Trimming of impellers can be an important way to change the characteristics of a centrifugal compressor. Sometimes in a centrifugal compressor performance test at the shop or site, performance does not meet expectations. A possible solution is slightly trimming one or some impellers to achieve reasonable compressor performance and avoid impeller redesign and refabrication. This trimming process can offer considerable commercial benefits since the impeller or compressor redesign and remanufacturing are expensive. Delays in compressor delivery and plant startup are extremely costly. Using properly designed impeller trimming to avoid weeks or months of delay could offer significant advantages.
Impellers used in centrifugal compressors are two-dimensional (2D) closed-type impellers, three-dimensional (3D) closed-type impellers and 3D semiopen impellers. Each impeller type requires a specific set of rules for trimming.
Impeller trimming can be an effective method of modifying the performance of a centrifugal compressor without significantly compromising the efficiency. Few works are published on impeller trimming topics. This article offers practical guidelines for the impeller trimming of centrifugal compressors.
For a given design, the impeller may be trimmed radially so the exit radius is reduced. This method can be used for nearly all types of compressor impellers. For 3D impellers – particularly 3D semiopen impellers or to a lesser extent for 3D closed-type designs – the impeller might be trimmed axially so the exit blade height is reduced, or along the entire meridional length of the blades so that the passage area is reduced.
Trimming: Simple patterns
Because of conservative engineering practices, inaccurate impeller designs or other errors in the engineering or fabrication of a compressor, the impeller flow or the impeller-generated head might be different than what they need to be. Sometimes the head is more than required, but often it is less than what it should be. Centrifugal compressor impellers can sometimes be oversized. For a fixed speed centrifugal compressor, limited options are available to deal with such an issue. Impeller trimming is usually the best or only option. In many variable speed compressors, the trimming of the impeller should also be studied and employed.
In a compressor casing or a compressor train where all impellers are operated with the same speed, too often some impellers generate more head than is required, while others produce head that is less than what is required. These may compensate each other. However, special circumstances such as compressors with side-streams and others where accurate head generation of each set of impellers may be present. For such a compressor casing, speed adjustment might not solve the operational problem and impeller trimming is necessary.
Trimming often involves machining the different parts and sections of an impeller to modify some dimensions or angles. Trimming should be limited to certain levels because excessive trimming can result in operational and reliability problems such as a mismatched impeller and casing. Trimming can affect the clearances between the impeller and casing, which could increase internal flow recirculation, cause head loss, lower overall efficiency, etc. Turbulences could increase at the vane tips as the impeller is trimmed because the shroud-to-casing clearance increases.
Impeller trimming is often used to change the performance characteristics (head-versus-flow) and is sometimes employed to limit driver loading. The goal is often to adjust the head and sometimes to reduce the head.
A simple trimming option is the trimming of the impeller radius to adjust the head. In this case, the prediction of the trimmed radius is crucial to a compressor impeller. As a rough indication, the formula “head” proportional to “(radius)2” can be used. The flow is not a constant after trimming and all these parameters are related. For a constant flow, speed adjustment might be required in addition to impeller trimming. In other words, a combined speed adjustment and impeller trimming could be employed to achieve head reduction while the flow is kept constant. Use caution; The Affinity Laws apply satisfactorily for pumps and fans (incompressible flow) but do not accurately describe the performance of turbocompressors.
Usually a sophisticated simulation should be applied in a trial-and-error procedure to find the outer rim of any impeller for even a simple radius trimming. Many factors make a single trimming difficult, so only comprehensive simulations with the trial-and-error procedure should be used to find trimming of different impellers and obtain the best result considering the flow, surge limits, efficiency and more.
Sometimes one gap, dimension or angle should be changed whereas other gaps or the distance should be reduced or kept constant. For example, for many years, machinery engineers have been machining the vane tips to reduce frequency vibrations passed by the vane (the clearance between the impeller vanes and the casing) while carefully maintaining the clearance between the impeller shrouds to the casing (or volute).
Excessive shroud to casing clearance and the resultant recirculation to the low-pressure side of the compressor could produce “eddy flows” around the impeller, causing low-frequency vibrations (usually axial vibrations) that can translate to seal problems or, in severe cases, to axial bearing issues. This can be a concern in large or high-pressure machines.
In many cases, trimming results in steeper curves and a narrower operating range. Some impeller trimming can change the way the gas exits the impeller. For instance, if the gas-exit angle is changed as the impeller is cut back, the head capacity curve might become steeper.
The exit angle of the gas might change the performance, resulting in a higher head at rated flow. Because of reductions in the wake of fluid exiting the vanes, the efficiency of the compressor should usually stay the same or just slightly be changed. The smaller the size of the impeller, the larger the effect. Usually no change in shut-off head is associated with such a trimming.
A particular range of specific speeds is most appropriate for each centrifugal compressor impeller design, and impeller radius trimming could increase the specific speed of the machine. Therefore, radially trimming an impeller of low specific speed could move the specific speed closer to an optimal value, while radially trimming a high specific speed impeller will move the design further from the range of optimal performance.
Trimming: Complex patterns
In a centrifugal compressor, trimming can be done on the impeller or impeller blades. Usually both options can be considered, which results in relatively complex patterns of trimming. A complex pattern of trimming can change the performance characteristics of an impeller. Several impeller trimming methods may be employed to change the impeller flow rate, the generated head, the pressure ratio or all these parameters. The blade and impeller trimming limitations and the effect on the flow fields should be thoroughly understood before any trimming is attempted. Computational fluid dynamics (CFD) simulations should be used to model the effects of different methods of blade and impeller trimming on compressor operation and reliability.
Each trimming method has been found to be limited at some point by surge or choke flow, particularly the choke flow (the limit of overload at the right-hand side of a compressor curve), which could be an issue for some trimming plans.
Modifications on impeller passage areas
Designs for lower flow rates are usually achieved by reducing the passage area. However, the passage area and impeller width are often difficult to trim. The passage area trimming can be done in a range of impeller designs, mainly 3D impellers and sometimes on 2D impellers. This method is also known as “flow trimming.” It is used to produce a modified impeller with the same pressure rise as the baseline design but with new flow rate characteristics. In other words, this method is used with the objective of changing the impeller flow rate but not the pressure ratio. Using this method, the blades and internal parts of an impeller are trimmed so the inlet and outlet areas are changed, usually by a given ratio. The blades are trimmed proportionally from inlet to outlet along the meridional length of the blade.
Considering the sources of loss in a centrifugal compressor, narrowing the passage would tend to increase wall frictions. On the other hand, larger passages might reduce wall frictions. However, compressor performance also has a strong dependency on the inlet relative Mach number. This need should also be considered in any trimming plan.
Trimming the passage area from inlet to outlet along the meridional length might change the flow rate of the impeller and will probably narrow the effective operating range. The head coefficient and efficiency relative to the choked flow coefficient usually remain unchanged for each trim area. A linear relationship could exist between the passage area and the resultant choked flow rate.
The flow trimming should be limited. Beyond these trim limits, the overall pressure ratio does not usually reach the design value. Other operational problems could also occur. As an indication, flow trimming of some 3D semiopen impellers should be limited to 30 percent of the original impeller blade height and to specific speeds of impellers as well.
Trimming 3D semiopen impellers
Trimming the impeller blades in the axial direction in 3D semiopen impellers can cause the head coefficient to be reduced while maintaining a constant flow coefficient. This trimming method is limited by choking in the radial portion of the passage, and the head coefficient for the impeller studied was reduced by up to 10 to 12 percent before further trimming limited the choked flow rate. Such a trimming method in 3D semiopen impellers could be a better option compared to trimming the impeller radius to achieve a lower head.
Shifting the original shroud profile axially and radially in proportion to the desired flow coefficient allowed the pressure ratio and efficiency of the original impeller to be maintained while changing the flow coefficient. Trimming the blades axially so that the original shroud profile is maintained could produce a change in pressure ratio while maintaining the original impeller flow coefficient.
One important goal in trimming is to maintain the efficiency of the original design as far as possible. In other words, all the process and trimming planned should be optimized to arrive at similar efficiencies (or just slightly lower efficiencies). A linear regression could explain approximately how much loss in efficiency can be assumed for a trimming process compared to the base design. Important considerations for keeping the efficiency are the studies and analyses for better inflow conditions into the diffuser. The greater the impeller trimming and the higher the specific speed of the impeller, the more the efficiency could be affected with impeller trimming. The impeller efficiency cannot usually be influenced significantly by the change of the shroud contour if the shroud contour was not a bad design originally. Therefore, a significant improvement cannot be expected. The diffuser performance, however, can be maintained or even improved, verified by higher pressure recovery and lower total pressure loss coefficients. Another important target is to maintain the surge margin of the impellers under trimming.
As the planned trimming and associated optimization are applied to the best point of the nominal speed (100 percent of nominal speed), the speed variation range influence should also be investigated. An optimization of the best efficiency point on the normal speed does not simultaneously keep the efficiency over the entire speed range. In some cases, properly designed trimming with good efficiency at nominal speed has resulted in lower efficiencies for an entire part-load speed range. For compressor applications with more frequent part-load operation, it may be necessary to optimize the trimming plan while considering the whole speed variation range of the machine. In other words, the trimming plan should depend on the speed variation range with respect to the compressor application.
Structural strength and stress patterns are sometimes important considerations when deciding on impeller trimming. For example, how much to reduce the vane diameter might be an important consideration because a large portion of unsupported shroud cannot be left unsupported. Experts recommend an oblique cut that will improve the vane exit flow and add some strength to the shrouds. These patterns require accurate CFD simulations and comprehensive stress analysis. In some specific cases, fluid-structure indications should be investigated. Sharp corners after a trimming can initiate cracks and eventual impeller failure.
Amin Almasi is a senior rotating machinery consultant. He is a chartered professional engineer of Engineers Australia and IMechE. Almasi is an active member of Engineers Australia, IMechE, ASME and SPE and has authored more than 100 papers and articles dealing with rotating equipment, condition monitoring, offshore, subsea and reliability.