Inspection and Remediation of Cryogenic Heat Exchangers in Gas Processing Facilities

S. Mark Wilhelm, Ph.D.

Mercury Technology Services

ABSTRACT

Plant shutdown and equipment inspection activities were undertaken to identify the cause of a high pressure differential in the primary cryogenic heat exchanger (cold box) in a major gas processing facility. Action was required because a process upset had occurred in which potassium carbonate from the Benfield acid gas removal unit carried over into downstream equipment and deposited in the cold box in some unknown quantity.

The presence of mercury in plant feeds and the intermittent performance of mercury removal equipment suggested the possibility of mercury deposition in the aluminum heat exchanger. Assessment of mercury deposition in the cold box was accomplished using a computational model and by monitoring the concentration of mercury in warm up gas exiting the cold box.

The plan to eliminate the flow restriction consisted of inspection of the cold box using radiography and visual (videoscope) observations, followed by a cleaning procedure. The remediation plan called for use of nitrogen back flow, solvent circulation and possibly a water flush depending upon the results of the inspection. A main concern was that any cleaning exercise would cause cracking of the susceptible welds in the cold box. Assessment of the risk of cracking depended in large part upon the results of the radiographic inspection in which susceptible locations were examined to determine if mercury has accumulated adjacent to susceptible welds. The main contingencies for cracking were cold box repair and/or replacement.

The radiographic investigation discovered mercury in critically susceptible locations adjacent to welds having microstructures prone to cracking in the presence of mercury. The visual inspection using the videoscope identified the cause of the blockage as potassium carbonate deposition having the appearance of a dry cake-like material that was estimated to block 40 to 50 percent of the total flow path in the feed gas pass of the cold box.

Risk Assessments of Cryogenic Equipment