Dynamic Analysis of Compressor Trips in the Sno̸hvit LNG Refrigerant Circuits

Abstract

Compressors are key components in the refrigerant circuits of the Snøhvit LNG plant and contain large amounts of mechanical energy. Thus it is imperative that the control system is able to keep the compressor out of surge in case of driver trip. A dynamic process simulator describing the total LNG plant has been developed by Kongsberg Process Simulation and the simulator has been applied in the engineering phase for the design and process verification. The simulator has also been used to verify the robustness of the closed loop refrigerant circuits and to verify that the refrigerant compressors are sufficiently protected after a driver trip. The presented work demonstrates the value of dynamic simulations for verification of compressor protection systems. In addition it shows the importance of using correct data for polar inertia of the rotating equipment, as well as the opening and dead times of the anti-surge valves. It is recommended to include a sensitivity analysis of these parameters as part of plant verification studies. INTRODUCTIONCompressors are key components in the refrigerant circuits of LNG plants. It is imperative that these compressors are properly protected against surge,, Compressors in surge can result in a severe damage to machinery and disturb plant regularity.One of the most serious incidents that can happen to a compressor system is a driver trip. Gas turbines and electrical motors are the most common types of compressor driver and a trip of the driver here is more serious than a process trip. This is because in a process trip the shutdown of the driver itself can be delayed until the surge protection valves have been opened. Important factors controlling the run down of the compressor after a trip are opening times of the anti-surge valves, piping layout, inlet and discharge volumes as well as the polar inertia of the rotating equipment including the compressor, gear and driver. It is essential to be able to validate that the compressor is properly protected during process upsets including the case of a driver trip.The use of dynamic models for analysing compressor transient behaviour has been addressed by several authors in various scientific journals and conference papers and has also been demonstrated by the supplier industry. Patel [1] discusses dynamic simulators as an important tool in the design and operation of compressor systems. A good overview of the scientific research within this area is given in [2].