Adaptive load sharing for parallel compressors

This thesis has been developed in collaboration with ABB at the Corporate Research Center in Baden (CH); it considers the adaptive load sharing optimization in natural gas boosting plant. The purpose of compressor stations along pipelines is to pump a quantity of natural gas at a desired pressure level. A compressor station usually consists of several compressor trains connected in series, parallel or mixed configurations. The life cycle analysis of turbo compressors indicates that the energy consumption takes up to 80% of the total life costs. The total energy consumption can therefore be greatly reduced by optimizing the operation of compressor stations. This will significantly impact the overall operation and maintenance costs of a gas compression station. Typically the compressor map and performance are tested periodically, but to a very limited extend, due to down-time minimization and expensiveness of testing. This is done to monitor and identify the stable operating region of each single compressor and is usually done every 3 to 5 years. Therefore, it is rather common to have inaccurate or wrong information on the compressor map and performance behavior. This lack of information makes it particularly difficult to optimize the compression system. In addition, especially the performance map of a compressor is subject to significant changes caused by external and unknown factors. Field testing of centrifugal compressors is not something to be taken lightly. Tests that use the field installed instrumentation can lead to highly unsatisfactory results. This is due to a combination of factors including poor maintenance, lack of proper calibration and the design range of the instrumentation. Therefore, using the conventional approach of equal set points for all compressor units in the station results in low efficiencies and poor exploitation of flexibility. These considerations point out, that load sharing optimization is very promising. Al- though, the main problem is the lack of information on the performance map of the individual compressors to optimize the system. Therefore, it is of great importance for load sharing optimization to have an up-to-date efficiency model, whose performance is monitored and whose parameters are updated if necessary.

Questa tesi `e stata sviluppata in collaborazione con ABB presso il CRC di Baden (CH). Le stazioni di pompaggio lungo le pipeline sono utilizzate per trasferire una certa quantit` a di gas ad una pressione fissata. Solitamente, questa, `e composta da pi`u compressori collegati in serie, parallelo o configurazioni miste. Il costo del carburante rappresenta pi`u dell’ 80% del totale dei costi del ciclo di vita dell’impianto e pu essere ridotto attraverso l’ottimizzazione dei carichi dei compressori. Sfortunatamente le mappe delle efficienze dei compressori non sono identificate molto spesso a causa dei costi. Per questo motivo le mappe utilizzate nell’impianto non rappresentano le attuali performance dei compressori. Inoltre le superfici dei rendimenti variano frequentemente in funzione delle condizioni ambientali, della manutenzione e dei periodi di lavoro. Senza un’accurata conoscenza delle prestazioni attuali il controllo convenzionale risulta inefficiente. Al fine di poter realizzare una gestione ottima dell’impianto `e evidente che `e necessario effettuare un’identificazione delle mappe delle efficienze basato sulle misure raccolte dal campo.