Liquid injected screw compressor, controller for the transition from an unloaded state to a loaded state of such a screw compressor and method applied therewith

A method for controlling a liquid injected screw compressor, comprising a compressor element with an inlet and a controllable inlet valve to be able to close the inlet;

• an outlet and a pressure pipe connected thereto that is connected to a downstream consumer network and a controllable blow-off valve for blowing off compressed gas to the inlet of the compressor element;
• a liquid circuit with an injector for injecting liquid into the compressor element;
• a liquid separator provided in the pressure pipe to separate liquid from the compressed gas and a pressure vessel to collect the separated liquid;
• an injection pipe that connects the pressure vessel to an injector for injecting liquid into the compressor element;
• a controller for controlling the inlet valve and the blow-off valve during a transition from an unloaded state to a loaded state when a pressure in the consumer network falls to a desired minimum network pressure, whereby in the unloaded state, the inlet valve is closed and the blow-off valve is open and maintains a compression pressure in the pressure vessel and in the loaded state, the inlet valve is open and the blow-off valve is closed, wherein during the transition from the unloaded state to the loaded state, the method comprises the following steps:
• determining the pressure in the consumer network;
• determining an injection pressure or a pressure in the pressure vessel at the time that the pressure in the consumer network falls to the minimum network pressure;
• if the injection pressure or the pressure in the pressure vessel at that time is greater than or equal to a minimum value, then the inlet valve is immediately opened;
• if the injection pressure or the pressure in the pressure vessel at that time is less than the minimum value which is lower than the compression pressure, then the inlet valve remains closed and is opened with a delay so that the compression pressure in the pressure vessel is gradually increased during this delay in the opening of the inlet valve;
• and,
• only fully opening the inlet valve when the injection pressure or the pressure in the pressure vessel reaches the minimum value,
• wherein the inlet valve and the blow-off valve can be controlled independently of one another and wherein the pressure in the pressure vessel during the transition from the unloaded state to the loaded state is increased by the Fact that the controller is such that during the transition the open blow-off valve is closed while the inlet valve remains closed during the aforementioned delay,
• wherein the pressure is increased by including an additional bypass with a calibrated passage to bypass the inlet valve or drawing in gas when the inlet valve is closed, whereby a controllable shut-off valve is provided in this bypass, whereby the controller is such that the shut-off valve is closed in an unloaded state and opened during the transition from the unloaded to loaded state.