Centrifugal compressors are considered to be one of the most critical equipments in the piping industry. For increasing the pressure of gaseous fluids centrifugal compressors are widely used in refineries and petrochemical complexes. From stress analysis point of view the compressor systems are critical because:
- Equipment being rotary it is prone to vibration.
- The allowable nozzle loads are very less. Normally combined suction and discharge nozzle analysis is required to be performed.
- As the pressure increases due to compression the pipe thickness increases which increases the rigidity of connected pipe and automatically the loads at nozzle increases.
However good part is that normally compressor connected pipes does not have much temperature.
In this article I will try to explain the procedure to analyse the method followed to analyse such systems.
Inputs required for analysis:
The following documents are required while analyzing a compressor connected piping system.
- Process P&ID.
- Line List for line parameters.
- Pipe Material Specification.
- Equipment Vendor GA drawing.
- Suction and discharge nozzle displacements in proper axes with direction.
- Line isometrics.
- Allowable nozzle loads.
Building the system in Caesar II:
In Fig. 1, a typical piping model connected to a centrifugal compressor and scrubber is shown. If we have to analyse such systems then the following steps we have to follow:
- Model the pipe from isometrics in similar way as you normally do for the other systems. Use Line parameters from PMS and Line List. If the line is insulated then take exact density of the insulating material as that value will impact the analysis. If fluid density is available then use it otherwise leave it blank (zero density as normally gas). A Caesar plot of the above system is shown below for your reference.
- It is very difficult to model the compressor using GA drawings and get the actual thermal displacements at the nozzle points. So vendor provides the thermal displacement values at operating temperature. So now find the compressor nozzle displacements from GA drawing or mail communications and input the same at compressor nozzle node as shown in Fig 3.
- Model the scrubbers taking data from Vendor GA.
- Now prepare all the load cases and run the system statically to check the results. Make the system stresses within code allowable limit. Make scrubber nozzles within vendor allowable. Refer the next paragraphs for compressor nozzle qualification.
In general practice compressor nozzles are to be qualified by API 617 or NEMA SM 23. There are three separate checking to ensure proper working of the compressor.
- Individual component forces (Fx, Fy, Fz and Mx, My, Mz, each component at each nozzle) at nozzle point must be within the limit specified in code. (For equations refer annex 2E of API 617)
- The resultant force and resultant moment of each nozzle (suction and discharge separately) must be within limit specified in code. (For equations refer annex 2E of API 617)
- The combined resultants of the forces and moments of the inlet, sidestream, and discharge connections resolved at the centerlines of the largest connection (resolution point) should not exceed the limit specified in code. (For equations refer annex 2E of API 617)
Always remember that Caesar axis and API axis may not be same. So you need to convert the allowable forces in proper Caesar axis and then compare. The API 617 axis system is reproduced in Fig. 4 for your reference.