Centrifugal Compressors


By Daniel Potter

Figure 1: The MSG LMAC centrifugal compressor is used in air separation. The red line indicates the transmission of net axial forces from the high-speed pinions to the low-speed bullgear shaft through the thrust collars and rider ring. Available in three models for ASUs ranging from 150 to 400 tons/day of oxygen production.

Much has been debated recently about compressor package standardization versus customization, particularly for integrally geared centrifugal (IGC) compressors. Depressed commodity prices and a scarcity of large-scale projects that traditionally demand custom engineering are driving engineering, procurement and construction companies (EPCs), end users and compressor original equipment manufacturers (OEMs) to search for avenues to save capital expenditure.

Many are evaluating the tradeoffs between standard package versus a custom- engineered solution. Standard compressor packages are typically lower in flow and pressure capability, as well as being lighter in scope. Custom compressor packages, on the other hand, are typically higher in flow and pressure, and meet more stringent specifications.

To adapt to changes in the market landscape, OEMs are developing products and compressor designs to bridge the divide. This is occurring from both ends of the product spectrum to apply the benefits of standardization to custom-designed packages, and to extend the capabilities of standard frames to perform duties traditionally served by custom designs.

Custom to standard The standardization of custom packages can be tricky due to the wide range of applications, each with their own specific performance and scope requirements. For this reason, many OEMs have taken the approach of standardizing solutions around specific applications. This allows for the benefits of standardization without sacrificing the scope, performance or reliability required by the target application.

One example of a standard main air compressor package designed for standard pressure (6-8.5 barg or 85-123 psig) in air separation units (ASU) is the Ingersoll Rand MSG LMAC. It maintains many of the robust design features of custom-engineered packages, such as a bullgear rider ring for axial thrust transmission (Figure 1), five-pad tilting pad bearings and conservative gear mesh limitations. It maintains customizable aerodynamic components, including backward- leaning impellers that are five-axis milled from stainless steel forgings.

A fuel gas booster (FGB) compressor design also has been standardized. It delivers discharge pressures suitable for most gas turbines, and the number of compression stages is configurable to accommodate lower pipeline pressures. It is delivered on a single base to simplify installation with pre-mounted accessories, such as the lubrication system and gas seal rack. FGB compressors are increasingly in demand as developing nations expand their power generating capacity.

Countries such as China, Russia and Bangladesh are building gas-fired plants. But they lack gas pipeline networks that can consistently deliver the elevated natural gas pressure required at the inlet of most gas turbines. They require FGB compressors to increase the available pressure to meet the operating requirements of the turbine (Figure 2).

Figure 2: A simplified diagram of a combined-cycle power generation process that highlights the FGB application of integrally geared centrifugal compressors

Standard to custom

Stretching the capabilities of standard compressor frames to serve custom applications is a way to achieve more with less, and for a smaller investment. The challenge is how to enable the frame to perform beyond its traditional limitations without sacrificing the presumed cost benefit.

First and foremost, efficiency must be close enough to that of a custom-designed machine to make the total cost of ownership evaluation favorable. The frame of the TURBO-AIR NX 12000, for example, has been updated to achieve discharge pressures up to 40 barg (580 psig). This is up from its design point of 15 barg (210 psig). The expanded discharge pressure capability accommodates elevated inlet pressure conditions, boosting process air or nitrogen to higher pressures.

These applications would often have been handled by a more maintenance-intensive reciprocating package or a more capital-intensive custom-engineered centrifugal design. Standard compressors can also be incorporated into package designs for API 617 and 672 service.

Author: Daniel Potter is Global Product Manager for MSG air separation and high flow process air compressors at Ingersoll Rand, which acquired the assets of Cameron’s centrifugal compression division. He has a B.S. in mechanical engineering and an MBA. For more information, visit http://centrifugal.ingersollrand.com or dan.potter@irco.com