by Steven Jagels
August 23, 2016

The modern oil and gas industry requires equipment manufacturers to build robust, safe and high-performance products that can deliver reliable results over extended periods of time and in the face of many challenging conditions. The environments in which equipment must perform are fraught with extremities, such as high and low pressures, extreme temperatures, and exposure to corrosive chemicals, all of which can compromise the integrity of mechanical parts. Additionally, the availability of routine repair and maintenance in remote access locations, such as those on the sea floor, is greatly marginalized.

For valve manufacturers in particular, understanding these challenges is critical. Valves are integral throughout the oil and gas production workflow, since they control the movement and flow of hydrocarbons. Without valves, no operating company would be able to deliver its service.

Valves are required to perform in subsea or even arctic environments where subzero temperatures and chemical interactions threaten the operational stability of the valve for a number of reasons. Enhanced oil recovery (EOR) operations such as steam, sour gas or carbon dioxide injection, can also subject valves to extreme conditions that could affect their performance. The key issue is that the sealing materials used within these crucial valves can, if not properly specified, become damaged and fail in situ.

This is highly problematic for any operating company because the cost of failure can be catastrophic, leading to severe financial loss or downtime. In the most extreme cases, it can even threaten the health and safety of personnel charged with delivering the repairs.

Although many manufacturers provide high-temperature resistance in materials for oil and gas valve applications, few offer a single elastomer that can seal low temperatures, sour gas, high-temperature steam and high-pressure gas (resist rapid gas decompression). This article will review these limitations of sealing materials as well as the potential impact these factors can have on working seals.

Valve Sealing

O-rings or chevron packings, which provide robust sealing for elastomeric materials, are common configurations used to seal valves both in the body and valve stem. Elastomers conform to and fill in the gaps between the mating surfaces of metal components, ensuring there are no leak paths. O-rings are typically energized by pressure to create an effective seal on a valve, but they also require the resiliency of the elastomer to react to pressure and temperature cycles that are part of a valve’s operational life.

Changes to the elasticity of the material, due to time or physical environment, can impact the reliability of the seal. It has been shown that chemical degradation of an elastomer will change its low-temperature performance.1

A valve with a long maintenance interval will need sealing materials that have little change in properties caused by thermal or chemical processes throughout the service life of the seal.

Types of Materials Used

A select few types of elastomers are used as seals in valves for the oil and gas industry. These elastomer types include the following:

  • Nitrile Butadiene Rubber (NBR)
  • Hydrogenated NBR (HNBR)
  • Fluoroelastomer (FKM)
  • Tetrafluoroethylene/propylene (FEPM)
  • Perfluoroelastomer (FFKM)

Some general performance properties are common to each group, such as temperature range, chemical resistance and glass transition (Tg) temperature (the temperature at which the elastomer will become brittle and unable to form an effective seal during cooling). However, within each of these elastomer families there are also variations differentiated by monomer ratios and cure chemistries.

Each of these materials will have trade-offs in performance in sour gas and low-temperature resilience as shown in Table 1. Three materials are fully resistant to sour gas (FEPM, FFKM and low-temperature FFKM), and, among the elastomer types shown, only low-temperature FFKMs have low-temperature capabilities below zero C.

The relative sour gas resistance of elastomer typesTable 1. The relative sour gas resistance of elastomer types and their respective glass transitions, approximate low- and high-temperature performance, and steam resistance. A rating of 1 for sour gas resistance indicates that the elastomer does not react to hydrogen sulfide. (Courtesy of PPE)

Specifications

Several factors can affect an O-ring’s low-temperature performance beyond the polymer’s glass transition, and a definitive relationship between laboratory results and service performance is not well-established. Additionally, the fluid in an application may permeate the polymer and improve low-temperature properties.2, 3, 4 The determination of the glass transition via differential scanning calorimetry (DSC) has become more prevalent as an analytical technique used to compare the relative low-temperature performance of different elastomeric materials in the laboratory.

Low-temperature performance on valves and wellhead equipment is important for meeting standards such as International Organization for Standardization (ISO) 10423 of minus 18 C, which requires low-temperature reliability to pass to pass the American Petroleum Institute (API) 6A PR2 test. While the low-temperature callout needs to be low enough for the valve to function both in cold climates or subsea with some safety margin, the elastomer often needs a greater low-temperature rating than the valve.

Factors such as cold set, temperature changes and response time to high pressure will require low-temperature performance to be lower than the valve’s rating. The low-temperature performance is in addition to the often high-temperature requirements observed in service.

Overcoming Low Temperature & Sour Gas Impacts

Sealing materials used in critical oil and gas valves should be proven to withstand all environmental conditions, exhibiting excellent low-temperature elasticity, high-temperature steam and sour gas resistance. Currently, only one material can meet all of the requirements, and that is low-temperature FFKM. This material has been proven to meet the demands of rigorous oil and gas operations and can withstand all of the anticipated temperature, pressure and chemical conditions common to the environment. Among the elastomers discussed in this article, only low-temperature FFKMs show no deterioration in sour conditions and have resilience at a temperature viable for subsea or arctic conditions.

Valve manufacturers should seek a specialist sealing partner to work toward the joint development of custom-made FFKM seals that are made to conform with the internal dimensions of the intended application, while delivering the performance characteristics previously mentioned.

References

  1. Tripathy, B., & Smith, K. (1998). The Myth About Low Temperature Performance of Fluoroelastomers in Oil Seal Applications. International Congress and Exhibition. 980850, p. 5. Detroit MI: SAE International.
  2. Warren, P. D. (2007). Low Temperature Sealing Capability of O-rings: The Relationship Between Laboratory Tests and Service Performance Under Varying Conditions. High Performance and Specialty Elastomers Fourth International Conference Proceedings. Smithers Rapra Technology.
  3. Thoman, R. A. (1989). Low Temperature Performance Characteristics of Elastomers. 40th Annual Earthmoving Conference. 890988, p. 4. Peroria, IL: SAE International.
  4. Stevens, R. D., & Thomas, E. W. (1990). Low Temperature Sealing Capabilities of Fluorelastomers. SAE International Congress and Exposition. 900194, pp. 7-8. Detroit, Mi: SAE International.

The author gratefully acknowledges the assistance in preparation of the manuscript by Eric Crawford and Mike Nelson of PPE LLC in Houston, Texas.