Centrifugal Compressors – Process Piping Arrangements

The inlet piping configuration is an important factor that must be carefully evaluated to ensure satisfactory compressor performance. Performance predictions are based on a smooth, undisturbed flow pattern into the eye of the first impeller. If the flow has any rotation or distortion as it enters the compressor, performance will be reduced.

Figure 200-67 may be used as a guideline to establish the minimum length of straight pipe run ahead of the compressor inlet.

Minimum Straight Pipe Run Ahead of Compressor Inlet

The nozzle loads, or forces and moments that the compressor can accommodate without misalignment are generally specified by the manufacturer.

API 617 specifies an arbitrary 1.85 times the limits defined by the NEMA SM-23 Standard. This results in limits which are not practical for all machine types. This criteria relates allowable loadings only to flange size. For example, a lightly constructed unit with 8-inch, 150-pound flanges would be expected to withstand the same loadings as a heavy barrel casing with 8-inch, 2500-pound flanges.

Specification, CMP-MS-1876, Centrifugal Compressors, specifies allowable loadings related to the weight of the machine. This approach provides limits which are generally accepted within the industry.

The design and location of piping supports, and the accommodation of thermal expansion, is generally left to the piping designer, although it should also be reviewed by the project or machinery engineer. This should be checked in detail during construction to ensure correct installation of piping, and that the location and setting of supports is in accordance with design drawings and specifications. Section 700 contains installation and precommissioning checklists which include piping installation review.

The following additional items should be considered when reviewing the overall compressor piping design.
1. High-velocity streams generate noise. Maximum velocity can be limited by the amount of noise that is allowed.
2. No side connections (such as the balance piston return line) should be put in the straight piping run ahead of the compressor inlet.
3. When a permanent strainer is used, specified compressor inlet pressure must include an allowance for strainer pressure drop.
4. To avoid problems prior to startup, the compressor manufacturer should be advised of the description and location of each strainer.
5. Woven wire mesh should not be used in strainers for centrifugal compressors. Wire mesh has the tendency to plug very rapidly, requiring frequent removal, and in some cases, it has been ingested into the compressor causing serious
internal damage.
6. Inlet strainers should be located in the first pair of flanges away from the compressor’s nozzle. Strainers should not be located right at the suction nozzle, since excessive flow distortion could result.

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