Production Separator Principles

Production Separator Principles, As the well fluids flow from high bottom hole pressure and temperature, they undergo pressure and temperature reducti

Petroleum, as produced at the wellhead, is a mixture of different compounds.  All have different densities, vapor pressures, and other physical characteristics.

Well fluids are high-velocity, turbulent, expanding mixtures of gases and liquids. These fluids are entrained with water vapor, free water, solids and debris.  As the well fluids flow from high bottom hole pressure and temperature, they undergo pressure and temperature reduction. Entrained gases separate from the liquids because of reduced pressure and gravity.

Field processing of the well fluids removes undesirable components and separates the saleable gas from liquids. This separation obtains the maximum amounts of each component at lowest cost.

The production separator is a vessel used to separate well fluids. A production separator separates the component fluids using a mechanical process at normal temperature levels. Low temperature separation and stabilization separation are not covered in this module.

Externally a separator is a cylindrical tank. Midpoint on the tank is the well stream inlet. Near the top or on top is the gas outlet. Also on top is either a relief valve or rupture disk (safety head). Some separators have both devices.

Near the bottom is a liquid-level control valve (dump valve) and a liquid-level controller. On the bottom is a drain for cleaning solids which accumulate over time. Gauges and sight glasses may also be on the exterior of the separator.

A production separator may be either two or three-phase. A two-phase separator separates gas from liquids; a three-phase separator sepa-rates gas, oil and water.

Separators are vertical, horizontal, or spherical. Two- and three-phase separators, as well as different separators, are covered in Section 2. 

Separator Components
The function of a separator is to separate free gases from the liquid stream at a specific pressure and temperature.  For efficient and stable operation over a wide range of conditions, a gas-liquid separator includes :  primary separation section, secondary separation section, liquid accumulation section, mist extraction section, control devices and safety devices.

Primary Separation Section
This section removes the bulk of liquid in the inlet stream. Slugs and large liquid particles are removed first to minimize gas turbulence and re-entrainment of liquid particles. To do this, the velocity and direction of flow is changed. Centrifugal force created by either an inlet baffle or internal piping allows for changes of flow direction and reduction of stream velocity.

Secondary Separation Section

The separation principle in this section is gravity settling of liquid from the gas after stream velocity has been reduced. The efficiency of this section depends on the gas and liquid properties, particle size and degree of gas turbulence. Some designs use straightening vanes to reduce turbulence. The vanes also act as droplet collectors.

Mist Extraction Section

The mist extraction section removes the very small droplets of liquid in a final separation step before the gas leaves the vessel. The mist extractor can be one of several designs, e.g., a series of vanes, woven-wire mesh pad or cycionic passage. More recent designs use the woven-wire mesh pad.

Liquid Accumulation Section
Liquids are collected in this section. By design the liquid has a minimum disturbance from the flowing gas stream. Two things determine the capacity of this section : the volume of well- stream surges and the time liquid must remain in this section for efficient break out of solution gas.

Control Devices

Liquid levels within the separator determine the vessel’s ability to perform the designed task. The liquid-level control valves (dump valves) are operated by liquid-level controllers.

Gas pressure must be controlled. Back pressure regulators on the gas outlet or downstream processes maintain the design pressure.

Separators also have sight glasses for monitoring liquid levels. These devices allow the operator to check levels, operation of the dump valves and proper control.

Safety Devices

Separators are equipped with safety devices. These devices may include  : Varec valve (vent), relief valve or rupture disk (safety head). All are mounted near the top of the vessel.

Both control and safety devices will be covered in more detail in Section 3.

Separator Flow

Well fluids enter the separator and flow around baffles at the inlet. The fluids change velocity and direction when they come in contact with the baffles.

The heavy fluids resist the change in direction and the light fluids flow relatively easily in the new direction. Some of the liquids are forced against the sides of the separator where they are caught in drains and channeled to the bottom of the separator.

Gas rise through the separator carrying some liquid in the form of mist. This mixture passes through the mist extractor. The extractor is a pad, usually made of woven wire mesh.  Liquid mist collects on the pad and as droplets form and become larger they drop into the liquid accumulation section. The gas leaves the separator through the gas outlet near the top of the separator.

Liquids and solids collect in the liquid accumulation section. Because the section is away from stream turbulence, gravity causesdense solids, e.g., sand, clay, etc., to settle on the separator bottom. These are removed periodically.

Liquids continue to collect until the level reaches the designed dump level. The liquid level controller causes the liquid-level control valve to open. Liquids flow from the separator to storage or other process facilities.

• [accordion]
• 1. Production Separator Principles
• 
• 2. Pressure Vessel ASME VIII Div1 2021
• 3. Pressure Vessel ASME VIII Div2 2021