Maximizing Compressor Performance

Abstract Maximizing Compressor Performance Process engineering services monitoring compressor performance is vital to successful maintenance of plant efficiency and energy use.  Process gas compression equipment is key in moving process fluids through chemical equipment as well as providing the necessary pressure to promote chemical reactions.  An Ammonia plant Process Air compressor was upgraded with high efficiency impellers, diffusers and improved seal technology with resultant performance improvement monitored for three years.  The compressor originally was designed employing 1970's technology, and the upgrade improved capacity about 15%, while improving stage efficiencies by about 6% Polytropic efficiency gain.  The article highlights the benefits of monitoring, improving and preserving compressor efficiency.
		Process gas compression equipment is essentially the "Heart" of plants, moving process fluids through chemical equipment as well as providing the necessary pressure to promote chemical reactions.  This article highlights maintaining and improving the performance of centrifugal compressors.  Centrifugal compressors by their design have inherent aerodynamic and thermodynamic performance and efficiency characteristics, dependent on size, shape and the number of impellers as well as the condition of shaft and inter-stage seals. Recent improvements over the past 10-15 years include new impeller shapes incorporating complex 3-D enhanced curve shapes for blading and the use of new materials and designs for shaft end and inter-stage seals.  These vendor improvements have boosted compressor efficiencies by 5-7 percentage points, reducing energy consumption while improving compressor capacity. An Ammonia plant Process Air compressor was upgraded with high efficiency impellers, diffusers and improved seal technology with resultant performance improvement monitored for three years.  The compressor originally was designed employing 1970's technology, and the upgrade improved capacity about 15%, while improving stage efficiencies by about 6% Polytropic efficiency gain.  The design basis and actual performance data is illustrated as Table 1, Process Air Compressor Data, with graphical representation of efficiencies shown in Figure 1.  The compressor equipment arrangement is illustrated in Figure 2, Compressor Flow Diagram.  The prior performance before retrofit is not shown, however, the compressor did achieve the capacity and efficiency improvements previously noted.
Analyzing Compressor Performance "Performance" thus involves several characteristics, most notably energy consumption of the compressor in this current age of energy short supply.  Performance is largely achieved by the equipment vendors, but fully maintained by the plant site. Damage to performance occurs naturally in normal "wear and tear" from long term running of the equipment, as well as from accidental or inadvertent damage from incidents such as compressor surge and vibration, due to speed or flow control problems and rotor/impeller imbalances. From the air compressor performance data shown, several points can be noted.  First, the compressor did fairly closely perform, but did not exactly perform according to design efficiencies at initial start-up.  A lot of considerations can come into play in analysis of these differences, including ambient conditions, required final discharge pressure, the condition of inter-coolers, etc.  The biggest point would be that the 4th stage never met design efficiency expectations.  (This was true of the original equipment before revamp too, thus the incremental efficiency improvement was achieved with the compressor retrofit project.) There are mild "inconsistencies" in the compressor performance data.  Efficiency of stages should not increase over time. Sometimes this appears to occur, particularly for two reasons.  First, of course, there may be slight error in the temperature and pressure test data that represents the operating efficiency of the machine.  Second, the compressor can operate in an "envelope" of varying efficiencies, dependent upon the operating capacity and impeller speed through individual sections of the machine.  (Example, Stage 2 at 1 service month)  The loss of compressor efficiency from normal wear and tear due to continuous operation, typically can amount to approximately 1 percentage point efficiency decrease per year.