Euro PM12-I

Case Study Regarding the Behaviour of the Structural Gradient from Various Composite Materials (WC-Co/WC-Ni), Used to Manufacture Ball Valves for the Oil Extraction Industry

I. Popescu2, R.M. Negriu1, S.G. Badea1, C. Besleaga1

1)Econet Prod Bucharest, Pades street no. 16, Bucharest, Romania2)Industrial Biogas Solution, Rokura Group, Bucharest, Romania

Abstract: Valves (ball and seat) equip pumps for extracting petroleum. The balls have to withstand complex erosive-abrasive wear in corrosive environments, combined with micro cutting and micro fatigue. The effect of high temperatures will also occur in the extraction through underground combustion. Experimental batches of balls with structural gradient were made using virgin and recovered materials and PM technologies. The experiments followed the recovery of material from worn-out balls, by removing the worn-out layer and completion with virgin material, and also making balls with structural gradient from dust with various qualities. We analyzed using the finite element the tension states and the life span of the balls was estimated. We present an analysis of the ways of deterioration, the used technologies, the control technologies, the experimental results, the behavior of these balls and new research paths, to optimize the process of producing balls with gradient.

Keywords: petroleum pumps, valves, balls, wear, recovered materials, PM technologies, C.A.D

1. Introduction

Piston pumps used for the extraction of oil contain closing/ opening valves to allow bringing the oil to the surface. Basically a pump has two valves, each comprising of a locking ball and valve seat respectively. In Fig. 1 such an extraction pump is shown.

Figure 1: Piston pumps used for the extraction of oil

These elements are mainly stressed by wear, corrosion caused by abrasives powder content and the substances contained in oil (eg. hydrogen sulfide) at the temperature from the oil layer, layer pressure and the column of oil to the surface, the thermo-mechanical stresses induced by the pumping conditions, the shocks produced by opening and closing valves etc.

The materials used to manufacture these components are steels with high mechanical and wear resistance and high alloys materials based on tungsten carbide with cobalt or nickel binder. The problem of increasing the life span of these components is important because, besides the cost of the components themselves, their replacement requires removing the pump from service during the entire operation and thus losing the production of corresponding quantities of oil.

The analysis of the causes of deterioration of the balls and seal seats revealed that wear by abrasion caused by solid grains (sand) in an aggressive environment is the main reason for removing these components from service. A series of presented articles from the specialized literature [1÷4] on the realization of objects with material gradient, using layers of 2 or more types of materials with different properties, have shown the efficiency of using sintering technologies of objects with a gradient of properties induced by the different composition and the related diffusion phenomena. The authors have aimed to make these balls from the valve of the pump used in oil extraction using a core of a CW-Co alloy (12% Co) and an outer layer of a CW-Ni alloy (12% Ni). The outer layer of WC-Ni was proposed because of the good properties of this material to the wear of abrasive particles, the temperature of the deposit and other aggressive substances present in the extracted oil. Engqvist and

co. have shown [5] that cobalt used as a binder in sintered carbides dissolves in low pH solutions. To counteract this phenomenon, it should be mixed with Ni or completely replaced by it. On the other hand, carbides with very little binder (almost only WC content) dissolve in solutions with high pH. The rate of wear is dependent on the ligament of CO (mean free patf, which is a measure of the thickness of the Co between the WC grains) and grain size of WC. Large sizes lead to a reduced wear resistance [6÷8]. Simple WC-Co alloys have a corrosion resistance to a medium with pH around 7. Because of this, the binder will be attacked from the surface leaving a skeletal structure that contains unsupported WC grains. These particles will be removed by the abrasive environment and will enable the emergence of a new binding surface to be corroded. When the area of the component is subjected to strong stress from corrosion/ wear, WC-Ni alloys are the most recommended to an environment of pH 2-3 and even, for some solutions, under this value. The choice through which the inner part is made of WC-Co and the outside, subject to corrosion/ wear is made of WC-Ni is recommended because Co and Ni are metals with similar characteristics and therefore the mechanical and thermal properties of the WC-Ni alloy are similar to those of the WC-Co alloy [9].

The authors have made a review of the phenomena of deterioration of the valve’s components from the piston pumps used in oil extraction [10]. Following this analysis, the authors concluded that there are two main types of deterioration: internal manufacturing defects that lead to breakage / separation of smaller or larger pieces and accelerated wear of the balls and seats resulting in the loss of sealing of the pumping chamber, thus resulting the inefficiency of the pump (there is the possibility that a piece of metal gets caught in the flow of oil and produces other collateral damage).

2. Experimental tests

First tests have demonstrated the need to develop a process to ensure concentricity of the two layers (core and outer) and to prepare proper pressing and sintering of the second layer. In the first experiments, the concentricity of the two layers, symmetric pressing and proper degassing could not be achieved. The temperature of the sintering and the sintering diagram was not adequate to achieve a sintering, an adherence to the base layer and a corresponding diffusion of the two addition materials (Co and Ni). Early results have led to the appearance of a detachment of the layer during the sintering operation, as seen in Figure 2. In this figure we can also observe the uneven thickness of the outer layer.

Figure 2: Defect of the ball: the uneven thickness of the outer layer

Another major defect in the first experiments was the pores occurring in large numbers at the interface of the two layers, as seen in Figure 3. In this figure we can see that, although the outer layer thickness was approximately constant, inappropriate technology has led to the appearance of large pores and to the lack of adhesion between layers. Pores have also occurred in the interior of the outer layer.

Fig. 3. Defect of the ball: the pores occurring in large numbers at the interface of the two layers

Theoretical and experimental analyses [10] showed that, for such parts, rapid deterioration problems occur even more strongly when the physical and mechanical properties of the layers’ materials are more different. Problems caused by different thermal deformations, by the difference in elasticity modules and, respectively, of the Poisson coefficient, and the different variation of these coefficients according to temperature, were analyzed and a technological process that minimizes the negative effects was proposed.

Through the verification of the used solutions, we proposed a new technology, in course of patenting [11], which is based on a special technological procedure that has the following features:

a. The making of the mold filling in which is placed the ball forming the base material with pow-der (virgin or recovered) is done in two stages by means of devices which are presented schematically in Figure 4. The preparation of the interface of the two semi-spherical calottes is important to eliminate cracks (lack of adherence) in this area;

b. The parameters that describe the filling and the compaction through vibrations are controlled carefully so as not to exceed the experimentally determined parameters;

c. The roughness and the cleaning of the ball forming the sintering base of the outer layer are strictly controlled. The roughness is correspondent to the granulation of the powder that fills the molds;

d. The rubber molds were made so that the pressing of the powder is closer to hydrostaticity;e. Hot isostatic pressing parameters, the final operation aimed at eliminating internal stress, pore

and micro-cracks reduction between the layers and from the material of the base ball.

Figure 4: Device for filling matrix from isoprene rubberDevices the making of the mold filling in which is placed the ball forming the base material with powder

(virgin or recovered): 1- ball; 2- pad; 3- vibrating plate; 4- filler (CW powder, virgin or recovered); 5- rubber sleeve; 6- stopper for filling hole; 7- shoulder for fastening of coating rubber; 8- conic shape; 9- separation surfaces prepared for bonding; 10- mounting ring; 11- pad; 12- rubber sleeve; 13- stopper;

14- filler (CW powder, virgin or recovered).

In Figure 5 we can see a coating of rubber sleeve with a ball of recovered material (disassembled and ready for isostatic pressing).

Figure 5: Coating of rubber sleeve with a ball of recovered material (disassembled and ready for isostatic pressing)

In Figure 6 the hardness measured on the section of a reconditioned ball is presented. Reconditioning is done on our own developed technology, the basis ball being recovered from balls made of a single layer of WC-Co and that were discarded due to advanced wear which no longer allowed the maintaining of the working pressure. The balls were grinded to a size where all visible traces of damage were removed and this was determined by visual inspection and by penetrant liquid inspections.

Figure 6: The hardness measured on the section of a reconditioned ball

The technological process imposed choice of WC-Co compositions and WC-Ni which satisfy the following conditions: a) the inner layer must ensure good mechanical resistance; b) the outer layer must provide good resistance to abrasion and corrosive environment; c) physical- mechanical properties of both materials must be similar so the tension produced in termobaric stresses due to differences of mechanical properties be minimal.

3. The simulation of stresses on the ball made with gradient of properties of material

The ball from the valve was modeled with the valve seat. The ball is modeled as consisting of two layers of material, the base material being WC-Co with 12% Co and the outer layer material consists of WC-Ni with 12% Ni. Because it is assumed that during sintering and then during hot

isostatic pressing all internal tensions are eliminated, they are not taken into account. It is also assumed that the manufacturing process is perfect and no defects of any kind arise. In order to simulate the material properties of the contact area between the two types of carbide is considered that these properties are uniform and are the average of the mechanical properties of the carbides used.

We made a complex simulation in which we took into consideration both the solicitations due of pressure and those produced by the temperature from deposit. In figure no. 7 is presented the meshing of model in finite elements. It can observe that the layers of WC-Ni and the diffusion layer were much finer mesh than other bodies to obtain more accurate results.

Figure 7: Meshing of the model in finite elements

In figure no. 8 is presented the main maximum stress in the assembly at temobaric stress. It can be observe that aren’t values of main maximum stresses which exceed the allowable values in compression or stretching.

Figure 8: The stresses state and the main maximum stresses in the assembly

4. Conclusion

Experiments and simulations have shown that the lifetime of the valve components is determined mainly by internal defects produced by a non-optimized technology, the wear caused by the pumped oil and the stress to fatigue of the ball and valve seat. The state of tensions produced by thermo-mechanical stresses for balls made of two layers may have a degree of hazard imposed by the different physical and mechanical characteristics of the layer’s materials and by the uneven thickness of the outer layer.

The strains state produced by the thermo mechanic stresses for the balls made from two layers can present a degree of danger imposed, on the one hand by the different physic and mechanic parameters of the materials of the layer and uneven thickness of the outer layer and on the other hand by the structural defects from contact zones between layers.

References:

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[10] Popescu I., Negriu R. M., Badea S. G., Besleaga Cr., Stefanescu M., Ways of deteriorating the balls with structural gradient from the valve used in the oil extraction industry, Proc. Int. Congress., New Trends in Fatigue and Fracture, NT2F12, Brasov, Romania, May, 2012;

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