Software can provide a powerful and easy tool to predict machine operation under different conditions.
by Massimiliano Di Febo and Pasquale Paganini
January 28, 2016
Figure 2. The discharge pressure for conditions marked as D1 and D2Figure 2. The discharge pressure for conditions marked as D1 and D2
Figure 3. The discharge temperature for conditions marked as D1 and D2Figure 3. The discharge temperature for conditions marked as D1 and D2
Figure 4. The polytropic efficiency for conditions marked as D1 and D2Figure 4. The polytropic efficiency for conditions marked as D1 and D2
Figure 5. The polytropic work for conditions marked as D1 and D2Figure 5. The polytropic work for conditions marked as D1 and D2

This confirms the capability of the proposed method to predict compressor performances aligned with design ratings.

The proposed method can be used on surge protection systems and for diagnostic purposes. Typical centrifugal compressor protection systems are based on simplified algorithms that use the inlet flow, suction and discharge pressures and temperatures. These algorithms are based on a basic surge locus equation using the ideal gas law, validity of hydraulic affinity laws, reference gas mix and inlet conditions.

These algorithms are consequently completely dependent on the suction thermodynamic parameters (pressure, temperature and gas mix composition). The protection action is linked to design suction parameters and remains valid only for the design specified gas mix. Therefore, these systems provide a limited surge protection valid in a small operative range around the design conditions.

The compressor protection is not correctly active when inlet conditions move to off-design values, especially when approaching the high-pressures range where introduced errors do not allow enough accurate predictions for more severe applications.

Consequently, the protection of these algorithms is partial and limited. When the compressor is requested to operate outside the design conditions, the protection action may become inefficient and potentially can damage the compressor. Surge limits are dependent on the gas mixture and the thermodynamic suction conditions (temperature and pressure), making it technically difficult to achieve effective and complete compressor protection from the surge phenomenon.

The proposed method for prediction of centrifugal performances can be profitably used to anticipate the centrifugal compressor's overall performances in every inlet condition and detect surge points in every operative condition.

The application of the proposed methods provides new diagnostic capabilities. The software can automatically store data and provide a real-time indication of compressor efficiency and comparison with expected values.

This can be a powerful feature with a continuous indication of how the machine's behavior is aligned with design expectations. These evaluations show historical trends and build pictures of the machine's status along the operation period.

Collected data will be useful to support decision-making on predictive maintenance plans and operations.

Conclusion

The methods proposed and described in this article can provide the following benefits to end users:

  • Predict the performances of a centrifugal compressor under varying thermodynamic conditions of the inlet gas. The prediction of compressor performances is accurate even at high pressures, where the ideal gas theory commonly used introduces considerable errors.
  • Analyze the performance of the compressor during operation and compare results with those expected provided by the manufacturer.
  • Implement advanced protection from a surge, overcoming limits of the current technology.
  • Deliver useful indications on the health of the compressor (diagnostics) based on the capability to analyze the performances and efficiency of the machine in a simple and immediate way.
  • Support decisions and planning of predictive maintenance and activities.

The methods and software tools have been validated compared to other compressor technical data and available scientific papers on thermodynamic gas theories developed in the hydrocarbons research field.

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