Case Study: Reciprocating Compressor Cracked Head Failures

Problem: A plant experienced multiple cracked head failures on a reciprocating compressor in a flare gas recovery unit. The plant needed to identify and address the root cause in order to prevent future cracked head failures.

 

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. However, the operating data showed that the knock-out drum upstream of the 1^st stage compressor experienced brief periods of rapid liquid level increase. Review of these events, with plant process personnel, determined that the increases in liquid to the low-pressure gas line, were occurring when some unit in the plant was rapidly shut-down and “steaming out” of equipment was necessary for protection. This resulted in a very significant surge of steam going into the gas plant line and resulted in condensed steam rapidly filling the knock-out drum.

KHE performed a design assessment of the knock-out drum and found that it was more than adequately sized, for normal operating conditions and probably even suitable for the surge conditions, assuming liquid in the drum was maintained at a reasonable level. However, due to the rapid and often unexpected nature of these events, drum levels would rise considerably before the automated drum drain pumps could address the rising liquid levels in the drum. In fact, there were two instances, each of which slightly preceded a cracked head failure, in which the liquid level in the drum reached the height of the gas inlet nozzle. KHE concluded that the interaction of the inlet stream with the high drum liquid level, during these events, resulted in liquid carryover from the knock-out drum. This liquid carryover eventually caused the cracked head failures, when the liquid reached the compressor.

 

Result: Due to the unplanned and unexpected nature of the events which caused the steam surges into the gas plant line and the fact that this steam serves to protect critical equipment, process changes to completely eliminate these events were, for the most part, not feasible. However, KHE recommended a review of emergency shutdown operations to determine if the severity of the steam surges could be reduced.

 

Knowing that steam surge events could not be entirely eliminated, KHE recommended increasing the protection of the compressor during such events by implementing the following:

 

• Reducing the knock-out drum liquid level, at which the compressor would be tripped.
• Monitoring and draining the compressor suctions bottles any time a steam surge event occurs to address any liquid that may have carried over from the knock-out drum during the event.

 

To date, there have been no compressor failures.