20xx Mirsky CCC CO2 Compressor Control in Urea Plants
To adequately protect these complex CO2 compressors from surge, a more aggressive approach than used in “design-by-tradition” is required. Singular recycle loop is not adequate to control these machines, and multiple loops are highly desirable, with a minimum of two — one for stages one and two, and one for stages three and four. For retrofits, the recommended and preferred solution is to modify the piping, and, in our experience, quite often the existing piping can be utilized very effectively. For retrofits where piping modification is too costly, even though this is a less desirable solution, a single valve may be used. But even for this we suggest utilizing multiple antisurge controllers and performance control, as discussed above, integrated with the antisurge controller. The valves should be sized adequately. The high pressure recycle valve must be placed as close to the hot compressor discharge line as possible to reduce the chance of forming liquids when the gas is expanded. This same valve should be designed and sized to handle liquids, and a knockout drum may need to be placed so that liquids will not be ingested into the compressor. Antisurge controllers must be provided for more than one stage. It is preferable to protect each stage separately, but a minimum of two antisurge controllers is required. Applying antisurge control across pairs of stages can cover both stages with sufficient precision (no stage is left unprotected), but variation in intercooling may still cause uncertainties of an acceptable magnitude in the location of the surge limit, requiring a larger but acceptable safety margin. A flow measurement device must be dedicated to each antisurge controller and be placed to accurately indicate the flow through the stage or stages being protected. Performance control manipulating the turbine speed set point is required in all scenarios for decoupling with the antisurge system.