California law requires facilities with certain process streams to limit the amount
of product “leaking” to the environment through gaskets and packings. The
released products are called “fugitive emissions” and exceeding permitted limits
can result in large fines. A facility had very large diameter valves which regularly
exceeded their fugitive emission limits. Valve replacement was considered to be
cost prohibitive and there was no guarantee new valves would meet the very
strict fugitive emission requirements. Several attempts were made to eliminate the problem with a trial-and-error approach following vendor recommendations. None were successful and one even resulted in a small fire. The facility engineer responsible for this unit realized he could not solve the fugitive emission problem if he did not fully understand the leakage mechanism.
Situation
Problems Cannot Be Solved If They Are Not Understood
Engineering Specialties Used
Finite Element
Fluid Mechanics
Heat Transfer
Thermal Stress
Solution
Finite Element Modeling and Closed-Form Calculations
A two-dimensional axisymmetric finite element model was built to
understand the temperature distributions and thermal displacements during
operation. The same model was then used to determine packing stresses, in particular, contact stresses between parts. A fluid analysis was performed using closed-form empirical equations to determine fugitive emission rates. These results compared favorably with the plant’s experience. A remediation design was prepared and tested using the same analytical approach to demonstrate compliance with regulatory limits. The parts were built and installed. The facility has not had a fugitive emission exceedence since. (Note: the analysis was also used to show how the parts passed a hydrotest in the shop but leaked in the field.)
Summary
Understand Physical Behavior With Mathematical Modeling
By understanding why the valves were exceeding fugitive emission limits during operation, a fix could be devised and tested analytically. Comparing the current configuration fugitive emission levels to calculated levels provided confidence in the analytical model. Argos Engineers is ready to partner with your facility engineers and maintenance personnel. Understand why something is not working properly before selecting a “fix”.
