Multiphase boosting in oil and gas production may typically be characterized by:Slug flow occurrence possible, andGVF at inlet conditions higher than 50 %. The physical challenge for the equipment in use is the strong variation of the GVF (Gas Void Fraction), leading to transient flow conditions at the pump's inlet. However, when the GVF during the entire operations constantly exceeds 98 %, control of heat generation becomes a further challenge. Suitable answers need to be found for the following questions likeHow can wet gas compression best be described?Is this an efficient way to compress wet gas?What about installation economics? A technical issue for wet gas compression is the shaft sealing. Available standard seals can either cope with liquid or gas, but not with varying fractions of both. Other limiting factors are differential pressure, lubrication characteristics of pumped liquids, solids, and circumferential velocity. Introduction During the last decade multiphase boosting in oil and gas production has considerably matured. It is widely recognized as a true alternative to traditional production scenarios. The twin-screw pump is playing the major roll, when the equipment has to be selected. Due to its volumetric character heavy slugging, varying water content and other typical multiphase operating challenges this pump type is well suited for this purpose. By its low speed the hydrocarbon is transported without considerable turbulence what strongly avoids emulsification whenever water is also present - a definite advantage for the later separation of the phases. The demand for an increased and more constant Gas Volume Fraction (GVF) only was a little step ahead, then to be called "wet gas compression". Once the average gas volume fraction increases to about 98 % and only few liquid slugs, if any do occur, multiphase pumping should be called "wet gas compression". At the same time customer's interest has turned from oil to gas, and the associated liquid phase is of low viscosity, i.e. condensate and/or water. As the flow regime to the pump still is transient with changing inlet pressure and temperature, the same mechanical design guidelines must be used for the wet gas compressor as for the multiphase pump. Thermal expansion and quick temperature changes especially have to be looked after. Definitions A typical working domain for a wet gas compressor may be described by the following set of parameters:Power installed: P = 1.0 MWDifferential pressure: ? p= 50 bar (5 MPa)Inlet pressure: pin = 50 barg (5 MPa)Total capacity: Qin = 1,000 m3/h, which is equivalent to 240 MSm3/d at 10 barg inlet pressure, or 1,000 MSm/d at 40 barg respectively. To make the applications harder, the viscosity range is around or even below 1 mm3/s. Technical Issues In a twin-screw pump two pairs of intermeshing screws, forming cavities between them are rotating in the housing. The fluid is thereby conveyed from inlet to discharge from both sides. Thus axial forces are fully balanced. As no metal-to-metal contact is permitted, the clearances occurring need to be sealed by liquid. The sealant characteristics increase with higher viscosities. With little liquid occurring in the wet gas stream, it needs to be made available. This can preferably be done by storing it in the pump housing itself with a cooling circuit attached. Hence no liquid is added to the production and does not need to be separated at a later stage. Heat Generation and Slug Flow Occurrence. Whenever liquid or gas is compressed, heat is generated. The main difference is the heat storage capability of the different fluids: good with liquid (Water = 4.2kJ/kg·K) and very poor with gas (= 1.0 kJ/kg·K), the difference in mass throughput has to be observed. The discharge temperature of the fluid is governed by the pressure ratio and the fluid parameters. The latter being described with the isentropic exponent ?. Heat Generation and Slug Flow Occurrence. Whenever liquid or gas is compressed, heat is generated. The main difference is the heat storage capability of the different fluids: good with liquid (Water = 4.2kJ/kg·K) and very poor with gas (= 1.0 kJ/kg·K), the difference in mass throughput has to be observed. The discharge temperature of the fluid is governed by the pressure ratio and the fluid parameters. The latter being described with the isentropic exponent ?.
TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractMultiphase boosting in oil and gas production may typically be characterized by:-Slug flow occurrence possible, and -GVF at inlet conditions higher than 50 %.The physical challenge for the equipment in use is the strong variation of the GVF (Gas Void Fraction), leading to transient flow conditions at the pump's inlet. However, when the GVF during the entire operations constantly exceeds 98 %, control of heat generation becomes a further challenge. Suitable answers need to be found for the following questions like -How can wet gas compression best be described? -Is this an efficient way to compress wet gas? -What about installation economics?A technical issue for wet gas compression is the shaft sealing. Available standard seals can either cope with liquid or gas, but not with varying fractions of both. Other limiting factors are differential pressure, lubrication characteristics of pumped liquids, solids, and circumferential velocity.
Multiphase Pumping is recognised as state-of-the-art technology. But still operators and manufacturers see themselves confronted by two major challenges which are first the set of operating parameters, and second the Mean Time Between Failure. The mechanical seal system strongly contributes to the MTBF, hence this has been subject to various brainstorming with all parties involved. Modern "Multiphase Boosters" have little in common with standard pumps or compressors as far as operating parameters are concerned. All parameters especially effecting the seals are of a transient character while those in pumps or compressors are quasi-static. This paper presents a new sealing technology, which keeps any process liquid inside the pump, with emphasis on experience gained in test-bed runs and field operations. 1. Introduction There is no doubt that potential users have recognised Multiphase Pumping as a mature technology to improve oil and gas production in general. Although the current pump population is only about 250, this is spread among 50 operating companies in 25 countries around the world! Most operators are "single users", but more and more installations can be seen with "repeat" customers as well as in parallel installations. There is no doubt that whatever pump type or working principle one is looking at, this "pumping" requires no further qualification as it is based on proven performance. In its basic application at the wellhead or field header this technique requires an extreme flexibility of the boosting equipment in use. The reason being typical hydrocarbon delivery from the well, separation effects in the flow lines, or even changing conditions over the field's lifetime. Twin- Screw Multiphase Pumps have proven to be able to withstand even the toughest conditions without the assistance of flow conditioning or other auxiliary equipment. However as the mechanical seal's contribution to the Mean Time Between Failure is significant, if sometimes not the only reason for a failure, they subsequently need further development to achieve the same degree of reliability. 2. Operations Looking onto the operational envelope of either standard pumps or compressors, one thing is obvious: the operational parameters do not change substantially so their character is of a quasi-static nature, although changes may occur during e.g. day and night times. Nevertheless it is often still a challenge for the design engineer to make the equipment fit for the intended use, and at the same time incorporate the latest technological advancements and observe commercial aspects in a highly competitive market. By various reasons the user unfortunately has to take into account that equipment tends to fail, and may it only be due to Murphy's (Sod's) law: "If things can go wrong, they will." Vendors have provided equipment for a task, which was thought to be impossible only about ten years ago: to boost a mixture of liquid and gas, no matter which portions of each one looking on. Together with their customers they have jumped into an environment where operational parameters are difficult to define and sometimes even unpredictable: oil and gas production. As multiphase flow has been a challenge for engineers since decades by now, the same goes for multiphase boosting with changingsuction pressure due to varying gas volume fractions (GVF),temperatures with the above,discharge pressure with the above,mechanical impact with the above. In general we are talking of a transient flow scheme (fig. 1) in a piece of equipment which originally was not designed for this. Subsequently this offers a lot of challenges for the design engineer in charge, but for the customer as well, as he is the one to most clearly and exactly define the task to be fulfilled. There is no doubt that equipment to be used in such an environment has to be thought over from the scratch, and simple adding of auxiliaries or non-reflected use of standard components is not acceptable. Besides the technological and operational experience of the manufacturer in standard applications, which account for the solution of the majority of challenges, sub-suppliers and customers have to be tied into this with a substantial exchange of information and operational (test and field) experience. There is no doubt that sealing technology needs improvement in this respect, thanks to everyone, who has already looked into it. He has thereby improved the Mean Time Between Failure (MTBF) of this type of equipment.
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