Gas turbine driven centrifugal compressors are widely used in the oil and gas industry. In evaluating the optimum selection of gas turbine drivers for centrifugal compressors, one of the main objectives should be to verify proper integration and matching of the centrifugal compressor to its gas turbine driver. Gas turbines are of standard designs, while centrifugal compressors are specifically designed to meet customer requirements. The purchaser should clearly specify process requirements and define possible operating scenarios for the entire life of the gas turbine driven centrifugal compressor train. Process requirements defined by the purchaser, will be used by the compressor designer to shape the aero-thermodynamic behavior of the compressor and characterize compressor performance. When designing a centrifugal compressor to be driven by a specific gas turbine, other design requirements are automatically introduced to centrifugal compressor design. Off-design performance, optimum power turbine speeds at site conditions as well as optimum power margin required for a future-oriented design must all be considered. Design and off-design performance of the selected gas turbine at site conditions influences the final selection of a properly matched centrifugal compressor design. In order to evaluate different designs and select the most technically viable solution, the purchaser should have a clear understanding of the factors influencing a proper match for a centrifugal compressor and its gas turbine driver. This paper discusses criteria for evaluating the most efficient combination of a centrifugal compressor and its gas turbine driver as an integral package from a purchaser’s viewpoint. It also addresses API standard requirements on gas turbine driven centrifugal compressors.

This paper presents a new improved approach to calculation of polytropic performance of centrifugal compressors. This rapid solution technique is based upon a constant efficiency, temperature-entropy polytropic path represented by cubic polynomials. New thermodynamic path slope constraints have been developed that yield highly accurate results while requiring fewer computing resources and reducing computing elapsed time. Applying this thermodynamically sound cubic polynomial model would improve accuracy and shorten compressor performance test duration at a vendor’s shop. A broad range of example case results verify the accuracy and ease of use of the method. The example cases confirm the cubic polynomial methods result in lower calculation uncertainty than other methods.

Turboexpanders provide the most efficient solution when it is required to reduce the pressure of a fluid stream. By expanding high pressure fluid, energy in the high pressure fluid entering the turboexpander can be efficiently used for either driving a booster compressor or for electrical power generation. While the plants are designed to operate without the need for the power produced by turboexpander, the work recovered from the expansion is a bonus, which increases the plant thermal efficiency. This paper is intended to explain the benefits of utilizing a turboexpander in LNG liquefaction applications. Also, in absence of a published API standard for a turboexpander-generator package, this paper provides recommendations on factory acceptance tests.

ASME PTC-10 (2009) provides a test procedure to determine the thermodynamic performance of centrifugal compressors for gases conforming to ideal gas laws and for real gases. It requires using real gas calculation methods where the compressibility values depart from the specified limits. ASME PTC-10 employs Schultz X and Y compressibility factors to calculate the polytropic exponent for real gas compression. Specific values of X and Y for the test gas at the test condition may be different from the values provided in ASME PTC-10 generalized charts. Therefore, special care should be taken to properly calculate X and Y factors for a test gas at specified conditions. In this paper, Schultz compressibility factors X and Y are derived as functions of reduced properties. These functions can be used with any equation of state to precisely calculate X and Y values for any gas composition at the specified operating conditions. By using the proposed method, Schultz X and Y compressibility factors for propane are graphically represented covering a reduced pressure range of 0.1 to 3 and a reduced temperature range of 1.05 to 2. Also, the rate of change of polytropic exponents for propane over a wide range of pressures and temperatures is graphically demonstrated.

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