LIMITATIONS OF HERMETICALLY SEALED CENTRIFUGAL COMPRESSORS

Over the last few years, a number of compressor manufacturers have started offering hermetically sealed, electric motor driven compressors where the driver and the centrifugal compressors are installed inside a single, perfectly sealed, and process gas pressurized enclosure. Specifically, the entire rotating shaft from driver side to driven equipment, including motor, coupling, all impellers, diaphragms, and bearings, and so on, are enclosed inside a pressure-containment vessel.

This eliminates any rotor-to-ambient air interfaces and thus the need for seals (dry gas, wet, or otherwise) between the process gas and the environment. Effectively, process gas leakage is eliminated, which is attractive from a performance and an environmental perspective.

The arrangement typically uses a highspeed motor driving the compressor directly without any gearboxes. Radial and axial thrust magnetic bearings are employed to levitate the motor and the compressor shaft, and consequently, a lube oil supply systemis not needed. Process gas can be used for internal casing thermal management and cooling of the motor. This approach is elegant and, as a first impression, appears to be advantageous over more conventional arrangements with separate driver and compressor casings. The hermetically sealed compressor is also the only viable technical option for sub-sea compression applications.

First model introduced

Man Diesel & Turbo introduced the first hermetically sealed compressor, the Mopico, in 1990. The motivation was to reduce the number of mechanical componentswhich should increase themechanical reliability and reduce the cost of the compressor. By using electric motors as drivers and eliminating dry gas seals, the compressor also avoids being a point source for greenhouse gas emissions.

Although other manufacturers have introduced similar units over the years, there are currently fewer than 100 hermetically sealed compressors in operation worldwide. Hermetically sealed compressors are currently offered for both upstream and midstream services, including re-injection, gas boost, and gas storage applications.

However, before deciding whether this type of compressor is appropriate for an oil and gas or process application, one should also carefully consider some of the disadvantages of the design:

• Cooling system: Process gas recycled from the discharge to the suction is utilized for motor cooling. This requires a complex control system including by-pass gas control valves and accurate motor temperature monitoring and control. Clearly, the gas that is recycled for cooling purposes is a direct compression efficiency loss and must be carefully controlled to maintain reasonable performance.

Also, because the process gas is at an elevated pressure and density, the electric driver motor aerodynamic windage losses can be substantial. For example, for natural gas at 1,000 psi and using a high-speed driver motor, the windage losses would reduce the total compressor system efficiency by about 4-5%. This number goes up proportional to gas density and pressure. The cooling flow recycle loss is about the same percentage. Thus, the hermetically sealed compressor has an inherent efficiency penalty of nearly 8-10% when compared with conventional compressor arrangements. Obviously, if the process gas used for cooling is not substantially dry and clean, the motor internals will foul and degrade rapidly.

• Construction: Although one would expect that the construction cost for a hermetically sealed compressor should be very low since seals, gears, and couplings are eliminated, the reality is that these compressors are currently priced much higher than conventional machines. Specifically, they are not just more expensive than conventional, low-speed-driven geared machines with fluid-film bearings but they are also much more costly than coupled, high-speed, direct-driven centrifugal compressors in separate casings.

This is probably due to the fact that the design of the hermetically sealed compressor is more complex, thus requiring more nonrecurring engineering costs, and that the pressure-containing vessel has to enclose both driver and compressor, which significantly adds to material costs. And, although high-speed motors and magnetic bearings have now reached a level of maturity and reliability where they are considered acceptable for tough, industrial duty applications, their cost is still about 2-3 times higher than similar size, low-speed geared motors and fluid-film bearings.

• Operation: One could argue that eliminating the gearbox and lube oil system should reduce maintenance costs but actual operator cost data does not yet bear this out and even seems to contradict this assumption. Apparently the added complexity of using magnetic bearings and high-speed motors eradicates any O&Mcost advantage. Because of the fully enclosed and highly compact design, hermetically sealed compressor internals tend to be more difficult to access for repair, overhaul, and maintenance activities. For the same reasons, removing and replacing compressor internals for restaging becomes more challenging.

• Installation: Finally, while the size of the high-speed compressor driver configuration is very attractive when compared to other driver-compressor arrangements, all hermetically sealed compressors require a Variable Frequency Drive (VFD) to transform grid frequency (50 or 60 Hz) to the high speedmotor running frequency, i.e., utilizing aVFD is not optional. VFD’s are large and heavy, require cooling, and thus often eliminate the advantages of a smaller compressor package.

There are certain applications, such as subsea compressions, where using a hermetically sealed compressor provides an excellent technical solution. However, its inherent design advantages, such as lower weight, compact size, no dry gas seals, are not relevant for the majority of on-shore compression applications. Most compression facilities have plenty of space available, are fully equipped to deal with lube-oil requirements, and modern dry gas seals have leakage rates that are below restrictive regulatory standards. This is aside from fundamental questions on grid stability that need to be addressed when even considering on-shore electric motor driven compressors.

Authors

Klaus Brun is the Machinery Program Director at Southwest Research Institute in San Antonio, Texas. He is also currently the Chair of the Board of Directors of the ASME International Gas Turbine Institute.

Rainer Kurz is the manager of systems analysis for Solar Turbines Incorporated in San Diego, CA. He is an ASME Fellow since 2003 and past chair of the IGTI Oil & Gas applications committee.