One of the main difficulties of analyzing the operative performance of centrifugal compressors involves how the data is presented. Expected performance is generally depicted using graphs of discharge pressures and temperatures, polytrophic heads, efficiencies and absorbed power related to the design inlet gas conditions (i.e. gas mix composition, inlet pressure and temperature).
However, actual performances are strictly related to field inlet gas conditions that are different from design conditions. The comparison of field measurements to expected values is complex because the expected performances must be adjusted to the actual suction field conditions. This article presents a method and tools for evaluating centrifugal compressor field performances and comparing them to design/expected performances.
The current practice is based on simplified theories, which are often neglected at the plant/machine level. This oversight can lead to a lack of analytical support capability for the evaluation of machine health and performance, which can negatively affect maintenance decision-making.
These simplified theories become progressively more approximate, especially when approaching the high-pressures range. In order to execute this advanced performance analysis, the research lab employing these evaluation methods used a dedicated software tool based on the most recent gas theories and centrifugal machine models. The software tool is designed to easily and quickly reveal centrifugal compressor performance evaluations. It offers centrifugal compressor analysts and industry users a powerful and easy tool to predict machine performance under different operative conditions.
For a centrifugal compressor, performance is strictly linked to the inlet gas conditions. Accurately comparing different inlet conditions is impossible. It is necessary to adjust and align performances to the same reference inlet conditions.
When attempting to evaluate performances by comparing field-measured parameters and expected parameters, the trick is to remember that one of the two performances must be manipulated and adjusted to the inlet conditions of the second one. In other words, inlet conditions must be homogeneous (see Figure 1).
Follow these steps to successfully conduct performance evaluations:
- Step 1: Adjust expected performances related to design inlet conditions to field suction conditions.
- Step 2: Compare field performances to adjusted expected performances.
The starting point is the availability of a centrifugal compressor performance curve, the relevant gas mix composition and thermodynamic conditions (pressure and temperature). Using this input data, the software performs calculations in a fully automated way. It also produces the expected compressor performances for inlet pressures, inlet temperatures and gas mix compositions different from design or reference performances. The software is intuitive, and it provides automatic data storage for quick consultation and historical trends. The software allows analysts and engineers to build their own machinery database and maintain records of all executed calculations.
Once the project is set up, the software has all necessary data to perform the computations. Beginning the calculation requires inputting the new gas mix before calculating the new inlet pressures and temperatures. Once completed, the software will display the new compressor performance map.
After this first calculation step, a performance analysis tool can compare field-measured data with expected data.
In the following example, a compressor was running under off design inlet conditions. An analysis provided a performance evaluation and comparison to the design values. A centrifugal compressor performance map in two different inlet conditions of pressure and temperature were available (see Table 1).
Starting from the curves of the D1 condition, the expected performance curves in the D2 condition have been calculated using the Cmap software. The obtained curves, D2calc, have then been compared to the D2 curves available as input.
The charts in Figures 2, 3, 4 and 5 compare the D2 calculations (X points), obtained with the Cmap software, versus the D2 (continuous line) performances curves. It should be noted how the curves almost overlap—the maximum error calculated is less than 0.2 percent (polytropic work at maximum speed). Measured errors have been considered tolerable for the purpose of evaluating compressor field performance.