In the energy sector, compressing gases and liquids facilitates easier storage, transportation, and processing. The average operating temperature of an industrial compressor used in oil and gas applications is typically between 93°C to 200°C (200°F-400°F), though this can depend on a range of factors such as ambient conditions, compression ratio, and the equipment’s size and efficiency.
When gases are processed, they typically experience a greater increase in temperature than liquids due to their higher molecular motion and compressibility. Petrochemical applications may handle particularly high-pressure gases, which increases the temperature range, while deepwater oil and gas fields may also have a need for high-pressure, high-temperature equipment due to the depth of their reservoirs. Enhanced oil recovery methods are a further example in which particularly high temperatures may be experienced.
Many industrial compressors are equipped with cooling technology to manage heat buildup and maintain optimal operating temperatures. In a multi-stage compressor, intercoolers help to cool the gas or fluid before it enters the next stage of compression. This can improve the overall efficiency and power consumption of the system while helping to ensure the internal temperature does not reach a level that could be damaging to critical components such as seals, bearings, and blades.
When it comes to these components, key considerations include thermal fatigue and potential changes in material properties. For example, at elevated temperatures, materials can become softer and increasingly prone to deformation. Additionally, tight clearances between components are critical for maintaining optimal efficiency in turbomachinery, but these clearances can be threatened by the effects of thermal expansion.
One area where this phenomenon can be noted is the labyrinth seal – a sealing device used in turbomachinery to minimise the leakage of gas or fluids between rotating and stationary components.
The impact on labyrinth seals
“Temperature is probably the biggest consideration and limitation that we face when considering a material for a labyrinth seal,” says Derrick Bauer, Manager, Materials Engineering Department at Elliot Group, an air compressor manufacturer specialising in oil-free, centrifugal technology.
The last few decades have seen a gradual evolution towards enhancing the properties and reliability of labyrinth seals, which have been traditionally made from metallic materials, through high-performance thermoplastics. Elliott Group has been a key proponent of this transition, opting for an abradable polymer engineered by Mitsubishi Chemical Group’s Advanced Materials division. Abradable seals allow for wear of the seal material, which can provide a closer tolerance between the rotating and stationary parts. As the industry standard for abradable polymer seals for over 40 years, Fluorosint 500 can be used continuously at temperatures up to 260°C (500°F).
“Fluorosint 500, an abradable seal material, is our standard seal selection for any new apparatus,” says Bauer. “We do have a limitation in terms of pressure and temperature, and when we exceed that we go to more of a metallic seal. We use [Fluorosint 500] when at all possible.”
Recent insights provided by John K Whalen, consulting engineer and author of numerous papers on thermoplastic labyrinth seals, have also emphasised both the important advantages but also the temperature-related considerations when designing labyrinth seals with polymeric materials, whether abradable or not.
“Efficiency gains are the greatest benefit,” states Whalen. “[Polymers] also have other benefits over the aluminium that is usually being replaced. For example, no or limited tooth tip wear/deformation and, in certain applications, better chemical compatibility.
“The challenges haven’t been that great,” Whalen adds. “There are concerns about thermal degradation over time, and I think more research should be done in this area.”
Comparing the data
While not suited to the abradable seal applications favoured by Elliott Group, other thermoplastics in Mitsubishi Chemical Group’s portfolio are designed for significantly higher temperature environments and can maintain impressive tensile strengths even after long periods of heat exposure.
The most durable of these is Duratron® T4540 PAI, which has a heat deflection temperature (HDT) of 279°C or 534°F. This figure is a single temperature value representing the point at which a material starts to deform. PAI can withstand significantly higher temperatures than other polymers, and is also regarded for its exceptional wear resistance, tensile strength, and low coefficient of friction. Ketron CA30 PEEK is another high-performance material not far behind on its temperature resistance, with a HDT of 232°C or 450°F.
The dynamic modulus is a measure of a material’s stiffness when under stress or strain caused by vibrations or cyclic stress, where a repetitive pattern of stress is applied to a material over time. In the chart below, dynamic mechanical analyser (DMA) temperature curves are compared for all four materials in MCAM’s current labyrinth seal portfolio. The data shows losses in the materials’ dynamic moduli as the temperature of the samples are gradually increased at the same time as cyclic stress is applied.
As the curves show, Duratron PAI maintains its stiffness for a significantly longer period in the face of increasing temperature, making it the optimal choice for thermoplastic seals in high-temperature oil and gas environments. Though Duratron outperforms them, Ketron CM 1030HT PEK and Ketron CM CA30 PEEK also provide impressive resistance to elevated temperatures.
Ultimately, while thermoplastics have potential to significantly improve labyrinth seal performance, selecting the right polymer for use in a high-temperature environment requires careful review of material properties. At Mitsbubishi Chemical Group’s Advanced Materials division, this is supported by a team of specialists who are ready to help, providing high-performance thermoplastic solutions alongside unparalleled material expertise.
To learn more about why compressor OEMs are replacing inefficient labyrinth seals with high-performance thermoplastics for energy customers, download the whitepaper below.
