Technologies for electrical driven gas compressors

This thesis, developed at the Italian ABB head quarter of Sesto San Giovanni (MI), is considered on the development of an electrical drives to control the surge instabilities of centrifugal compressor. These days, turbomachines are widely applied in the Oil & Gas Industry, where is required to pump a quantity of natural gas, through pipelines, at a desired pressure level and flow rate. Compressors are an integral part of the production process; they are used to increase the line pressure from the gathering system so that gas can be delivered to the processing plants and/or sales lines. Gas compression is one of the most energy-intensive production processes. For this reason, it is important to investigate the most efficient and appropriate options According to the particular type of turbomachine, the centrifugal compressor, and due to they play critical role in the Oil & Gas production chain, their continuously performance is considered vital from the plant operation standpoint. The surge phenomena create dangerous situation both for compressor and production chain, it is important to design effective anti-surge control units. Compressor Anti-Surge protection is required on all continuous flow (centrifugal and axial) compressors. These units are used in natural gas pipeline, LNG refrigerant, and chemical (petrochemical) processes. Sizes range from small electric motor driven units to those driven by large gas turbines. Anti-Surge protection is provided by a control valve that opens allowing gas from the compressor discharge (outlet) to recirculate to the suction (inlet). Compressor surge happens when the required downstream flow changes and not be matched the current compressor operating point. The flow control system will change the driver (turbine or motor) speed if possible to adjust to the new flow rate, nevertheless the speed of the large rotating mass (driver + compressor) often cannot be changed quickly enough to match the change in operating point. When the compressor speed and corresponding design flow rate do not match the actual system demand, flow reversal can occur in the compressor. Without outside intervention the flow reversal cycle will continue placing seals and other internal parts under great stress. Complete compressor failure is possible. Compressors may be driven by reciprocating engines, gas turbines or electric motors. Reciprocating engines are commonly fired by natural gas; essentially internal combustion engines, they comprise a chamber filled with natural gas which is ignited to drive a piston. Low-speed and high-speed engines are matched with compressors of corresponding speed. Gas turbines rely on the hot exhaust gas discharged by a gas generator to drive a power turbine. The output power from the turbine shaft is used to drive the pipeline gas compressor. Finally, electric motors use an electromagnet to produce movement.Reciprocating compressors are typically driven by natural gas-powered reciprocating engines or electric motors, while centrifugal compressors are usually driven by gas turbines or electric motor. A centrifugal compressor driven by an electrical motor is studied, and the drive itself is used for surge control, thus eliminating the need for additional actuators.

Questa tesi, sviluppata presso la sede ABB di Sesto San Giovanni (MI), è uno studio relativo alle tecnologie usate per I compressori a gas industriali azionati da motore elettrico. La tesi analizza i componenti principali di un treno di compressione industriale, ovvero il compressore, il sistema di controllo, il motore elettrico e l’azionamento elettrico a velocità variabile. Lo studio copre anche l’uso dei compressori di processo all’interno dell’industria petrolifera. Il compressore di processo è analizzato in dettaglio, in particolare si è studiato il processo termodinamico di compressione del gas, i componenti interni principali e i fenomeni legati all’instabilità operativa. Si è posta un’attenzione particolare al sistema di controllo del compressore, che può garantire un funzionamento sicuro e affidabile della macchine, attraverso i sistemi di controllo e rilevamento del pompaggio (surge). L’azionamento elettrico è stato studiato in dettaglio, sia nel caso di motore sincrono che asincrono, connesso direttamente alla rete o con un azionamento a velocità variabile.