Tribological Wear of Ball
Valve Seats
MANE 6960- Friction, Wear, and Lubrication of
Materials
Nathaniel R. Anker
5/16/2015
Table of Contents
Abstract ......................................................................................................................................................... 2
Introduction ................................................................................................................................................... 2
Exxon Case Study ......................................................................................................................................... 3
Ben Jemaa Tribometer .................................................................................................................................. 4
Finite Element Model ................................................................................................................................... 5
Full Valve Model ...................................................................................................................................... 6
Geometry............................................................................................................................................... 6
Material ................................................................................................................................................. 7
Results ................................................................................................................................................... 8
Micro-Structure Model ........................................................................................................................... 10
Geometry............................................................................................................................................. 10
Results ................................................................................................................................................. 10
Conclusions ................................................................................................................................................. 12
References ................................................................................................................................................... 12
Abstract
The study of tribology is crucial in the design of valves. Due to the interaction between the
sealing elements of the valve, wear occurs that directly affects the life cycle of a valve in service.
Specifically, in ball valves a seal is created between the valve ball, usual a metal material, and
the valve seats made of a softer material. Due to the mismatch in material hardness, the valve
seats exhibit wear. In this paper several different methods of examining the loading
configuration and the affect it has on the wear the valve components will be studied.
Introduction
Valves are essential type of fitting to any system as they regulate control and regulate fluid flow
within that system. There are numerous different types of valves such as ball valves, globe
valves, gate valves, butterfly valves, and many more. However, most valves can be broken down
into the same list of general components. The typical components found in the valve include the
valve body, a plug, seats, a stem, and packing to seal the valve.
Figure 1- Valve Components (Ref (a))
As a valve is designed to regulate flow it is critical that the plug properly contacts the seat
surface to create a seal that is leak-proof. Seats are classified in two separate categories: hard
seats and soft seats. Hard seats, made of metal and integral to the valve body, are typically found
in globe and gate valves. Soft seats, made of plastics or elastomers, are typically found in ball
and butterfly valves.
Hard seat ball valves are not typically used as it is difficult to obtain a leak proof seal due to the
metal on metal contact between the seat and ball. However, with the improvements of modern
machining using CNC devices, hard seat ball valves now have applications in power plants
where soft seat ball valves fail due to high temperature and pressure. Soft seat ball valves are
often advantageous due to low cost as well as being small in nature they are easy to design into a
system. As these are seats are made of a much softer material than the metal ball, the design life
of the valve seats must be taken into consideration due to seat wear. In this paper the various
methods used to study ball valve seat wear will be reviewed.
Exxon Case Study
ExxonMobil was experiencing an issue with valves failing in services due to heavy abrasive
wear on the ball valve seats that saw a high cycle rate. A valve that had a historic service life of
two years was failing in 8 months. The sealing pressure had been increased from 1000 psi to
4000 psi. To make up for this change in sealing pressure the seat material hardness was
increased from 371 Bhn to 475 Bhn. Due to the failures seen in use from these changes it was
recommended to increase the hardness to 613 Bhn while reducing the sealing pressure to 2000
psi. Test data did not exist to verify if this would increase the design life of the valve so the
following model was created.
Figure 2- Wear Model (Ref (b))
In the model, as seen in Figure 2, the wear component is the softer material (valve seat) and the
wear volume is the harder material (ball). Where S is sliding distance, 𝜎is the applied pressure
load, V is the sliding velocity, and 𝛿 is the amount of wear that occurs. The following equation is
derived from the model to solve for the amount of time in years, t, for a given amount of wear, 𝛿 .
𝑡=
𝑡𝑡
10𝑡𝑡𝑡𝑡
Where H is the Brinnell hardness of the wear component, K is the wear coefficient, D is a
geometrical factor in inches, and N is the number of cycles. Based on this model is was
calculated an additional design life of 1.65 years. Based on this data, the system went into a
testing phase where the model was verified after running successfully for three years.
Ben Jemaa Tribometer
The above model is convenient to study the life cycle of a given valve configuration, however
due to its simplicity it does not provide insight into the loading seen in the valve seat. Actual
performance testing of the valve materials to understand how the material tribological properties
is the only way to truly understand the valve’s life cycle. One way to do this is by creating a
tribometer as seen in Reference (b). A tribometer is a device that models the contact
configuration between the ball and seat. Reference (b) examines a case study in which a
tribometer is used to examine the contact area between a brass ball between two
polytetrafluorethylene (PTFE) seats.
In order for the tribometer to accurately portray the wear found in a ball valve, it is designed to
meet several important criteria of found in the interaction between the valve seats and the ball.
The configuration of the tribometer mimics the contact of the ball and seats in an actual ball
valve. The loading of the tribometer allows for the clamping loads applied to valve seats and the
sliding of the ball between the seats. A seat clamping force anywhere from 100-1000 Newtons
can be achieved. The sliding frequency to simulate cycling of the ball valve can operate
anywhere from 0.5-2 Hertz. The tribometer measures several variables during operation
including the seat clamping force and operating friction torque. Measuring this variables allows
for studying several important features such as creep and relaxation of the valve materials.
PTFE is a commonly used seat material in ball valves due to its many advantageous properties.
PTFE has a high chemical resistivity allowing it to be used in valves for various fluids.
Additionally it has a low coefficient of friction that allows the ball valve to be operated at lower
torques even at greater seat clamping forces. Additionally PTFE has a higher melting
temperature than other plastic materials. Despite this, a major disadvantage of PTFE is that it
has a low fatigue life due to poor wear performance. As the seats wear a leak path past the ball is
created rendering the valve useless. In the study a brass ball was used. Brass is a typical
material used in ball valves and is much harder than PTFE, and therefore will not wear as
rapidly. The ball was coated in chromium with an average roughness of 0.02 micrometers.
Chromium is another material found in ball valves due to its high corrosion resistance and
hardness.
Finite Element Model
As mentioned above, the best way to understand any tribology system is through testing. In the
tribometer created in the Ben Jemaa case study certain assumptions were made to replicate the
interaction in a ball valve between the ball and the seat material. The rest of this paper will
describe a model using the finite element method in COMSOL used to validate some of the
results found in the Ben Jemaa case study.
Full Valve Model
Geometry
The contact between the seats and the ball within the valve can be simply described as the
interaction between a cylinder and a sphere; where the seat is represented by the cylinder and the
valve ball by a sphere. In order to validate the Ben Jemaa case study it is also crucial to
dimension the components similarly. The following parameters were defined in COMSOL.
Name Expression Description
r_b
11.5e-2
Ball radius
r_s
1.5e-2
Seat radius
Table 1- COMSOL Geometry Parameters
To model the geometry a 2D axisymmetric domain is chosen. A quarter circle with a radius of
r_b is created at the center. Due to symmetry along the z and r axes only a quarter of the ball
needs to be models, allowing for a much faster computing time. Another circle was created 45
degrees from the z axis with a radius of r_s to represent the valve seat. To complete the model
geometry a normal density mesh was generated in COMSOL. The geometry and mesh can be
seen below in Figure _.
Figure 3- Geometry
Material
The material was also selected to recreate the Ben Jemaa Case Study. The ball was represented
by the following properties per Reference (a) to represent brass.
Name
Value Unit
Young's modulus 97e9
Pa
Poisson's ratio
0.31
1
Density
8490
kg/m^3
Table 2-Brass Material Properties
To represent the PTFE seats the properties from COMSOL’s material library were used. As the
wear of the seats is being studied, additional properties were added to allow for plastic
deformation of the PTFE seats. The yield stress of PTFE per Reference (a) is used.
Assumptions were made on isotropic tangent modulus based on the study of stress-strain curves
of PTFE.
Name
Value
Unit
Density
2200[kg/m^3] kg/m^3
Young's modulus
0.4e9[Pa]
Pa
Poisson's ratio
0.48
1
Initial yield stress
12e6
Pa
Isotropic tangent modulus 0.4e4
Pa
Table 3- PTFE Material Properties
Loading Conditions
In the model each surface must be loaded correctly to ensure the valve is constrained properly.
As described in the Geometry, due to axial symmetry only a quarter of the valve ball is modeled.
Both flat edges of the quarter circle were constrained by symmetry boundary conditions.
Between the two surfaces a contact pair is created to ensure there is adequate interferences
between the two bodies. In addition to the contact, COMSOL allows for friction to be accounted
for at the contact pair. The contact pair can be seen below in Figure 4.
Figure 4 -Contact Pair
Finally, a boundary load is applied to top half of the seat to simulate the clamping load applied to
the seat by the valve body. Various boundary loads were used to compare the results found in
the Ben Jemaa Case Study.
Results
A surface fringe plot of the Von Mises stress can be found below. Under a boundary load of
1000 N, the peak stress was approximately 1.1 MPa, well below the PFTE yield stress of 12
MPa. As such the seats did not undergo plastic deformation in this load condition.
Figure 5- Surface Von Mises Stress
As the loading was within the elastic region, the seat did not see stresses beyond yield as such
there was no plastic strain present. As expected the peak strains can be found in the area of
contact between the seat and the ball. A fringe plot of the strain can be found below.
Figure 6- Surface Strain
As the values for stress and strain were found to be low in the nominal case several other finite
element runs were performed for various boundary loads. As expected, with a larger clamping
load imparted on the seat, a greater stress is found. For typically used clamping loads no plastic
deformation occurs. In order to find tangible plastic strains an unrealistic clamping load of
20,000 N was used. While a clamping load this high may create a strong seal between the seats
and ball it is highly disadvantageous as the valve operating torque becomes unreasonably high as
well. Data for the different boundary loads can be found in Table 4.
Load
100
1000
2000
20000
Strain
Stress
Strain
Displacement (plastic)
1.24E+05 2.03E-04
1.04E-05
N/A
1.23E+06
0.00204
1.05E-04
N/A
2.46E+06
0.0041
2.12E-04
N/A
1.34E+07
0.04781
0.00218
0.02441
Table 4- Boundary Load vs Stress/Strain/Displacement
Micro-Structure Model
The above macro model does not describe the entire story in regards to the wear seen in a ball
valve. As designed the loading is well below the yield strength of both materials allowing for a
low risk of failure. However, even a failure on the microscopic level can create a leak path past
valve seats. To verify this a second finite element model was created at a much smaller scale.
Geometry
As mentioned in the Ben Jemaa study, the ball has an average roughness of 0.02 micrometers.
The model below created to equilateral triangles with a height of the average roughness value.
Figure 7-Microstructure Geometry
The upper material is the PTFE seat while the lower material represents the brass ball. The
lower edge of the brass was fully contstrained. The upper edge of the PTFE seat was given a
downward displacement into the sphere.
Results
Seen below is the surface Von Mises stress in the contact area between the two asperities. It is
noted the peak stresses are found in the brass. However the peak stress, approximately 71 MPa,
is below the yield stresses found in brass, 124 MPa.
Figure 8 - Microstructure Asperity Von Mises Stress
It is observed that the stresses found in the area of contact in the PTFE are approximately equal
to the material yield strength. However, this is due to nature of PTFE in plastic deformation, as
the tangential elastic modulus is much lower compared to the Young’s modulus.
Figure 9- Plastic Strain
In the areas above the PTFE yield stress deformation occurs by way of plastic strain. This is due
to the fact that the PTFE is a weaker and softer material than brass. The data discussed here is
for a single static displacement. As the valve cycles potentially thousands of times in a life cycle
contact of this nature will occur, creating larger values of strain. Eventually, fractures will occur
creating a leak path through the lost material. Additionally wear issues could occur such as
erosion due to the debris or metal asperities ploughing through the seat material.
Conclusions
Based on the results of the finite element analysis, it can be shown the models used to investigate
valve seat wear adequately predict stresses and strains found in operation. The theoretical and
finite element models accurately predict preliminary stresses and strains for different material
combinations for seat and ball materials. It is also advantageous to use this methods during
preliminary design as they can often be cheaper than testing. However, testing offers the most
accurate results as certain assumptions will be made in any model. Additionally, the finite
element model used in this study provided a reasonable baseline for the material combination in
this study. However several improvements can be made to the model. While the model used a
non-linear solver due to contact and plastic material, including additional, viscoelastic properties
for the seat material could be used. While this will help the accuracy of the results, it comes at
the cost of much higher computing power needed to run the model. In conclusion, the finite
element model used in this study quantified the results to validate the materials chosen to
proceed with cycle testing the valve.
References
a) Jemaa, M. C. Ben, R. Mnif, K. Fehri, and R. Elleuch. "Design of a New Tribometer for
Tribological and Viscoelasticity Studies of PTFE Valve Seats." Tribol Lett Tribology
Letters 45.1 (2011): 177-84.
b) T. Sofronas, "Case 11: Excessive ball valve seat wear," Hydrocarbon Processing, 2002.
c) http://www.ezlok.com/TechnicalInfo/MPBrass.html