Under Pressure? Here’s why you need to check the glass transition temperature of your seal’s material

by:Seema Gangatirkar

The pressure exerted in downhole conditions can be a key factor in selecting your seal’s material, especially if your application needs sealing components to work at low temperatures. Here’s why: Pressure changes the glass transition temperature – the temperature above which the amorphous component of a polymer transitions from a hard state to a more rubbery state.

Elastomers are typically used above their glass transition temperature when they are softer and more flexible. However, as pressure increases in an application, it shifts the glass transition temperature upwards. For each 725 psi (50 bar) increase in pressure, the glass transition temperature of an elastomer shifts upwards by a factor of 1.8 on the Fahrenheit scale (1 °C). That means that at 15,000 psi, the glass transition temperature has shifted upwards by approximately 37 °F. At 30,000 psi, the glass transition temperature has increased by more than 74°F (41°C). In addition to assessing chemical compatibility and temperature, a Greene Tweed engineer will consider this glass transition shift under pressure when recommending a material.

For example, if an application calls for 30,000 psi, a Greene Tweed engineer may suggest a material that has a low glass transition temperature such as Chemraz® 678. Chemraz® 678 has a glass transition temperature of -31 °F (-35 °C) at atmospheric pressure, which would enable a sealing temperature of 43 °F (6°C) and higher at 30,000 psi. A similar perfluoroelastomer (FFKM) with a glass transition temperature of 2 °F (-17 °C) would have a minimum sealing temperature of 77 °F (24 °C) at 30,000 psi. These use temperatures are estimates based on the glass transition temperature, but depending on application, there may be other tests that could also give insight into the appropriate use temperature.

In addition to its exceptional reliability to form a seal at high pressure, Chemraz® 678 provides excellent chemical resistance to the majority of downhole fluids and best-in-class Rapid Gas Decompression (RGD) as qualified by a third-party laboratory. RGD is generally found in high-pressure conditions where gas is present. During operation, gas permeates the elastomer, and when pressure is released, the possibility exists that the gas escapes from the elastomer at a rate that could damage the seal. The ISO 23936-2:2011 protocol, Petroleum, petrochemical and natural gas industries — Non-metallic materials in contact with media related to oil and gas production — Part 2: Elastomer, provides testing guidance that can help characterize how RGD-resistant a material is.

Akron Rubber Development Lab, a leading testing laboratory specializing in rubber, plastic and latex, has independently qualified Chemraz® 678 to the ISO 23936 global standard for RGD resistance. Under the standard’s stringent testing conditions, Chemraz® 678 passed with the best possible score of “0000”; this score means that after the conclusion of testing, no cracks were observed in the material, thus decreasing the possibility of a seal failure due to an RGD event.

While temperature and chemical compatibility are key factors in determining the class of material to be used, the impact of high pressure on a seal is also a determining factor in material selection. Backed by an extensive portfolio of materials, Greene Tweed’s field experienced application engineers use additional diligence and take all application parameters into consideration when recommending a material and seal geometry for applications in high-pressure conditions.