Piping

Piping is required to bring the gas to the compressor and take it away. If it were not for this somewhat basic need, compressors would do well without it. Piping brings with it pipe strain. If one were to ask the maintenance department why pipes are used, the answer would undoubtedly be that piping was installed to the compressor to destroy the fine alignment just completed by the millwright.

Excessive pipe strains on equipment can cause both internal as well as external alignment problems. Some of the compressor designs are more vulnerable to internal misalignment problems from external forces than others. If the internal components become misaligned, rubbing can occur. Accelerated wear and early failure may occur. The effects of external misalignment may not be as obvious as those from internal problems but will in time reduce the length of operations. The vibration levels increase as the couplings become misaligned and the high vibration trip system may cause an unscheduled outage. Extended operation at high levels of misalignment may cause coupling failures and possibly bearing damage or, at the worst, a catastrophic failure.

The original equipment supplier should be contacted for the recommended allowable forces and moments. Equipment purchased to API standards specify a design level for allowable external forces and moments to be used in the equipment design. These values are frequently used by piping designers as field allowable values. Only when the equipment supplier echoes these values should they be used in piping design. Even under these circumstances, the entire value should not be used for piping design. Most of the piping stress programs do not account for field tolerances in fabrication, such as flange squareness and length variations. While not totally in concert with normal field fabrication practice, some of the tolerance problems can be minimized by piping away from the compressor and using some field-fit piping. It is very important to check the piping for strains by aligning the equipment and then connect ing the piping while observing the shaft-to-shaft alignment change. An alignment change of no more than 0.002 inch is considered acceptable for most helical-lobe and dynamic compressors.

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