Finite element analysis has enabled engineers to virtually predict material and product performance, eliminating non-functional possibilities and establishing the best material and design without the cost and time of full-scale production testing.
FEA can be used to test an ever-expanding range of materials in new ways by using innovative models. Improvements in the virtual testing process continually come about, resulting in more accurate modeling and confidence on the part of the customer.
Simple 2-D models to complex 3-D models can be created, and each has specific applications. Two-dimensional models are typically used for axi-symmetric analysis, allowing engineers to learn how a product will install, grow from thermal expansion, and react to forces applied.
Elements of a Successful Model
There are several well-established elements in creating a successful model for FEA.
1. A clear and comprehensive plan of boundary conditions such as hardware interaction with the product or seal.
How will the product or seal fit into the surrounding hardware? Does it need to be stretched or contorted for installation? Making sure the product or seal is accurately modeled before other parameters are applied is extremely important.
2. A clear and concise understanding of what forces or stresses the product or seal will be subjected to.
Will the product or seal be exposed to temperature fluctuations? Will the product or seal be exposed to pressure? What about dynamic movement of the hardware surrounding the product or seal? All conditions the seal will experience must be identified and accounted for in the model.
3. A library of accurately modeled materials, both metallic and non-metallic.
We perform almost all of our analysis on non-metallic materials, which require non-linear modeling. This non-linear modeling is dependent upon the quality of the material properties used in the model. Therefore, all materials must be tested in the real world to determine stress-strain curves, Poisson's ratios, and all physical material properties. It is critical to test the materials at various temperatures, as non-metallic materials have non-linear properties. Having accurately tested material properties will greatly increase the quality of the output data.
4. A robust FEA program.
It is crucial to select a testing partner that offers the latest in modern technology and understands how to use it. Flexibility is also crucial to allow for adaptation or a new direction in analysis.
5. A team of analysts representing a range of specialties.
No FEA is routine, and a team of experienced people can provide valuable input and ideas to circumvent potential pitfalls. Criteria include a high level of experience in FEA among the team leaders and engineers involved in the design of the product or seal. An FEA analyst may not fully understand the complexity of an application, and product engineers can fill that void.
New developments in sealing technology require the use of advanced design techniques, and a complete solution requires pre-processing, solution and post-processing for effective nonlinear FEA.
FEA teams need to have access to the latest software for contact, large strain, and multi-physics analysis available today to solve static and dynamic nonlinear problems.
Finite element analysis has become a critical part of the seal development process. Many end-users claim to have linear analysis capabilities, but few are able to consistently and reliably solve nonlinear sealing problems involving changing contact conditions between components and/or large strain (plasticity or elastomeric behavior, for example).
Because of these limitations, there are many aspects of seal behavior that are not well understood until physical prototypes are available. It must be understood that FEA results are not definitive proof of a product or seal's performance. It is an analysis and design tool to be used alongside traditional design tools; FEA should complement the complete design process. Product and seal testing in the lab or field is still needed to prove performance, and using FEA results alone to qualify a product or seal is insufficient. Not fully qualifying the product or seal leads to late and expensive design changes, product failure in the field, and, sometimes, safety issues.
Matching FEA to Sealing Requirements
FEA capabilities make possible:
- Seal failure prediction from cracks, leaks, or deformation, allowing a better-designed product or reverse-engineering of existing problems to discover root causes.
- Assembly force requirements to install the seal, allowing end-users to determine whether specific tooling is needed.
- Temperature simulation to determine thermal effects on various seal materials.
- Thermo-viscoelasticity simulation to analyze the effects of creep and viscoelasticity over a period of time, determining sealing load long after installation. Elastomers can be tested for properties such as hyperelasticity and Mullins effect.
- Friction analysis to determine breakout torque and running torque of rotary seals, or actuation force of linear seals.
Modeling space should include 2-D models for axisymmetric, plane strain, and plane stress analysis as well as 3-D modeling of both seals and complex client hardware with exact materials and geometry in order to determine loading or recommend design changes. Ideally FEA modeling should be able to be performed on composites, such as an elastomer with fabric or resin with fabric.
Real-World FEA Applications
To show how FEA lends itself to the real world, here is a scenario of a high-pressure (15,000 psi) and high-temperature (300 degrees F) hydraulic actuator used in a ram-type blow-out preventer. The seal that the customer chose was a venerable v-stack using a "face-to-face" bidirectional design and a PEEK hat ring to separate the two halves of the seal assembly.
As seen in the first graphic, the PEEK hat ring began to buckle at only 7,150 psi.
FEA revealed that a stronger material was needed for the center hat ring, and stainless steel was chosen. A secondary analysis revealed that the hat ring supported the v-stack up to the required 15,000 psi at 300F and that was what helped to convince the customer to purchase the seal assembly.
A real-world test ensued in the customer's lab and the seal stack passed API testing. Without the aid of FEA, considerable expense and time would have been wasted to discover the PEEK hat ring was insufficient. Analyzing the seal assembly in the virtual world allowed the customer to qualify the seal in one test, solidifying confidence in the sealing solution.
Another such example of the power of FEA took place in looking for a solution that could handle extreme forces for a high-pressure wellhead cap application. The pressure would be 22,500 psi, which caused the customer's hardware to balloon and distort, opening a bigger-than-preferred extrusion gap. Due to the nature of this application, a seal with an angled PEEK back-up ring was proposed. The customer did have a key question that FEA would help to determine, wanting to know if the seal could accommodate a larger extrusion gap at 22,500 psi, as shown in the "loose" graphic below.
FEA revealed that the seal was able to hold the pressure in both the designed and "loose" state, while the backup did not suffer significant extrusion. FEA also allowed engineers to visualize the internal material stresses to confirm that the seal and back-up materials were not overstressed, as shown in the below graphic.
Even though FEA indicated satisfactory performance, verification of the virtual results in the real world is always needed. After a test at the customer's lab, the seal was confirmed to hold pressure and not extrude. Given that the resources for lab testing can be exorbitant, FEA gave the customer the confidence needed to move forward with testing and helped to ensure a successful application.
FEA offers important advantages. It shortens the design optimization process, while improving design and seal performance through integrated software simulation. In addition, it provides reliable seal analysis capabilities to reduce development costs and lead times. Developers can greatly enhance and add value to their end-product through the use of FEA modeling.[images via Trelleborg Sealing Solutions]
Eric Bucci is segment manager Oil & Gas Americas, Trelleborg Sealing Solutions. He has a BSME degree from the University of Texas and started working for a Houston seal company as a seal design engineer in 1992. After four years he moved to Busak+Shamban (purchased by Trelleborg in 2007) as an applications engineer and has now worked there for 20 years. His responsibilities include market development and identification of future growth trends for Trelleborg Sealing Solutions products.
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