With the discoveries of new natural gas deposits, including shale formations, competition is intensifying for LNG supply contracts, making it imperative that LNG operators achieve the lowest delivered costs by optimizing reliability and attaining larger economies of scale.
Advancements in technology are supporting the pursuit of these goals, allowing producers and shippers to rely on a new generation of extremely reliable centrifugal compressors typically equipped with mechanical dry gas seals developed and sized specifically for refrigerant and boil-off compressors used in the LNG sector.
At onshore terminals, new, ultra-large capacity processing equipment is being used to increase production rates and reduce the percentage of natural gas diverted to power the liquefaction process. Compressors as large as 132 tons have been developed for this application, in turn requiring the development of a 15.35-inch dry gas seal to accommodate an increase in maximum compressor shaft sizes from 13 to 13.8 inches.
Good shaft sealing technology is critical to compressor efficiency and reliability. Without a highly effective and consistently reliable shaft seal, the compressor would lose efficiency and in the worst case, suffer costly unscheduled shutdowns.
A dry gas seal used in everyday industry cannot simply be upsized for LNG compressors, the design and materials selection must anticipate all operating conditions in order to function properly.
Likewise, the testing and manufacturing is extremely demanding and critical to safety and reliable performance during extended operations. As a rule of thumb, a seal should require no maintenance.
It is common practice to replace dry gas seals at scheduled overhauls of the compressor - sometimes left to the operator’s discretion, but typically occuring at five-year intervals. LNG mechanical seals are typically designed to function reliably in temperatures ranging from- 274°F up to +446°F.
The major challenges for designing LNG seals are:
- Selecting the best materials for very low temperatures
- Accounting for temperature fluctuation on sealing materials with different thermal expansion coefficients
- Assuring safety and reliability in all operating conditions
Dry gas seals used in onshore liquefaction are normally operated in the range of 104°F. Their low temperature tolerance is required only for special operating conditions.
In LNG terminals, the 15.35-inch seals being installed on the new ultra-large horizontal split barrel type compressors have an outer diameter of up to 21.65 inches and fit a shaft diameter of 13.8 inches.
For this product, EagleBurgmann employed a safety-critical dry gas seal design using two single seals in tandem arrangement - a primary and secondary with a labyrinth in between - to ensure zero natural gas leakage to the atmosphere where it could ignite.
The secondary seal is capable of operating under the same conditions as the primary. To avoid any oil immigration from the bearing into the seal, a carbon ring seal as oil separation was used.
The smaller compressor / expander plants installed onboard LNG vessels typically employ geared-type compressors.
The mechanical seals used in the compressor stages are subjected to moderate temperatures and don’t require special materials, but must be capable of withstanding other operating stresses like vibration and rough seas.
Furthermore, the overhang design of the compressor requires that the dry gas seal be as short as possible, so a design employing a single seal plus oil separation seal is used.
Expander stage seals present the bigger challenge in sealing gases at extreme low temperatures. The seals used on the expansion turbines are exposed to extreme cool-down to -348°F in regular operation.
Usually, nitrogen is the process gas used in the secondary cooling cycle. For these dry gas seals, special metal materials and elastomer-free secondary sealing elements were selected.
The EagleBurgmann dry gas seals for LNG re-liquefaction compressors / expanders were designed for varying conditions.
When considering the materials selection criteria for low-temperature operations, EagleBurgmann ensures that for all metal parts and secondary-sealing elements, there are properties to cope with temperatures as low as -320.8°F.
All secondary-sealing elements are made of polytetrafluoroethylene (PTFE) with special filler material to withstand low temperatures.
For the primary sealing elements, the rotating seat and stationary face employ EagleBurgmann's proven standard solution used since 1992: silicon carbide against silicon carbide. These faces are especially large in a 15.35-inch seal, so compensating for temperature deformation is especially important.
The rotating seat is turning together with the compressor shaft while the ceramic stationary face is fixed by torque pins to the metal housing. The balance sleeve is shrink-fitted to the housing. Tungsten carbide is used for both the balance sleeve and supporting ring for the sealing element, being the preference for low temperature coefficient and because, when polished, tungsten carbide gives a very high quality, smooth surface.
The stationary face has to be sealed reliably against the balance sleeve to avoid any leakage bypass and the stationary face must move axially to compensate for any shaft movements.
During extreme temperature fluctuation, the gap will vary dramatically. If it's too large, good sealing function is not ensured, and if it gets too small or even disappears, it loses axial flexibility.
This required special attention to cope with the temperature extremes and the -211°F standard operating temperatures for expander seals. The EagleBurgmann PDGS design of the polymer cup seals ensures minimum sliding forces combined with wear-free operation.
Proper tapering of the seal face is another critical part of designing a dry gas seal for extreme temperatures.
It begins with predicting the seal face deformation. The solution is to size the seal face taper at room temperature for optimal geometry under ordinary operating conditions, applying EagleBurgmann’s validated calculation codes and extensive experience to get the optimum result for extreme heat or cold temperatures, then fine tuning the result through extensive testing.
The rotating seat of a dry gas seal compressor seal is grooved to ensure the gas separation film forms and stabilizes during dynamic operation, including compressor start-ups. These proven 3D grooves ensure optimal pressurization within the sealing gap (between rotating seat and stationary face).
EagleBurgmann engineers simulated operating conditions to generate an optimum operational sealing gap below 5 μm and ensure reliability of the gap under all operating conditions.
Every single LNG mechanical seal is subjected to rigorous cryogenic testing under simulated operating conditions at an EagleBurgmann test center. Liquid nitrogen is used to cool down the seals to -274°F prior to the dynamic test.
The master tests for each seal also simulate particular operational issues of LNG compressors that represent major challenges for dry gas seals such as:
- The impact of rotor deformation that might occur in variable or low-speed operations
- Compressor coast down from shaft misalignment, simulated by reproducing a forced plant shutdown more than 120 times
Very large seals, like the 15.35-inch LNG seals, are the most challenging to manufacture because the geometrical tolerances have to be the same as smaller seals. Geometrical tolerances have little to do with the size of the seal.
Since EagleBurgmann delivered its first 2.4 - 2.75-inch LNG seals for maritime transport applications in 1999, we have built up an installed base of more than 350 seals worldwide.
Then in 2005, we undertook to develop 15.35-inch cryogenic seals for onshore LNG operations for one of the world's leading compressor manufacturers. As a result of that success, the OEM commissioned EagleBurgmann to develop a version of the same 15.35-inch design for a high-temperature environment - a furnace gas recompression train in a new cogeneration plant at a major steel mill in Wuhan, China.
The 15.35-inch seal underwent further testing up to a 482°F bulk seal temperature. With the design robustness and the sophisticated gap control solutions of EagleBurgmann, no significant design modifications for high heat were necessary.
Those seals have been in continuous service since the cogeneration plant started up in 2009. The feedback from the compressor OEM has been positive, a testament to the versatility of the original design to adapt to all temperature extremes and fluctuations.
To learn more about EagleBurgmann dry gas seals, contact us at firstname.lastname@example.org.