Impulse bladed turbines, which historically did not produce significant thrust loads, have used reaction blades in the last three stages since the 1980s for increased turbine efficiency. The use of reaction blading in the last stages has significantly increased thrust loads – to the surprise of both end users and designers. To avoid higher than expected thrust pad temperatures in impulse turbines, consider undercutting the turbine shaft to act as a balance drum, and reduce the load on the thrust bearing during the project pre-bid phase (before vendor final priced proposals are submitted).
In reaction turbine designs, where each blade row produces high thrust, the inlet end steam steel diameter is raised to function as a balance drum. Vendor balance drum calculations should be audited during the pre-bid project phase to ensure proper thrust balance under all load and steam conditions.
A balance drum or opposed impeller design reduces thrust force. The total impeller force is the sum of the forces from the individual impellers. If the suction side of the impellers is opposed, the thrust force will be significantly reduced and can approach zero. If the suction side of all impellers are the same (in series), the total impeller thrust force can be very high and may exceed the thrust bearing rating. If this is the case, a balance drum must be mounted on the rotor. The balance drum face area is varied such that the opposing force generated by the balance drum reduces the thrust bearing load to an acceptable value.
The opposing thrust force results from the differential between compressor discharge pressure and the compressor suction pressure since the area behind the balanced drum is usually referenced to the suction of the compressor. This is accomplished by a pipe that connects the chamber to the compressor suction. This line is typically called the balance line.
It is very important to note that a balance drum is used only where the thrust bearing does not have sufficient capacity to absorb the total compressor axial load. And the effectiveness of the balance drum depends directly on the balance drum seal. Fail the seal (open clearance significantly), and thrust bearing failure can result.
A common misunderstanding associated with balance drum systems is that a balance drum always reduces the rotor thrust to zero. This statement may or may not be true, depending on the thrust balance system design. And even if it is, the thrust is zero only at one set of operating conditions. Another misconception regarding thrust balance systems is the normal or active direction of the thrust. In many cases, the active thrust is assumed to be towards the suction of the compressor.
The active direction can change when the turbo compressor has a balance drum or is an opposed design. It is recommended that the use of active thrust be avoided where possible and that axial displacement monitors be labeled to allow determination of the thrust direction at all times.
These monitors detect thrust position by targeting the shaft end, thrust collar or other collar on the rotor. Usually, two or three probes (multiple voting arrangement) are provided to eliminate unnecessary compressor trips. The output of the probes is noted on the monitor as either + (normal) or – (counter). However, this information gives no direct indication of the axial direction of the thrust collar. The following procedure is recommended:
With compressor shutdown, push rotor towards the suction and note direction of displacement indicator.
Label indicator to show direction towards suction of compressor.
Knowing the actual direction of the thrust can be very useful during troubleshooting exercises in determining the root cause of thrust position changes. Many impulse steam turbines designed in the 1980s and 1990s have encountered high thrust bearing pad temperatures. This restricted turbine power and required multiple modifications to reduce thrust pad temperatures to an acceptable level. When vendor modifications helped reduce thrust loads in impulse turbines it was not until rotor modifications were made their values were reduced to acceptable levels (below 110 degrees C or 230 degrees F maximum).
This best practice has been used since the 1990s to finally reduce impulse turbine thrust pad temperatures to acceptable levels without the use of “band aid” modifications (questionable fixes that may not last). This has produced steam turbines with trouble-free thrust bearing operation and thrust bearing MTBFs exceeding 100 months.