A REVIEW OF GAS-LIQUID CYLINDRICAL CYCLONE (GLCC) TECHNOLOGY by G. E. Kouba

Economic pressures continue to force the petroleum industry to seek less expensive alternatives to conventional gravity based separation. The gas-liquid cylindrical cyclone (GLCC) is a simple, compact, low-cost separator that can provide an economically attractive alternative to conventional separators over a wide range of applications. Although cyclones have long been used for liquid/liquid, solid/liquid, and gas/solid separation, they have experienced only limited use in full range gas/liquid separation applications. The biggest impediment to the wide spread use of GLCCs has been the lack of reliable performance prediction tools in order to know where they can and cannot be successfully applied. This paper presents the status of the development of the GLCC, the state-of-the-art with respect to modeling the GLCC, and discusses current installations and potential applications. INTRODUCTION The GLCC is a simple, compact, low-weight and low-cost separator that is rapidly gaining popularity as an alternative to conventional gravity based separators. Shown in Fig 1 is a simple GLCC consisting of a vertical pipe with a tangential inlet and outlets for gas and liquid. The tangential flow from the inlet to the body of the GLCC causes the flow to swirl with sufficient tangential velocity to produce centripetal forces on the entrained gas which are an order of magnitude higher than the force of gravity. The combination of gravitational and centrifugal forces pushes the liquid radially outward and downward toward the liquid exit, while the gas is driven inward and upward toward the gas exit. The performance of a GLCC is characterized by it’s operational envelope which is bounded by lines of constant liquid carry-over in the gas stream and constant gas carryunder in the liquid stream. The onset to liquid carry-over is identified by the smallest flow of liquid observed in the gas stream. Similarly, the first observable bubbles in the liquid underflow mark the onset of gas carry-under. Despite the long history of cyclone technology and the seemingly simple design and operation of the GLCC, these cyclones have not been widely used for full-range gas-liquid separation. Part of the reluctance to use GLCCs must be attributed to the uncertainty in predicting performance of the GLCC over a full range of gas-liquid flows. The difficulty in developing accurate performance predictions is largely due to the variety of complex flow patterns that can occur in the GLCC. The flow patterns above the inlet can include bubble, slug, churn, mist and liquid ribbon. Below the inlet the flow generally consists of a liquid vortex with a gas core. At lower liquid levels, a region of annular swirl flow may exist between the inlet and the vortex. Further refinements of flow pattern definition below the inlet have not been made. This difficulty in predicting the hydrodynamic performance of the GLCC has been the single largest obstruction to broader usage of the GLCC. Even without tried and tested performance predictions, several successful applications of GLCCs have been reported. The development of reliable performance prediction tools will govern the speed and extent to which the GLCC technology can spread in existing and new applications. APPLICATIONS The GLCC has a distinct advantage over conventional gravity based separators when compactness, weight, and cost are the overriding considerations for separator selection. There are a variety of applications where requirements may vary from partial to complete gas-liquid separation. Below are some of the current installations and potential applications for the GLCC. Multiphase Measurement Loop: Figure 2 shows a GLCC in a multiphase metering loop configuration. This type of measurement loop configuration affords several advantages over either conventional separation and single phase measurement or nonseparating multiphase meters. The loop configuration is somewhat selfregulating which can reduce or even eliminate the need for active level control. The compactness of the GLCC allows the measurement loop to weigh less, occupy less space, and maintain less hydrocarbon inventory than a conventional test separator. Furthermore, complete or even partial gasliquid separation can improve the accuracy of each phase rate measurement in a multiphase metering system. When complete gas-liquid separation is achieved in the GLCC, several liquid metering options are available, e.g., bulk liquid metering and proportional sampling. Chevron has several multiphase metering loops in operation that use this standard liquid metering approach on the liquid leg of a GLCC. This is a very low cost option for multiphase measurement although sampling can be labor intensive. Two-phase liquid-liquid meters are also available for the liquid leg. Liu and Kouba have shown that Coriolis meters with the net oil computer (NOC) option can simultaneously measure oil and water flow rates with excellent accuracy for production allocation applications, such as well testing, provided there is no gas present in the meter. Chevron has deployed several multiphase metering loops with Coriolis NOCs on the liquid leg as shown in Fig.3. One of the main limitations of the Coriolis NOC in the measurement loop is the sensitivity of the Coriolis NOC to small amounts of gas that may carry-under with the liquid. The Accuflow multiphase measurement system, shown in Fig. 4, utilizes a second stage horizontal pipe separator between the GLCC and the Coriolis meter to prevent gas carry-under from reaching the Coriolis meter. When gas carry-under cannot be prevented, a three-phase metering system is required on the liquid leg. In general, the accuracy of a multiphase meter on the liquid leg will benefit significantly from removing some of the gas. Most multiphase meters have an upper limit on the gas volume fraction allowed through the meter in order to maintain their accuracy specifications. Beyond improved accuracy, partial gas separation provides the additional benefit of utilizing a smaller, less expensive, multiphase meter. For some multiphase meters whose price scales directly with size, the cost savings of using a smaller meter was over 4 times the cost of the GLCC. The effect of partial gas separation on multiphase metering can be so pronounced that several multiphase meter manufacturers are configuring their meters in multiphase measurement loops utilizing compact gas-liquid separation. At least two manufacturers are supporting further research on GLCCs. Preseparation: A compact GLCC is often very appropriate for applications where only partial separation of gas from liquid is required. One such application is the partial separation of raw gas from high pressure wells to use for gas lift of low pressure wells. Weingarten et al. developed a gas-liquid cyclone separator with an auger internal for downhole and surface separation of raw gas. They showed that the auger cyclone could successfully separate up to 80% of the gas without significant liquid carry-over into raw lift gas stream. The cost of the auger separator was reported to be about 2% of a conventional separator. The real savings in this sort of application comes from reducing or eliminating gas compression facilities. Separating a significant portion of the gas will reduce fluctuations in the liquid flow and may result in improved performance of other downstream separation devices. Krebs Petroleum Technologies is investigating the use of a GLCC in series with other compact separation devices such as a wellhead desanding hydrocyclone and a free water knockout hydrocyclone. Chevron is investigating the series combination of a GLCC with a freewater knockout hydrocyclone and a deoiling hydrocyclone in an effort to improve discharge water quality, as shown in Fig. 5. Arato and Barnes investigated the use of GLCC to control GLR to a multiphase pump to improve pumping efficiency. Sarshar et al showed several combinations of GLCC and jet pumps that could be used to extract energy from high pressure multiphase wells to enhance production from low pressure wells. Production Separation: Vertical separators with tangential inlets are fairly common in the oil field. These predecessors of the GLCC are often big and bulky with low velocity perpendicular tangential pipe inlets. The tangential velocities are usually so low that gravitational and centrifugal forces contribute roughly equally to separation. Technological developments (discussed in later sections) in both GLCC hardware and software should reduce the size and improve the performance of vertical separators. One challenge in optimizing the size of a GLCC for production separation is designing a system that can respond quickly to surges without serious upsets. Cyclone separation has already proven useful in internal separation devices for large horizontal separators. The GLCC may also provide a useful external preseparation device to enhance performance of existing horizontal separators, as in Fig. 6. By separating part of the gas, the separator level might be raised to increase residence time without encountering the mist flow regime in the vessel. The biggest impact to the petroleum industry from GLCC technology may be in subsea separation applications. Baker and Entress have concluded that “wellhead separation and pumping is the most thermodynamically efficient method for wellstream transfer over long distances, particularly from deep water”. No doubt, the development of marginal offshore fields will depend upon developing efficient and profit effective technologies. Subsea applications demand a high degree of confidence in separator design and performance while demanding that the equipment be simple, compact, robust and economical. Here again the virtues of the GLCC should place it in good standing among competing technologies. DEVELOPMENTS Few systematic studies of design configurations of different GLCC physical features have been conducted. La