Choose the Correct Turbo-Compressor

A variety of factors affect the selection of the most-appropriate machine

By Amin Almasi, rotating equipment consultant

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Turbo-compressors are flexible and efficient, and have a wide range of application. Evaluating and purchasing such a machine should involve a comprehensive optimization process. Otherwise, the turbo-compressor may not meet desired targets for performance, reliability, maintainability, availability, commercial terms or power requirements. So, this article will look at factors to consider in an optimization.

An important decision for every turbo-machine is whether it should be located indoors or outdoors. At first glance, an indoor installation seems safer, more reliable and better. However, design and operating experiences suggest examining an outdoor installation first. It can make operation and maintenance much easier, and cost far less.

An indoor installation can pose conflicting requirements for enclosure design, noise control, the heating/ventilating/air conditioning (HVAC) system, foundation and support design, and piping arrangement, as well as access for various maintenance activities and cranes. This could result in an extremely costly system and enclosure.

In many chemical-processing projects, the turbo-compressor is ordered based on indoor installation, assuming future design of an enclosure for noise control and protection (with overhead crane, HVAC, proper area classification, etc.). To keep costs down, the enclosure volume should be minimized — however, this compromises access and other requirements. The heat-generation calculation and proper ventilation usually are complex issues. Most importantly the access requirements for installation and major overhaul could be problematic. An indoor installation also introduces various health, safety and environmental issues.

That’s why I generally recommend an outdoor installation, initially without a shelter. Base required noise protection on an initial noise study, include an acceptable value (with a proper margin) in the vendor scope, and allow the vendor to suggest some machine-localized noise protection. My experience indicates an 83–85-dB noise limit could be a good range for the start. Use suitable mobile cranes for all maintenance (routine, overhaul, etc.). Later on, installation of a shelter (preferably a removable one) will improve the package’s reliability. Of course, an outdoor installation isn’t always best but certainly consider it first.

Factors to check include inter-stage pressures, auxiliaries, coupling selection, piping and auxiliary vessels, and provisions for condition monitoring.

Inter-stage pressures. Optimization requires considering the performance and investment for both the compressor and the inter-stage facilities (heat exchangers, drums, etc.). If both compressor and inter-stage facilities are in the vendor scope (the package design), it’s best to leave optimization to the vendor. However, in the majority of cases, the inter-stage facilities are supplied separately; the compressor vendor’s suggested inter-stage pressures for the compressor alone may not be justified and often aren’t optimum. So, as a practical matter, determine the inter-stage pressures (identified at the basic design phase) by proper optimization. Inter-stage pressures also may increase during different operating scenarios. Approximate pressure-drop values for inter-stage facilities are:
• around 0.5–1.5% of the pressure for the drums and vessels; and
• approximately 0.70–1.5 bar for the inter-cooler and after-cooler.

Auxiliaries. The oil system should include (at least) two oil pumps, both sized for 20% extra oil flow, dual removable-bundle shell-and-tube oil coolers (alternatively, plate exchangers or air-coolers), double (duplex) oil filters with removable elements and stainless steel piping.

For cooling water systems (particularly the electric-motor cooling water system, the converter cooling system, etc.), first calculate the generated heat and then adjust the temperature rise. To optimize the selection of centrifugal cooling-water pumps, pay special attention to the operating curve slope, ensuring you can get a continuous rise to the shutoff and a proper shutoff pressure. Some extra flow margin is necessary for reliability and to allow the machine to cope with situations other than normal operation, overload and future expansion (if applicable).

Figure 1 shows extensive auxiliaries for a modern turbo-compressor at a chemical processing plant.

Coupling selection. Usually, the potential exists for torsional resonance and, thus, torsional fatigue failure. The coupling is the best available means to modify and tune the system. Options include:
• a high-torsional-stiffness coupling (optimum if allowed by the torsional analysis);
• a direct forged-flanged rigid connection (no coupling), if permitted by alignment procedure(s); and
• a flexible coupling (which can provide more elasticity and dampening but at the cost of more maintenance).

Torsional vibration problems continue to occur in turbo-compressor trains. The main reasons are:
• lack of comprehensive torsional vibration analysis;
• improper application (mainly of flexible couplings);
• insufficient maintenance; and
• lack of proper monitoring.

As a rule of thumb, for shafts coupled together the electric motor’s shaft diameter should equal or exceed that of the turbo-compressor or the gear unit. (The turbo-compressor or gear unit shaft usually is fabricated from a higher grade material than the electric motor shaft.)

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