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Design analysis for screw compressors and roots blowers

Screw compressors and roots blowers are frequently used in many applications. They generate pulsations that can cause a number of problems, such as severe vibration, fatigue failure, noise, reduction in efficiency, hammering of valves and errors in flow measurement. TNO has the expertise and tools to evaluate the dynamic behavior of the compressor system and to recommend cost-effective measures to mitigate vibration problems.

Screw compressors are positive displacement machines. To achieve compression, the gas is periodically compressed mechanically by the helical movement of two or more screws. Upon discharge, a pocket of gas is delivered to the high-pressure side, leading to a strongly pulsating flow. Especially when the internal pressure ratio of the compressor does not match the external pressure ratio of the system, high-amplitude pulsations will be generated, that contain high-frequency components. The pulsation-induced forces may excite mechanical resonance frequencies in the piping and supports. Also, the high-frequent pulsations may cause excessive noise radiation to the environment. Normal mitigation measures, such as standard orifice plates, may not be effective at the higher frequencies. Furthermore, special care must be taken in the orifice design to avoid problems with lube oil.

Simulation tools

TNO has a dedicated software to model the pulsating flow of the compressor. This pulsation sources is integrated in our software tool PULSIM, which has set a standard in modeling of propagation of pulsations in pipe systems over the past 40 years. The effects of the multiphase flow (gas/oil) are included in the simulations. To mitigate pulsation levels in the piping, special multi-bore restriction orifices (MBROs) are applied. These are needed due to the high frequencies of the screw compressor pulsations. Prediction of the dynamic behavior of MBROs requires special tools. TNO has an extensive experience in designing MBROs, based on simulations and measurements. Pulsation-induced forces are evaluated in Ansys, to compute the resulting vibrations and cyclic stresses of the structure. Finally, TNO has the appropriate tools to model the noise radiation from the piping and vessels and produce noise contours for the installation.

Trouble shooting

When problems in the field are encountered, TNO is the right partner to assist in finding the solution. We have an extensive experience in measuring in the field, including pulsation, vibration and noise measurements. Our engineers have all the required safety instructions and permits to operate in your plant, including offshore installations. Based on the measurements, a robust root-cause analysis will be made. Effective reduction measures will be formulated and validated by simulations. This approach ensures the most (cost-)effective solution strategy.

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