Special Multiphase Applications

Screw pumps have been used with gas-entrained applications for many years, but recent process changes in oil field technologies have created requirements for pumping multiphase fluids, containing more than just nominal amounts of gases. In many oil well applications, the liquid oil flow eventually degenerates into all sorts of difficult multiphase mixtures of oil, gas, water, and sand. In the past, it was common for the gas to be separated and flared off at the well head with only the liquid product to be retained for further processing. If the gas is to be processed as well, separators, compressors, and dual pipelines are required to handle the gas phase. Therefore, a pump, which can handle these difficult liquids with high gas content, can save significant equipment costs as well as operating costs. Under various conditions, the well output can vary from 100 percent liquid to 100 percent gas and all possible combinations. The applications also require the pumping equipment to be able to switch rapidly between the extremes or to handle slugs of liquid or gas, while maintaining the full discharge pressure. The timed two-screw type of pump has proven capable of pumping these multiphase products.1

Oil well applications can include traditional on-shore sites as well as off-shore platform installations. Subsea installations are also being used to reduce the high costs of equipment and operation of traditional oil platforms. In these applications, the pumping equipment is mounted on the sea bed with piping running to on-shore gathering facilities. Although the actual pumping conditions are similar to surface installations, the installation and operating environments are far more challenging.

When pumping multiphase products with high gas void fractions (GVF), the pump must be designed with a small pitch to provide a sufficient number of locks. The key to

FIGURE 20 Special screw pump for multiphase applications. (Flowserve Corporation)

pumping multiphase products is to ensure that some liquid is always available to seal the screw clearances and reduce the slip. Even a small amount of recirculated liquid is sufficient to provide this seal and enable the screw pump to operate with GVFs approaching 100 percent. Depending on a number of factors, the volume of liquid required to seal and cool the screws can be three to six percent of the total inlet volume flow rate. In order to ensure that sufficient liquid is available at conditions of high GVFs, a separate liquid flush can be provided or a separator type of pump body can be used. This type of body includes a special chamber that can separate some liquid from the multiphase mixture being pumped. This liquid can be recirculated back to the screws and mechanical seals to provide sealing and cooling liquid at times when the product is almost all gas. Figure 20 shows a special screw pump designed for multiphase applications with a separating chamber built into the body.2

When pumping liquids, the slip through the internal clearances is proportional to the differential pressure and inversely proportional to the viscosity. However, in multiphase applications, as the GVF increases, the slip decreases until the inlet volume flow rate is equal to the pump displacement. This results in an almost constant inlet volume flow rate, regardless of the differential pressure. This performance can be explained by examining the pressure drop of the multiphase product across the finite clearances. This theoretical analysis confirms that a small amount of liquid in the clearances will effectively seal these clearances and reduce the slip to near zero. Figure 21 shows the typical performance characteristics for a screw pump in a multiphase application when pumping mixtures of air and water at various GVF values.3

It should be emphasized that screw pumps must be sized for the inlet volume conditions. Since the gas portion of the multiphase product is compressible, the inlet pressure and temperature conditions must be known in order to calculate the gas volume. The pump will ingest a fixed volume of product and the amount of liquid being pumped will depend on how much of this volume is being displaced by gas at the inlet to the screws.

The newest area of application for multiphase pumps is subsea. With special modifications, the rotary screw multiphase pumps can be coupled to submersible motors and mounted on the sea bed, instead of on surface platforms. The idea of pumping multiphase products directly from a subsea well head to shore facilities by means of submersible multiphase pumps has significant potential savings in separating equipment and platforms.4 Figure 22

FIGURE 21 Typical performance of a screw pump on multiphase products (bar = psi/14.504).

shows a special, timed, two-screw pump, configured for subsea multiphase applications, with special, product-lubricated outboard bearings and pressure-compensated, lube oil chambers.

Survival Treasure

Survival Treasure

This is a collection of 3 guides all about survival. Within this collection you find the following titles: Outdoor Survival Skills, Survival Basics and The Wilderness Survival Guide.

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