Development of a Centrifugal Hydrogen Pipeline Gas Compressor

Written by Francis A. Di Bella, PE and Dr. Colin Osborne of Concepts NREC - Tuesday, January 12 2010

Engineering Approach

The engineering approach used by CN to accomplish these goals is to utilize state-of-the-art aerodynamic/structural analyses to develop a high-performance centrifugal compressor system for pipeline service. The centrifugal-type compressor is able to provide high pressure ratios under acceptable material stresses for relatively high capacities (flow rates that are not usually higher than what a piston compressor can provide). CN’s technical approach also includes the decision to utilize commercially available, and thus proven, bearings and seal technology to reduce developmental risk and increase system reliability at a competitive cost. CN has researched the use of a material that is compatible with hydrogen and that can enable the highest possible impeller tip speeds, which results in the reduction of the number of required stages while meeting DOE’s goals for overall pressure ratio and efficiency.

In order to minimize the development time and ensure industrial acceptance of the design for the new pipeline compressor system, CN has assembled a project team to assist in the advanced engineering of the compressor while also preparing an implementation plan that can provide for near-term industrial pipeline applications. The team consists of Praxair (to provide industrial user experience and “hands-on” experience), Texas A&M University (to provide materials science expertise and testing), and HyGen Industries (to provide experience in hydrogen fueling infrastructure).

The engineering challenge to implement this technical approach is to design a compressor stage that can achieve the highest acceptable pressure ratio and thermodynamic efficiency per stage, but also by using as few stages as possible and the smallest diameter. For centrifugal-type compressors, the pressure ratio is proportional to rpm² and radius². Thus, even a small increase in tip speed or impeller radius results in significant increases in pressure. The aerodynamic design challenge in reducing the number of stages is to maximize the tip speed of the centrifugal compressor impeller, while also staying within acceptable design safety levels of the strength limitations of material and utilizing advanced diffuser systems to maximize recovery of dynamic head into static pressure. However, material stresses also increase proportional to the square of the rotor speed (rpm²) and radius (radius²) and also by material density. Ultimately, the major constraint is imposed by the limitations of the maximum stress capability of the impeller material. This constraint is further aggravated by the need for the material selection to consider the effects of hydrogen embrittlement on the strength of the material.