PRACTICAL NOTES ON TURBOMACHINERY PIPING

There are certain piping, support, nozzle load and stress analysis fundamentals that can have an impact on turbomachinery efficiency, alignment, operation and component life. Important practical notes on turbomachinery piping are necessary to retain a high performance and reliable piping system.

Flexibility studies are intended to verify that stresses in piping as well as stresses and deflections in turbomachinery components (forces and moments at the turbomachinery nozzles) are within acceptable limits through all anticipated phases of normal (including startup, shutdown, and so on) and abnormal (emergency shutdowns and upsets) operation.

Piping support systems are generally designed according to two major rules:

• The support locations are determined by the guidance of themaximumallowable spans, corrected by natural frequency considerations

•The support types are selected based on the expected thermal displacement and dynamic load considerations

Piping should be designed and supported so that turbomachinery can be dismantled or removed without adding temporary supports (only with removing spool pieces). For parallel turbomachinery, piping of two trains may be mirror imaged in order to get easy access to common maintenance areas.

In some units, both turbomachinery and adjacent equipment such as the vessel or cooler are located on either side of the pipe rack. In this case, the connecting piping will move toward equipment due to thermal loads. This arrangement induces marginally more loading on the nozzle compared to having equipment on the same side of the pipe rack.

Steam turbine or turboexpander turbines (whether cryogenic or hot gas type) are strain sensitive and operate at high-temperature differences with relatively low allowable nozzle loads. Usually piping flexibility should be assured. Even a good piping layout can create problems if proper supports are not considered.

Recommendations for turbine piping, therefore, are as follows:

• A flexible piping arrangement could encircle the turbine

• Line stops should be located in such a way that they match the turbine centerlines. This line stop arrangement could nullify the thermalmovement effects and help in reducing nozzle loads

• Supports near turbine nozzles should have low friction. Spring supports with minimum variability are recommended. Piping can be supported with hanger rods and rigid struts that reduce the friction at the support.

Sometimes support devices with high damping such as snubbers (either hydraulic ormechanical) are required to provide to the piping systems protection from different potential excitation sources such as shock, or dynamic loads. These snubbers or dampeners are expensive, require constant maintenance and have built-in play.

A friction restraint may be better for some turbomachinery piping. By using frictional sway braces adjusted to suitable forces, the braces could act as quasi-rigid stops during shock events, while accommodating proper thermal expansion and other loads. These solutions (snubbers, dampeners, frictional supports or similar) should always be approached with a great care.

Themost obvious effect of turbomachinery vibration on the piping is fatigue failure, especially at critical high-bending stress regions. In reality, it is not vibration amplitudes which are problematic, but rather the dynamic stress level in the piping system which should be limited below a destructive level. Stress level in piping is mainly a function of physical distortion.

Special considerations should be given to the configuration and dimensions of the piping system. Obviously, the dynamic stress generated in a cantilever span due to vibratory deflection is less than that generated in a complex configuration (such as “L” shape, “U” shape, loop shape, and so forth) by an equal vibratory deflection.

Variation in dynamic stress per unit of vibratory deflection tracks rather directly with the natural frequency for a given piping. Vibration velocity criteria could effectively be specified. The stress concentration factor and other reduction factors, such as correction for concentrated weights, non-ideal end condition, changes in pipe diameter, and effects of vibration mode shapes, should be taken into considerations.

As a rule of thumb, the vibration velocity limit could be mentioned as 7-10 mm/s and 3-7mm/s for long piping span and short complex piping (such as “L” shape, “U” shape, loop shape, and so on), respectively. It is just an indication, and higher vibration values may be acceptable depending on a case-by-case study.

Author

Amin Almasi is a registered professional engineer in Australia and Queensland (M.Sc. and B.Sc. in mechanical engineering). He is a consultant specializing in rotating equipment, condition monitoring and reliability.