PRACTICAL NOTES ON INTEGRALLY GEARED TURBOCOMPRESSORS

An integrally geared centrifugal compressor usually offers a lower initial price, higher efficiency and a compact package compared with a conventional centrifugal compressor configuration. There are successful references for integrally geared turbocompressors across a range of applications. Different performance, mechanical and reliability aspects of integrally geared centrifugal compressors are described in this article, as well as risks and challenges. The main concern for the user, though, is correct selection between an integrally geared machine and a conventional- type compressor based on application and plant requirements to obtain the lowest total cost of ownership.

An integrally geared centrifugal compressor is basically an overhung style compressor installed on a gear system and uses the gear pinion shaft extensions to mount impellers. The individual compressor casing is attached to the gear system main casing. In other words, an integrally geared machine uses a main gear to rotate several pinions, each coupled to one or two overhung centrifugal impellers.

One advantage of integrally geared configuration is that the pinion shafts can operate at different speeds allowing each stage to run at its optimum speed. The most common form of integrally geared compressors has been packaged plant air compressor (covered by API 672). However, over the last two decades integrally geared centrifugal compressors have been introduced for process applications as well (recognized by API 617).

In the bidding stage for a centrifugal compressor, an early concern should be the correct selection between the classic, conventional compressor type (driver, gear unit and multi-casing machine) and the new, compact and relatively cheaper integrally geared machine. Selection should always be based on the correct evaluation of technical and commercial aspects of offers to make sure the best compressor is purchased.

Although conventional designs with proven in-service history and more references may be preferred by some operators, this should not preclude the introduction of new technology of process integrally geared compressors, particularly where advantages can be gained in cost, effectiveness, efficiency, operational flexibility, and reduced footprint.

Pressure coefficients are higher for semi-open overhung three-dimensional impellers, which are usually used in integrally geared machines. The maximum tip speed of semi-open impellers is around 30% more than the closed-type impellers. However, semi-open impellers offer higher pressure ratio capability (around 2-3 times more than closed impellers). This means they can reach the same discharge pressure using fewer impellers compared with conventional compressors. It is economically desirable to use the most compact and the smallest possible compressor.

Varying conditions

The variable (inlet) guide-vane system is the preferred method of capacity control of an integrally geared compressor to meet varying operating conditions. One of the main applications of integrally geared machines is small-size and medium-size, constantspeed electric motor-driven centrifugal compressors (where a variable-speed drive is not feasible).

Integrally geared compressors are not preferred with variable-speed drivers, VSD electric motors, gas turbines or steam turbines. Dynamic responses are unnecessarily complex. Integrally geared centrifugal compressors could be around 15-30% less expensive and require around 5-15% shorter fabrication time. For some specific applications (such as in some revamp projects where available space and commercial issues are major limits) integrally geared machines can be the only solution. One of main disadvantages of integrally geared compressor, though, is that each impeller should be sealed, so applications where state-of-the-art seals are required (usually for many process gases) should be dealt with caution.

An integrally geared compressor typically involves more complex dynamic behaviors and may introduce more vibration, noise and dynamic issues. It is more sensitive to the unbalance compared with a conventional-type compressor. If impeller fouling is anticipated, the integrally geared design may not be suitable. Integrally geared machines usually are employed for a molecular weight below 50. Also the operation range of an integrally geared compressor is usually limited compared to conventional ones.

Special care should be taken when an integrally geared machine has:

1 - Any gear mesh with a ratio of 7:1 or greater (these gear sets are rated in accordance with Annex 3F of API 617, based on AGMA 2101 with safety factor 1.25 to provide increased reliability)

2 - Gear pitch line velocities of more than 125 m/s (design features such as windage baffles, false bottoms, adequate sump depth, additional full-size drains, and others, could be required)

3 - The preferred gear set design is the wear-out before the breaking (a failure occurs due to the pitting rather than the bending)

4 - Hunting tooth combinations are strongly preferred (if necessary, vendor adjusts the exact gear ratio)

Forces and moments allowed on integrally geared compressor nozzles are generally less than values permitted on conventional type compressors. API does not specify these loads since limiting criteria are the gear-contact pattern and the impeller-stator gap, which depend on various complex parameters (the volute design, the compressor and gear system design, and others). In many packages, expansion joints are installed on integrally geared compressor nozzles to minimize the loads.