By John An – Polymer Concepts Technologies, Inc. (PCTI)
The nature of valves used in extremely demanding applications earns them the designation of severe-service valves. The function of severe-service valves is defined and driven by higher temperature and pressure requirements to satisfy the industry’s desire for increased production efficiency. Valves and their sealing components work harder as the industry goes deeper on land and offshore.
In a natural gas environment where cryogenic temperatures are the norm, it is critically important that polymer seals are compatible with the task. There is a heavy focus on controlling fugitive emissions, so natural gas valves and their sealing components face even greater scrutiny.
Let us review three common valve seal applications, and then answer four questions about what to consider when selecting them.
Subsea
API-standard valves that perform at 15-25 ksi and 350-400°F are best suited for subsea applications as the combination of high pressure and temperature with corrosive and toxic fluids rapidly degrades the valve material. Since there is no tolerance for leakage, designing the right sealing system for the valve stem is a top priority.
Natural Gas
The seal materials used in the cryogenic valves tend to become brittle and shrink making them difficult to seal over time. Materials soften in high temperature and pressure environments. Material degradation can cause product integrity challenges. Valves designed to API 6A and ISO 15848 standards offer the most comprehensive temperature, pressure, and fugitive emission protection.
Oil and Gas
Flammable fluids are a real hazard for the oil and gas industry, so fire-safe valves and valve sealing materials are worth considering. Products designed to API 6FA specification meet the test and evaluation requirements for valves exposed to clearly-defined fire conditions.
Attention to each of the valve seal applications mentioned above can help reduce the risk of equipment failure. Here are a few more things to consider.
1. What is the role of hardware and surface finish in valve seal performance?
It is very important to have proper gland designs to achieve optimum seal performance. The hardware conditions are critical to seal performance, such as gland styles and sizes, tolerances, extrusion gap, lead-in chamfers, surface finish, and surface hardness. A proper lead-in chamber will prevent seal damage during installation. When sealing at high pressures and/or temperatures, the extrusion gap behind the seal becomes critical. It should be held to a minimum to prevent or minimize extrusion of the seal material. Having a smooth sealing surface significantly reduces abrasive wear and enhances sealing ability. We recommend the 8 RMS and 16 RMS for the dynamic and static surfaces, respectively. High surface hardness (55 to 65 Rc) is recommended to make the sealing surface impervious to erosion and abrasion. Hardware can be either coated or plated to achieve the proper surface hardness.
2. How are oil and gas valve seals tested?
Testing is critical because of the cost of failure. The valves and sealing materials used in oil and gas applications must be tested and validated. Validation begins with establishing a high level of competence via finite element analysis (FEA), which is a simulation of how the seal design and material behave in the application based when subjected to the pressure and temperatures they will experience in the field. The FEA analysis is backed by in-house or third-party testing in conjunction with OEM testing. Finally, application experience provides an additional layer of performance assurance. Many seal manufacturers have a long history of working with some of the most difficult and challenging applications in other severe-service industries, such as space exploration, with similar seal design parameters.
3. What should a design engineer know about seals and sealing materials for valves on wellhead equipment?
The common type of valves used in the oil and gas industry are ball valves, control valves, butterfly valves, safety valves, cryogenic valves, and gate valves. The seal materials are determined based on the operating conditions, such as media, temperature, speed, and pressure. The seal performance can be significantly improved by considering or developing the materials with low friction and excellent heat and wear properties. Designing highly engineered bi-directional stem packing assemblies with proper seal materials should meet the leakage, friction, and seal life requirements under extreme conditions; e.g., 25,000 psi, +500°F, abrasive and corrosive wellbore fluids, and reciprocating shaft.
4. Are there any technological advances on the horizon for valve seal manufacturing?
We see a general industry trend in the use of a combination of newly developed materials and custom designed seals with certain configuration. The seal configurations can be face seals, radial seals, and fully-contained bump seals (or anti-blowout seals) which are used for static or dynamic conditions. The use of energizers and custom spring designs are also on the rise. PCTI has a proprietary Polycon material technology and process. This technology allows the company to provide sealing solutions for extreme operating environments characterized by temperatures ranging from -450° to 600°F, system pressures up to 30,000 psi, and intensely abrasive and corrosive media.
Regardless of the application, severe-service valves and seals are important for oil and gas industry. When equipment fails, it has a significant impact on human safety, the environment, and the total cost of downtime and repair. Valve-related equipment failure can be significantly reduced or eliminated with the proper seal selections.