By D. Band, T. Benten & J. Stahlhut, Balcke-Dürr GmbH, Germany
In Germany, high-efficiency coal fired power plants are traditionally fitted with header-type heaters. Low-pressure heaters were introduced in the 1920s, but high-pressure heaters did not come into operation until the between 1940 and 1955, following the construction of unit-type power plants. The low-pressure heater was based on a shell-and-tube condenser, while designers of the first high-pressure heater also took their bearings from the design of water-tube boilers.
Header-type heater for DONG Energy’s Avedöre 2 450 MW CHP plant in Denmark, which was commissioned in 2000
Two types of heater with different fields-of-use developed from these origins in Germany. The U-type heater, which is used for low tube-side pressures and flow rates and the header-type heaters. which are predominantly used for high pressures and flow rates. A heater design with header pipes instead of tubesheets allowed a much better thermoelastic operating behavior during start-up, turbine trip or cycling power plant operation. Thus, header-type heaters have established themselves as the technology of choice in the high-pressure range of German power plants
Triggered by the imminent coming into force of the National Allocation Plan 2008-2012, around 30 coal fired power plant units, with nominal outputs of 600 MW to 1100 MW, and a total output of 26 GW are currently being built in Germany. The new power plants will set new standards with efficiencies of over 45 per cent while offering flexible power plant operation and high availability, and all of them will be fitted with header-type heaters.
A complete high-pressure heater train usually comprises three header-type heaters and a separate desuperheater. They are usually installed upright, seldom horizontally, and normally have three or four water passes depending on the piping system. The world’s biggest header-type heaters are used in both 1100 MW units F and G of the lignite fired power plant in Neurath (BoA 2 and 3).
Global technology trend
High-pressure header-type heaters have also been used for some time in Denmark. Coal fired power plants with unit outputs of 300 MW to 600 MW, such as Avedöre, Ensted or Nefo have been operated successfully with header-type heaters for decades.
The situation in South Africa is similar to that in Central and Northern Europe, where many conventional power plants are traditionally equipped with header-type heaters. Unlike the normal arrangement with only one high-pressure heater train, the South African header-type heaters are installed in two parallel piping trains with two high-pressure heaters each. This is because of historical reasons.
One of the first large-scale power plants to be equipped with header-type heaters was Duvha, which went into operation in 1975 with an overall capacity of 6 x 600 MW. This example was followed, amongst others, by the power plants in Matla, Lethabo, Majuba and Kendal, which with its 6 x 686 MW is currently the biggest coal fired power plant in the world.
The dynamic economic growth of the past few years with an unexpectedly high demand for electricity has greatly depleted the South African power plant capacity reserves. Because of imminent power failures, a government resolution was passed to build new coal fired power plants between 2008 and 2018. These new plants, with an overall capacity of over 20 GW, will be planned with header-type heaters because of the good operational experience gained with this technology.
In contrast, high-pressure header-type heater technology was largely unknown in the USA for many years. Coal fired power plants there were operated exclusively with U-type feedwater heaters up until the 1990s. However, serious damage occurred in several U-type heaters after only 12-15 years of operation, exacerbated by the cycling operation of the power plant. This was caused by stress-induced cracks in components with thick walls, with a particular trouble spot being the connection between the tubesheet and waterbox. The damage led to reduced output, numerous unscheduled downtimes for repairs and often the replacement of the heaters.
With this in mind, a study was launched in 1990 by the Electric Power Research Institute (EPRI) with the aim of identifying ways of avoiding damage, and therefoire significantly improving the availability of the USA’s coal fired power plants, with a simultaneous reduction in operating costs.
The EPRI study underlined the technical and long-term economic advantages of header technology and described the benefits of a conversion to header-type heaters. The first U-type heaters in the 100 MW Glennwood 5 power plant in New York were replaced by header-type heaters in 1991. Other heaters followed in the 500 MW Sioux 2 power plant in Missouri in 1996.
Supported by the Energy Policy Act 2005 (Energy Bill), around 120 new coal fired power plant projects were applied for in the United States. Plant concepts based on header technology exist for some of them. However, it is doubtful whether these concepts will catch on because the manufacturers of feedwater heaters in the USA have relatively little header technology know-how, and are therefore more likely to defend the traditional U-type heaters in their own economic interests.
Two countries that are experiencing a steep rise in demand for electricity, and are greatly influenced by American power plant technology are China and South Korea. Here, however, there is a noticeable change in the way of thinking, with a growing interest in header technology. The Korea Power Engineering Company (KOPEC), for example, the planning company for the South Korean power plant operator KEPCO, describes the technical and economic advantages of header-type heaters over U-type heaters in great detail in its 2006 study “Next Generation Thermal Power Plant”.
Header types offer technical advantages
The two types of heater, U-type and header-type, are fundamentally different in regard to the separation of the tube and shell sides and the design of the heat exchanger tubes. In the U-type heater, the feedwater and heating steam are separated by the tubesheet and a bundle of U-tubes. In header-type heaters they are separated by headers and a bundle of snake-shaped tubes. The feedwater in U-type heaters passes through the inlet side of the waterbox, the U-shaped tubes and finally the outlet side of the waterbox, while the feedwater in header-type heaters initially enters the inlet header, passes through the snake-shaped tubes in three or four passes and leaves the unit via the outlet header.
High-pressure heaters have to withstand an internal pressure of between 300-400 bar on their tube side in supercritical steam power plants. The mass flow of the feedwater ranges from 400 kg/s to 800 kg/s. Under these boundary conditions, the pressure-bearing components of the heater tend to have very thick walls that react quite sluggishly to short-term changes in temperature. This leads to secondary stresses, which rise with an increasing and differing wall thickness of the components.
In a U-type heater the tubesheets have the highest wall thickness. In high-output power plants the tubesheet thickness can be between 400 mm and 800 mm depending on the design data. Tests have shown that wall thicknesses over 500 mm are regarded as critical.
Very high peak stresses are induced through transient temperature gradients at the connection points between the thick tubesheet and the relatively thin waterbox shell, with wall thicknesses above the critical value. The consequences are mostly cracks in the areas of the transition between the tubesheet and waterbox. This type of damage has been investigated by metallography and FE analysis. The results of the metallographic tests and FE analysis confirm that the cracking is caused by “expansion-induced corrosion”.
The thickness of the tubesheet wall has to be reduced to reduce the stresses. This is why U-type heaters are arranged in two trains in the high-pressure area, in other words twice the number of units in two parallel piping trains. The feedwater mass flow through the heater is halved, and the necessary shell diameter and tubesheet thickness reduced.
Nontheless the thermoelasticity of the U-type heater remains relatively low. Maximum temperature gradients of 5 K to 10 K per minute only allow a very slow start-up and shutdown of the power plant, and a very limited number of possible load changes. Thus, operation in the cycling mode should be avoided. U-type heaters in supercritical power plants often have a service life of only 10 to 15 years, and have to be replaced up to three times with an average power plant lifetime of 40 years.
In comparison, header-type heaters with their excellent thermoelasticity have a number of advantages. Unlike the tubesheets in U-type heaters, the inlet and outlet header of a header-type heater have walls that are between 70-120 mm thick. Accordingly, the necessary header wall thickness under the same basic conditions is only around 15 per cent of the tubesheet thickness.
Geometric discontinuities in the transitional areas between the header and shell that favour the occurrence of unacceptable stresses are completely avoided through the low wall thicknesses and wall thickness differences, thus negating the risk of damage through thermal cracks. As shown in the comparison in Figure 1, lower stresses occur at the transition header/shell of header-type heaters than in the critical transition area of the tubesheet in U-type heaters.
Figure 1: Thermal stresses in the critical transition areas
The thermoelastic design of high-pressure header-type heaters allows a flexible mode of operation for power plants with a large number of load changes. Maximum temperature gradients of up to 25 K per minute permit a fast start-up and shutdown of the power plant with service lives of between 35-50 years. In addition, the heater section does not necessarily have to be designed as a two train system.
Return on investment
An investment in a header-type heater pays off, as shown in Figure 2, in a power plant with an output of around 500 MW. Both header-type and U-type heaters are designed as single-train systems below this power plant output. Header-type heaters can also be used with higher power plant outputs (currently up to 1100 MW) in a single-train arrangement, while U-type heaters are usually designed as two-train systems with power plant outputs of over 500 MW because of the increasing risk of damage due to thick-walled tubesheets. The double piping, double the number of U-type heaters and increased space requirements of the two-train design create additional investment costs. The investment costs for U-type heaters escalate because of these additional components, and normally exceed those of equivalent header-type heaters.
Figure 2: Heater arrangement depending on investment costs and power plant output
The operating and maintenance costs of header-type heaters are also much lower with higher power plant outputs. The lower failure rate and up to four times longer service life of header-type heaters produce a higher availability of the power plant and much lower maintenance costs and work. This is also confirmed by the results of an EPRI study, shown in Figure 3, which investigated the frequency and causes of damage in almost 200 high-pressure heaters in 51 power plants (35 in the USA and 16 in Europe). European power plants displayed much better results because of the widespread use of header-type heaters. The high vulnerability of U-type heaters in comparison called for frequent repairs and often complete replacements.
Figure 3: Cause and frequency of heater damage in the USA and Europe (source: EPRI)
Global shift towards header type
Header-type heaters are mainly found in Central and Northern Europe and South Africa, but are being increasingly considered in the planning concepts for new supercritical power plants in the USA, China and South Korea. Around 30 coal fired power plant units, ranging from 600 MW to 1100 MW are currently being planned or built in Germany with header-type heaters.
Header-type heaters allow a much more flexible mode of operation of the plant with higher load changes than U-type heaters, and are by far the more cost-effective solution in high-output power plants in terms of both the investment as well as operating and maintenance costs.