Fulfilled R&D projects

Ensuring reliability and safety of maintenance of thermo-mechanical and electro technical equipment while increasing power of power-generating units of the nuclear power plant. Strength analysis and the substantiation of the separator-superheater of the Smolensk’s nuclear power plant periodic examination without hydraulic tests of casings

Keywords Separator-superheater, turbine equipment, nuclear power plant (NPP)
Programs in use SolidWorks, ANSYS

Computational mechanics laboratory (CompMechLab) by "ROSENERGOATOM" Corp. request executes multivariate analysis of 3D thermal and thermal-stressstate of SSH-500-1 separator-superheater.

For execution of this work CompMechLab has special capabilities:

  • License of the Federal service on ecological, technological and nuclear supervision «Possibility of maintenance of blocks of nuclear plants with regard to works' performance and to services accordance to the maintaining organization»
     
  • Commercial license for finite element analysis system (CAE-system) ANSYS (we should remind that ANSYS is termless certified in scientific and technological centre on nuclear and radiating safety of Russian Federal supervision on nuclear and radiating safety (software registration number - 490, date of registration - 9/10/2002, software certification passport registration number - 450, date of issue - 10/31/2002). 

Separator-superheateris one of the major components of nuclear power plant. It is intended for dewatering and super-heating of steam arriving there after the turbine’s high-pressure cylinder, working on saturated steam.

SSH-500-1 separator-superheater’s 3D geometrical model
SSH-500-1 separator-superheater’s 3D geometrical model

The plant consists of two basic elements: a separator and a superheater.

The separator is a welded cylindrical vessel (4367 mm height) with external diameter 4160 mm. The basic units of the separator are: casing consisting of a sidewalland a cylinder head welded into it, two branch pipes (welded into the cylinder head) - one for steam discharge (the fourth steam discharge in theturbine), another for safety valves joining. On the casing’s sidewall there are two branch pipes for separat dischargeand one branch pipe for a wet steam supply.  

In the top part of the separator-superheater case are located:

  • central pipe transforming in the top part to a transitive output branch pipe where the superheated steam from separator-superheateris pipe-bended;
  • separation device;
  • cylindrical gasket with lens compensator.

The distance piece is located between separation device and the central pipe.

Separation device consists of 20 standard modules located radially in annular space between the casing sidewall and the central pipe. In separation device moisture separation is realized on the separating elements' surface. Separating elements - leaves - are undulatory bent sheets located vertically and assembled in packages. Leaves’ packages are attached to vertical frame pillars. Each package has a trough in the bottom part. Thus the interflowing separat is turned off to the center of the device. Each standard module consists of three vertically located packages. For uniform package distribution of a steam flow there are vanes in front of each package and perforated shield behind of it. The neighboring blocks are separated from each other with radially located curved sheet. This sheet and the front block are forming narrowed to the bottom wedge-shaped channel for the steam distribution over packages, and with the back block the similar channel, but extending to the bottom for the exit of dry steam.

Superheater contains following basic units:

  • casing;
  • heat exchange device of 1-st and 2-nd steps of overheat;
  • four heating steam supply chambers;
  • four condensate of heating steam removal chambers.

The superheater's casing consists from sidewalland from the bottom bearing skirt welded to it. The bottom part of the bearing skirt is a slab (thickness135 mm).  The superheater's bottom is welded in a bearing skirt.There is a branch pipe for the separator-superheater emptying. On the casing sidewall there are 8 branch pipes where chambers of heating steam supply and of drain water removalare located.

 Heat exchanging device of 1-st and 2-nd steps of the overheat consists from separate shell-and-tube heat exchangers of the same type, the so-called modules - pipes where tube plates are welded. In tube platestubes 25x2are expandedand welded to the center of the top tube plate of each module. In the top part of a branch pipe theunion is welded (for noncondensable gas removal), in the bottom part - union (for the drain water removal).

During maintenance the plant is under mechanical loads(internal pressure, dead weight and weight of internal elements), and temperature stress caused by variable modes of operation.

Within the limits of this work 3D geometrical, finite element (FE) and mathematical models of the plant are developed and investigated in the laboratory. These models are considering all basic elements of the design, including: separationblocks, branch pipes and attached pipelines, superheater, entrance and drain chambers, bearings and internal devices.

Computational model of the plant. NDF = 2 257 000
Computational model of the plant. NDF = 2 257 000

The basic computational loading factors on separator-supeheater's casing are:

  • hydro-testing pressure;
  • separator-superheater's temperature influences and pressurein operating condition;
  • dead weight of the separator-superheater's casing and weight loadings of separator-superheater's pipe system, modeled by means of effective density;
  • attached pipelines loads and compensation moments.

Computational FE model fragments
Computational FE model fragments

On the basis of researched FE models:

3D thermal stress state analysis of plant elements is made for various operating modes, satisfying strength criteria according to special normative documents.

 

 

Module of a vector of displacements distribution in the hydro-testing mode  Intensity of the reduced stresses distribution under normal operation conditions

Module of a vector of displacements distribution in the hydro-testing mode

 Intensity of the reduced stresses distribution under normal operation conditions

The executed analysis proves that static and cyclic strength of the separator-superheater's casing is provided according tonormative documents.

Like a substantiation of hydraulic tests canceling (during periodic technical examination) a crack-like defect modeling discontinuity is considered. This defect is modeled by spatially oriented elliptic cut (crack). The maximum possible linear dimension of the considered elliptic cut is chosen according to normative documents, and is located inside of most loaded zone (by results of separator-superheater's stress state analysis) of the casing wall,  in control inaccessible (during SSH-500-1 technical survey) places.

The scheme of single and accumulation of crack-like defects, modeling discontinuities inside of the casing wall
The scheme of single and accumulation of crack-like defects, modeling discontinuities inside of the casing wall

The executed analysis proves that cyclic strength of the separator-superheater's casing is provided according to normative documents.

On the basis of the 3D stress state detailed analysis pressure intensity coefficients KI werecalculated (using the multilevel submodeling method). It was made taking possible 3D contact interaction of cracks' (defects') faces in the neighborhood of 3D curvilinear cracks' fronts in the most dangerous areas into consideration.On the basis of Paris equation possible cracks fronts growths (during 25 years after putting into operation) are calculated. For this in Paris equation for chromium-nickel rust-resisting steel of the austenitic class 18-8 and welded joints we used the factors considering environment corrosion influence.

Calculations of crack size increase under static and cyclic loadingof separator-superheater's casing (when basic casing's metal possible defects at conservative defect schematization like crack-formed are assigned in accordance with RC) prove that:

  • cracks increaseunder static loadingis minor; 
  • maximum crack increase takes place in the most loaded part of the separator-superheater's casing and (taking calculation modes and hydro tests during 25 years exploitation into consideration) makes Dа 0,00007mm (if to choose maximum safety factor  10 by Russian Codes[2]). It means, that under separator-superheater's working conditionoperating modes and periodic hydro tests do not influence on the basic casing's metal internal defect size increases.