Why some turbomachinery applications need expansion joints

This column is about nozzles such as the suction nozzles and discharge nozzles connecting piping to compressors, steam turbines, and pumps. In other words, nozzles on turbomachinery casings that connect to the piping. Nozzle loads are loads exerted from piping to the turbomachinery.

Turbomachinery’s capacity to tolerate piping loads is limited. Its effectiveness depends on near-perfect shaft alignment, balanced rotating parts, and proper clearance. Even a small shift in load can challenge this finely tuned condition.

To give you an idea of the significance of relatively low allowable loads and moments specified in turbomachinery codes (such as NEMA and API), the maximum stress in piping when meeting the piping nozzle loads of the turbomachinery is noted for comparison with ordinary piping (those not connected to turbomachinery). For 12-inch (DN300) piping nozzles, the allowable moment corresponds to piping stresses around 15 Mpa. These stress values are even smaller for sensitive machinery such as integrally geared compressors or steam turbines.

For example, for 12-inch (DN300) piping, the allowable moment of a steam turbine corresponds to piping stress of only about 7 Mpa. In comparison, a piping system that does not connect to any machinery can have a thermal stress plus sustained stress of more than 120 Mpa (usually more than 150 Mpa) for most commonly used piping materials.

Large sizes

When size increases, allowable nozzle loads slowly increase. However, the reaction loads of piping increase rapidly with the increase of the piping size. Therefore, it is more challenging to deal with the issue of nozzle loads for large turbomachinery. As a rough indication, piping loads might be assumed to be proportional to the cube of the piping diameter. But in commonly used turbomachinery codes and standards (API, NEMA, etc.), allowable loads are increased less than proportionally to the nozzle diameter. In line with actual numbers, the gap of understanding between piping engineers and turbomachinery manufacturers grows wider as the piping size becomes bigger.

The extremely low allowable nozzle load on large turbomachinery such as steam turbines or integrally geared compressors makes it a challenge to connect them to piping systems. A perfect piping layout is needed combined with a well-planned support and restraint system that includes special components. But this is not always enough. All parties involved, including the designers, engineers, operators, and project managers, should be aware of challenges of piping nozzle loads frequently encountered in the piping systems of delicate turbomachinery applications.

In many cases, expansion joints are prohibited due to maintenance and operational problems. After all, expansion joints can be expensive pieces of equipment and should only be purchased after specific engineering works. They need special care for correct installation, operation and maintenance.

However, when there is no way to make the piping load meet the allowable limit, an expansion joint may be considered. Although an expansion joint is a specialty item requiring special engineering, it is preferable to a system that will not work. Due to high temperature differences and large piping sizes, it may be impossible to provide a piping system that will meet the allowable load, even with loops or other options. This is particularly applicable to some high temperature units such as large steam turbines or extremely low temperature compressors.

The low nozzle loads required by certain turbomachinery applications such as steam turbines or integrally geared compressors, may need expansion joints when piping sizes of 12 inch (DN300) or greater are connected to them.

However, problems have been reported due to improper application or installation of expansion joints. The most direct method of expansion joint installation is to have an anchor to take the entire piping load and to place the expansion joint in between the anchor and the turbomachinery nozzle. This may work well in some cases but not in others. Great care is needed for such an arrangement.

One problem with this arrangement might be the pressure thrust force in the bellows (expansion joint). In order to meet the piping load limitation, the maximum (permitted) internal pressure should be low. Tie-rods can sometimes be added to the expansion joint to deal with this specific issue. However, tie-rods may create other difficulties so they are not popular. ■