Abstract

Among the technologies that have been developed for oil and gas production in the last years, Multiphase Pumping can be seen as one of the most promising. The significant reduction of capital investment, together with an increased production rate has led to a growing demand on systems to be installed. The biggest challenge that has to be overcome by a piece of rotating equipment in multiphase service is the sealing of media that varies from 100% liquid to 100% gas surges, slugs of liquid, gas locks and sand content. A multiphase pump, and particularly it's mechanical seals, must be able to withstand all these rather harsh operating conditions.

This paper describes various successful Mechanical Seal arrangements in positive displacement twin screw pumps must often used in multiphase applications.

Introduction

Among the technologies having been developed for oil and gas production in the last 15 years Multiphase Pumping can be seen as one of the most promising. The significant reduction of capital investment either on- or offshore together with an increased production rate has led to a growing demand on systems to be installed. Meanwhile the following advantages for using Multiphase Pumps are in the meantime well known:

  • reduced equipment capital cost and Complexity

  • elimination of separate oil/water and gas lines

  • profitable production of marginal fields

  • increased production rates

  • reduced weight and space requirements (for offshore applications)

  • prolonged reservoir life

  • reduced wellhead back pressure.

The biggest challenge to be overcome by a piece of rotating equipment in multiphase service is the variation from 100% liquid to 100% gas at the inlet of the fluid to be moved.

This fluid provides therefore very difficult operating conditions for the pump, but also for the mechanical seals, since this variation is unpredictable. The transient nature of the wellhead stream creates pressure surges, slugs of liquid and gas locks. A multiphase pump must be able to withstand all these rather harsh operating conditions. This kind of pump must be insensitive to changing liquid density and gravity and resistant to corrosive effects of the well fluid impurities, such as H2 S,C02, CaC03 (=scaling) and wax. The sand content of the fluid is an additional challenge for the pump.

Multiphase pumps can be subdivided into two general groups, each with their own benefits and limitations. These are rotodynamic and positive displacement pumps. Rotodynamic pumps relies on the concept of helico-axial hydraulics. A multistage rotodynamic pump can employ up to 15 compression cells on a single shaft. Each compression cell comprises a rotating helico-axial flow impeller and a stationary diffuser. The produced effluent can thus be boosted over long distances without the need for prior separation and with no limitation regarding gas void fractions. These kind of pumps are compact, produce high flow rates and have high sand handling capacity. Industrial rotodynamic multiphase pumps are available through two licensed manufactures, namely Sulzer Pumps and Framo Engineering.

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