Slowdown valves must be installed in centrifugal compressors for the same reasons as in reciprocating compressors. They must be designed with more care than those on reciprocating units, since centrifugal compressors have oil film seals where the shaft goes through the case. These seals only work if the shaft is rotating. If the compressor shuts down, pressure must be relieved from the case before the shaft speed decreases to the point where the seal no longer will contain pressure. This requires careful attention to manufacturer furnished data as well as overall flare system design.