Cantrifugal Air Compressors

Elliott 90P Centrifugal Compressor Case

ECRS was contacted after this compressor apparently went into a surge condition, causing the rotor element to fail. This event caused serious damage to the cast iron compressor case and inlet housing. After inspecting the damage, ECRS engineers were confident that a mechanical repair could be designed and installed that would return the case and inlet housing to service.

survey of damages
The case of the compressor was essentially split in half along a plane perpendicular to the rotor centerline. The total length of cracking was approximately 700 linear inches.

FEA analysis
A Finite Element Computer Model was used to help determine the stress patterns in the affected areas, and to evaluate the proposed repair design. The loading and boundary conditions were applied to simulate normal operating conditions. From here, sophisticated software was able to compute the operating stresses in the pump case. Some of these results are shown below.

repair procedures performed
The 700 inches of crack was repaired from the inside as well as the outside of the case. Masterlock, metalock®, and metalace components were all incorporated into the mechanical repair of this crack. The ends of the crack were stop drilled to hinder any further propagation. Following is a picture of these repairs.

installation of reinforcement
In addition to the repair components mentioned above that were actually embedded into the compressor wall, external reinforcements were designed and installed. The primary purpose for this external reinforcement was to prevent any flexing of the main housing. Custom radial ribs were designed, and the FEA modeling confirmed the design to be sufficient. Following is one rib providing reinforcement to a section of the case. There were 13 ribs attached to the case in all.

repair complete
As a foot note, regarding the structural integrity of this repaired unit, the rotor failed again a short time after this unit was returned to service. The resulting damage to the case was considerably less than the damage suffered during the original rotor failure. In fact, the majority of the damage was sustained in areas that had not been reinforced by us.

ECRS was contacted when the rotor on this compressor also threw a blade. Damages were similar to the job above. There were a series of smaller cracks rather than one large crack. The total length of cracks was approximately 540 inches. These were repaired using masterlock, metalock®, and metalace components. In addition to the cracking, there was a piece missing entirely from the case which had to be fabricated and then fit into the vacant area of the housing. This replacement is shown in the second picture below. As in most cases there was also a need for reinforcement. The same design as the above job was used. There were 14 ribs installed around the perimeter in all. Following are before and after pictures of the repair.

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