Design and Analysis of a Multiphase Turbine for Compact Gas-Liquid Separation
- C.H. Rawlins (Kvaerner Process Systems US) | G.D. Ross (Multiphase Technologies)
- Document ID
- Society of Petroleum Engineers
- SPE Production & Facilities
- Publication Date
- February 2002
- Document Type
- Journal Paper
- 47 - 52
- 2002. Society of Petroleum Engineers
- 4.1.6 Compressors, Engines and Turbines, 5.8.9 HP/HT reservoirs, 5.3.4 Integration of geomechanics in models, 4.1.9 Tanks and storage systems, 4.6 Natural Gas, 4.1.2 Separation and Treating, 5.9.2 Geothermal Resources, 2.4.3 Sand/Solids Control, 4.5.5 Installation Equipment and Techniques, 4.1.5 Processing Equipment, 4.5 Offshore Facilities and Subsea Systems
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The rotary separator turbine (RST) uses unique technology to provide compact separation of oil and gas while recovering energy wasted in conventional pressure-reduction operations. The technology has been used for industrial refrigeration and geothermal applications since the early 1980s and has recently finished a field evaluation and assessment on a deepwater, tension-leg platform (TLP) in the Gulf of Mexico. The focus of this paper is to cover the operating principles and mechanisms of multiphase turbine technology and the design parameters for installation as a separation or power-generation system in new developments or in debottlenecking applications. Operational experience pertaining to separation efficiency, mechanical reliability, and power output in a live-fluid environment are highlighted.
Conventional technology for separation of liquids and gases is well developed in the petroleum industry. Reliable separation technologies based on gravity settling have been available for many years. However, the changing economics of oil and gas production, deriving from fields with smaller recoverable reserves and/or deepwater environment, require improvement over the methods currently used to reduce the cost of development and operation.
The large footprint and weight characteristics of conventional gravity-based separators require a substantial support structure when used offshore and are expensive to transport and install. Additionally, the potential energy available in the high-pressure, multiphase fluids that are treated by these separators is significant. Current oil-and-gas separation systems waste this potential energy through dissipation as pressure drop across a choke or other reduction valves. Recovery of this energy, in whole or in part, can offset power-generation costs and is free from greenhouse emissions.
The RST provides compact and efficient gas/liquid separation while recovering the pressure-drop energy. This energy can be put to use for liquid pressurization, shaft power, or a combination of the two. Separation efficiency of the gas and liquid meets or exceeds that of a conventional vessel separator with only 25 to 40% of the weight or volume. Additional benefits of the RST stemming from the centrifugal separation include foam reduction and operability improvement because of motion insensitivity.
Rotary Separator Turbine
The RST is an outgrowth of high-velocity two-phase flow research at the Jet Propulsion Laboratory in Pasadena, California.1 Extensive experimentation on two-phase nozzles and experience with stationary separators and two-phase diffusers established the technical foundations for RSTs. The first turbines were tested in 1974 with compressed air and water at the 2 kW (2.7 hp) level. Subsequent demonstration turbines, designed and tested for geothermal power applications, operated on brine and steam. These demonstration units ranged from a 30-in. (750-mm) turbine operating at 21 kW (28.2 hp) in 1979 to a 54-in. (1350-mm) turbine operating at 1569 kW (2,104 hp) at Roosevelt Hot Springs, Utah, in 1983. The larger unit operated for 4,000 hours in a reliability test, exhibiting 100% mechanical availability and no erosion.
Simplification of the rotor design and addition of gas blading was introduced, and a new design prototype was tested at Coso Hot Springs, California, in 1993. This unit operated for 700 hours at 42 kW (56.3 hp) with no measurable erosion. As a result, a full-size commercial unit was installed in August 1998 at Cerro Prieto, Mexico, for a commercial demonstration project. The 30-in.- diameter commercial unit was designed for a power output of 2 MW (2,682 hp).
The first oil-and-gas application involved the use of the RST as a centrifugal foam breaker for an offshore application. An aqueous foamy mixture (beer) was successfully tested in a lab unit and recycled by the test crew. A reaction-type subscale RST was built and operated with natural gas and foamy crude at a gas plant. Complete foam breakdown was observed resulting in a liquid carry-over of only 30 ppm. A full commercial prototype was manufactured in 1984 for 100,000 BD (662.4 m3/h) flow at 1,000 psia (69 bara) inlet pressure; however, because of changes in the target test site, the unit was never placed in service.
Further test work was done in 1996 with a drag RST for the separation of inert gases and crude oil during shuttle-tanker loading. This unit was short-term tested in a pilot plant on an oil tanker in the North Sea. The unit exhibited good separation, and performance was unaffected by the ship's motion. A joint industry program was formed in 1997 to test a second RST at a major oil-andgas test facility.
RSTs have been installed in waste heat-recovery applications and have enjoyed commercial success in industrial refrigeration applications. More than 100 commercial chillers (up to 2,500 tons or 8,793 kW) are operating with RSTs.
The basic principle of operation and the liquid and gas flow path are shown in Fig. 1. The four main components of a rotary separator turbine are as follows.2
Two-phase nozzle to convert fluid pressure into a high-velocity homogeneous stream.
Rotor drum for separation and energy transfer.
Diffuser scoop or reaction jets for liquid extraction.
Gas blading for power extraction.
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