Finite Element Analysis of Vapor Recovery Unit (VRU) Base Skid

Three-Dimensional Finite Element Analyses Performed on Base Skid for Various Load Cases

We performed finite element analysis on a Vapor Recovery Unit (VRU) base skid to ensure structural integrity to ASME Boiler & Pressure Vessel Code.

The skid package is used to recover process vapor for use as fuel gas – for a plant near Fort McMurray, North Alberta Canada. The equipment included motors, compressors, a separator, heat exchanger, instrumentation and controls, piping and valves, and miscellaneous components. The VRU base skid will also support the utility building in which the system will be located.

The combined VRU Base Skid and Utility Building structure is approximately 55 ft in length, 23 ft in width and 18 ft in height. The VRU Base Skid is a welded steel structure comprised primarily of American Standard W18x50 wide flange beams. The beams are mostly covered with 1/2-inch and ¾-inch thick steel partially connected (stitch welded in most locations) diamond plating. The diamond floor plates have not been designed to act compositely with the steel beams.

An overhead crane with associated support structure interfaces with transverse members of the base skid. The crane can access half of the overall skid, servicing the electric motors and compressors for the VRU system. Accommodation has been made for a transport vehicle such as a fork truck to enter the side of the utility building for retrieval and delivery of periodic maintenance components such as the motors and compressors. Design criteria for the base skid structure was obtained in part from the 2006 Alberta Building Code.

Three-dimensional finite element analyses were performed for various individual and combined load cases. The resulting axial tensile and compressive forces and stresses as well as bending moments and bending stresses from these analyses were compared to corresponding allowable stresses according to design criteria. Modal analysis was also performed, excluding the piping and the surrounding building.

Piping is considered in the skid analysis with its weight contribution and other loads that apply to the skid (reactions to external loads and thermal loads). Piping materials are either carbon steel or 316 SS.

Three-dimensional structural analyses of the VRU base skid were completed for various individual and combined load cases. Resulting axial tensile and compressive forces and stresses, as well as bending stresses from these analyses were compared to corresponding allowable stresses according to the skid design criteria. The final design for the base skid was found to satisfy the specifications and design requirements.

A modal analysis was performed on the base skid, excluding the piping spools and the building. The response of the skid in the region of the motor/compressor was investigated – in particular, the potential for the electric motor or compressors to excite natural frequencies in the skid structure. No excitable skid modes of vibration were found.

Piping analyses for the VRU Package were also performed, which included various thermal and structural load cases. The results determined that additional pipe supports were needed. The final design was within allowable stress criteria and satisfied the required specifications.

O’Donnell Consulting performs design & analysis of skids and related components to codes including API, AWS and ASME.