Gas compressor control system preventing vibration damage

A method of compressing a gas comprising the steps of:

• (i) compressing the gas in a series of compression stages from a lower pressure to a higher pressure, each compression stage having a compressor driven by a variable speed motor having a restricted speed zone, and wherein two of the compression stages are driven by a first variable speed motor;
• (ii) adjusting the speed of the first variable speed motor to increase or decrease the flow rate of the compressed gas from each compressor toward desired flow rates and wherein the speed of the first variable speed motor remains situated above or remains situated below the restricted speed zone;
• and
• (iii) further adjusting the flow rate of the gas to one or both compressors by adjusting inlet guide vanes associated with the respective compression stages to avoid the speed of the first variable speed motor being situated within the restricted speed zone;
• wherein, when the speed of the first variable speed motor is above the restricted speed zone and the inlet guide vanes are adjusted to a position not equal to a 0° swirl angle and a new speed of the first variable speed motor, below the restricted speed zone, is required to obtain a new desired flow rate, the method further comprises the steps of:
• (iv) decreasing the speed of the first variable speed motor in decreasing speed steps until a lower speed of the first variable speed motor, below the restricted speed zone, is obtained;
• (v) successively opening the inlet guide vanes during each of said decreasing speed steps such that the inlet guide vanes are set at the 0° swirl angle at the lower speed, below the restricted speed zone;
• and
• (vi) further decreasing the speed of the first variable speed motor until the new speed is obtained below the restricted speed zone.

A method of compressing a gas comprising the steps of:

• (i) compressing the gas in a series of compression stages from a lower pressure to a higher pressure, each compression stage having a compressor driven by a variable speed motor having a restricted speed zone, and wherein two of the compression stages are driven by a first variable speed motor;
• (ii) adjusting the speed of the first variable speed motor to increase or decrease the flow rate of the compressed gas from each compressor toward desired flow rates and wherein the speed of the first variable speed motor is situated above or below the restricted speed zone;
• and
• (iii) further adjusting the flow rate of the gas to one or both compressors by adjusting inlet guide vanes associated with the respective compression stages to avoid the speed of the first variable speed motor being situated within the restricted speed zone;
• wherein, when the speed of the first variable speed motor is below the restricted speed zone and the inlet guide vanes are opened and a new speed of the first variable speed motor, above the restricted speed zone, is required to obtain a new desired flow rate, the method further comprises the steps of:
• (iv) increasing the speed of the first variable speed motor in increasing speed steps until an upper speed of the first variable speed motor, above the restricted speed zone, is obtained;
• and
• (v) successively closing the inlet guide vanes during each of said increasing speed steps such that the inlet guide vanes are set at the position required to obtain the new desired flow rate when the upper speed of the first variable speed motor, above the restricted speed zone is reached.