During the past two weeks, this blog has discussed the use of level control instrumentation to improve the safety of applications at the beginning and middle of the crude oil production process. This week, we’ll review the level measurement challenges that occur during the final phase of the crude stream, including water processing, storage tanks and vapor recovery units.
During water processing, produced water, wash-down water and collected rainwater are treated before being disposed of or reused for reservoir flooding. This is required because water collected from process operations contains hydrocarbon concentrations that are too high for safe discharge. In addition, suspended hydrocarbon droplets in water can hinder well-injection.
The equipment used during water treatment is similar to three-phase separators except that water is the main product. While level control is essential for skim tanks, precipitators, coalescers, flotation units, collection tanks and sumps, interface level measurement is required to ensure proper draining of clean water and removal of residual oil.
An ultrasonic gap switch, such as the Echotel® Model 961, or a float-actuated external cage level switch can provide point level control for this application. For continuous level control, a guided wave radar (GWR) or displacer transmitter is recommended.
Process and Storage Tanks
Refineries use process and storage tanks to store crude and water in oil fields. In contrast to midstream tank farms at terminals and refineries, field storage uses smaller vessels for oil and water processing. These tanks may also be used to store diesel generator fuel, potable water and fire water.
Tank level monitoring is required for overflow control, shutdown pumps and alarm systems. Interface controls can also sense the beginning of an oil/water interface during tank dewatering and control the water draw-off. In addition, new API 2350 recommended practices for above-ground storage tanks that receive Class I (flammable) liquids state that level controls can effectively protect against tank overfills.
A GWR transmitter with a flexible probe, non-contact pulse burst radar transmitter or displacer level transmitter can be an effective continuous level control for this application. A float-actuated level switch is recommended for point level control.
Vapor Recovery Unit
Allowing hydrocarbon vapors to escape into the atmosphere can create fire hazards, pollution problems and diminish income through loss of hydrocarbon volume. That’s why refineries use a Vapor Recovery Unit (VRU) to collect vapors from storage and loading facilities, reliquefy the vapors and return the liquid hydrocarbons back to storage. Common vapor recovery methods include absorption, condensation, adsorption and simple cooling.
The use of a VRU is a simple, economical process that provides EPA compliance and improves operating economies by capturing up to 95% of fugitive emissions. A critical component of the VRU is the flash drum, where vapors are reliquefied. It is essential to control the liquid level within the flash drum.
Effective continuous level controls for the vapor recovery unit include GWR and displacer transmitters, while a float-actuated switch or float-actuated external cage level switch can provide point level control.
For More Information
To learn more about level instrumentation for crude oil processing applications, please download the Magnetrol® Crude Oil Processing Technology bulletin.