Myth: Going bigger with steam turbines

Engineers have a tendency to overdesign and oversize rotating equipment. In past articles, we discussed the impact of unnecessary oversizing for compressors and gas turbines. But in terms of performance and cost, the steam turbine (ST) is most impacted by unnecessary oversizing.

For those not familiar with STs: Steam generated in a boiler enters the turbine at an inlet and chest area and flows through parallel governor valves. The flow is distributed by a flow ring, allowing for partial admission before it enters the control stage.

Subsequently, steam flows through several impulse or reaction stages to extract energy efficiently and leaves through a back-end stage. All of these stages consist of stationary and rotating blades. STs can have side streams. Their discharge pressure can be a defined backpressure or the pressure at the steam condensation temperature.

STs designed for API 612 applications must provide 10% margin above the rated driven equipment power with the lowest possible steam demand. This, as well as the desire to build in safety and degradation margins, often leads to oversizing.

It also reduces the efficiency at normal operating points. An ST driving a compressor must be able to produce at least 110% of the rated power at steam conditions coincident with the minimum inlet, and maximum exhaust temperature and the pressure conditions specified.

An example: The boiler could be dropping off, and the main condenser losing vacuum while the plant process is still trying to maintain 110% of the compressor-rated condition at rated speed. What does this mean for the various components and their sizing?

Inlet & Steam Chest Area

To meet design velocity limits entering the ST, all operating points must be considered, including the rated power at worst case conditions. This maximum flow case coincides with the minimum inlet condition, which has a higher specific volume, resulting in the need for a larger inlet flange and steam chest area to pass the flow at or below inlet velocity limits. This increase in volume area raises the internal surface area, which produces larger hoop stresses, requiring increased wall thickness and other structural design challenges.

Governor Valves

When designing the governor valve opening scheme for a multi-valve ST, it is preferable for the normal operating point (or guarantee point) to be on a valve point to reduce throttling losses.

The valve point is when the valve is almost completely open and immediately before the next valve is about to crack or lift from the valve seat. To meet the rated operating point, the governor valve opening schematic must be modified to ensure all valves can pass the required rated flow at minimum conditions to make the turbine-rated power. This change in opening creates a throttling condition for the normal operating points, reducing the ST’s front-end efficiency.

Control Stage

To mitigate governor valve throttling losses at normal operating points, flow distribution must be improved by manipulating the area in the nozzle ring to provide a distribution favoring a valve point.

This is accomplished by increasing the nozzle height, thereby forcing a lower arc of admission. Additionally, the throat opening can be increased, which moves the velocity triangles away from optimum design and often requires adjusting blade angles.

In both cases, this creates a larger pressure ratio on the control stage and an increase in active arc losses for normal operating points. The larger pressure ratio produces higher dynamic blade stresses from nozzle wakes and partial admission stimulus when entering and exiting the active arc. This requires more robust staging that reduces operating efficiency.

Back-End Staging

A large change in flow rate can have an impact on the volume flow area needed in the condensing section of the LP staging. Since the ST starts with the lowest available inlet steam conditions, this reduces the energy that can be extracted by increasing the exhaust pressure.

Thus, the flow required to make rated power becomes substantially larger. Meeting the required flow rate requires changes to the design of the back-end stages. For example, the last stage will either need to be gauged open or its height increased.

But either option negatively impacts efficiency. A taller blade height also increases centrifugal and bending stresses. Alternatively, gauging open the nozzle and bucket passages reduces stage efficiency.

The impact of these parameters varies based on the type of machine, the power balance and operating conditions. Oversizing is different for straight-through condensing, back-pressure, or extraction- and induction-type turbines. Straight-through turbines are impacted across all design conditions, which can result in flow requirements up to 60% above the guarantee condition.

For an extraction turbine, impact splits performance losses in each section. Extraction/induction machines can see front-end flow oversizing requirements from 10% to 30%, and back-end flow oversizing from 30% to 50%. Clearly, this negatively impacts efficiency. A performance drop of more than 5% is not uncommon.

It is sometimes appropriate to conservatively oversize power margin per API requirements and account for future degradation. But oversizing an ST usually results in an exponential increase in hardware costs and a significant reduction in design performance efficiency. (Adam Neil of Elliott Group significantly contributed to this article). ■