|Lifetime monitoring provisions for piping in power plants are essential
As modern thermal power plants push for ever-greater efficiency, the need for lifetime monitoring of piping systems is more vital than ever, writes Paschalis Grammenoudis of Technip
Lifetime monitoring of HT/HP piping systems in conventional thermal power plants plays an important role.
Current new power plants have significantly high steam parameters and thus a low design reserve. The trend of future power plant projects goes towards increasing these steam parameters in order to achieve even higher efficiency and also to reduce existing conservatism in the design by using new dimensioning concepts.
Consequently, for the safe operation of plants, lifetime monitoring provisions are essential for piping components operating at high temperature range.
The background of the monitoring is the limited lifetime of these components due to creep damage and low cycle fatigue. These damage effects can be quantified by the calculation of the degree of exhaustion which was previously performed in accordance with TRD 301/508. These calculation rules have been replaced by the new European standard DIN EN 12952. The lifetime calculation is made on the basis of the Tresca equivalent stress. This procedure is justified whenever the Tresca equivalent stress is not affected by system loads.
For checking these boundary conditions it is necessary to monitor the piping system. A long-term monitoring of the system behaviour of the piping is performed by means of force and displacement measurements on selected piping points.
The efficiency of hard coal- and brown coal-fired power stations has been improved, among other things, by increasing the parameters of steam pressure and temperature. Figure 1 provides an overview of the pipeline systems with their typical operating parameters and the materials used in the current generation of power plants.
|Figure 1: Example of the HT/HP piping systems in a new power station|
The operating pressures of the new power stations are around 300 bar for the live steam lines and 70 bar for the hot reheat lines. The operating temperatures in these power stations refer to a maximum of 610°C for the live steam and 615°C for the hot reheat lines.
At the same time, a higher flexibility due to the modulation of the power grid is required, so in the future, startup and shutdown processes are expected more frequently.
Because of the low creep strength of the materials used, the high steam parameters cause high demands on the proper functioning of the pipeline system in order to guarantee even allowable creep rupture strength values and limit the exhaustion due to fatigue.
Stresses on piping components arise mainly from internal pressure, dead weight and restrained thermal expansion. In view of the elevated temperature and pressure levels, incomplete relaxation is of major importance as are other possible adverse influences, such as increased friction in pipe supports, which quickly use up the remaining small stress reserves.
If adverse influences are not noticed and are allowed to continue on a long-term basis, premature damage to the highly stressed components of the power station is likely. On-line monitoring of the degrees of exhaustion of thick-walled components, the relaxation of the piping system and the functionality of the hanger system is thus an important precondition for maintaining operational reliability, establishing maintenance and inspection intervals and the safe long-term operation of new power plants.
The objective of the service life monitoring of highly stressed power plant components is to check whether the lifetime on which the design is based is actually attained, or whether reserves for continued operation beyond the design lifetime are available.
Positive influences (increasing service life) and negative influences (decreasing service life) must be taken into consideration. The fact that the actual operating parameters of pressure and temperature are, in general, below the design parameters has a positive effect. Deviations from the ideal component geometry as well as additional loads from the connecting piping system have a negative effect.
The temperature transients occurring during load cycles in the plant (startup and shutdown) also influence its service life in negative or positive ways.
As previously mentioned, these components have limited lifetimes in the high temperature ranges of the new power stations. The lifetime of an individual component is determined on one hand by creep damage/creep exhaustion which occurs as the result of mechanical loading at temperatures of 350°C and higher (depending on the particular material) and which leads to irreversible changes in the material via the stages of micropores, chains of pores and cracks up to the failure of the component in the extreme case.
On the other hand, component lifetime is determined by low cycle fatigue (LCF) caused by thermal stresses in the walls of components during startup and shutdown processes. Damage to the material of the component depends on the level of the stresses and the frequency of the processes producing them.
The effects of creep and fatigue can be quantified by calculating the degree of exhaustion. From records of the operating variables of pressure and temperature, the theoretical component exhaustion due to creep and cyclic stress can be calculated on the basis of the calculation algorithms contained in TRD 301/508 and DIN EN 12952 respectively. The two components, creep and low cycle fatigue, are calculated separately and added to the overall degree of exhaustion due to the linear damage accumulation hypothesis (see Figure 2).
|Figure 2: Example of the calculated degree of exhaustion for a monitored component under operating conditions|
But without a correlation between exhaustion due to creep (as calculated) and damage due to creep (as a measure of the changes in the material), calculation of the degree of exhaustion is not sufficient.
Recent research results also show that this approach is not conservative in any case. Thus, in the context of periodic inspections but at the latest when 60 per cent total exhaustion has been attained, structural examinations and creep measurements must be performed on components subject to creep.
By means of calculated service life analyses which are as exact as possible, the scheduling of the required testing can be optimised, i.e., the date can be delayed as far as possible. This reduces the costs of outages and testing activities.
Creep damage and LCF
The service life of components which are operated in the creep range is limited even under static loading. Above certain temperatures in steel components, creep damage occurs due to thermally induced changes in the microstructure.
The creep range, for the materials used in the power plants currently under construction, starts at temperatures above 525°C. Since the intended use is in this range, the corresponding components must be monitored.
In addition to stresses caused by static pressure and dynamic loads due to pressure surge, etc, especially thick-walled components subjected to varying load conditions undergo stresses which lead to the so-called low cycle fatigue (LCF).
As a result of changes in the pressure, but in particular temperature changes in the component wall, stresses are induced, which in some cases are above the yield strength and may have a significant impact on component lifetime and hence must be monitored.
Therefore, for thick-walled components, the required number of startups and shutdowns and other significant load changes, and the consequently required load change rates (pressure and temperature changes), must be known and taken into account.
Displacement measurements (see Figure 3)supply representative information concerning piping displacements under various operating conditions, and thus allow the operator to check the behaviour of the piping.
|Figure 3: Displacement measurement on a constant pipe support|
For this reason, displacement sensors are attached at points with corresponding significance – large heat expansion or high flexibility – for the movement resulting from the calculations of the pipe system.
As a rule, about two thirds of the sensors monitor the vertical movement and one third the horizontal directions. For each system – main steam and hot reheat – approximately 25 displacement measurements are required. The measurements are carried out via cable line potentiometers as sensors.
The force measurements (see Figure 4) are used to monitor the hanger loads in relation to the components’ dead weight and restrained thermal expansion.
|Figure 4: Force sensor at rigid strut|
In addition, force measurements also made it possible to evaluate the operating hysteresis of the (constant) pipe supports, the friction forces in the guides, and the relaxation conditions of the piping under various circumstances.
The most important X, Y and Z stops are equipped with measurements (approximately 20 measurements in each system). A force measurement includes equipping and calibration of the rods with strain gages in a bending- and temperature-compensating manner.
In order to evaluate the measured hanger load and movements of the pipes, it is necessary to make a comparison of the calculated and predicted values (see Figure 5). Coefficients were derived from the developed algorithms from load calculations (e.g., with ROHR2) and parameter studies, so that the expected values for piping forces and movements could be determined for each operating condition.
|Figure 5: Example of visualisation of measured and calculated values. The green field depicts the expected range|
The values allocated to the system status are, as a rule, the piping pressures and temperatures as well as representative boiler temperatures and any mass flows. In addition to these expected values, tolerance ranges can be determined through error calculation which provides the permissible deviations from ideal behaviour. The following were considered in system calculations: vertical pipe supports without friction influences, pipe supports and guides with friction influences, hanger angularity, boiler house expansion, etc. Piping monitoring makes it possible to constantly optimise the behaviour of the piping, either through targeted servicing measures or by changes in the mode of operation.
Monitoring of piping
In the meantime, there has been comprehensive experience with the monitoring of piping systems in several power stations.
Operating conditions in modern power stations are characterised by increased steam and temperature values. From this it follows that there is a danger of water accumulation in the lower part of the pipe while superheated steam flows through the upper part. This leads to considerable temperature differences over the pipe circumference which sometimes exceed 200°C. This phenomenon is not observed, e.g., in the thermal stress monitoring used so far, because only the temperature difference through the wall thickness of selected components is considered.
Also, very high thermal stress can occur during the injection of cold water. In some cases, the pipes and thick-walled components are subjected to a thermal shock, especially when steam condensation occurs on their inner surface or when fresh water at a lower temperature enters the installation. Because the power stations are subjected to irregular and fast temperature changes, it is necessary to monitor the operation, especially during startup and shutdown.
In power stations, shutdowns are frequently associated with thermal shocks due to small amounts of steam at a lower temperature seeping into a hot pipe. These arise after the ‘fire-out’ and trickle into the connected steam piping.
The piping reacts to small amounts of steam at a different temperature of this type with severe deformations. The amount of deformation is a measure for the global bending stress on the piping; it likewise allows conclusions to be drawn concerning the transient temperature field, and thus the local stresses applied.
Dr Paschalis Grammenoudis is Head of Technical Calculations at Technip Germany GmbH.
This article is based on a Best Paper Award winner at POWER-GEN Europe 2014.
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