Loss of Flow In a Process Water Pump

The BASF-FINA Limited Partnership (BFLP) operates an ethylene cracker in Port Arthur, Texas.  The plant's Process Water Pump sends a side-stream of stripped quench water to the Dilution Steam Generators.  This pump lost flow on several occasions in early 2008.  The operators reported that it behaved as though it were ‘starved for suction flow.'  Such symptoms had never been seen previously.  The first such episode resulted in an extended unplanned downtime.  Operations learned to anticipate loss of flow in the pump and quickly start its spare, but the cause remained obscure and the threat of additional downtime remained.

Site personnel considered a long list of possible causes.  Vapor lock was one of the few that could not be ruled out, and attention focused on how it might arise.  Various sources of gas and vapor were ruled out, but a sample return blow case connected to the pump's suction could not be.  The blow case was driven by nitrogen gas; calculations showed that its valves might leak enough to cause vapor lock in the water pump.  No obvious abnormalities were found when the blow case was torn down.  However, the process water pump suffered no further difficulty after the blow case's nitrogen supply was blocked.  The sample system has been permanently isolated from the pump's suction, and the pump continues to run normally.