Case Study: Reciprocating Compressor Liquid Carryover Analysis

Problem: A plant was experiencing chronic liquid accumulation in the suction bottles, upstream of a reciprocating compressor, in a hydrogen desulfurization unit. In order to prevent liquid getting pulled into the compressor and causing damage, operations was draining the bottles several times a shift. Sole reliance upon operators for protection of a critical piece of process equipment is not ideal. The plant wanted to determine and eliminate the cause of the liquid getting into the suction bottles.  

Solution: KHE applied its Integrated Systems Approach to this project, focusing primarily on the process and mechanical aspects of the problem. KHE reviewed historical operating data for both the compressor and associated knock-out drums, as well as compressor design data and past performance assessments. KHE did not find any notable mechanical issues with the compressor.

There were two vapor-liquid separation vessels upstream of the compressor suction bottles: a knock-out drum, followed by a second separation vessel, that was added at some point after the initial construction of the unit. Analysis of operating data indicated that operating conditions, for these vessels, were fairly consistent (no excursions) with slightly higher liquid filling rates in the knock-out drum during cooler months. KHE performed a vapor-liquid separation efficiency analysis on both of the vessels at the typical operating conditions. This assessment determined that, while the knock-out drum’s design was marginal at best for the typical operating conditions, the downstream second separation vessel was appropriately sized and should provide near total liquid separation under the normal operating conditions. Therefore, KHE concluded that the source of the liquid was likely condensation in the overhead lines between the separation vessels and the suction bottles. A dew point assessment of the process stream confirmed this as a possible source for the liquid.

Result: KHE recommended that the plant address the potential for condensation by heat tracing the overhead lines. KHE also recommended that the plant continue to monitor the suction bottles for the presence of liquid following implementation of steam tracing to verify that the modification was successful in eliminating the source of the liquid. To date, this solution seems to be working.