Analysis of superheater tubes failure

Failures of boiler pressure parts, which working in high temperature and pressure conditions are often caused by overheating or corrosion. These two parameters are decisive, but not the only ones. Local stress concentration also depends on the type of headers support and external loads from pipelines. Boiler pressure parts subjected to all loads mentioned before are steam superheaters. Thermal expansion, high pressure and temperature lead to shortening superheaters lifetime. In the places with significant stress caused by all load combinations it is difficult to predict creep strains and material structure changes. This paper shows superheater in which considering external loads from pipeline and their influence on the stress concentration in the superheater tubes. This article also shows steel S304H creep analysis for 100k [h] results and creep equation with experimental developed constants.


INTRODUCTION
Many boiler pressure part damages were caused by unpredictable working conditions, e.g. material overheating, corrosion and erosion. There are not only cracking and rupture causes. Most of boiler pressure parts work in predictable conditions. Main design parameters determining pressure part durability are temperature and pressure. In the predictable cases, other loads were omitted. Examples of these are additional loads from piping, unbalance loads caused by mistakes during maintenance, simplifications during the design process [2]. Effect of those loads often causes local concentration of stress. Boiler elements subjected to the all combination are just steam superheaters [3]. These elements of boiler are characterized by work in high pressure and temperature parameters. In addition, they are subjected to external loads from pipelines, which affect their lifetime shortening. One of the most weakened part of superheaters are coil tubes.
In those tubes it is very hard to predict strain value due to creep phenomenon, and to predict changes in the material structure and lifetime [3].
Exemplary damage coil was shown in figure 1. Tubes rupture occurred in the place of crossing closure plate of superheater chamber. Tubes in the outside of chamber show significant deflections.
Due to the influence of external forces on the work of superheater tubes, it is necessary at the design stage to take into account all additional loads that may cause creeping intensification. Also during operation, pipeline working conditions should be examined to avoid any other tensions in superheaters area. Neglecting these works can cause additional loads in coils and headers, often larger than in design phase.
In this paper it will be shown analysis of the pipeline system affecting the superheater, influence of external forces on stress concentration in superheater tubes and creep analysis results for material S304H after 100,000h working time. Also for this purpose, the paper will propose a creep equation whose constants have been determined experimentally [3].

ANALYZED SYSTEM DESCRIPTION
The analyzed superheater is exposed to loads from outlet pipeline supplying steam to the turbine. For analysis purposes, a part of pipeline (with boundary conditions) with the biggest impact on the loads on the outlet header of superheater has been taken into account.

STEEL S304H MATERIAL PROPERTIES
Steel 18Cr-9Ni-3Cu-Nb-N (according ASME SA-213) , X10-Cr-Ni-Cu-Nb-18-9-3 (according VdTUV 550) also known as S304H is the material for steam superheater due to its good strength properties [4]. Chemical composition is shown in tab. 1. The use of this material for the superheater tubes affects the possibility of loading them with high temperature and pressure. Creep strength is one of the parameters that affects the trouble-free operation of superheaters.

BOUNDARY CONDITIONS AND LOADS
The global analysis was performed in the AutoPipe software [6], including pipelines and suspension with the greatest impact on the forces at the point of connection with superheater header. This kind of software uses 1D finite elements. This approach allows quick model verification and identification of the most loaded parts of the superheater.

STEAM SUPERHEATER ANALYSIS
In the first stage of the analysis, a standard approach to the calculation of cylindrical elements (such as superheater tubes) subjected only to internal pressure was taken into account. In accordance with the requirements of standards EN 12952-3 [7] and EN 13480-3 [8], the basic calculations are limited to the dimensioning of tubes due to the circumferential stresses due to pressure.
The hoop stresses for the Ø51x11 coil can be calculated from the converted formula on the maximum pressure in the cylindrical body (1)  what mean that the thickness of the tube was selected correctly due to the assumed pressure load. These calculations do not take into account additional loads from pipeline. In the second stage of the analysis, the work of pipelines with optimal settings of the constant hanger suspensions was taken into account. This calculations were also made in the Auto Pipe software. The results of these analyses showed that the most stressed elements for working conditions are the superheater tubes in close area of closure plate. Fig. 5 shows the equivalent stresses for the described operating state.
The maximum reduced stresses in tubes are σHMH = 147 [MPa] and are lower than the yield stress Re650 = 160MPa. This means that the strain of the tubes is at the level of 92% for the elastic state.
Taking into consideration the long boiler operation time and creep of the S304H material, in steam superheaters, there may be significant strains of the tubes, which consequently excludes the superheater from further work. The value of equivalent stresses is higher than the creep strength for 100,000 [h], therefore it requires further analysis using the ANSYS software [9] using the proposed creep equation (2) [3,10,11,12]:  The Solid 185 type finite elements were used to build the model in the ANSYS software. The preliminary analysis in the ANSYS software was based on a typical solution for the theory of elasticity. Due to the narrow scope of analyses (only one tube with the greatest strain), the submodelling technique [13] was used to build the model, in which boundary conditions were assumed in the form of reaction obtained from analyses in the Auto Pipe software as in Fig. 6.
The results of the analyzes for the time t0 = 0 [h] are shown in Fig. 7 and

CONCLUSIONS
The article presents the impact of the steam pipeline on the superheater, in which concentration of stress occurred in the tubes with high pressure and temperature. Performed analyses for the initial time showed that additional forces cause an increase in stresses by 71 [%] in relation to the circumferential stresses calculated in accordance with EN 13480-3. It has also been shown that omitting additional loads in the analyses may result in incorrect estimation of the trouble-free operation of superheater. It has been shown that stresses for 100,000 [h] do not meet the requirements of EN 13480-3.
The paper presents a creep curve prepared on the basis of analyses using the proposed creep equation. The results of these analyses showed that the material S304H for time t1 = 100 000 [h] is in the second period of creep, and extending the service life to 200,000 [h] may cause the destruction of the tubes due to entry into the third creep stage.