Lifetime Assessment of a Steam Pipeline

The aim of this paper is to design a method for assessing the life of steam pipes for Czech power plants. The most widely-used material in Czech power plants is steel 15 128. Our findings may also be applied for international equivalents of this steel. The paper shows the classification of cavitation damage and microstructure classification status, based on the German VGB Act, with references to EPRI law in the USA. Calculations of remaining life on the basis of Russian experience are also shown. The possibility of applying this method to increase the operating parameters for power plants is discussed.


Introduction
A switchover to high-parametric power plants requires an assessment of the remaining life of existing steam piping.In creep conditions, this requires a large amount of information on operating conditions (temperature, pressure, time), material characteristics (microstructure, creep strength, creep strain rate, etc.)It is also important to monitor the stress in exposed parts and weld joints.The greatest influence on component life is the formation and joining of internal defects, i.e. cavities.It is therefore necessary to provide a sophisticated method for observing and evaluating internal defects.This paper deals mainly with steel type 15 128, because the vast majority of the steam piping currently operating in the Czech Republic is made from this material.
The main use of this steel is for steam piping, superheaters and boiler tubes operating at temperatures up to 580 • C.
The microstructure of steel in the initial state depends on the heat treatment that it has undergone.The initial state of 15 128.5 has a ferritic-bainitic microstructure with fine dispersion of globular carbides of M 4 C 3 or MC type, respectively, precipitated in the ferritic matrix.The initial state of 15 128.9 is formed by a fine carbide-bainitic microstructure.The two initial states differ in their mechanical properties; see Table 3 [1].

Density of inclusions
Size and type of grain

Monitoring the operating parameters, service life management
The main parameters that should be monitored over time are the temperature and the pressure of steam in the steam pipeline.Regular monitoring of the operating parameters several times a year is most advantageous in terms of creep lifetime.

According to epri (USA) [2]
A three-stage approach is used for evaluating the lifetime of steam pipelines.At each stage, the estimated remaining life and the desired service life of the steam pipeline are compared.STAGE 1: Includes general calculations based on operating history and especially exploring the pos-sibility of degradation of components.The main contents of this stage are the relevant technical drawings, material properties, operating hours and cycles, history of inspections and maintenance, failure history (details of failures and repairs to failures), operational parameters and their maximum values (temperature and pressure).
STAGE 2: Includes non-destructive testing of components, the results of which can improve the evaluation of the lifetime in STAGE 1.For tests carried out at this stage, see Table 4. Visual examination includes observation of geometric inaccuracies (e.g.buckling).This includes geometry measurements (wall thickness, ovality) and measurements of the position of selected hinges and supports.If cracks are found by capillary tests, an ultrasonic examination is to be made.The replica method provides a preliminary estimate of the lifetime of the steam piping (welded joints).4. The high cost of this type of evaluation needs to be taken into account.(For example, is it better to replace or repair a part of the steam piping, or is it better to operate under lower conditions?) According to [3], the replica method can be used on properly prepared surfaces.Two to three replicas should be taken from each site.Microstructure and cavitation (creep) damage is evaluated on the basis of an evaluation of the replicas.Figure 1 shows a creep curve with states of cavitation damage marked on it.The structural condition is assessed according to the etalons.Resistance to corrosion attack can also be assessed with the use of electrochemical polarization measurements.

Operation and inspection
On the straight parts of steam pipelines operating at temperatures from 450 to 545 • C it is necessary to measure the residual deformation 200 thousand hours after the pipeline came into operation.For steam pipelines operating at temperatures from 546 to 570 • C, analogous measurements should be performed after 150 thousand hours.If the residual de-formation exceeds 0.75 %, an assessment of the material is made in terms of mechanical properties and chemical composition.
On the bent parts of the steam piping, measurements of the residual deformation, magnetic powder and ultrasonic flaw detection must be performed after 150 thousand hours.In operating temperature ranges from 546 to 570 • C, this measurement is performed after 100 thousand hours.
For welded joints, the degree of fatigue life τ ho /τ lp is evaluated in accordance with the range of structures and their microdamange for welded joints (τ hohours of operation; τ ls -limit service life at the stage of microcrack discovery).The residual service life (τ rs ) can be calculated from the difference τ rs = τ ls − τ ho [4].Typical damaged places of welded joints in the creep region are found mainly along the outside of the heat-affected zone of the material.

Service life calculation
The lifetime of a steam pipeline operated in the creep region is assessed according to the degree of microdamage within the structure of the material.The basic parameter for calculating the lifetime is the number of micropores in a unit area of the metallographic scratch pattern (replica).Figure 2 shows an example of data processing for steel 12Ch1MF at 600 • C. For a reliable extrapolation of long-term strength at 100 000 hrs, it is necessary to start from an experiment planned for at least 4 000-5 000 hrs.In [5], the calculation was based on 50-300 thousand hrs in creep conditions, steam piping energy blocks 250-800 MW at 515-560 • C and 3.7 to 25.5 MPa.

Creep time
The creep time from pore creation to the development of a macro-crack for 12Ch1MF can be one half of the total operating time, which is (1-3) × 10 5 hrs (in the course of a year the plant works for (7-8) × 10 3 hrs).Information on the degree of harmfulness of the metal therefore allows the uptime capacity to be calculated.At the present time, the diagnostics is carried out mostly by metallographic methods when the energy blocks are shut down, or during an overhaul.The high work difficulty involved in preparing the metallographic sample reduces the control performance, and extensive repairs are needed.It is therefore necessary to introduce more progressive express methods.

Increasing the operating parameters
The Russian literature [4,7], presents examples of service life extension by up to an additional 100 thousand hours after appropriate repairs to the welds on the basis of microstructure analysis and degree of damage.Cyclic heat treatment (3 to 10 cycles) with heating up to 980 Other specific cases for increasing the operational parameters are very difficult to access.Most of the results and experience gained in relation to this issue are covered by trade secrecy.However, it is obvious that even a small change in operating parameters (e.g. a temperature increase) has a significantly negative effect on the residual service life.

Assessment of microstructure damage
In our proposal for the metallographic evaluation of a structure, we utilize the classification degrees used by Neubauer, ISQ, and VGB Nordtest, a detailed description of which, and a comparison, are given in Table 5 [3]. Figure 3 shows various states of material 14MoV6-3 damage.

Figure 1 :STAGE 3 :
Figure 1: Creep curve with states of cavitation damage marked on it [3]

Figure 2 :
Figure 2: Example of experimental data processing in double logarithmic coordinates after tests on steel 12Ch1MF for term strength at 600 • C [6]

Table 1 :
Equivalent materials according to international standards

Table 2 :
Chemical composition of the material according to the standard

Table 3 :
Mechanical properties of steel 15 128 in dependence on heat treatment