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Oil & Gas

Maintaining sealing integrity at low temperatures is of vital importance to Oil & Gas operators for two main reasons.

First is exploration and production activities in fields in arctic-type regions where ambient temperatures of -40°C (-40°F) are common.

Second is the adiabatic process that occurs during rapid gas decompression (RGD), where a sudden gas expansion in a closed system can cause the temperature to fall by many tens of degrees Celsius — a process similar to the refrigeration cycle in domestic freezers.     

We have carried out decades of research with elastomers at low temperatures, and developed RGD-resistant materials that work efficiently under such conditions. The table shows the capabilities of our five leading elastomers for Oil & Gas field duties:

Why is low temperature an issue with elastomers?

When elastomers are cooled to sufficiently low temperatures they exhibit the characteristics of glass — they become hard, stiff and brittle. In this state they cannot deform readily and are of little use for fluid sealing. But, as the temperature is raised, the segments of the polymer chain gain sufficient energy to rotate and vibrate, until at a high enough temperature the elastomer behaves in its normal ‘rubbery’ way again.

The low temperature at which an elastomer starts to stiffen is known as the glass transition temperature (Tg). This is also the temperature at which the coefficient of thermal expansion starts to change.

Tg is measured practically by monitoring torsion modulus or temperature reaction, with decreasing temperatures. Above Tg, the motion of chain segments, characteristic of the rubbery state, requires more free volume than the atomic vibrations in the glassy state. (There is also an intermediate ‘leathery’ phase, where free volume reduces and the elastomer becomes increasingly sluggish as brittleness approaches).

Conventional theory claims that the free volume of an elastomer is constant at any particular temperature. This is why rubber is generally considered to be incompressible — it changes shape rather than reducing in volume.

This conventional theory breaks down when considering high applied pressures, because the free volume can be reduced. This manifests itself as a Tg shift which, as a rule of thumb, is in the order of 1°C (1.8°F) for every 5.2MPa (750psi) of applied pressure. (There is also a pressure threshold where the intermolecular forces resist the tendency to a free volume reduction.)

Low temperature testing

         
Torsion modulus

The Gehman test is used to measure the torsion modulus by twisting a strip test piece, at room temperature and several reduced temperatures, to give a torsion modulus curve. The result is often quoted as the temperature is two, five, ten or 50 times the value at room temperature.

However, BS 903 Pt A13 refers to the temperature at which the modulus increases to a specific value, typically 70MPa, which corresponds to the loss of technically usable flexibility. This is partly because measurements of the very low modulus at room temperature have proved unreliable.

Temperature reaction

This is determined by elongating a test specimen and freezing it in the elongated position. It is then allowed to retract freely while the temperature is raised at a uniform rate. Percentage retraction is calculated at any temperature from the data obtained.

In practice, the temperature for 30% retraction (TR30) roughly correlates to ther limit of usable flexibility.

James Walker product-configured test regimes

Our experience with BS 903 Pt A13 shows that the results obtained give an indication of torsion modulus, rather than an accurate representation of seal behaviour at low temperatures.

To overcome this issue, we developed alternative test regimes to assess low temperature functionality. Based on product-configured testing, they replicate more accurately the service conditions found in the field by taking account of seal shape and sealing surface interaction, in addition to material behaviour.

Using our test regime TR2076, we evaluated five of our RGD-resistant materials, and produced the Minimum usable temperature values shown in the table near the top of this page.

With this testing capability, we can simulate accurately the required sealing parameters, and thus optimise seal performance to meet customers’ specific applications.

Click here for a pdf copy of our guide to Elastomeric seals & components for the Oil & Gas Industry.