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Every compressor is connected via two or more nozzles with flanges to its suction, discharge, and side-load piping. These pipes, due to flange misalignment or thermal pipe expansion/contraction, can exert significant loads on the compressor flanges. Flange loads, in the form of directly acting static and dynamic multi-directional forces and moments, can cause excessive strain on the nozzles, deflection of the casing, shaft misalignment, and even excessive stress on the equipment and foundation bolts.
For very stiff compressor casings, excessive flange loads usually result in shaft misalignment between the compressor and the driver since the loads ultimately must be carried by the skid bolting – the less stiff interface between the foundation and the dynamic equipment. For lighter compressor casings with lower stiffness, critical internal clearances between rotating and stationary components can be affected. In these cases, flange loads on the nozzles can lead to internal casing or bundle deformation and failure of single-digit-mil clearance components such as dry gas seals and bearings. They may even result in impeller rub.
Compressor vendors often use multiple times the allowable NEMA or API requirements as their design standard. These requirements include NEMA SM-21, the subsequent NEMA SM-24 and the calculation method published in Appendix E of American Petroleum Institute 617. A common multiplier is 3.0 times API 617 (equal to 5.55 times NEMA) as allowable load limits. Similarly, API flange load limits are equal to 1.85 times NEMA. Although NEMA is a steam turbine standard, it is commonly used for flange load calculations on other rotating machinery.
Alternatively, rather than relying on the somewhat arbitrary use of fixed multiples of API or NEMA, many vendors ask the equipment end user to supply pipe flange loads for review and approval prior to design and fabrication of the equipment. However, it is often difficult to accurately predict flange loads before final construction of the compression piping system. Though good pipe design software is commercially available, as-built construction errors and pipe installation misalignments are common.
Regardless, the aim of both the vendor and the user of rotating machinery should be to design equipment and piping that reduces the actual load on the compressor nozzles and casing. The compressor should not be an anchor point for poorly designed piping. Compressors are expensive and complex dynamic machines that are not intended to take the function of relatively inexpensive pipe supports/loops or correct for piping system design errors. API 617 states, “The design of each compressor body must allow for limited piping loads on the various casing nozzles. For maximum system reliability, nozzle loads imposed by piping should be as low as possible regardless of the compressor’s machines’ load-carrying capability.”