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№ 10 (October 2012)

Compressors with Quasi-Isothermal Cycle – New Trend in Multiphase Pumping of Hydrocarbon Material

   Severed requirements for ecology and cost reduction for oil and gas field infrastructure development force production companies to use more advanced technologies, among which multiphase pumping of hydrocarbon material is currently distinguished.

By A.I. Rabinovich, adviser director general on new facilities, NOVOMET-Perm I.S. Pyatov, chairman of board of directors, REAM-RTI, LLC

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The core of such technology is multiphase pumping unit.

   Functions of the multiphase unit include receiving of multiphase medium (immiscible liquids with various viscosity, gases, gas-liquid emulsions, mechanical particles) and pressure increase up to values that allow to transport this medium from the collection location to the processing point.
Due to instability of pumped multiphase medium features, the design of the pump and compressor unit shall provide efficient operation both in liquid and gaseous media.

   As a rule, differential pressure developed in the multiphase unit is up to 3.5 (up to 5.0) MPa.

   For these purposes pumps of rotodynamic type (for example, pumps made by Sulzer) and volumetric type (for example, pumps made by Bornemann, Leistritz Corporation, Framo Engineering, Warren Pumps, Inc., Weatherford International, Ltd., NETZSCH, Moyno, Can-K, etc.) are used.

   Disadvantage of these known multiphase pumps is limitation of operation time without liquid at the pump inlet.

   Thereupon of interest is the use of the concept of single-shaft volumetric machines without internal medium compression in working chambers capable of long-term operation as the compressor on the "wet gas".

   In 2004 by the technical design assignment of NOVOMET-Perm, JSC and according to the technical requirements of TNK-BP for multiphase medium transportation Russian сompany REAM-RTI, LLC developed a project of the pump-compressor positive-displacement module of gerotor type.

   Gerotor designs differ from gear mechanisms with internal engagement in that their external rotor has one less tab ("tooth") than the internal rotor.

   Simplicity and compactness of gerotor mechanisms, rigidity of basiс layout, possibility to connect these mechanisms on one drive shaft (implementation of modularity principle) make them the most attractive for creation of positive-displacement machines without compression of pumped value. Thus, in relation to gas pumping it is allowed approaching quasi-isothermal compression process and, therefore, reduction of energy consumption per drive.

   Similar approach in relation to compression of gas or vaporous medium in the compressor is implemented by Star Rotor Corp.

   However, designers of this company have not set themselves a task to pump aggressive multiphase medium containing incompressible liquids.

   The task of pumping liquid, gas-liquid medium and gas is solved in the design of multiphase pump/compressor of gerotor type of REAM-RTI, LLC,  where the principle two-in-one has been used. The purpose of the project is to create the base module for the pumping units family. The family consists of unified section modules (up to 4 and more on one shaft).

   The basic kinematic scheme of the multiphase pump/compressor made by REAM-RTI, LLC/NOVOMET-Perm, JSC is shown in Fig.1.

   Based on their preliminary calculations and experience REAM-RTI specialists designed and in 2005 NOVOMET-Perm specialists manufactured a full-size pilot of gerotor multiphase pump/compressor NPM 200/600 (Fig. 2).

   At shaft rotation speed n = 3,000 min-1 and at differential pressure of liquid of  3.5 MPa liquid supply is 120 m³/h.
Overall dimensions:
Unit assembly: diameter – 204 mm, length – 198 mm;
base module: length – 415 mm;
Gerotor unit  is shown in Fig. 3.

   Fig. 4 shows comparison of characteristics of liquid flow rate due differential pressure at various specified rotation speeds for pump NPМ 200/600 and two-screw multiphase pump of Bornemann type.

   Multiphase pump/compressor NPМ 200/600 pilot module has confirmed the main design solutions. There have been obtained the expected figures of liquid flow rate and load characteristics (liquid flow rate / differential pressure); operation in the gas medium (air) without liquid has been confirmed.
For the gerotor pump (Fig. 4) low internal media leakages are specific, that conditions low sensitivity of the gerotor pump to back-pressure at outlet with the two-screw pump (dotted curve).

   Interpretation of these results and strength parameters for the single-section multiphase pump/compressor NPМ 200/600 allow to obtain differential pressure over 5.0 MPa. The basic elements design will provide the resource of 40 000 h minimum.

   According to main technical solutions having novelty RF patents are issued:
Patenting is provided abroad.

   Developed base version of gerotor pump allows to create the unified family of positive-displacement machines both for liquid/gas media transportation - pumps, compressors and for liquid/gas (vapor) pressure energy conversion – hydraulic motors, pressure-reducing valves.

   Technical solutions of REAM-RTI’s, LLC, used in the design of the positive-displacement machine of gerotor type, allow to connect the gas compressor and the hydraulic pump on two-in-one principle and provide new possibilities for multiphase pumping technologies in order to improve environmental friendliness and energy efficiency.

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