A Project on Wear and its Relation to Ball Valves. A project for MANE 6960 By Robert Sayre Submitted 12/9/2013

Transcription

1 A Project on Wear and its Relation to Ball Valves A project for MANE 6960 By Robert Sayre Submitted 12/9/2013

2 Introduction Ball valves have many possible applications; they typically have low pressure drops and a high reliability.the tend to be more reliable when sitting idle, compared to other valves such as globe or gate valves[1]. When ball valves are used in situations with high cycling, the valve experiences wear. This wear of the seats due to the ball acting on it, continually wears away at the seats until leakage is deemed unacceptable or the seat and ball tolerances are below spec. For this project 2-way, 2-posiiton ball valves will be examined, these types of ball vales rotate 90 degrees from full open to full shut. For simplicity a 1-inch diameter ball will be used. A cutaway of an end-loaded ball valve is shown in Figure 1 below. From left to right there is the port, a seat, a (red) sealing o-ring, the ball, the other seat, and the opposite port. The end is also the tailpiece. Tightening this tailpiece in the body creates an interference fit between the body, the seats and the ball. This compressing of the seats, creates the sealing force on the ball, and ideally creates a leaktight seal. Figure 1: Ball Valve Cutaway [2] The seal force simply reduces to a ring on a sphere, since a ball valve is symmetric, the valve can be modeled using symmetry and even axial symmetry to decrease calculation times. When a ball valve is brand new, the seal force is the highest, a minimum circumferential seal force in lbf/in or N/m must be maintained or the fluid pressure will push past the seal. On the other end of the spectrum if the seal force is too high, the valve will be harder to operate, and assemble, and more importantly the wear rate of the seats and possibly the coating on the ball will wear away rapidly. Archard s classic wear equation,

3 of the form K=(V w *L)/(Fn/H), where K= wear rate, V w =worn volume, L=sliding length, Fn=normal force, H=hardness of the softer material[3]. The Archard equation is derived on the basis of single asperities sliding past each other. Looking at this equation one can estimate increasing the hardness of the softer material, or decreasing the normal force should decrease the wear. Therefore increasing the normal force on the seats beyond what is required for sealing can negatively affect wear. In order to define the rest of the problem, one needs to identify several more parameters, through empirical testing, it is assumed is takes the user 1 second to rotate the valve from open to shut or vice versa. The motion is assumed to be constant velocity for the sake of modeling. The valve is assumed to turn exactly 90 degrees. The friction between the seats and the ball also directly affects the torque required to turn the ball valve. Nominal dimensions of the ball valve are used in the model, only the ball and seats are modeled. Finishes within Ra < 0.5µm surface finish are commercially viable. The ball is assumed to be made of CRES 316L and the seats are made of PEEK plastic. These are materials used in commercial ball valves in fluid systems. These are assumed to be low pressure ball valves. The material properties for PEEK are shown in Table 1. Table 1: PEEK material Properties[4],[5] Property Value Units Specific Gravity 1310 Kg/m 3 Hardness, Rockwell R 126 Hardness, Shore D 100 Tensile Strength 110 MPa Elongation at Break 40% % Elastic Modulus GPa Flexural Yield Strength MPa Flexural Modulus MPa Compressive Strength MPa Compressive Modulus GPa Shear Strength Mpa Izod Impact, Notched J/cm Coefficient of Friction 0.32 Unitless K (wear) Factor 755 x 10-8 mm 3 /N-M Limiting Pressure Velocity MPa-m/sec Thermal Conductivity W/m-K Melting Point 340 ºC Glass transition 144 ºC temperature Poisson ratio.39 unitless

4 PEEK can have different materials (such as carbon or glass to increase the strength and stiffness, or PTFE) added for additional strength and/or lubrication property enhancement. A variation of PEEK, PEAK can even be used in medical implants[6]. For simplicity material properties for Quadrant EPP Keytron 1000 PEEK unfilled, extruded was used, with the gaps filled in with a general PEEK material data sheet filling in any needed property unknowns. The CRES ball properties are shown below in Table 2. Table 2: Ball Properties (CRES 316)[7] Property Value Units Hardness, Brinell 146 Hardness, Rockwell B 79 Hardness, Vickers 152 Tensile Strength, Ultimate 560 MPa Tensile Strength, Yield 235 MPa Elongation at Break 55% % Modulus of Elasticity 193 GPa Izod Impact 150 J Charpy Impact 103 J Specific Heat Capacity J/g-ºC Thermal Conductivity W/m-K In a case study of excessive ball valve wear by Exxon, they investigated increasing the hardness of the materials and increasing the seal force from 1,000 lbf/in 2 to 2,000 lbf/in 2[8]. In a ball valve, a minimum seal force is required to prevent leakage, however excessive circumferential seal force can cause accelerated wear and increase the torque requirements. This reference translates the spherical coordinates of the ball into a wear equation to give an equation defining the wear in terms of time as: t=(h*d)/(10*k*d*ns), wheret=time, H=hardness, d= wear rate, N= cycles per hour (constant), s = stress (lb/in^2), D = length in, K= wear rate. In this case study they wanted to compare increasing the effects of the hardness of the seats and the seal force to analytically calculate the life of the valve. They calculated the life of the valve would indeed increase to increased seat hardness, and the valve exceeded their expected life calculation. A study of the PEEK rubbing on CF-30 steel[9], which is relevant to the problem at hand, showed Contact stress showed little effect on the friction coefficient. They also discovered increasing the temp to 150C

5 leads to a higher specific wear for all materials. Counterface roughness and sliding velocity play a role in the wear. They re experiment mentions Hanchi and Eiss (1997) studied the friction and wear of PEEK- CF30 at elevated temperatures. The sliding experiments were carried out on a pin-on-disc tribometer at selected temperatures in the range of C.As temp increased from below to above, the wear performance decreased, and friction increased. For these experiments, the PEEK material used had a glass transition temperature of 143ºC. As one can see in Figure 2 and Figure 3, the results of the experiment in reference[9], clearly show and accelerated wear rate with increasing PV (product of apparent contact pressure and sliding velocity), and increased wear rate as the material is heated, especially when it nears the glass transition temperature. Therefore one can conclude at elevated temperatures, the thermal effect should be considered. - Figure 2: PEEK pin on disk test

6 - Figure 3: Weight loss in pin on disk experiment

7 Theory and Methodology The configurations of a ball valve are a ring on a seat, this type of model can be reduced to aaxisymmetric model for the simplicity of modeling. Figure 4and Table 3show the cross section of the model for axi-symmetric modeling. This model simplifies the shape of the seat to a ball since the ball to seat interface is just a ball on a ball. The 3d representation of this is shown in Figure 7. Figure 4: Model Dimensions Table 3: Model Dimensions Variable Value Units r M r M a M B M Since this is an end loaded ball valve the model constrains the seat to move in the x-direction only, for the purposes of simplicity and the lack of easily available ball valve schematics, the seat is approximated as a ball. These material properties listed above, were input into COMSOL 4.3. In the model, the seat was given plastic properties, while the steel ball was not, as the steel ball should not exceed yield. This will be checked in the model. Since in an axi-symmetric model, the ball cannot rotate within the seat, a 2D model was built as well, using a flat rectangular plate, longer than the circumference of the ball, and a half ball for the seat.using a preload of 100 lbf/in minimum (444.82N/m). Knowing the valve turns 90 degrees in 1 second, the sliding distance L, the velocity of the seat V, and model thickness can be calculated. The model is run as plane strain. The plastic properties were calculated using the data in Table 1. The strain x-coordinate was calculated at a 2% yield offset for simplicity, and a tangent modulus

8 was calculated using the ultimate break strength. Since this was the only non-commerically free for use data available, it was used in lieu of conditional data. The plot of these material properties are shown in Figure 5. A summary of the model inputs are shown in Table 4. A figure of the starting geometry is shown in Figure 6. Table 4: 2D Model Inputs Variable Value Units Rectangle width, w M Rectangle height, h M Seat radius M Seat linear travel M 2D Model thickness M Acceptable material loss M PEEK Elastic Modulus 4.34 GPa PEEK Tangent modulus 52 MPa PEEK Yield Strength 100 MPa Friction Coefficient 0.32 Seat Velocity M/sec Stress (Pa) PEEK Properties 1.40E E E E E E E E Strain Figure 5: PEEK Properties

9 Figure 6: 2D Model Layout Figure 7: Axi-symmetric 3D Representation A quick calculation of the heat dissipated to get the temperature ein the specimen is using Q=M*Cp*T, where M=mass, Cp=specific heat, T if the temperature change, and Q is the heat. If we input the kinetic energy of the sliding ring as all the energy gets turned into heat. The kinetic energy is 7.721x10-8 J, then the temperature increase is about T=0.01C. Therefore the heat effects should be negligible, and can be disregarded.

10 The potential modes of wear are adhesion and transfer, corrosion film wear, cutting, plastic deformation, surface fracture, surface reactions, tearing, melting, electromechanical, fatigue. The most likely candidates for wear in this situation is adhesion, cutting, and tearing. Analytically looking at the wear rate of the seat, using a wear rate found in the material specs from [MATWEB], of K=755x10-8 mm 3 /N-M, a circumferential seal force of 100 lbf/in on the seat parameters in Table 4, yields a normal force of 6.69N, and a sliding distance of m per cycle (1 cycle is back and forth). This yields a lost volume of material for inch ( m) of linear material lost as 5.192*10-8 m 3 of material lost before sealing is lost. Solving for the sliding length yields L= m, which translates into 2576 cycles before sealing is lost in the valve, and the seats will need replacement. This assumes constant hardness, and constant normal force. In reality the normal force of the valve seat will decrease with wear, since the force is created due to the compression of the seat between the ball and the body/tailpiece. The model was run in multiple time steps to attempt to capture the velocity of the seat. Results and Discussion Results are to include the results of analytical methods with their assumptions and comparisons to model executed in Abaqus and/or COMSOL. The wear rates will be correlated where possible to test data to form some sort of a basis. In the models the von mises stress in the steel plate did not exceed the yield strength of the material (193 GPa), so the assumption of no plastic deformations in the steel plate was correct. One optimistic goal was to capture particle wear, and use advanced FEA techniques, such as adaptive remeshing, however computational difficulties (computer was not fast enough), made this goal unobtainable for the timeframe. In order to work around the adaptive remeshing issue, several geometries were made with a worn amount removed from the The time stepping proved somewhat challenging, multiple steps must be taken with contact pairs, first a small gap must be between the seat and ball, then they have to be pushed together slightly, then the preload apply (calculated from the ball and seat dimensions, and circumferential seal load) applied. Finally a velocity is applied for a time step of 1 to get the total motion. For the sake of simplicity, The seat model with no wear exhibits a max von mises stresss 0f MPa. The model is shown in Figure 8. An intermediate step of ~20% wear is shown in Figure 9 with a von mises stress of MPa. The seat was modeled down to 50 thousands of an inch of wear. This is typically considered acceptable

11 wear, but as shown in Figure 10, there may be leakage at this point depending, the max von mises stress is x10 5 Pa. A summary of the results comparing the wear, which is the horizontal amount of material removed from the seat, akin to sanding a cylinder, or what you would measure in the shop with a micrometer is shown in Figure 11. Figure 8: No Wear

12 Figure 9: Intermediate wear Figure 10: Full Wear

13 7.00E E+06 Von mises stress vs wear Von Mises Stress (Pa) 5.00E E E E E E Wear (m) Figure 11: Von Mises Stress vs Wear

14 Works Cited [1R.W. Zappe Peter Smith, Valve selection handbook: engineering fundamentals for selecting the right valve design for every industrial flow application. [2] [3] J. F. Archard, "Wear Theory and Mechanicsms," New York,. [4] Quadrant EPP Ketron 1000 PEEK Polyetheretherketone, unfilled, extruded. [Online]. [5] Overview of materials for Polyetheretherketone, Unreinforced. [Online]. [6] Steven M. Kurtz, PEEK Biomaterials Handbook.: William Andrew, [7] Material Properties for AISI Type 316L Stainless Steel, annealed plate. [Online]. [8] T. Sofronas, "Case 11: Excessive ball valve seat wear," Hydrocarbon Processing, [9] "Thermo-mechanical model to predict the tribological behaviour of the composite PEEK-CF30/steel pair in dry sliding using multiple regression analysis".