Lars Brenne

Performance Evaluation Of A Centrifugal Compressor Operating Under Wet Gas Conditions.

This paper presents the results of performance testing of a single-stage centrifugal compressor operating under wet gas conditions. The test was performed at an oil and gas operator’s test facility and was executed at full-load and full-pressure conditions using a mixture of hydrocarbon gas and hydrocarbon condensate. The effect of liquid was investigated by changing the gas-volume fraction between 1.0 and 0.97, which covers the range encountered by the operator during regular gas/condensate field production in the North Sea. Other parameters that were evaluated include the 111 PERFORMANCE EVALUATION OF A CENTRIFUGAL COMPRESSOR OPERATING UNDER WET GAS CONDITIONS by Lars Brenne Staff Engineer Tor Bjorge Staff Engineer Statoil ASA Trondheim, Norway Jose L. Gilarranz Senior Aero/Thermodynamics Engineer Jay M. Koch Staff Engineer, Aero/Thermodynamics and Harry Miller Product Manager, Marketing Dresser-Rand Company Olean, New York compressor test speed, the suction pressure, and two different liquid injection patterns. During the tests, the machine flowrate was varied from near surge to choke conditions; hence, the evaluation covered the entire operating range of the machine. Although the test was primarily intended to evaluate the effects of the wet gas on the thermodynamic performance of the machine, the mechanical performance was also investigated by measuring the machine vibration levels and noise signature during the baseline dry gas tests as well as during the tests with liquid injection. INTRODUCTION Centrifugal compressor packages utilized for upstream gas processing often must operate under wet gas conditions in which the fluid handled by the compression package contains a mixture of liquid and gaseous phases. Typically, the liquid components of the mixture are separated from the gas stream before they enter the compressor by the use of scrubbers and separators located upstream of the compressor inlet. These devices are very large and heavy, requiring a large “footprint” (amount of floor space) as compared to the gas compression package. A compressor with the ability to directly handle wet gas without the need for separation equipment is very attractive from an economic standpoint, as it would drastically reduce the size, weight, and cost of the gas compression package. For the case of future subsea compression systems, this capability is even more attractive because of the high costs of deploying a compressor train and all of its associated equipment under water. Wet gas compression (WGC) technology represents new opportunities for enhanced, cost-effective production from existing and future gas/condensate fields. Many oil and gas operators face future challenges in tail-end production, unmanned operation, and improved recovery from topside and subsea wells. This emphasizes the need to develop more robust compression systems, which can be designed for remote operation in unmanned topside installations, or could be designed for subsea operation for reinjection and/or transport boosting. The use of this technology for subsea boosting represents a new and exciting application for rotating equipment, which will allow new gas/condensate field production opportunities as well as enhanced recovery of existing gas/condensate fields and cost-effective production from marginal gas fields. As mentioned above, these wet gas compression systems could be based on the use of a liquid tolerant dry gas compressor, which could boost a coarsely separated (via a scrubber) well-stream, however, an even more attractive solution would be the development of compression systems that can boost the well-stream directly. Many research projects and product qualification programs are currently underway to develop such a system either by modifying existing multiphase pump technology or by the adaptation of currently available gas compression technologies (Scott, 2004). Regardless of the choice of concept, the compressor solution should be able to tolerate liquid ingestion for an extended time without failure. For the case of subsea applications, the high cost associated with the retrieval of the compressor from the sea floor accentuates the importance of a reliable design. The work presented herein served as an initial test to verify the multiphase boosting capabilities of a centrifugal compressor as well as to provide an oil and gas operator with data to compare the performance of this technology with other available wet gas compression concepts. It is important to state that the test compressor used for this investigation was not originally designed for wet gas boosting, nonetheless it provided an economically viable test bed for centrifugal compressor technology. DESCRIPTION OF TEST VEHICLE The test vehicle used for this work was a barrel-type, singlestage compressor, manufactured by the coauthors’ company. Said compressor was equipped with a high-head impeller, with a diameter of 0.384 m (1.26 ft), and a design flow coefficient of 0.02380. The compressor was originally designed to handle an inlet flow of 4332 Kg/min [2167 Am3/hr (76,526.88 ft3/hr)] of dry hydrocarbon gas (molecular weight of 18.49), with an inlet pressure of 130.2 bar (1888.4 psi)and a discharge pressure of 161.8 bar (2346.7 psi). Figure 1 shows a cross-section of the test compressor; the inlet and discharge nozzles are located at a 45 degree angle with respect to the top dead center of the machine. The original design of this machine, which dates to 1986, was not intended for wet gas service, and hence the internal geometry was not optimal. Nevertheless, in order to increase the reliability of the machine, the original rotor design was modified to accommodate an electron-beam welded and vacuum furnace brazed impeller with a shrink fit to the shaft. The rest of the machine remained the same (i.e., casing and stationary components). This compressor was equipped with a vaneless diffuser configuration. Figure 1. Cross-Section of the Test Compressor. The compressor was driven by a 2.8 MW synchronous electric motor, through a speed increasing gearbox, with a gear ratio of 6.607. A variable speed drive permitted the operation of the compressor within its speed range of 6000 to 13,000 rpm. The test compressor is utilized in the coauthor’s closed loop test facility, and was equipped to simulate the conditions expected for a centrifugal compressor operating under wet gas conditions. Figure 2 shows a schematic diagram of the test loop that was used for the evaluations. The major components of the test loop included a scrubber, the test compressor, a pump, a cooler, and a liquid injection module (mixer). The scrubber, here called guard separator, was used to separate the dry gas (saturated hydrocarbon mixture) from the liquid (hydrocarbon condensate) in order to permit accurate measurement of the massflow of each stream (liquid and gas). The liquid stream was measured with a Coriolis flowmeter while the gas stream was measured with a calibrated orifice plate. Figure 2. Schematic Diagram of the Wet Gas Test Loop. PROCEEDINGS OF THE THIRTY-FOURTH TURBOMACHINERY SYMPOSIUM • 2005 112 Variable Speed Electric Motor (MW) Gas Flow 2 Phase Flow Condensate

Wet Gas Performance of a Single Stage Centrifugal Compressor

This paper evaluates the performance analysis of wet gas compression. It reports the performance of a single stage gas centrifugal compressor tested on wet gas. These tests were performed at design operating range with real hydrocarbon mixtures. The gas volume fraction was varied from 0.97 to 1.00, with alternation in suction pressure. The range is representative for many of the gas/condensate fields encountered in the North Sea. The machine flow rate was varied to cover the entire operating range. The compressor was also tested on a hydrocarbon gas and water mixture to evaluate the impact of liquid properties on performance. No performance and test standards currently exist for wet gas compressors. To ensure nominated flow under varying fluid flow conditions, a complete understanding of compressor performance is essential. This paper gives an evaluation of real hydrocarbon multiphase flow and performance parameters as well as a wet gas performance analysis. The results clearly demonstrate that liquid properties influence compressor performance to a high degree. A shift in compressor characteristics is observed under different liquid level conditions. The results in this paper confirm the need for improved fundamental understanding of liquid impact on wet gas compression. The evaluation demonstrates that dry gas performance parameters are not applicable for wet gas performance analysis. Wet gas performance parameters verified against results from the tested compressor is presented.Copyright

Prospects for Sub Sea Wet Gas Compression

Wet gas compression technology renders possible new opportunities for future gas/condensate fields by means of sub sea boosting and increased recovery for fields in tail-end production. In the paper arguments for the wet gas compression concept are given. At present no commercial wet gas compressor for the petroleum sector is available. StatoilHydro projects are currently investigating the wet gas compressors suitability to be used and integrated in gas field production. The centrifugal compressor is known as a robust concept and the use is dominant in the oil and gas industry. It has therefore been of specific interest to evaluate its capability of handling wet hydrocarbon fluids. Statoil initiated a wet gas test of a 2.8 MW single-stage compressor in 2003. A full load and pressure test was performed using a mixture of hydrocarbon gas and condensate or water. Results from these tests are presented. A reduction in compressor performance is evident as fluid liquid content is increased. The introduction of wet gas and the use of sub sea solutions make more stringent demands for the compressor corrosion and erosion tolerance. The mechanical stress of the impeller increases when handling wet gas fluids due to an increased mass flow rate. Testing of different impeller materials and coatings has been an important part of the Statoil wet gas compressor development program. Testing of full scale (6–8 MW) sub sea integrated motor-compressors (dry gas centrifugal machines) will begin in 2008. Program sponsor is the Asgard Licence in the North Sea and the testing takes place at K-lab, Norway. Shallow water testing of a full scale sub sea compressor station (12.5 MW) will begin in 2010 (2 years testing planned). Program sponsor is the Ormen Lange Licence.Copyright

Effects of raw natural gas on the aging of high-voltage electrical machine mainwall insulation

The total recovery of natural gas from subsea wells can be significantly increased with a compressor installed near the wellheads. The compressor is powered by a high-speed induction motor integrated in the same casing. The process gas flows through the motor and acts as a cooling medium. The insulation system of the motor is in direct contact with the gas and must be resistant to it. The gas mixture contains hydrocarbons, water, and monoethylene glycol. The effects of the gas mixture and its individual components on the properties of a high-voltage machine mainwall insulation consisting of mica, glass, and epoxy are obtained by experimental tests with raw natural gas at accelerated conditions. The tests at high pressures and temperatures indicate that heavy hydrocarbon compounds cause similar effects to plasticizers inside the bisphenol A epoxy resin, but such compounds do not penetrate easily into epoxy novolac resin. The plasticizing effect is seen as increased weight and volume, decreased mechanical strength and E-modulus, and reduced glass transition temperature. The polymers did not decompose chemically. The mainwall insulation is vulnerable to delamination, which is initiated by the detachment of glass strains and epoxy resin. Water causes dielectric loss peak at very low frequencies, while the heavy hydrocarbons produce a loss peak in higher frequency range.

An Experimental Investigation of Airfoil Performance in Wet Gas Flow

Development of wet gas compressors is challenging due to the liquid phase impact on performance. Experimental investigation of airfoil behavior in wet condition contributes to a revised compressor design and increased understanding of multiphase flow mechanisms. The performance of an airfoil was investigated in wet gas flow. An air-water mixture was used as the experimental fluid. The influence of wet gas flow on airfoil performance was investigated at different angles of incidence and gas volume fractions. A qualitative description of the complex physical process observed when liquid is introduced into the flow field is given. Airfoil performance was degraded at increased liquid mass flow rate owing to premature boundary layer separation. The initiation of separation was observed as a local film thickening, followed by increased liquid film fluctuations. A continuity wave was observed surrounding the airfoil, forming a U shape of increased liquid concentration. The wave was initiated by deposited droplets and the formation of secondary droplets. The investigation reveals that compressor operating range, surge and stall margins are affected by the wet gas fluid. Reviewed literature and experiments confirm a reduced stall and surge margin when a compressor is exposed to wet gas. Further investigation will involve sub-scale impeller tests to determine the effects on the performance and stability ranges.Copyright

High voltage machine mainwall insulation material behavior when exposed to raw natural gas

Natural gas recovery from subsea fields is increased by use of artificial lift, utilizing subsea compression technology to boost the gas flow. Such machines consist of a high-speed, high voltage electrical motor integrated into the same casing as the compressor and where the motor is cooled by the process gas injected from the compressor itself. Since the insulation system is in direct contact with the process gas it has to possess excellent chemical resistance and mechanical strength to withstand rapid decompression. Various types of mica-based mainwall insulations were exposed to an environment consisting of hydrocarbon gas, condensate and a mixture of water and monoethylene glycol (MEG) at 150 bar and 130 °C. Physical and mechanical parameters of the samples were evaluated. The process gas ageing tests revealed that the mechanical strength of the mainwall insulation is mainly dependent on the selected resin and the degree of resin saturation inside the insulation.

Fortifying Subsea Wells: A Comparative Analysis of Mainwall Insulation Materials Used in Natural Gas Production

Natural gas recovery from subsea wells is increased by an artificial lift, which can be provided by a compressor installed at the wellheads. Such a machine is driven by a high-speed, high-voltage electrical motor that is integrated into the same casing as the compressor. The motor is cooled by the process gas bled from the compressor itself. Because the insulation system is in direct contact with the process gas, it must have good chemical resistance and high mechanical strength to withstand the environment.