A Fitness-for-Service Assessment of Thermal Dehydrators in NGL Plants – Fatigue Life Extension

Introduction

Propane-butane dehydrator vessel internal attachment welds had repeatedly failed due to fatigue cracks caused by thermal cycling of the vessels. This had been a chronic issue since it had been identified in several past operating runs of the NGL plant. Despite the increasing severity, these internal cracks were repaired extensively and repeatedly, and the vessels were put back into service. A fitness-for-service (FFS) assessment was performed to establish the remaining life of the vessels that have been subject to accumulated fatigue damage in accordance with API-579-1/ASME FFS-1 Level-III FFS Methodology. The FFS was performed for both the existing design and the modifications proposed to the current design. The calculated fatigue life of the structure provided an accurate correspondence with the level of stresses and fatigue damage initiation within the cyclic operation period and correlated well with the thermal fatigue cracks that were observed in the dehydrator vessel. The existing design discrepancies of the vessel internals have been identified, and the relevant design fix for eliminating the repeated failures on a permanent basis has been recommended to enhance the mechanical integrity.

Background

The NGL plant was designed around 1970 as a natural gas liquids fractionating and treating plant to recover propane, butane, and pentane (natural gasoline) products. The block flow scheme is shown in Figure 1. The filtered propane contains moisture in excess of 20 ppm and is routed to the three dehydrators, as shown in Figure 1. After over 45 years in operation, two dryers have always been in service, while the third has been regenerated or ready for use. The wet propane/butane enters the respective dryer at the bottom and flows upward. The regeneration is done using a hot dry product, propane for the propane system, and butane for the butane system. The dryer regeneration consists of two phases: a heating cycle and a cooling cycle. Each dryer is switched from drying to heating and then to cooling. The heating cycle lasts 9 hours and 58 minutes on average for both plants. This cooling cycle flow will continue for 2 hours and 21 minutes on average for both plants when the dryer is cool enough to return to drying service.