Mechanical Sealing Technology Used in Multiphase Production
Authors: Necker N. 
Published: July 2010

This paper describes various successful mechanical seal arrangements in positive displacement twin screw pumps most 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 rule has led 10 a growing demand for systems to be installed. Meanwhile the following advantages for using multi phase pumps are well known:reduced equipment capital cost and complexity:elimination of separate oil/water und gas lines:profitable production of marginal fields;increased production rates:reduced weight and space requirements (for offshore applications);prolonged reservoir life:, and,reduced wellhead hack pressure.

The bigges1 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.

FIGURE: 1: Example of compression cell.(Available in full paper)

This variation provides very difficult operating conditions for the pump, as well as for the mechanical seals, since this content 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 the corrosive effects of the well fluid impurities, such as H2S.CO2 (= 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 rely on the concept of helico-axial hydraulics. A multistage rotodynamic pump can employ up to 15 compression cells on a single shift. Each compression cell (Figure 1) 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 manufacturers: Sulzer Pumps and Framo Engineering.

Positive displacement pumps, known as "twin screw pumps" (Figure 2) could already handle gas void fractions up to 70%. In this respect, only some minor improvements had to be made to an existing pump design. The Twin Rotor Screw Pump consists of two contra-rotating screws. A certain amount of liquid is needed to seal the gaps between the screws and the liner. These kind of pumps can handle large amounts of gas and are less susceptible to pressure fluctuations. Industrial screw pumps are available from Bornemann, Leistritz, or Flowserve.

FIGURE 2: Example of twin screw pump design.(Available in full paper)