Designers often underestimate the sizing factors for rotating machinery trains (particularly drivers), leading to installation of marginally adequate units. Then, once in service, components suffer the inevitable effects of ageing (fouling, wear and other forms of degradation). Because all elements in a train impact its operation, degradation in any component affects total performance. As a result, plants often must contend with equipment that no longer can provide sufficient capacity and, thus, has become a bottleneck. The output power limitations of compressor drivers in particular are serious issues in many cases.
Washing or cleaning can address some effects of ageing but can’t recover a large portion of the lost capacity. So, plants usually must consider revamping, renovating and uprating rotating machines such as pumps, compressors, steam turbines and gas turbines. Revamp and renovation projects generally include uprating drivers, re-wheeling of some compressors, installing larger pump impellers, modifying control valves and making various piping changes.
In this article, we’ll look at various factors to consider in a revamp and upgrade.
First, conduct a performance test on the particular unit being considered for revamping or upgrading. The results can assist in defining the extent of performance degradation and the scope for upgrading; these machinery performance data also will allow you to assess the improvement provided by the renovation.
Machinery upgrading usually is feasible and, indeed, routinely takes place at many chemical plants. Whenever possible, retain the existing casing and only replace the internals — rotors, impellers, diffusers and similar. (Sometimes, this is termed “re-rotoring” or “re-wheeling.”) A rule of thumb (with some exceptions) calls for changing only the internals if the cost of retrofit parts doesn’t exceed 45% of the purchase price of a new machine. The advantages of not replacing the entire machinery system are rather obvious. Only minimum changes must be made to external components such as the piping, foundation and baseplate. Also, less time is needed for getting revamp parts than for delivery of a new machine. More importantly, a revamp requires a considerably shorter shutdown (and, therefore, provides a huge production benefit). Figure 1 shows the dismantling of a steam turbine for repair/renovation. Figure 2 shows the main shaft of a compressor being removed during a revamp.
A speed change possibly can upgrade both compressor/pump and driver. In some instances, replacement gearing — to give a different ratio and, thus, a speed change — will fit in the existing gearbox (gear unit) casing. Together with selective replacement of some (but not all) compressor or pump components (even selective replacement of stages), this could reduce greatly the investment, revamp project time and upgrading workload.
A plant also may gain efficiencies from internal design improvements made since the current equipment was installed. These include reduced parasitic losses, tighter manufacturing tolerances and use of modern seals. Today, compact and very efficient internal designs that can be fitted into old casings are available. So, ask your vendor to evaluate components that you want to retain (such as the casing) and to provide a suitable guarantee. And always subject new internal/rotor systems to the full range of testing.
SEAL SYSTEM UPGRADING
Nowadays, turbo-compressors most commonly rely on tandem dry gas seals. These seals considerably reduce power consumption and increase reliability. So, every turbo-compressor upgrade and renovation program (especially for old compressors using oil seals) should include the latest dry-gas-seal technology. The primary vent of today’s dry gas seals generally will be piped to a flare because venting to the atmosphere normally isn’t permitted. The secondary vent may release a small amount of inert gas (such as nitrogen) to the atmosphere.
Upgrading and renovating dry gas seal systems is a well-known option for many compressors at chemical plants. Two options usually are discussed:
• Improving the seal-gas filter system. Usually, original (manufacturer standard) filters are small in capacity and have a large mesh size. In most cases, it makes sense to switch to filters with 4–6 times more capacity and 5–8 times smaller mesh size.
In a case study for a turbo-compressor in a chemical processing unit, the original seal filter mesh was 3–5 microns. The upgraded seal filter has a 1-micron mesh size and dimensions six times bigger than the original filter.
• Upgrading seal system instruments. Current instruments may not have been selected properly. Sometimes, the instrument range isn’t sufficient. Often, seal system flow meters have been sized improperly.
GAS TURBINE UPRATING
Unlike previous generations, the latest gas turbines use single crystal blades, better blade-tip clearance control, advanced metallurgy, new thermal barrier coatings, advanced manufacturing techniques, modern three-dimensional flow analysis, accurate heat-transfer studies and gas-flow-improvement programs.
Performance improvement (uprating) packages take advantage of these advances to enhance both heavy industrial gas turbines (sometimes known as “frame” units) and aero-derivative gas turbines. For example, the uprating of a frame gas turbine at one plant increased production capacity to around 112% of normal. In this case, the main parts of the uprating package included the first stage nozzles, the thermal-barrier coated combustion chamber liners, the splash-plate crossfire tubes, the high-pressure wheel buckets and new sets of thrust bearings as well as a new control system and modern condition monitoring. Installation of such a package can allow an increase in the turbine firing temperature. For aero-derivative gas turbines, an uprating package can result in higher power output (up to 13% more for some turbine models), greater energy efficiency and lower NOx emissions (if suitable new technologies are part of the package).
Use proven-in-service technologies in a gas turbine renovation/uprating project. Sometimes, the turbine components may get overheated as result of a high firing temperature (a usual option in an uprating project) and this could result in a costly field modification.
AMIN ALMASI is a rotating equipment consultant based in Brisbane, Australia. E-mail him at email@example.com.