Using seals as a diagnostic tool

January 5, 2010

Elastomer seals and their wear can tell the offshore equipment maintenance engineer a lot about how well equipment is operating and provide early warning of potential problems. By visually examining the seals as part of a preventative maintenance schedule, problems such as overheating equipment, excessive pressures, poor fitting seals and incompatibility of elastomer, can be spotted early before they lead to equipment failure. Precision Polymer Engineering's Steve Jagels explains.

Elastomer seals work by being compressed between two surfaces to create a mechanical barrier that physically separates two fluids through the use of elastic force. It also creates a differential pressure between the external environment, the seal and internal equipment atmosphere, which is used by the seal to generate additional sealing force against the mating surfaces. Through this mechanism elastomer seals can provide effective sealing to very high pressures. Elastomer seals are preferred over metal or plastic seals as they are more compliant and able to conform into much more rough, uneven surfaces. Elastomer seals are often selected over PTFE seals due to the ease by which PTFE seals may be scratched and damaged in dirty environments.

The role of o-ring seals within offshore equipment is widespread and varied with sizes ranging from only a few millimeters in downhole tools to meters in diameter for pumps and compressors. It is common for seals to be exposed to a wide range of aggressive chemicals, high pressures and elevated temperatures. These include carbon dioxide, methane, hydrogen sulphide and nitrogen in various combinations, temperatures from –40°C up to 230°C, and pressures up to 30,000psi. This combination means that seals are exposed to gases and gas mixtures with extraordinary chemical and solvent power. Add to this steam and acids, aggressive amine inhibitors and alcohols, rapid pressure changes, and the mechanical damage arising from abrasive muds; and it's easy to see why seal material selection and component design are critical to the efficient operation of the seal.

Seal inspection and replacement is a feature of a piece of equipment's preventative maintenance schedule. Moreover, visual inspection of a seal can also help the maintenance engineer relate the condition of the seal to the performance of the equipment. Equipment that is running too hot, for example, will be easily detectable by the seal's degraded appearance. It will also show instances where the wrong elastomer has been selected or the seal may have been fitted incorrectly, both factors will have an effect on overall equipment performance.

Diagnostic indicators
The most common seal diagnostic indicators are: thermal, chemical, explosive decompression and extrusion under pressure. The immediate cause of damage to the seal should be apparent from its location in the equipment being maintained.

Thermal seal damage: Thermal degradation of the rubber can take a number of different forms, for example with silicone rubbers, as the polymer is degraded; the seal feels 'sticky'. With other elastomers, the material typically increases in hardness and rigidity, and takes on a 'compression set', where the seal does not recover when it is removed from the application.

If thermal degradation of the seals is occurring, two solutions can be considered. Switch to an alternative higher temperature resistant material; Figure 1 (below) shows typical maximum service temperatures for different elastomer grades.

Alternatively, there may have been changes to the seal's operational conditions caused by a wider equipment problem. By utilising thermal analysis techniques such as a Thermogravimetric Analyser (TGA), Differential Scanning Calorimeter (DSC) and Fourier Transform Infra-Red (FTIR), it is possible to evaluate a failed seal and correctly determine its failure mechanism. This might indicate that the elastomer is correct; however 'hot spots' may be occurring close to the seal suggesting an equipment problem that needs resolving such as faulty cooling pump.

Chemical damage: Signs of chemical damage to a seal are hardening or softening. In extreme cases the plasticisers and other process aids used to manufacture the elastomer may, as the result of chemical attack, leach out producing an overall volume loss and less flexible seal.

Seal hardening or softening is the result of chemical attack that is progressively either increasing the crosslinking (vulcanization) of the polymer to produce a more rigid seal or unzipping the polymer's crosslinks to produce a softer seal as is the case with some grades of fluoroelastomer.

Fluoroelastomers (FKM) seals are widely used in oil and gas equipment due to their resistance to a wide range of chemicals. However this type of elastomer typically shows poor resistance to hot water and steam. Therefore, maintenance engineers have to replace FKM seals more frequently than perhaps they would like, if seal failure is to be prevented. The issue can be easily avoided when it is noted that not all fluoroelastomers materials are the same.

Within the FKM family of materials, there are differing 'cure systems' – the chemical cross-linking reaction which occurs to join the polymer chains together. Common FKM sealing materials are of two types: co-polymer and terpolymer. The widely used traditional, bisphenol-cured, co-polymer fluoroelastomers are cured by utilizing a 'condensation' reaction where, water is generated during the cure process. When the fluoroelastomer is exposed to high temperature water and steam environments, the cure is reversed, breaking down the cross-links of the material, leading to premature failure of the seal.

Peroxide-cured, terpolymer fluoroelastomers, on the other hand, do not suffer the reverse condensation reaction. Instead, the peroxide cure system is a 'free-radical' reaction, and as such, provides superior water and steam resistance. This, and other compounding techniques, now enable FKM elastomers to withstand steam up to 200°C.

Effect of pressure: High pressure offshore applications such as pumps and compressors place the most arduous physical demands on elastomer seals. Seals are prone to extrusion and explosive decompression, and thus visual inspection is particularly important if equipment failure is to be avoided.

Elastomer extrusion occurs when the seal is exposed to high pressure and temperatures and begins to soften and extrude through the clearance gap between the seal's mating surfaces. Signs of extrusion are when the seal begins to show signs of 'flash' or a thin film of sealing material where the seal has flowed into the gap. To prevent extrusion either switch to another elastomer better suited to the operating environment or use a PTFE back ring on the outside of the seal groove that acts as a barrier preventing the seal extruding through the gap.

Explosive decompression (ED) can occur when gas permeates into an elastomer under high pressure and the pressure is released rapidly. The pressurised gas comes out of solution and expands suddenly which can rupture the elastomer in a catastrophic manner. Telltale signs of explosive decompression damage are partial cracks and blisters. Take these warning signs seriously, ED is occurring but so far the seal may not be leaking, the next stage could be catastrophic failure of the seal.

The extent of a seal's ED resistance is a mix of the choice of elastomer, design and manufacture. Design is important in ensuring the seal fits, and can be fitted, correctly. A poorly designed seal will introduce stresses that could weaken the polymer structure making it more vulnerable to explosive decompression or chemical attack. Similarly high quality, seal moulding processes are essential for seals exposed to high pressure to prevent micro cavities being formed that can weaken the elastomer structure, during seal manufacture.

One way to reduce the incidence of explosive decompression is to allow for longer decompression periods, enabling gas to exit the elastomer more slowly. Another approach is to modify the seal design to improve the mechanical performance of the seals under pressure.

For example, two 4-cylinder and four 6-cylinder HOS reciprocating compressors, located on three floating platforms, operating at 150°C, at a pressure of 200 bar with a decompression to 12 bar in one minute, would suffer explosive decompression failure on the valve head seals (from 3000 revs to stop in six seconds) after just 10 cycles, resulting in gas leaks. Due to HSE safety concerns, the compressors were required to achieve at least 30 cycles before failure.

Using a combination of an L-shaped ED FKM seal with PEEK back-up ring to provide high levels of resistance to explosive decompression, the compressors consistently operated without a single leak occurring for 30+ cycles. On removing the FKM seals after use they were found to be completely intact, showing little sign of explosive decompression.

Increased efficiency
The outcome of using elastomer seals as part of the maintenance engineer's repertoire of equipment diagnostic techniques will almost certainly be an improvement in seal performance and, as a result, a positive benefit to equipment efficiency overall. OE

About the author
Steve Jagels is a materials technologist with Blackburn, UK based oil and gas seals maker Precision Polymer Engineering. An authority on the development of high performance polymers, Jagels was recently appointed the company's US business development manager, based in PPE's US east coast office in New Jersey.

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