The present invention relates, in general, to surge control systems and, in particular, to a new and useful method and apparatus for controlling surge in a centrifugal compressor.
The centrifugal compressor is one of the most commonly used means of gas compression. It is used in many fields, such as the petroleum, chemical and synthetic fuel industries.
It is known that the operation of a centrifugal compressor can become unstable due to changes in various operating conditions such as flow rate or pressure. This causes rapid pulsations in flow which is called surge.
When operated into the surge region, the head flow characteristics of a centrifugal compressor actually reverse slope developing a negative resistance characteristic as shown in the characteristic curves of FIG. 1. As flow is reduced, discharge pressure falls, so that flow and pressure are further reduced. When discharge pressure falls below that in the surge line 10, a momentary reversal of flow occurs and line pressure starts to fall. This condition creates demand for more flow causing flow to reverse again. This pulsation continues until either a control action is applied to force the compressor out of the surge region or until compressor linings or other structures are damaged.
In the current state of the art, surge control systems are based upon differential pressure measurement across an orifice plate which is installed in the compressor suction line. See, for example, Compressor Handbook for the Hydrocarbon Industries, Gulf Publishing Co., Houston, Tex., (1979). In many installations, however, such as gas recovery-compressors in a fluid catalytic cracking unit, it is not possible to measure the orifice differential pressure in the compressor suction line due to difficulties in installation. (See the compressor Handbook referred to above).