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In a recent article, (Volume 57, Number 7, page 18), Turbomachinery International discussed the trend from customized to standardized compressors. The opinions are still flying whether standardization will actually bring cost savings or whether advanced requirements will force more customization.
While certain areas of the industry, such as air compressors, are highly standardized, others cling to highly customized designs. What does it mean when we talk about standardized compressors? Certain areas must be considered: Aerodynamics, rotor dynamics, mechanical design and packaging. So let’s review these areas.
A standardized compressor stage only exists if the entire stage is not modified. A compressor stage consists of the impeller, diffusor and return channel. For the last impeller in a compressor, the discharge system must be included, and for the first impeller, the inlet system must be included. Any change to the configuration, be it the modification of the diffusor or the change of the shroud contour of the impeller, makes that stage nonstandard. Based on this requirement, very few compressors use standard aerodynamic components. It should also be noted that due to the different process requirements, the combination of stages will be different for virtually any compressor built.
Rotordynamic rules would not allow mixing and matching when we want to define a standardized machine. Even changing the bearing span, modifying shaft diameters, or adding damper bearings, damper seals or swirl brakes will change the rotordynamic behavior of the system. The manufacturer may know from test data how the rotordynamic system will behave, and components, such as bearings or seals can be standardized. But in many instances, the rotordynamic system is customized for every application.
Standardizing mechanical design would particularly affect the assembly and disassembly of machines. Items like dry gas seals are components that are critical to the availability of a compressor, and a significant number of problems are actually related to incorrect assembly of the seals in the compressor.
This is the area that could be standardized very easily. But one question is whether the standard should be driven by the manufacturer or the user? The solution is often a compromise, and thus may only be somewhat standardized.
Requirements are often driven by experiences and the attempt to avoid past problems. The question is whether the past experiences and past problems are applicable to the design and application. This is one area where true cost savings are possible. It also needs to be acknowledged that applicable standards do not design turbomachinery packages, but rather should only be looked upon as guidelines.
One should also discuss the philosophical need for standardizing a product. Most centrifugal compressors operate in unique and often changing conditions. The optimal design for their application has a great impact on the compressor’s efficiency, operating capabilities, life span, maintenance requirements and overall operating costs.
However, many of these arguments only apply if the specified operating conditions are close to the actual condition it sees in operation. Otherwise, the flexibility and adaptability of a machine becomes very important. In addition, the impact of standardization on maintenance cost has to be evaluated.
So, while users and manufacturers often tout standardized designs as a goal and an important feature, we feel that the concept is often misunderstood, abused or misapplied. It is acknowledged that for certain applications a custom design will be required.
This may be the case if a new technical solution is required, for example, very high-pressure applications. However, probably more than 90% of compressor applications do not fall into that category and an open discussion on what standardization means and what it can do to reduce cost, project completion time requirements, and increase availability would certainly be helpful.