Stress Analysis of Centrifugal Compressor Connected Piping Systems Using Caesar II

Stress Analysis of Centrifugal Compressor Connected Piping Systems using Caesar II: Part 2 Continued from Part 1 Sometimes vendor allows using some factor. Always check with vendor at early stage of project and agree on this. Resolution point is the point with respect which all forces and moments of suction and discharge nozzles will be resolved for combined study. As per code it is the flange face of largest compressor nozzle (normally the suction nozzle). Measure the position (dimensions) of the other nozzle with respect to this point for combined study. A typical diagram showing suction and discharge nozzles from a GA drawing is reproduced in Fig. 5 for your understanding.

Fig. 5: Typical GA drawing showing suction and discharge nozzle.

Qualification of Compressor Nozzles: From the above paragraph it is clear that to qualify the compressor nozzles you need to model and analyse the discharge nozzle and connected piping system (discharge side piping is shown in Fig. 9). So follow the same procedure as mentioned for suction nozzle and analyse. Compressor nozzle loads are checked in NEMA SM 23 module of Caesar. To perform the same click on Analysis and then NEMA SM23 as shown in Fig 6.

Fig. 6: Opening NEMA SM 23 module in Caesar II The module will open the spreadsheet (Shown in fig. 7) where you have to enter the parameters of suction and discharge nozzles, loads at suction and discharge nozzle and distance from resolution point. Enter the factor of allowable increase if vendor allows to do so. Normally as per API 617 the value is 1.85

Fig. 7: NEMA SM 23 spreadsheet in Caesar II If you provide node number Caesar will automatically select the loads from your static analysis upon clicking the proper load case.  After loads are provided for both suction and discharge nozzle click on the run butt on to see the analysis results. It will show the results as passed or failed. If failed check the reason for failing and reduce the same force or moment value to get it qualified. A typical output report is shown in Fig. 8 for your reference.

Fig.8: Typical Caesar Output of NEMA SM 23 module Additional considerations for Compressor Piping Analysis: 









 

Perform alignment check (Anchor free analysis) in a separate file and keep nozzle movements within limit as specified in API RP 686. In most of the cases supports near compressor suction and discharge nozzles are Spring hangers. So create an additional load case as WNC+H- (SUS) type and qualify the nozzles in that case too. Normally Compressor nozzles are qualified in operating temperature (and sustained) only. However if client asks you have to do so in design temperature case. If variable springs are used to qualify Compressor nozzles keep the variability as low as possible (less than 10%). Normally modal frequency analysis is not required for centrifugal compressor connected systems. In general, Compressor nozzles are not required to qualify in occasional load cases. Don’t forget to request nozzle displacements at early stage of project.

Fig. 9: Compressor Discharge side piping of a typical system