Optimum Design of Turbo-Compressor: Inter-stage Pressures, Stage Curve and Auxiliaries

1   INTRODUCTION

Turbo-compressors are flexible, efficient, and have wide range of application. Optimum configuration of turbo-compressors regarding inter-stage pressures, stage curve and auxiliaries are discussed.

2   TURBO-COMPRESSOR INTER-STAGE PRESSURES

Optimum inter-stage pressures should be obtained by formulation and optimization of performance and investment for both compressor and inter-stage facilities (heat exchangers, drums, etc). If both compressor and inter-stage facilities are in vendor scope (package design), it is best to leave optimization to vendor. In the majority of cases, inter-stage facilities are supplied separately. Vendor offered inter-stage pressures just for the compressor, may not be justified (it is often not optimum). Practically inter-stage pressures (identified at the basic design phase) should be obtained by proper optimization.

Inter-stage pressures may also increase during different operating scenarios.

Approximate pressure drop values for inter-stage facilities:

1-Drums: maximum 1% pressure.

2-Inter-cooler and after-cooler: around 0.70-1 bar.

The use of orifice plates may contribute to significant pressure drops.

3   STAGE CURVE

A review of the steepness of the proposed compressor stage curves can quickly identify which stage (and where) is quite steep in nature, and thus small changes in pressure can cause significant changes in flow (and load). Often a steep curve indicates improper sizing (or unsuitable selection) of the turbo-compressor stage. Units with a steep curve could be difficult to control, automate and tune.

4   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 oil filters with removable element and stainless steel piping.

Liquids should never form inside the machine. For many applications (particularly dirty, wet, or saturated gas services), a good sized suction drum with a drain provision should be provided for each stage.

For cooling water systems (particularly electric motor cooling water system, converter cooling system, etc), firstly, generated heat should be calculated, then temperature rise needs to be adjusted. For centrifugal type cooling water pump selection, special care should be given to the operating curve slope (proper slope), continuous rise to shut-off and proper shut-off pressure. Some extra flow margin is necessary for reliability reasons and the ability of the machine to cope with situations other than normal operation, overload and future expansion (if applicable). Optimization should be done for pump selection.

Amin Almasi

Amin Almasi is senior rotating equipment consultant in Brisbane, Queensland, Australia.

Email: amin_almassi@yahoo.com , amin.almasi@worleyparsons.com