Engineering High-Performance Compression: Centrifugal Compressors and Dry Gas Seals

Why Compressors Are Essential to Industry

For hundreds of years, energy and processing applications have been powered by compression equipment. Today, a growing shift toward sustainability is reshaping the industry, with a clear emphasis on decarbonisation, circularity and long-term environmental responsibility. And compressors will continue to play an integral role.

Looking at it from a purely functional perspective, compressors add pressure to gases, such as air, natural gas, hydrogen, carbon dioxide and nitrogen. Centrifugal compressors use rotating impellers to add kinetic energy to a process gas; this is sometimes referred to as radial or dynamic compression. The added kinetic energy is converted into velocity, then the gas slows in the downstream diffuser which results in increased pressure. When applied, this fundamental principle powers a wide range of complex and critical applications across traditional global process and energy industries, including oil and gas and power generation, to cleaner technologies, such as carbon capture, hydrogen, biofuels and sustainable aviation fuels (SAF).

The Fundamentals of Single-Shaft Centrifugal Compression

Powered by a turbine or motor, a single-shaft compressor spins a central impeller to accelerate and compress process gas in one continuous motion. As the process gas enters the rotating vanes, or blades, the gas increases its velocity and discharges at an angle away from the axis of rotation. Single-shaft compressors have a between-bearing design with one or more impellers supported between two bearings, one bearing at each end of the rotor. As the process gas passes through sequential impellers along the shaft, gas pressure increases incrementally.

The Engineering Behind Integrally Geared (IG) Centrifugal Compressors

An integrally geared (IG) compressor is a widely used centrifugal compressor design, which is valued for its advanced efficiency and flexibility. Unlike single-shaft compressors with impellers arranged along one shaft, integrally geared compressors use multiple shafts, each with impellers at one or both ends. These shafts are driven by a central bull gear, allowing each one and its impellers to rotate at an optimised speed, based on its specific gear ratio. Each shaft and impeller “set” is considered a compression stage. The number of impellers accounts for classifying the equipment as single-stage or multi-stage. IG compressors can have multiple impellers, each adding a stage of compression to reach the required pressure. The compressor manufacturer usually determines how many stages are needed, depending on the application and pressure gain or compression level required. Since stages do not have an inline configuration, an integrally geared compressor can have a smaller footprint than a single shaft unit.

As with single-shaft technology, impeller design is key to efficient, reliable operation because it controls how energy is transferred to the gas. However, the gear design adds complexity for integrally geared compressors: a component design for each stage differs from achieving the required compression.

How Mechanical Seals Protect and Optimise Compression Systems

The efficiency and longevity of rotating equipment are critical in industrial equipment. A vital component that plays a significant role in achieving these goals is the compressor mechanical seal. These seals are engineered to seal the gases within the compressors effectively, preventing leakage into the environment. Comprising of a stationary and a moving part, compressor mechanical seals are designed to maintain sealing integrity under the dynamic conditions of operation.

Mechanical Seal Types Used in Compressor Applications

When it comes to compressor seals, one size does not fit all. Different applications require different types of mechanical seals, each with their unique features and benefits:

• Wet Seals: Although their use has declined, wet seals are a traditional method for sealing compressors. They rely on a liquid film - oil - to prevent gas from escaping and necessitate constant and complex fluid management.
• Dry Gas Seals: Dry gas seals have set a new standard in the compressor seal industry. These seals use a gas barrier to achieve sealing efficiency. Valued in the industry for their reliability and operational efficiency, dry gas seals are the epitome of modern compressor mechanical seal technology.
• Separation Seals: Designed to keep bearing oil and the sealing mechanism separate, separation seals are crucial for preventing oil migration toward the dry gas seal thus allowing the mechanical seal to continue to maintain its non-contacting regime - safeguarding the compressor equipment.

The Value of Dry Gas Seals in Compression Systems

Choosing dry gas seals for compressors brings numerous benefits:

• Reduced Energy Consumption and OPEX: These seals are designed for zero-contacting operation, leading to lower energy consumption and operational costs.
• Increased Reliability: The durability of dry gas seals means less frequent failures and more consistent operation.
• Enhanced Safety: By significantly reducing leakage, they also lower the risk of hazardous exposures, making the operating environment safer.
• Ease of Maintenance: Dry gas seals are simpler to maintain than traditional wet mechanical seals, which saves cost and time.
• Longer Mean Time Between Repairs (MTBR): The reliability of dry gas seals leads to extended operating periods between maintenance or repair schedules.
• Digital Supervision: New digital systems allow an in-situ supervision of the dry gas seal to review its performance, which can support customer's preventative maintenance strategies and total cost of ownership goals.

Sustainability: The Forefront of Compressor Seal Design

The emphasis on sustainability is paramount in the current industrial landscape. It is becoming increasingly important as the industry strives to meet sustainability targets, such as decarbonisation goals and fugitive emission control. Compressor sealing solutions can be tailored to meet specific American Petroleum Institute (API) regulations, adhering to stringent quality and safety standards. Compared to traditional mechanical seals, dry gas seals offer superior sustainability. They boast longer lifespans, lower leakage rates and energy efficiency, contributing to more sustainable industrial processes.

Safety Considerations

Optimised compressor sealing solutions are available for nearly all types of gases, including toxic or chloride-containing substances. These solutions ensure safe application in a broad spectrum of industries.

Compressors In Clean Energy Applications

As temperatures across the globe climb, industrial emissions reduction initiatives are also on the rise. The Oxford Net Zero team notes that specific targets referenced in national decarbonisation policies have grown from 10% to 75%, indicating that “net zero” has moved from ambition to implementation. Meanwhile, the International Energy Agency (IEA) estimates a capital flow of $3.3 trillion to the energy sector despite elevated geopolitical tensions and economic uncertainty. Compressors have long been pivotal to the energy economy, and their role will only become more pronounced amid the energy transition. Achieving net zero will require operators to leverage both established and emerging technologies. Centrifugal compressors, which have been used throughout the industry for decades, are fundamental to the processes that help enable new energy progress.

Hydrogen Production, Storage and Transportation

The use of hydrogen as an energy carrier is gaining momentum as the world looks for ways to achieve ambitious decarbonisation goals. Compression equipment, including compressor seals that help increase operational efficiency and reduce emissions, will be necessary to support and scale the hydrogen value chain. Hydrogen is a low-density gas that occupies significant space in its natural state, and compression is required to reduce its volume for handling. Compression also drives secondary processes for hydrogen management, involving oxygen, carbon dioxide, ammonia and other gases. When developing hydrogen solutions, many open questions and challenges persist. Material selection is a concern, as are leakage, safety, cost and energy efficiency. Manufacturers have an opportunity to support industry leaders by engineering compressor units for advanced integrity and efficient performance.

Carbon Capture, Utilisation and Storage (CCUS)

Sequestering CO₂ from industrial processes so it can be transported, stored or used is referred to as CCUS. As a key approach for tackling decarbonisation in hard-to-abate industrial sectors like cement, steel and chemical production, CCUS deployment is increasing. A multitude of CO₂ handling processes are expected to be implemented in the near future. CO₂ compression is well understood, but the gas is challenging to handle, as the baseline at which it turns from liquid to gas is relatively low. This means that a phase change can occur during processing, creating problems for turbomachinery.

Decarbonisation is a priority across the CCUS value chain, including the fugitive emissions from compressor seals involved in processing CO₂. Single-shaft and integrally geared compressors that handle carbon dioxide have a range of operational requirements due to CO₂'s low-temperature phase change. Preventing fugitive emissions with seals requires precision engineering. This is particularly important for integrally geared compressors with floating carbon rings, which are prone to leakage at medium-to-high pressures. Besides liquid and gas phases, CO₂ can also reach a pressure and temperature that causes it to change to a supercritical phase during operation. Special considerations for supercritical CO₂, which has the multiphase properties of both a gas and a liquid, are required for these types of operations.

How John Crane Supports the Future of Energy

The energy industry is gearing up to meet the world's increasing energy demands while also accelerating decarbonisation to achieve climate-neutral goals by 2050. John Crane is committed to engineering solutions to meet the evolving needs of our OEM customers and end-users as they scale up new energy and enhance sustainability. John Crane has a long history of enhancing compressor reliability and sustainability. In 1968, we patented spiral groove seal design technology, and we haven't stopped innovating since. John Crane's products are used in many sites where CO₂ is injected underground across each stage of the CCUS value chain and liquid, gaseous and supercritical phases. We're committed to investing in innovative solutions that will help solidify our industry's secure future.

Energy Transition Opportunities for Compressor Manufacturers

The demand for high-performance compressors is poised to increase, along with the implementation of clean energy technologies, such as hydrogen and carbon capture. Compressors that handle diverse gas compositions are essential to energy transition applications, including low-carbon fuel production and transportation, and CCUS processes that support decarbonisation.

There are two ways for a process owner to approach compressor optimisation:

1. Retrofitting an existing unit with dry gas seal technology can help reduce energy use and improve reliability, as demonstrated in a Texas pipeline case study that showed a significant increase in mean time between failures.
2. Install a new unit and components to increase energy efficiency and reduce emissions.

Compressor manufacturers that support the energy transition with sustainable technologies have a competitive advantage in a complex, evolving industrial landscape.

Energy Efficiency

Equipment purchasing decisions are increasingly focused on efficiency due to rising electricity costs and stringent environmental regulations. In the context of the energy transition, the IEA calls energy efficiency the first fuel because it plays an important role in enhancing energy security and affordability, and in establishing low-carbon alternatives. Beyond its fundamental role in the energy transition, efficiency also delivers clear productivity benefits. Efficient compressor operation maximises resources and streamlines unit performance for energy savings. Increased efficiency also goes together with increased reliability for a longer mean time between repairs (MTBR). The longer the MTBR, the lower the maintenance costs over a unit's lifetime due to fewer part replacements, less costly downtime and fewer technician hours to sustain operation.

Reducing Emissions Through Smarter Compression

As the energy transition continues its journey, emissions reduction has become a rapid, economically viable strategy for decarbonisation. Reducing GHG emissions is a key lever for achieving net zero, and there is a growing demand from key decision makers and purchasing functions for compressors that help to protect the environment. Decarbonising compressor operation may involve technology and components that capture fugitive emissions before they are released into the atmosphere or prevent fugitive leaks in the first place.

Compressors with dry gas seals reduce fugitive emissions, improve process efficiency and safety and reduce an operation's carbon footprint. However, to fully realise the sustainability benefits of dry gas seal technology, the design must be engineered to the specific process it supports, making the right technology and service partner essential.