly tied to the operating conditions provided by the customer.
Key inputs include:
These parameters determine the compressor's aerodynamic design, stage configuration, power requirements, and operating range.
Accurate flow and gas property data are essential for proper compressor selection. Variations in operating conditions, molecular weight, or future capacity requirements can significantly influence performance, efficiency, and power consumption.
FS-Elliott engineers routinely assist customers in validating process conditions during specification development, helping identify potential performance challenges before they affect equipment selection or project execution as well as making recommendations to improve performance. By reviewing operating requirements early, project teams can often avoid costly modifications later in the project lifecycle and better align their RFQ to actual project needs.
When process conditions are incomplete or inaccurate, performance issues often emerge later in the project when modifications become more difficult and expensive to implement.
While compressor performance often gets the most attention, driver selection has an equally significant impact on project execution and long-term operation. The specification should define driver type and power requirements, voltage and electrical characteristics, starting method, hazardous area classification, and auxiliary system requirements. Electric motors are often the package's longest lead-time component, so changes to motor specs, voltage, or area certifications discovered late can create schedule impacts.
Beyond API 672, API 614, and API 617, several other API standards commonly apply to individual components and sub-systems within a compressor package, including API 541 and 547 for electric motors, API 611 and 612 for steam turbine drivers, API 670 for machinery protection systems, API 671 for couplings, and API 661 for air-cooled heat exchangers.
Area classifications and auxiliary system requirements should be evaluated early, as they can significantly affect equipment selection, package complexity, lead times, and overall project costs.
Motor size is also an integral point of consideration. While many standards require oversizing the motor based on certain conditions, it is very common for that oversize to not offer any real benefit based on customer performance leading to lowered packaged efficiency and higher package cost.
Modern centrifugal compressors rely on sophisticated control systems to maintain safe and efficient operation. Control philosophy decisions should be established early in the project to ensure alignment between the compressor package and overall plant automation strategy.
Important considerations include:
Among these, anti-surge protection remains one of the most critical functions. Surge can cause severe compressor instability and potential equipment damage, making proper anti-surge system design essential. However, the appropriate type of anti-surge or surge avoidance system can change depending on the application. While gas compressors typically require a sophisticated dedicated anti-surge system, instrument air compressors do not require these same systems to maintain the same equipment protection. Response times, instrumentation accuracy, and unloading or recycle valve performance all play important roles in protecting the compressor throughout its operating range.
Communication protocols, cybersecurity requirements, operator interface expectations, and alarm management strategies should be clearly defined to ensure proper plant integration.
FS-Elliott's engineered products team has extensive experience integrating compressor control systems into refinery, petrochemical, industrial gas, and other critical process applications. Through its Regulus® R2000 and R3000 platforms, FS-Elliott provides configurable control systems featuring anti-surge control, Integrated Compressor Control (ICC), Pressure Band Optimization (PBO), Energy Advisor, Maintenance Notification Systems, and FS-Connect® remote monitoring.
For demanding applications, the R3000 can incorporate redundant power and communications, advanced machine monitoring, alarm and trip voting logic, auxiliary system controls, and enclosure options including NEMA 4X stainless steel and hazardous-area-rated designs. Together, these capabilities help improve reliability, operational visibility, and compressor performance in complex API-compliant installations.
One of the most effective ways to reduce project risk is to identify potential challenges before the specification is finalized. Changes to area classifications, drivers, controls, auxiliary systems, or site-specific requirements after equipment selection has begun can result in engineering rework, increased costs, procurement delays, and commissioning challenges. Early identification of these issues helps minimize project risk and schedule impacts.
Many of these challenges can be avoided through early collaboration between the owner, EPC contractor, and compressor manufacturer during the specification development phase. Identifying potential issues before the specification is finalized allows project teams to validate operating conditions, resolve conflicting requirements, and ensure compliance with applicable industry standards before they affect project execution.
A brief technical review during specification development can often prevent weeks of clarification efforts during proposal evaluation and eliminate costly late-stage changes.
For more than 60 years, FS-Elliott has supported critical oil and gas, petrochemical, industrial gas, and refinery applications through its PAP Plus® compressor product line. Our engineering teams work closely with EPC contractors and specification authors to navigate evolving industry standards, complex project requirements, and demanding process applications.
This experience provides valuable insight into developing effective compressor specifications that support long-term reliability. By participating early in the specification process, FS-Elliott helps identify potential challenges before they impact project schedules, budgets, or equipment performance.
Our goal is to provide a product that truly meets the customer’s needs without unnecessary scope, testing, or documentation requirements and the accompanying expenses.
FS-Elliott has extensive experience with multiple API specifications and can provide detailed guidance on how our equipment meets these standards and your specific project requirements. We also have experience with the IOGP's JIP 33 standard, which builds upon the API standards to further standardize project requirements across the industry.
Our approach extends beyond equipment supply. We collaborate with project stakeholders to validate process conditions, evaluate operating requirements, identify specification conflicts, and optimize compressor designs for reliability, maintainability, energy efficiency, and long-term performance. This collaborative approach helps stakeholders specify the equipment and support the project truly requires.
A centrifugal compressor specification is more than a procurement document. It is the engineering foundation for equipment performance, reliability, controls integration, project costs, and long-term operation.
By accurately defining process conditions, applying the appropriate standards, evaluating equipment requirements, and engaging experienced manufacturers early, project teams can reduce risk and improve outcomes. Combined with custom aerodynamic design and advanced controls, a well-developed specification supports greater efficiency, reliability, and long-term performance.
Successful compressor installations begin long before startup, with the right specification.
To learn more about FS-Elliott’s centrifugal compressor solutions or discuss your project needs, contact our team today.
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