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Drivers for turbocompressor trains used in LNG refrigeration are sometimes marginally sized during the design stage. And these sizing factors are often underestimated.
A review of many gas turbine drivers shows that the effects of aging, such as fouling, wear and other degradation effects are significant over time. While some of these aging effects may be recovered by washing or cleaning, a large portion of lost capacity cannot be recovered. Revamp, renovation and up-rating is required.
Turbomachinery upgrades have been routinely engineered in many facilities. Whenever possible, the casing is retained and only internals, such as rotors, impellers, and diffusers are slated for replacement. This is also called “re-rotoring.”
A rule of thumb (with some exceptions) calls for changing only the internals if the cost of retrofit parts does not exceed 40% of the purchase price of new equipment. With a revamp, only minimum changes are made to external components, such as piping, foundation and base plate. Revamp is the faster option.
The bottleneck Refrigeration compressor power usually becomes the bottleneck in any LNG plant. In particular, the output power limitations of compressor drivers are serious issues. Note, however, that margins and degradation patterns are different in various refrigeration compressor trains. Also up-rating packages could affect refrigeration compressors in different ways.
Usually, then, LNG plant de-bottlenecking is based on the redistribution of the gas cooling load between various refrigeration cycles. For example, shifting a part of a mixed refrigerant load to a propane cycle is a common option for some LNG unit renovation projects.
It is normal, too, that some refrigeration compressor rotors will need to be “rewheeled.” Re-wheeling of the low-pressure propane-refrigeration compressor is often required. Sometimes a portion of the endflash loads should be shifted to an LNG boiloff gas (BOG) compressor system. These revamp and renovation projects generally include driver up-rating, the re-wheeling of some compressors and the larger LNG pump impellers, some control valve modifications, and various piping changes.
As an example, a renovation feasibility study to increase an LNG plant’s capacity from 5.0 mtpa to 5.5 mtpa had an estimated budget of $40 to $60 million. About 40% of the total amount would normally be for gas turbine driver up-rating and about 35% for new compressor rotor assemblies.
The good news is that there are a variety of performance-improvement packages (up-rating packages) available that can effectively up-rate the gas turbine drivers of LNG refrigeration compressors. Successful examples exist for both heavy industrial frame gas turbines and aeroderivative gas turbines.
As a result of up-rating frame gas turbines, for example, the production capacity of one LNG plant increased to around 112% of normal plant capacity. In this case, the main parts of the up-rate package included first-stage nozzles, thermal barrier coated combustion chamber liners, splash plate cross-fire tubes, high-pressure wheel buckets, a new control system, modern condition monitoring and new sets of thrust bearings. Such upgrades can also increase the turbine’s firing temperature.
In a different aeroderivative example, the up-rate led to a higher gas turbine power output, lower emissions and greater energy efficiency. The produced power was raised by more than 13%. NOx emissions came down from around 180 ppm to less than 25 ppm and CO emissions to less than 25 ppm using DLE (Dry Low Emission) technology installed as part of the renovation package. Typically, it takes four-to-eight weeks to perform such a major upgrade on a large gas turbine.
Compared with previous gas turbine generations, newer models tend to use single crystal blades, better blade-tip-clearance control, more advanced metallurgy, newer thermal barrier coatings (TBCs), advanced manufacturing techniques, modern three-dimensional flow analysis, more accurate heat transfer studies, and a variety of gas-flow improvement programs.
That said, when it comes to upgrades of older equipment, only proven-in-service technologies should be used in any gas turbine renovation project. Bear in mind that some turbine components may overheat as a result of the higher firing temperatures typically associated with up-rating projects. This could then necessitate a costly field modification to fix the resulting problems.
Looking at the bigger picture, however, efficiencies have been improved in modern compressor internal designs due to reductions in parasitic losses, advanced modeling techniques, the latest manufacturing methods, modern dry gas seals and more. Compact internal designs are now available that can be fitted into older casings.
Vendors should be asked to evaluate and assess the components which will remain in order to provide a suitable guarantee. Finally, new internal and rotor systems should be subjected to a full range of testing and verification.