Compressor nozzle loads (API 617)
The piping, restraints and soil stiffness have a direct relationship to the stress in the pipe as well as the loads on the compressor nozzle. If the nearby restraints are rigid, the piping grows thermally towards the compressor and increases the nozzle loads. A very flexible system would result in low compressor nozzle loads, but would be very susceptible to vibration. This balance between thermal flexibility and mechanical stiffness is a critical factor that should be accounted for in a station piping design.
API 617 is commonly used to determine acceptability of the compressor nozzle loads on centrifugal compressors. The API 617 allowable values are equal to 1.85 * NEMA SM23. Nine different allowable values are calculated based on the “equivalent” nozzle diameter. An allowable multiplier is typically applied (often 3.0 * API 617). The calculated moments are resolved about a common point, which can and does affect the acceptability of the predicted loads. The summation of forces and moments are compared to the code allowable values along with the three combined loads shown below.
Individual Nozzle Loads
• Suction Flange 3 * Fr + Mr
• Discharge Flange 3 * Fr + Mr
Nozzle Load Summations
• SFx, SFy, SFz, SMx, SMy, SMz (six different allowable values)
• 2 * Fc + Mc
Figure 3 presents a sample calculation of the API 617 compressor nozzle allowable values.
Oftentimes, resolving the loads about a single point will result in the highest value. This can be due to high thermal loads that should be lowered to ensure safe operation. For larger diameter piping, however, resolving the load about the discharge nozzle often results in a large moment about the X-axis as shown in Figure 4. The weight of the suction flange (FY) is multiplied by the length between the compressor nozzles (DZ) and creates a high moment about the discharge nozzle. This is not due to any thermal growth of the piping or unit; it is due to the weight of the flange.
The predicted loads and code check should be discussed with the original equipment manufacturer (OEM) to determine if the loads are acceptable to the vendor. A good understanding of the loads and their sources is necessary to understand whether the loads are due to a thermal growth issue or if it is a result of the resolved loads calculation. Good design practice can often lower nozzle loads to acceptable levels during the design phase and is preferred to validating high loads and having a review by the OEM.
As discussed later in this paper, improper modeling techniques and temperature range selection can lead to underpredicting equipment loads. At a new compressor station, large diameter piping (36 in. [915 mm] suction, 30 in. [762 mm] discharge) and the compressor were analyzed during the design phase of the project.
The goal of this analysis was to evaluate various pipe routing and ensure that equipment loads were within acceptable limits. In this case, using 45° elbows on the suction piping was preferred (in order to keep the centerline of the compressor as low to the ground as possible). A thermal stress analysis was performed during the design phase and this pipe routing (and the predicted equipment loads) was deemed acceptable.
During the commissioning of the compressor package, several issues were noted with the piping. A new piping thermal model was independently built and followed the modeling techniques cited herein. These results showed equipment loads in excess of allowable API values and much closer to those limits established by the OEM. Using this updated model, design changes were identified that would lower the stresses in the piping as well as lower the predicted equipment loads. These costly changes were then implemented with much better results.