Recent technological advances in mechanical and aerodynamic design of the turbine steam path are resulting in higher reliability and efficiency.
By Kenneth Potter, P.E., and Diana Olear, P.E., General Physics Corp.
Historically, utility steam turbine design has been driven by the need for high reliability and efficiency. This remains true today, as the current marketplace creates a financial incentive for still higher efficiencies in new units and upgrades to existing equipment. The desire for greater efficiency and capacity often justifies OEM development of new features that achieve increased efficiency and capacity without compromising reliability. Recent technological advances in mechanical and aerodynamic design of the turbine steam path make these improvements possible.
A recent study conducted on a 390 MW, pulverized coal-fired unit revealed just how much these new technological advancements can improve efficiency and output. The empirical study showed that the turbine upgrade raised high pressure (HP) turbine efficiency by 5 percent, intermediate pressure (IP) turbine efficiency by 4 percent, and low pressure (LP) turbine efficiency by 2.5 percent. In addition, the unit’s highest achievable gross generation increased from 360 MW to 371 MW.
HP/IP Dense Pack
Drawing on gas turbine and aircraft engine technology, engineers developed a new steam turbine design with higher reaction, lower pitch diameters, longer blades and often an increased exhaust annulus area. In addition, the designers increased stage count, generally without increasing the overall length of the machine, giving rise to the name “dense pack” (Figure 1).
Figure 1. Baseline (left) and Dense Pack (right) Test Rotors. Photos compliments of GE.
These changes lead to a significant reduction in flow-through velocity with a corresponding drop in profile losses, while the improved blade aspect ratio reduces overall secondary losses. The lower velocities allow smaller pitch diameters, which are made possible by advances in rotor-dynamic technology. Developments in aerodynamics, manufacturing processes, advanced seals and leakage flow control also increase turbine efficiency.
Last Stage Buckets
As steam expands through the turbine and energy is extracted, steam specific volume increases and greater exhaust annulus areas are required to efficiently pass the required flow to the condenser. This leads to larger bucket heights to correspond with larger nozzle sizes to the point where constant axial velocity no longer applies. Because the velocity triangles change along the radial height of the buckets, the last stage blades are contoured for the optimal inlet angle and pressure distribution along the entire height.
In the 1960s, a mathematical technique known as the “Schwartz-Christoffel Hodographic Laminar Incompressible Conformal Transformation” was used to design a class of profiles called SCHLICT buckets. These taller buckets are referred to as SCHLICT Vortex or SV buckets. Older designed tall buckets were double-taper type or DT buckets, and nearly all of the old DT style buckets have a modern replacement SV equivalent that offers increased efficiency. To gain the maximum benefit of the SV bucket, these stages should be accompanied by a nozzle with similar vortex characteristics, such as accounting for radial changes in pressure distribution and axial velocity.
The last stage of the LP turbine produces about 10 percent of the turbine power output. Unfortunately, because the LP turbine is subjected to wet steam conditions, erosion has been a major problem. The latest LP turbine designs use better moisture removal devices to reduce this problem while simultaneously improving efficiency and availability.
The study discussed here is based on a utility that recently upgraded one of its unit’s HP/IP turbines to a dense pack and installed a new LP rotor with last stage buckets. In this case, the existing 26-inch last stage bucket was replaced with a new 26-inch bucket with a more efficient profile.
The scope of the study included examining and comparing eight to 10 days of high-load process and calculated plant data, recorded shortly before and after the turbine upgrade. The data were recorded, calculated, stored and retrieved using General Physics’ EtaPRO Performance Monitoring System. Using EtaPRO, empirical data were collected and analyzed to compare turbine performance before and after the upgrade.
Figure 2 clearly illustrates that the upgraded turbine uses less steam per MW of power produced. This reduced steam rate translates into an equivalent reduction in heat rate. At 360 MW, the original turbine used 2,586 kilo pounds per hour (kPPH) of throttle steam. However, the upgraded turbine uses only 2,546 kPPH of steam to produce the same 360 MW. This is more than a 1.5 percent reduction in steam production required to make 360 MW, which translates into a 1.5 percent reduction in turbine cycle heat rate. With net turbine cycle heat rate approximately 9,000 Btu/kWh, generating 360 MW required 3,240 MMBtu/hr from the boiler. With a boiler efficiency of 89 percent, this mandates 3,640 MMBtu/hr from the fuel. After the upgrade, 1.5 percent less steam is required, which is 3,191 MMBtu/hr from the boiler and 3,586 MMBtu/hr from fuel, which is 54 MMBtu/hr lower than before the upgrade. Assuming a fuel cost of $3/MMBtu, this translates into a savings of $162 per hour. If the unit is available 85 percent of the year and the average load is 90 percent of maximum capacity, the annual fuel savings are greater than $1 million.
Not only is the turbine using less steam to make the same power, but it is also making more power with the maximum steam flow. The boiler for this unit can produce roughly 2,600 kPPH of throttle steam at maximum capacity, which enables the turbine to produce about 360 MW. After the conversion, the same 2,600 kPPH (and likewise the same amount of fuel) produces 371 MW. Assuming a sale price of $45/MWh, this results in an additional $495 per hour of revenue. Using the same assumptions as above (85 percent availability and 90 percent load factor), this upgrade results in a potential $3.3 million increase in annual revenue. With a capital investment of roughly $5 million, this upgrade should pay for itself in about a year and a half.
There were marked increases in both HP and IP turbine efficiencies after the upgrade. The original HP turbine was designed to operate with an efficiency of 84 percent at valves wide open, while the upgraded HP turbine is close to 90 percent efficient. The IP turbine, originally designed to be 89 percent efficient, is now operating close to 97 percent efficient. (Both IP turbine efficiencies are uncorrected for the leakage flow of first stage steam to the IP turbine bowl through the shaft packing between the HP and IP sections of this common-casing turbine.)
Figure 3 illustrates that turbine cycle heat rate improved substantially across the load range after the upgrade. Not only does heat rate improve, but the optimum heat rate occurs at a higher flowrate. This trend indicates that optimal heat rate before the upgrade occurred at a throttle flow of about 2,250 kPPH, which corresponds to about 320 MW of gross generation (Figure 2). The trend after the upgrade indicates that the optimal heat rate occurs around 2,700 kPPH of throttle steam, but the boiler cannot quite meet that demand at present. The increased potential for greater power output requires greater steam generation from the boiler. In many cases, this increased turbine capability is unused due to capacity limitations of the existing boiler.
As such, this particular utility has already scheduled boiler modifications during the next outage to increase capacity and deliver more steam to the turbine cycle. Although the mills and burners can currently handle the increased demand, targeted areas of the boiler upgrade include superheater and reheater tube bundles, and increased ID fan capacity.
The dense pack upgrade focuses on design changes that are most likely to achieve increased capacity and efficiency. Key features include more stages with smaller blade pitch diameters, but with a negligible reduction in rotor shaft diameter. Stage reaction is optimized for high efficiency while minimizing internal leakage. Blade exit angle is also slightly decreased to increase blade height.
A comparison of individual loss mechanisms shows how the decreased velocity and improved aspect ratio have directly improved the profile and secondary losses. Advances in mechanical design and analysis allow these features to be implemented without compromising mechanical strength or reliability. The design is suitable for steam turbines covering a wide range of applications. Steam turbine dense pack units are suitable for both the combined-cycle and the large fossil-fired markets.
The steam turbine upgrade discussed here was clearly a good investment. While improved efficiency clearly results in fuel savings, the additional benefit of increased generating capacity provides the potential for increased revenue, often at lucrative times. Not only does this upgrade have a quick return-on-investment, but it also increases the reliability and longevity of the turbine, according to the manufacturer.
Kenneth Potter has a master’s degree in mechanical engineering from Penn State University. Mr. Potter is the application engineering supervisor in General Physics’ Performance Engineering Business Group. Mr. Potter is a project manager and a key developer of engineering methodologies used by General Physics’ EtaPRO Performance Monitoring System. Mr. Potter is a Gulf War Veteran of the U.S. Navy and a registered Professional Engineer in New York.
Diana Olear has a bachelor’s of science degree in mechanical engineering from the Polytechnic Institute of Minsk, Belarus. Ms. Olear is a senior application engineer, also in General Physics’ Performance Engineering Business Group. Ms. Olear has more than 20 years of experience in power plant design, commissioning and performance monitoring. Ms. Olear is a registered Professional Engineer in New York.