5. Intensification of impurity removal from SG
4.1.6 Steam generator design
The relative impact of tube degradation mechanisms on overall PWR steam generator performance depends strongly on the plant and SG design. Inadequate geometries such as gaps and crevices
0
01.03 31.03 01.05 31.05 01.07 31.07 31.08 30.09
Fe3O4 [kg/SG] ; Power [%]
promote the accumulation of corrosion products and this must be avoided at the maximum extent by design.
There are the three locations in the heat transfer areas of steam generators common to all steam generators together with the important features that affect the performance of such crevices (see Figure 4.32):
Tube support structures or tube support plate (TSP).
Top of tube sheet crevices at the tube to TS intersections (TTS).
Sludge/deposits on tube sheet.
Figure 4.32. Geometries that produce heat transfer crevices involving tubing in steam generators [40].
The tube sheet (TS) deposits are the only that cannot be effectively decreased by design owing to the vertical design of the recirculation SGs. Therefore, the tube sheet is to be considered as the weakest point of the SG from the corrosion point of view and TS cleanliness is a high priority objective to retard SG ageing.
4.1.6.1 Tube support structures
The design of the tube support structures has been proved to be a key factor or a good SG performance.
The majority of the reported SG tube corrosion problems after conversion to AVT - chemistry occurred in the tube to TSP crevices.
Basically there were three types of tube support devices (see Figure 4.33):
Drilled hole tube support plates (DH-TSP).
Broached trefoil and broached quatrefoil tube support plates (BH-TSP).
Egg crate tube support grids (EC-TSG).
The evaluation of the different tube support plate geometries since the beginning of the nuclear business are summarized in Figure 4.34. The first used geometry was the drilled hole tube support plate designed by Westinghouse and Combustion Engineering in the early 1960s in USA. This tube support plate design was replaced later in 1970s and 1980s by the broached hole design in Westinghouse designed SGs and by the egg-grate structure in Combustion Engineering designed SGs.
Babcock-Wilcox (USA) used only broached trefoil TSPs for their once through SGs, whereas Babcock-Wilcox (Canada) had started to use egg crate tube support structure in their recirculation SGs in 1990s. For Siemens SGs the egg crate design was the only tube support structure from beginning on in 1960s.
Figure 4.33. Geometries of tube supports: (a) drilled hole typical of early Westinghouse designs; (b) Egg crate typical of Siemens and Combustion Engineering; (c) broached trefoil typical of EDF and Babcock and Wilcox; (d) broached quatrefoil typical of later Westinghouse designs [40].
Figure 4.34. Evolution of tube support geometries and materials used by four nuclear steam supply system vendors for steam generators. [40].
As licensee of Westinghouse, Framatome (now AREVA SAS) and Mitsubishi Heavy Industry (MHI) have started to change to quatrefoil broached hole design in their SGs; whereas finally AREVA SAS moved to a trefoil broached hole design with convex tube lends, as shown in Figure 4.36.
Drilled hole tube support plates
They consist on a perforated plate with a thickness of about 20 mm where the holes diameters through which the tubes are inserted have a tolerance of +200 µm to +300 µm referred to the tube outer diameter (see Figure 4.35).
The drilled hole tube support plate is the worst design known and the main cause of tube degradation at tube support levels in the 80s and 90s. Corrosion product deposition inside the crevice will rapidly take place, causing:
Strong increase of the local temperature due to lack of water circulation outer side of the tube.
Local boiling conditions leading to dry-out of the area and enrichment of impurities in there.
Corrosion.
Figure 4.35. Drilled hole tube support plate design.
Figure 4.36. Broached hole tube support plate design.
Broach hole tube support plates
This design eliminates the local overheating and reduces the presence of crevices:
Less or no enrichment of impurities.
Better corrosion behaviour.
However, broach hole support plates have, depending on the design, more or less susceptibility to deposit accumulation and eventually clogging of the broached holes. The design features (quatrefoil or trefoil; Concave, straight or convex surfaces in contact with the tube) are described by Figure 4.36.
The main problem observed with this design is clogging of the broach holes as shown in Figure 4.37.
The clogging of the broached holes causes various difficulties:
Perturbation of the SG internal hydrodynamics,
Change of the recirculation rate,
Increase of local flow velocities,
Flow induced fretting,
Flow accelerated corrosion (carbon steel plates).
Figure 4.37. Broached hole clogging.
Filled crevice, impurity enrichment, CORROSION Overehating, clogging with
corrosion products
Convex lend Straight lend
Concave lend
CP accumulation Good design
Quatrefoil, square pitch Trefoil, triangular pitch
Convex lend Straight lend
Concave lend
CP accumulation Good design
Quatrefoil, square pitch Trefoil, triangular pitch
Tube support grids of the egg crate type
This design reduces at the maximum the overheating and crevice problem. The Siemens designed egg crates are made of stainless steel and its design principle is given by Figure 4.38. The egg crate design is from the chemistry point of view the best choice due to the following reasons:
There are only two points in contact with the tube at a given level, with exception of the high bar intersections (only 4 tubes from 100 having a closed geometry around).
The tube surfaces are better exposed to the SG bulk water, no overheating and/or deposit accumulation.
No or negligible corrosion damage in more than 30 operation years is reported. Three plants report moderate initiation of OD damage after 31–34 years operation at the high bar intersections, i.e.
affecting only a small fraction of the tubes.
Figure 4.38. Siemens tube support egg crate design.
There is also another design type of egg crate grids designed by Combustion Engineering, which are made of carbon steel and have four contact points with the tubes at a given level (see Figure 4.39) [40].
In contrast to Siemens designed egg crates, this type of egg crate tube supports provides less recirculation round the tube resulting in less tube cooling and increased corrosion product deposits. In fact, SG tube corrosion was experienced at field within the crevices of tube to this type of egg crates.
4.1.6.2 Tube sheet design
The tube sheet area is the weakest point in the SGs of vertical arrangement. Corrosion products will eventually accumulate on the tube sheet surface. Accumulation of corrosion products on the tube sheet can be minimized by adequate design of the steam generator:
Flow distribution baffle to maximize the radial flow velocity to reduce deposition of corrosion products at the maximum extent (see Figure 4.40). This device has a limited effect.
Tube arrangement enabling efficient tube sheet cleaning. Once deposited, the corrosion products must be removed by high pressure tube sheet (TS) lancing each shutdown. A tube arrangement with square pitch cannot be cleaned as thoroughly as those arranged with triangular pitch (see Figure 4.41). In the latter case, with an adequate cleaning tool the perimeter of the tube can be cleaned almost completely.
Figure 4.39. Combustion Engineering designed egg crate tube support grids [3].
Figure 4.40. Flow distribution baffle to minimize the tube sheet deposits.
Figure 4.41. Influence of the tube arrangement on the tube sheet cleaning efficiency.
Tube to tube sheet intersections
The crevice between tube sheet and tube may represent a serious SG tube integrity problem.
Different techniques have been applied (see Figure 4.42). In the early years a mechanical expansion was performed only at the bottom, letting a deep crevice open towards the secondary side. This arrangement had as consequence the creation of differently aggressive local environments leading to numerous corrosion problems.
Fully expanded technique is now state of the art (see Figure 4.42 b, and Figure 4.43). A small crevice of 3 mm to 6 mm depth and a crevice having ~150 µm cannot be avoided for technical reasons (expansion tolerance at the top of tube sheet to avoid damage by over expansions).
The expansion technique can be hydraulic, mechanical, or a combination of both.
The minimization of the crevice depth becomes essential to avoid degradation problems like tube support denting and stress corrosion cracking.
If impurities accumulate in these crevices, oxidation of the carbon steel of the tube sheet will occur, generating less dense Fe oxides that, occluded inside the gap, will press and deform (dent) the tube (Figure 4.44).
This type of denting has been found under hard-caked deposits, and not necessarily implied later appearance of damage of outer diameter (Figure 4.45). In many cases the denting stopped its evolution after one or two cycles.
(a) partially expanded (b) fully expanded
(c) fully expanded with ‘top kiss roll’
Figure 4.42. Tube to tube sheet geometries [3].
Tube Lane Tube Lane
Figure 4.43. Advanced tube to tube sheet expansion technique.
Figure 4.44. Tube sheet denting.
Figure 4.45. Damage of tube outer diameter following denting at top of tube sheet level.
4.1.6.3 Blowdown internal design Two basic designs have been used:
• Blowdown collector pipes at the tube lane.
• Peripheral groove.
A typical design for collector pipe design is shown in Figure 4.46.
This is the most widely used SG blowdown design. The blowdown is taken from the middle of the SG and therefore it comes out very close to saturation. Saturated fluid may cause cavitation problems by pressure drop at the blowdown transport line outside the SG, requiring a rapid cool down or a tempering by mixing the blowdown with a small fraction of feedwater.
C-steel corrosion Tube wall Ferritic particles
Tube sheet
3-6 mm
0,1 - 0,2 mm
Tube wall
Tube sheet Expansion due to density reduction
Dent
Corrosion Product
accumulation Hardening SG Tube Corrosion
Impurity enrichment, risk of tube corrosion Local acidic,
oxidative conditions C-steel corrosion
Denting
The design of the blowdown with perimetral groove (gutter) is shown in Figure 4.47. This design enables a better accessibility to the tube lane, and since it takes water from the periphery the water is subcooled because of the flow coming from the downcomer, which is mixed with feedwater.
The efficiency for removal of impurities and corrosion products is comparable.
Figure 4.46. SG blowdown arrangement with internal perforated blowdown tubes (typical).
Figure 4.47. Blowdown with perimetral groove.
The external gutter provides the possibility of a better collection and removal of corrosion products being lanced towards the outer perimeter of the tube bundle during tube sheet lancing (easier aspiration of TSL waste).