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Increase in the life of submersible centrifugal pumps of the ETsN type

  • E. Kh. Isakaev
  • V. B. Mordynskii
  • V. M. Gusev
  • M. G. Frolova
Safety, Diagnosis, and Repair

A group of materials and their deposition method on the inner surface of a pump diffuser for restoration and/or strengthening in place of traditional “bushing” technology is studied as applied to the operating conditions of submersible centrifugal pumps of the ETsN (electrical centrifugal pump) type. Wear-resistant pseudo-alloy metallized coatings are developed based on 12Kh13 + 08G2S, distinguished by high service properties, economy of application and the possibility of machining with a cutting tool. Equipment is created for implementing the production process. Results are provided for bench tests and industrial tests on coated components.

Keywords

Wear Resistance Chromium Steel Pump Diffuser Pneumatic Cylinder Intermediate Support 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Wear and corrosion, and also a combination of them play the main role in reducing the service life of various assemblies and components.

Analysis and experience shows that most vulnerable from the point of view of failure in operation of ETsN type pumps is the friction pair, formed by the pump diffuser (PD) and the protective sleeve of the shaft, whose proportion is about 20% of pump failures. Currently in order to manufacture PD modified cast iron is used, and brass L63 is used for the sleeve. Depending on the oil deposit, the content of oxygen, nitrogen and sulfur compounds and suspended abrasive particles change over wide limits. Special corrosion and erosion-resistant and wear-resistant alloys are used in manufacturing PD for the most unfavorable deposits, which considerably increases their cost.

In order to prolong the PD service life during pump repair it is traditional to use the method of “bushing,” i.e., pressing in a bush of high-hardness steel ShKh15. Another solution to the problem of increasing PD life may be application of wear-resistant coatings by gasothermal deposition (GTD), both to the a new component, and in carrying out repair work. The GTD method may be used to restore components even with a very thin boss wall, which breaks during bushing.

The volume of output of new PD and yearly repair of equipment in the Russian Federation may be estimated at 10–15 million items/yr, and therefore it is necessary that coating materials are inexpensive and mass produced, and methods for their application are highly productive, well recorded in general production process, with a required level of mechanization. In addition, the process of final component machining with wear-resistant coatings should not their renovation markedly complicate technology.

In developing wear-resistant coatings for strengthening and restoring a PD boss, 36 grades of extensively known materials have been studied with increased resistant to wear and corrosion, consisting of three groups:
  • the first group is self-fluxing hard alloys based on nickel (type PG-SR, SNGN, PG-12N-01, VSNGN), and the wear resistance with respect to hardened steel 45 is ε′ = 2.3–5.5;

  • the second group is cerment composite materials and intermetallics (P20-40Kh13, PRKh18N9, type KhVS, PGS-1+4%Al and FBKh6-2+4%Al, etc.), ε′ = 2.2–3.9; and

  • the third group is metal coatings based on iron.

The application methods were proven gas flame and plasma deposition, and electric-arc metallization (EAM).

Adhesion of coatings applied to an inner surface may not provide their sufficient strength. In order to reduce the risk of coating separation there is often use of preliminary preheating to 250–300°C, addition to the coating composition of ductile components, and conformity of linear thermal expansion coefficients for the component and coating materials. A considerable amount of research has predetermined the behavior of previously evaluated test coatings under laboratory conditions. The evaluation criteria for potential use of materials and methods of their application selected are coating phase composition and microhardness, their wear resistance, determined in SMTs-2 and KhB-4 machines, adhesion and cohesion properties of deposited layers.

The first group of coatings had a matrix of γ-solid solution with a microhardness of 3800–4500 MPa and crystals of carbides (borocarbides) of variable composition with a microhardness of 7000–11000 MPa. Depending on the grade of self-fluxing powder, hardness varies within the limits 35–62 HRC. The wear resistance of these coatings in a surfaced state is higher by a factor of 5.5 than for hardened steel 45 with a hardness of 52–55 HRC. A disadvantage of these materials is the high value of their thermal expansion coefficient. Stresses, which arise after their deposition, even by the plasma method, at the boundary of a coating with the substrate, often lead to its separation during subsequent machining. An increase in the adhesive strength of coatings with surfacing at 1050–1150°C leads to PD warping. For this reason, coatings of this class were excluded from studies in the preliminary stage.

The second group of coatings was applied by gas-flame and plasma methods. Cermet coatings after gas-flame deposition consist of a metal matrix and weakly deformed ceramic inclusions. Apparently the temperature of the oxyacetylene flame is insufficient for melting refractory ceramic inclusions. With a change-over to plasma deposition heating of refractory ceramic components is considerably better, and they also formed a wavy structure typical for gasothermal coatings. The microhardness of the metal matrix was at the level of 1000–1800 MPa, the hardness of oxide inclusions was 9000–15000 MPa, and for carbide components it was 9000–20000 MPa.

The uniformity of strengthening phase distribution and strength of fastening in a matrix determines the high wear resistance of these coatings. In tests they appeared to be higher by a factor of 4.4–6.1 than modified cast iron, from which PD are manufactured. The relative wear resistance of intermetallics was ε′ = 2.9 for plasma coatings of powder PN55T45 and ε′ = 2.1 for powders PN70Yu30. Due to the high hardness coatings of the second group excluded machining with a cutting tool and required grinding. For this reason, and also in view of the high cost of the starting materials and significant expenditure in plasma deposition, they were excluded from use as a PD coating.

The third group is application of a coating chromium steel by the EAM method, i.e., the most promising from an economic and production point of view. Steel with 13% chromium, exhibiting a positive potential, does not rust or oxidize in air, in water and a number of acids, alkalis and salts. With a carbon content of the steel of less than 0.1%, they form a ferrite-pearlite structure, and with an increase in carbon content to 0.4% they form a martensitic structure. The hardness and wear resistance of chromium steels generally depend on the volume of residual austenite and martensite hardness. With a reduction in temperature of structural transformations and an increase in the stress level, the rate and volume of martensitic transformations increase. A high cooling rate for fuzed particles, from which a coating forms, and their deformation on impact with the substrate promote preparation of a quenched structure without additional coating heat treatment.

In addition, by changing the deposition regime parameters it is possible to affect the thermal state, with which a layer forms, and by means of this to affect the physicomechanical properties of a coating. Martensitic structures, due to considerable distortion of the crystal lattice, have very low ductility and are inclined towards forming cold cracks. A pearlitic structure, retaining sufficiently high hardness, exhibits greater ductility and a capacity to relax peak stresses, retaining high wear resistance.

The thermal expansion coefficient of chromium steel coatings is almost independent of the carbon content in the original material and deposition regime. It differs somewhat from corresponding values of compacted materials, but agrees quite well with cast iron properties. The microhardness of a coating increases with an increase in carbon content. Physicomechanical properties of metallized coatings from chromium steels are provided in Table 1.
Table 1

 

Properties

Starting material

12Kh13

20Kh13

30Kh13

40Kh13

12Kh13+08G2S

20Kh13+AMg

Microhardness, MPa

2950

4500

5500

6120

2960

2120

Hardness, HRC

28–30

32–33

36–43

41–50

30–32

Porosity, %

8.7/13.5

7.0/16

7.4/16

6.7/12.3

Density, g/cm3

6.73

6.45

6.47

6.47

6.51

5.03

Linear expansion coefficient, ×10–6

12.2–13.0

12.2–13.3

12.4–13.8

17.4–18.3

Thermal conductivity coefficient, W/(m·deg)

10.9

10.4

10.4

49.5

Notes. Microhardness is shown for average values; porosity: values of open porosity shown in the numerator, total porosity in the denominator.

Simultaneously, in studying the effect of production parameters on the EDM process on coating properties of steel 30Kh13 (Table 2) and chemical composition of chromium steels on coating hardness in relation to deposition distance (Table 3), on adhesion and wear resistance, i.e., the main service properties of a coating, an estimate was made of the possibility of their machining by cutting tools.
Table 2

 

Arc parameters

Content of elements, %

Adhesion, MPa

Current, A

Voltage, V

Power, kW

C

Cr

Fe

90

25

2.25

0.15

11.1

88

30.6

75

30

2.25

0.16

11.2

88

30.7

65

35

2.28

0.09

11.1

88

23.5

Table 3

 

Coating

Deposition distance, mm

Microhardness, MPa, with air pressure, MPa

Hardness, HRC

0.3

0.5

0.7

12Kh13

50

2410

80

2900

3080

2900

28–30

150

2460

26–28

20Kh13

50

3250

3860

3180

32–35

80

3250

3500

3220

33–35

150

3250

3260

3300

32–34

30Kh13

50

4120

40–44

80

4800

6400

5500

42–44

150

4700

3500

36–40

40Kh13

50

6060

6400

80

6400

6480

51–55

150

4060

6070

523

48–51

12Kh13+08G2S

50

80

2800

3200

3100

32–33

150

2700

3000

3000

31–32

It was established that machining of coatings with martensitic structures, even with hard alloy cutters, may be carried out at slow revolutions (less than 200 rpm) and cutting depths of 0.1 mm. However, even with these limitations 10–15% of components had coating chips.

Consequently in considering the expediency of increasing hardness and wear resistance of coatings, taking account of subsequent machining in mass production, a compromise solution is necessary. As one of the versions of coatings the composition of pseudo-alloy 20Kh13 + AMg was checked, prepared by deposition of wire of the same diameter (1.6 mm) and the same speed of feed. Thermal conductivity of these coatings is markedly higher, which should point to thermal stresses at the point of contact of coating rubbing surfaces (see Table 1). According to published data, these coatings also have a low friction coefficient. However, for coating application technology for pseudo-alloys there are marked disadvantages with breakdown of the stability of EAM process itself with a different wire melting rate. This requires development of special measures (use of wire with a different diameter or feed at different rates, removal of oxide film from aluminum wire), which considerably complicates the production process and limits the possibility of controlling pseudo-alloy composition. The limitations in question make it difficult to use this technology in mass production for strengthening or restoring PD, although the promise of research in this area in developing shaft protective sleeve materials should be recognized.

Taking this into account in the present work experience of applying pseudo-alloy coatings [5] has been used. The best combination of wear resistance and the action of shock mechanical and thermal loads, and also machinability with a cutting tool are exhibited by composite coatings consisting of uniformly distributed solid and soft phases. The solid phase provides an increase in wear resistance, and soft phases dampen shocks and provide heat removal from rubbing surfaces. The optimum combination of properties is exhibited by coatings containing from 60–80% (vol.) phases with high hardness, and the rest of the volume is phases forming an elastic binder. It is necessary that these phases are uniformly distributed throughout the volume of a coating and the structure is formed with mutual penetration of components.

Whereas with deposition of coatings from powder materials there is the possibility of creating a composite with any volume ratio of components due to controlling them in the original composition, with EAM these possibilities are limited by conditions of arc burning stability between wire electrodes. The amount of heat liberated with burning of an arc at a positive electrode is twice as large than at a negative electrode, and therefore consumption of the anode wire with use of two wires with the same of similar thermophysical properties should be twice as great as consumption of the cathode wire. Consequently with use of wires of different materials their consumption is determined by thermophysical properties of the materials, primarily density, heat capacity, and melting temperature. By designating the product of three values of γCT m as the melting difficulty factor K m for materials, and considering the distribution of thermal fluxes in the cathode and anode, we obtain the required ratio of wire volumes providing stable arc burning:
$$ 2{P_1}{K_1} = {P_2}{K_2}, $$
(1)
where P 1 and P 2 are weight consumption of each of the wires in a unit of time; K 1 and K 2 are their melting difficulty factors.
With an identical wire supply rate, this ratio is fulfilled due to the choice of their diameters:
$$ D_1^2{K_1} = D_2^2{K_2}. $$
(2)
Whence we obtain the wire diameter for the soft component:
$$ {D_1} = { }0.{7}{D_2}{\left( {{{{{K_2}}} \left/ {{{K_1}}} \right.}} \right)^{{{{1}} \left/ {{2}} \right.}}}. $$
(3)

By selecting electric arc deposition materials and regimes in developing composite wear-resistant coatings, it is necessary to observe fulfillment of the requirement of arc burning stability and the volume ratio of components in a coating. This is fulfilled quite well with the use as an anode of carbon rod combined with a cathode wire manufactured low-carbon steel or nickel. In these cases the volume of soft component in coatings is 30–33%. With the use of copper as a cathode material its proportion in a coating is not less than 42%, and in steel-aluminum composites the volume fraction of soft phase is 64%, which is much more than the optimum version. Attempts to reduce the content of soft component in coatings, by making wire consumption less than shown in relationships (1)–(3), causes instability of equipment operation and as a result of forming arc pulsations uniformity of material distribution in a deposited layer is disrupted.

For all the parameters studied, good indices were obtained for pseudo-alloy 12Kh13 + 08G2S, prepared from wire of diameter 1.6 and 1.2–1.4 mm, respectively. These coatings have a ferrite-pearlite and martensitic structure with a small amount of carbides, they are quite dense (porosity 6–7%), and consist of uniformly distributed structural components throughout a layer volume (Fig. 1).
Fig. 1

Microstructure of 12Kh13+08G2S coating after etching with nitric acid (microhardness of light phases 4590 MPa, dark phases 5150 MPa); ×400.

Coatings machine well with a cutting tool even with a rate of 6 m/sec (600 rpm). The chromium content in these coatings is somewhat reduced and is about 8%, but nonetheless they exhibit satisfactory corrosion resistance and comparatively high wear resistance.

On the basis of these studies, a process has been developed for electric-arc deposition of wear-resistant coatings on PD bosses taking account of their structural features, and production equipment has been created for its implementation (Fig. 2) within which there is a device for continuous abrasive treatment of a surface (CATU), i.e., part of the deposition device, equipped with a modernized series electrometallizer EM-12 (EAMU).
Fig. 2

Layout of production complex for restoring or strengthening the internal dimensions of control equipment of submersible pumps of the ETsN type. CATU: 1) chamber; 2) charging window; 3) components; 4) component supply tray; 5) component unloading window; 6) assembly plate; 7) component intercept; 8) intermediate support; 9) fixing support during treatment; 10) control cabinet (11 is electric motor, 12 is reducer, 13 is time relay, 14 is pneumatic distributor, 15 is pneumatic cylinder); 16) drive roller; 17) pneumatic gun. EAMU: 1) chamber; 2) component; 3) intermediate support; 4) connecting tray (transporter); 5) pusher; 6) spring-loaded support; 7) drive roller; 8) idling roller; 9) upper pressing roller; 10) time relay; 11) pneumatic distributor; 12) pneumatic cylinder; 13) component discharge tray.

The CATU is a chamber 1, and it consists of abrasive equipment 17, a device for feeding components into the working zone 69, a device for retaining components in the form of a three-pronged yoke with supports, connected with a control unit 1014. Components 3 are loaded into a tray 4 and rest on an intercept 7. The treatment time is prescribed by a time relay 13, after which a signal is fed to a pneumatic distributor 14, from which compressed air entering pneumatic cylinder 15 removes the temporary intercept 7, and sets up intermediate support 8. The first component rests on the intermediate support 8, and after a command from the time relay pneumatic cylinder 15 returns to the original position. A component rolls into the treatment position (friction roller 16) and is slowed down by support 9. In this position, the rest of the components are held back by intercept 7. During a prescribed time, there is abrasive treatment, at the end of which the pneumatic cylinder removes support 7 and the next component slides to support 8, and the cycle is repeated. Simultaneously support 9 is removed and the treated component slides over the sloped tray through window 5.

After treatment, a component from window 5 enters the sloping tray 4 into the EAMU deposition chamber 1 to intermediate support 3 and spring-loaded support 6. On command from the time relay 10 pneumatic cylinder 12 operates on a pneumatic distributor 11, and pusher 5 through the component inclines the spring-loaded support 9, installing a component in the deposition position between three rollers 69. At this time, pusher 5 prevents further advance of the rest of the components, and on returning to its original position the next component enters the intermediate area 3 and is held by support 6. Deposition is carried out for a prescribed time, while the component is between rollers 69. With entry of a new signal, the pusher inclines support 6, installs it into the working position for the next component, pushing the spray-coated component on to the sloping tray, and it slides into the finished product container.

Thus, deposition of coatings is performed simultaneously on the inner surface of the boss and on the small shoulder, and spot movement over the surface being strengthened is provided by rotating a component in a special spring-loaded three-shaft grip, and component loading and unloading into the deposition position is provided by a pneumatic pusher, whose operating cycle is controlled by a timer, which is provided after loading a component into the storage tray of the unit in an automatic regime.

The thickness of the layer applied is controlled by the rate of wire feed and time of component exposure in the treatment zone. In the deposition regime chosen, the thickness of the layer in 1 second is 0.1 mm, which provides automatic heating of the surface being coating, does not lead to excess component overheating, and is convenient for controlling the rate of its feed into the deposition zone. The device has undergone successful industrial testing in restoring a batch of PD of more than 100,000 items. The diameter of the restored PD boss was 21.9 ± 0.1 mm, taking of a requirement for subsequent machining of an opening Ø21 N9 of a coating deposited with a tolerance.

Bench testing of restored PD was carried out in special units (benches) developed by Kriogenmash company and Gubkin Russian State University of Oil and Gas with a shaft rotation rate of 3000 rpm with an electrocorundum content from 8 to 5 g/liter. Results of testing confirmed an increase in wear resistance due to coating application and an increased life of the PD boss – shaft protective sleeve pair (Table 4). The material for the protective sleeve in all test versions used was series components of brass L63 (average values of wear resistance were derived for three points).
Table 4

 

PD boss material

Friction pair elements

PD

Protective shaft bush

Pair

Modified cast iron (series version)

1

1

1

12Kh13 (coating)

2.88

1.78

2.36

Pseudo-alloy 12Kh13 + 08G2S (coating)

2.56

1.6

2.04

Industrial examination of a test batch was carried out in the oilfields of Surgutneftegaz company. No coating wear was detected after operation for 150 days, and wear of brass sleeves in contact with them did not exceed the standard.

The technology developed may be used not only for restoring worn, but also the manufacture of new PD with the aim of increasing their life and reliability.

In addition, EAM is effective for restoring touching areas on various shafts. Coatings of simple and inexpensive low-carbon welding rod with respect to wear resistance considerably surpass hardened layers on carbon steels, which is important not only in the repair of components. These metallized coatings operate well on the journals of shaft with different packing, when access of abrasive articles to the contact surface is possible, which has been observed by us over a prolonged time in repairing wagon reducing gears type TK-2 and TRKP.

Thus, it is possible to draw the following conclusions:
  1. 1.

    Provision of high wear resistance for submersible centrifugal pumps of the ETsN type for oil recovery is a serious scientific, technical, and economic problem. The main ways of controlling friction and wear processes is choice of materials and their combination in friction pairs, and also use of different forms of strengthening treatment for joined surfaces.

     
  2. 2.

    A group of wear-resistant materials and methods for their gasothermal deposition on the inner surface of pump diffusers have been studied for their restoration and/or strengthening instead of traditional “bushing.”

     
  3. 3.

    The wear-resistant pseudo-alloy coatings developed based on 12Kh13 + 08G2S are distinguished by high service properties, economy in application and the possibility of machining with a cutting tool. Production equipment has been created for implementing the coating application which after loading a component operates in an automatic regime.

     
  4. 4.

    The promising nature of the measures developed is supported by bench and industrial tests of components with coatings. The service life of a restored pump diffusers considerably exceeds similar indices for new units and provides an increase in friction life as a whole.

     

Notes

The work was carried out with financial support of the Russian Foundation for Basic Research (Grant Nos. 09.08.00086 and 10.08.00030).

References

  1. 1.
    G. B. Stroganov, “Coatings, a highly effective area of resource saving technology and tasks for implementing them in the national economy,” in: Protective Coatings in Engineering: Proc. 22nd All-Union Conf., Naukova Dumka, Kiev (1987).Google Scholar
  2. 2.
    A. Hasui and O. Moriaki, Surfacing and Deposition [Russian translation], Mashinostroenie, Moscow (1985).Google Scholar
  3. 3.
    V. V. Kudinov and G. V. Bobrov, Coating Application by Deposition. Theory, Technology, Equipment [in Russian], Metallurgiya, Moscow (1992).Google Scholar
  4. 4.
    L. Kh. Baldeev (ed.), Gasothermal Deposition [in Russian], Market DS, Moscow (2007).Google Scholar
  5. 5.
    E. Kh. Isakaev, V. M. Gusev, V. B. Mordynskii, M. V. Gusev, and A. V. Marichev, RF Patent 2386720, “Electric-arc metallizing method,” publ. 04.20.1010, Byull. Izobr., No. 11.Google Scholar

Copyright information

© Springer Science+Business Media, Inc. 2011

Authors and Affiliations

  • E. Kh. Isakaev
    • 1
  • V. B. Mordynskii
    • 1
  • V. M. Gusev
    • 2
  • M. G. Frolova
    • 3
  1. 1.Joint Institute of High TemperaturesRussian Academy of Sciences (OIVT RAN)MoscowRussia
  2. 2.Gubkin Russian State University of Oil and GasMoscowRussia
  3. 3.All-Russia Institute of Scientific and Technical InformationRussian Academy of Sciences (VINITI RAN)MoscowRussia

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