Compressors: Why they don't operate as designed?

One of the main difficulties of analyzing the performance of centrifugal compressors begins with how the data are presented. Expected performance is generally described in terms of graphs of discharge pressures, discharge temperatures, polytrophic heads, efficiencies and absorbed power related to design inlet gas conditions, i.e., gas mixture composition, inlet pressure and temperature. However, actual performance is strictly related to field inlet gas conditions that are often different from design conditions.

The comparison of field measurements to expected values becomes complicated as expected performance needs to be adjusted to the actual suction field conditions. This requires the availability of calculation algorithms that can predict machine behavior and thermodynamic gas properties under different operating conditions.

The current practice relies on basic formulas. However, higher pressure ranges and other severe conditions tend to make these approximations unworkable. In these cases more rigorous calculations are necessary.

For a centrifugal compressor, performance is strictly linked to the inlet gas condition. Also note that it is not possible to accurately compare different inlet conditions. It is necessary to adjust and align performances to the same reference inlet conditions.

The starting point is the availability of a centrifugal compressor performance curve, the relevant gas composition and thermodynamic conditions (pressure and temperature). Having these inputs, it is possible to perform all calculations in an automated fashion.

The most common way to set up compressor reference data is to enter performances curves expressed in terms of delivery pressure versus inlet flow rate, and delivery temperature versus inlet flow rate. In addition, it is necessary to input the considered gas composition, and the considered pressure and temperature.

(More on this topic in the September/October,2012 issue of Turbomachinery International magazine)