Sulzer's prime quality and best-performing centrifugal pumps
A long history of high pressure
Since 1857, centrifugal pumps have been part of Sulzer’s portfolio and are therefore the company’s longest standing product line. Boiler feed pumps, which drive the feed water circuit in thermal power plants, are essential to the stations’ availability. Progress in the design of steam turbines and boilers has subsequently led to major innovations of boiler feed pumps. For the design of high-performance boiler feed pumps, Sulzer Pumps employs a consistent process from product development to manufacturing.
In the last 100 years, changes that were carried out in the design and operation of thermal power plants to
improve efficiency have led to significantly elevated flow rates and pressure levels in feed water circuits as well as more frequent load changes than in the past. Where reliability, efficiency, and operational flexibility are important in power plants, boiler feed pumps (BFPs) are essential. The drive power for BFPs can reach up to 45 MW, making them the biggest single consumer of auxiliary power in the plant. Even though the power consumption of the BFP is small compared with the output of a thermal power plant, this pump is crucial for the plant’s availability.
Increased power concentration
Historically, the development of BFPs [Figures 1 & 2] has substantially contributed to the increase of power output and efficiency of thermal power plants. The capacity for higher head and flow rate, which together lead to an increased power density in the pump, has been crucial to making larger power plants possible. As the power output of the plants rose, the BFPs had to be kept as small as possible, resulting in higher stage pressure and an additional optimization against vaporization of the feed water at the impeller inlet. This performance improvement was only possible by throughly investigating all aspects of hydraulics and mechanics by addressing fluid dynamics, vibration, and mechanical integrity using the most modern tools
and reliable processes.
Building pumps capable of meeting the exacting demands of modern power plants requires not only the best
hydraulic performance but also reliable design features such as high-speed rotor design, axial thrust balancing device, bearings, shaft seals, and static seals. The innovative spirit has been a long tradition
at Sulzer. The company owns the original patent for the disc/counter-disc thrust balancing system, which was
installed in Sulzer BFPs as early as 1905.
One of the world's largest power plants
The efficiency of a thermodynamic cycle increases with operating temperature. Steam power plants that operate with higher water temperatures need to be specially designed. These plants are called supercritical and once-through plants, because water only circulates one time in the boiler cycle. New supercritical designs have efficiencies of up to 48%, whereas subcritical fossil-fuel power plants achieve 36–38% efficiency. Thermal power plants that are more efficient consume less fuel and have lower emissions of carbon dioxide.
For its 2×1100-MW plant in Neurath (Germany), the utility RWE has ordered two BFPs [Figure 3] from Sulzer Pumps with an input of 47.3MW each, which makes them the biggest such pumps in Europe. The lignite-fired Neurath power plant has a high efficiency and considerably lower carbon dioxide emissions per kWh compared with existing lignite-fired power plants. The two once-through steam generators are the largest lignitefired
boilers in the world. They also operate with the highest steam mass flows and temperatures ever reached for lignite. The feed water temperature in the pump can reach 187 °C.
Extremely reliable pumps
New, standardized power plants no longer use redundant equipment, which means that only one main feed pump provides the water for the boiler. In the event of the pump’s failure, the plant can only operate at reduced load using the two start-up pumps. The reliability of BFPs depends on proper application, sound hydraulic and mechanical design, precise manufacturing, shop testing, field installation, pump operation training, and routine maintenance work. To meet the exacting demands relating to dependability, Sulzer performs factory pump tests of all critical-service pumps to correct any potential problems before installation.
The main challenges to BFPs in coalfired power plants are the high pressures and temperatures combined with high flow rates. Sulzer Pumps designed the BFPs for the Neurath plant specifically for operation under such conditions. The pump design features robust impellers; diffusers with thick shrouds; shrunk-on rotor components; a straight balance drum combined with a high-capacity thrust bearing to balance axial thrust; a
full pull-out cartridge for fast change out; and a rugged barrel casing to withstand thermal load cycles, high flow velocities, and external loads.
Specifically designed units
Whereas in the past, thermal power plants used to provide base load power, cycling, or load following operation, has recently become a standard requirement. This operation involves rapid variations of BFP flow and temperature, as well as frequent stops and start-ups. To fulfill these requirements, BFPs must be specifically designed and built to be tolerant of changing operating conditions.
In order to build pumps that push these limits, it is imperative that the designers acquire detailed knowledge of cavitation, prediction of hydraulic performance, structural strength, and deformation as well as dynamics of rotor and structure. Leading-edge design tools, which contribute greatly to the success of advanced pump designs, paired with a wealth of experience from pumps in operation, help Sulzer meet these requirements. Modern numerical tools, such as computational fluid dynamics (CFD) [Figure 4] or finite element mechanics (FEM), support design and evaluation of a new pump early in the product development phase. CFD predicts hydraulic performance, including losses, operational behavior, and cavitation. Cavitation of a fluid is the formation of vapor bubbles in a region where the pressure falls below the vapor pressure. These bubbles can block the flow channel, compromising the pump performance, or they can collapse in regions of higher
pressure and cause significant damage to the pump. Flow quality at the impeller inlet strongly influences the suction performance of large multistage BFPs. Engineers optimize these elements using 3D parametric models coupled with modern CFD codes. Designers can, for example, determine the best size, location, and
number of ribs in the inlet casing in order to obtain the most homogeneous flow possible with minimum losses at the inlet of the suction impeller. Flow visualization in virtual reality validates that the flow pattern is homogenous over the entire area of the impeller eye before the casing is manufactured.
The design of modern high-performance pumps involves pushing the physical envelope. One of these limits is cavitation development, which is associated with erosion problems and impairment of pump performance. Precise and reliable methods for the prediction of cavitation are therefore a must. All modern CFD codes have cavitation analysis capability. However, the methods used in these codes rely on two-phase flow models, which require an unsteady approach. This method is very time consuming and therefore not suitable for the design phase of new suction stages. Hence, Sulzer has developed a more efficient and faster means of cavitation prediction. This methodology considers that the coupling between main flow and cavitation is negligible, which is valid for moderate cavitation development.
A validation of this approach showed that this faster method gives results very close to those of the commercial code with regard to location of the cavitation onset and length of the attached cavity. Accurate prediction of the cavity length also allows the potential associated erosion to be forecasted. It is therefore possible to pre-estimate the impeller life for given suction pressure and material combinations on the basis of the numerical prediction of the cavitation development.
In addition to confirming the overall hydraulic performance of the pump, numerical prediction of the flow helps the hydraulic designer assess the interaction of rotor and stator and associated pressure pulsations. These pulsations can be very strong and may cause damage to the impeller or the diffuser vanes. Pressure fluctuations can additionally be the source of noise and vibration. It is therefore essential that design engineers have the capability to analyze unsteady flow in the full stage.
Models rapidly built and tested
Scale models of the newly developed hydraulics are systematically tested to verify performance before the first proto-type is build. So that Sulzer can achieve short development times and reliable designs, the model pumps are produced from aluminum plates using multi axis CNC machining [Figure 5]. Through a window in the pump casing, engineers can fully observe cavitation development and thus confirm the cavitation inception predicted by CFD. It is also common to machine some impeller vanes out of plexiglas to facilitate observation of cavitation at the vane pressure side, which would otherwise not be possible. Pressure-side cavitation is very aggressive at over-load operation. Once the design is proven by testing, the hydraulic profile is transferred to the mechanical design group—a straightforward process, as all hydraulic-design tools have full 3D capability.
3D mechanical analysis
High pressure in the pump casing leads to high deformation forces. The integrity of the bolted delivery cover is critical to safe and reliable operation. Generally accepted design codes provide guidelines for wall thickness, flange thickness, and bolt size. However, these codes, even when combined with the designers’ experience, do not provide enough infor-mation about stresses and deformations in critical areas. With increasing pump size and operating pressure, this information is crucial to the success of the design. Therefore, a FEM analysis is required. Because the cover is cyclically symmetric, only a sector of the circle is modeled in 3D for the finite element calculation. The cover and associated parts—like bolts, nuts, and balance drum liner—are analyzed for stresses and deformations. The calculated values are then compared with the allowable values for the design load and all other specified operating conditions.
Ensuring secure static seals
Deformation in the area of the profile seal between the barrel and the cover is of particular interest. The extrusion gap and metal-to-metal contact at design conditions determine if integrity is achieved during long-term operation.
Another point of interest is the influenceof deformation on the clearance between the balance drum and the balance-drum liner at operating, start-up, and shut down conditions. The minimum clearance and its distribution over the balance drum length need to be checked; this analysis may lead to design changes if the results fail to meet allowable values.
Start-up, shutdown, and load changes are critical operating conditions for boiler feed pumps, because the pump has to run through thermal transients and speed changes. Plant system design and operating schemes determine whether a warm-up or a combined warm-up cooldown system should be installed. In large modern power plants with sliding pressure de-aerators, pumps are often actively cooled during a load trip to avoid thermally induced water hammer in the pump suction system. This additional safety feature can be achieved with a combined warm-up and cooldown system, which allows the forced cooling on trip or shutdown as well as the slow warming of the suction system, booster, and main boiler feed pump prior to startup.
Nevertheless, some pumps have to undergo severe thermal shock conditions such as start-ups, in which hot boiler feed water enters the cold pump. In such cases, temperature jumps from 20 °C to 180 °C are possible. Thermal and mechanical finite element analyses [Figure 6] help the designer to understand how the pump behaves under such extreme circumstances and allow optimization of the design ensuring safe operation. Sulzer has developed innovative solutions for the next generation of boiler feed pumps.
Mastering thermal shock
The reduction of the narrow clearances in the suction-side labyrinths of impeller and balance drum during thermal shock is of particular interest. The impeller temperature follows the hot feed water temperature while the casing is still cold. The difference in thermal expansion thus reduces the running clearance. A Sulzer patented elastic design integrated into the support of the stationary wear rings allows fast thermal expansion with little restriction.
The opposite situation occurs in the interstage labyrinth between the impeller and diffuser. During a steep downward temperature transient, the temperature drops faster in the labyrinth, which is shrunk into the thin-walled diffuser, than in the impeller, which is connected to the thick shaft. Consequently, the clearances decrease at this location during cooldown. The shrink fits of the impellers and the balance drum also have to be examined regarding their behavior during temperature changes. It must be ensured that these parts do not loosen during upwards thermal shock and that the stresses do not exceed the allowable limits during cooldown. Exceeding the yield strength in these mounted parts must be avoided, as plastic deformation would reduce the shrink fit, and the parts might become loose. Finally, all stresses, especially those due to differential thermal expansion, are checked.
To meet the requirement for high availability, factory-based pump tests are conducted prior to shipping [Figure 7]. Engineers test the pumping equipment for many operating conditions, such as reduced suction pressure, full-train operation at variable speeds, and hot transient conditions. Hydraulic performance, bearing temperatures, vibration, and noise levels are measured to discover any potential problems and prevent them from occurring in the field. Full test capabilities are essential services to prove performance and reliability of pumping equipment. The Sulzer test bed in Leeds is the largest in Europe. With a 30-MW gas turbine drive, almost any pump can be tested at full capacity and full speed using the same drivers—gas turbines or electric motors—as in the plant.
Simulation ensures safety
Today, modern simulation tools and consistent methodology are fundamental when developing high-performance pumps. These tools can be of maximum benefit if they are based on design experience and are used by experienced pump engineers. Another key advantage of these tools is that they can be applied at an early stage of the project, whenadaptation of the hydraulic or mechanical pump design is still possible. A leaky cover connection detected on the test bed would cause weeks of delay, as it takes considerable time to develop and manufacture a solution for such big casings. Discovering unacceptable cavitation in the suction impeller on the test bed would cause even longer delays. It could easily take months to design and manufacture a new suction impeller with a corrected hydraulic.
Finally, modern tools can reduce the risk of failure. Performance-critical units, such as boiler feed pumps in thermal power plants, have to operate reliably over an extended period of time. To ensure this operating behavior, all possible load cases, even of those that cannot be studied on the test bed, must be examined. Modern simulation tools allow client and designer to acquire maximum levels of information about a new pump prior to manufacturing and commissioning.
These modern design tools are therefore essential for the design of reliable tailor-made, high-performance pumps both today and into the future.
Gerdes, Ralf et al. "A Long History of High Pressure." Sulzer Technical Review 2+3/2009: 9-15.