There are basically two methods to clean a fouled process compressor; on-line and off-line cleaning.
On-line cleaning is the preferred cleaning method for process compressors. The advantage of on-line cleaning is that the process compressor can be cleaned during normal operation. There is no need to shut down the unit and the cleaning operation can be performed in a matter of minutes.
A suitable cleaning chemical will be injected by means of a specially designed nozzle system installed in the process compressor suction line. The atomized cleaning chemical will be entrained by the process gas thereby wetting all the process compressor internals. On-line cleaning will not only clean the gas passages, but also between the impellers and diaphragms.
Depending on the application and fouling level, on-line cleaning can be performed periodically or on a continuous basis. For process compressors where buildup on the diaphragm surface is a problem, continuous flushing with a cleaning chemical may be required.
Given the fact that the atomized cleaning chemical is evenly distributed inside the process compressor casing, there is no risk of flow instability. Any material buildup will be gradually removed thereby minimizing the risk of increased vibration levels and/or accumulation in downstream process equipment.
The positive effect of on-line cleaning will be visible instantly. The process parameters can be monitored and the cleaning operation can be adjusted as required.
To enable off-line cleaning, the process compressor must be shut down and isolated. During off-line cleaning, the process compressor casing will be filled with a suitable cleaning chemical. Care shall be taken not to overfill the casing; the maximum liquid level shall remain below the shaft end seal level. Furthermore, the shaft end seals must remain pressurized to prevent liquid from entering the shaft end seal cavities and bearing areas.
While injecting the cleaning chemical into the process compressor casing, the rotor should be rotated slowly (<20 rpm). The process compressor internals should be soak washed this way for at least 15 minutes. When completed, the liquid can be drained from the casing and a sample should be taken to determine the amount of suspended and dissolved particles. Depending on the results, the procedure should be repeated as required.
Off-line cleaning is an alternative option to on-line cleaning. However, a major disadvantage is that the process compressor must be taken out of service which, in case of no backup, results in a complete plant shutdown. The costs associated with such an unscheduled shutdown could be enormous and adversely affect plant profitability.
Another negative aspect of off-line cleaning is its effectiveness. The process compressor casing can only be filled below the shaft end seal level. The top half of the casing, containing the top half of the diaphragms, cannot be soak washed. Given the fact that any material buildup in this area most likely cannot be removed, off-line cleaning could result in the additional risk of flow instability which potentially could cause increased vibration levels. Furthermore, chunks of dirt may eventually dislodge from the top half and move within the process compressor thereby aggravating the risk of increased vibration. Some will accumulate in the aftercooler inlets or knockout drums resulting in high pressure drops.