Carryover Losses – Steam and Gas Turbines

A number of loss prediction methods exist in turbomachines. Concerning axial turbines, there are at least seven methods just for cascade losses! But there are also loss models developed to predict individual loss components such as secondary, seal and tip clearance losses and more.

Of course depending on the machine and application type, some of the models are more or less applicable to specific cases. But ff the different types of auxiliary losses, which are losses that do not belong to blade cascades and can be classified as whole stage, there are carryover losses.

What Are Carryover Losses in Steam Turbines?

A carryover loss, also called “leaving loss”, “exhaust loss” or “leaving-velocity loss”, (about 3%) occurs on certain stages when the kinetic energy of the steam leaving the rotating blades cannot be recovered by the following stage because of a difference in stage diameters or a large axial space between adjacent stages.

When and Why Carryover Losses Occur

Typically this happens in control stages and in the last stages of non-condensing sections and is due to formation of eddies in annular space between the nozzle and moving blades. It is the most important single loss in a condensing steam turbine and increases at high loads.

Condensing turbines are frequently “frame sized” by last-stage blade height. It is sometimes economical to size the unit with exhaust loss equal to 5% deterioration in overall turbine performance at the design point (valves wide-open throttle flow and 5 kPa [abs] exhaust pressure) when the normal expected exhaust pressure will be higher or the unit will be operating at part load for a large part of the time.