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Level Control Considerations for Crude Oil Desalting, Dehydration and Degassing Applications

Throughout the crude oil production process, there are many opportunities to measure and improve the safety of your operations. In last week’s blog post, we discussed how you can use level control instrumentation to improve the safety of applications at the beginning of the crude oil production process, including production fluid storage, chemical injection and wellstream separators. This week, our discussion will focus on the complex measurement challenges that occur during the next phases of this process, including crude oil desalting, dehydration and degassing.

Crude Oil Desalting
When salt is present in the crude stream, it can create serious corrosion and scaling problems, and must be removed; however, salt can be dissolved within the remnant brine of the crude oil. The crude oil desalting process removes the salt as well as the residual free water. Field desalting is also necessary due to pipeline requirements.

Level instrumentation is vital for single and two-stage desalting systems, multiple orifice plate mixers, and the settler tank of a chemical desalter. In addition, interface level control stops free water from contacting the desalter electrodes – and prevents expensive damage. You must also keep the interface level constant to prevent electrical field changes from disturbing electrical coalescence.

A displacer transmitter – such as the E3 Modulevel® – or a guided wave radar (GWR) transmitter with an enlarged coaxial probe can provide continuous level control for this application. For point level control, a thermal dispersion switch or float-actuated external cage level switch is recommended.

Crude Dehydration
During the crude dehydration process, the first stage of gravity separation does not remove all water from the oil. In fact, separated crude can contain up to 15% water, which exists in an emulsified form that is difficult for a separator to remove. The oil and water emulsion must be broken down using chemical agents, such as glycol and heat, so that the water can be removed before the crude is shipped.

Level control is required for two-phase and three-phase water knockout drums, as well as heater treaters and chemelectric dehydrators. Interface measurement is also critical in dehydration because it keeps the water-emulsified oil from flowing over the separator weir.

Effective continuous level and interface level controls for the crude dehydration process include a displacer transmitter or GWR, while a float-actuated external cage level switch can provide point level control.

Crude Degassing
The crude stabilization and sweetening processes diminish safety and corrosion problems by using a stabilizer to remove dissolved gases and hydrogen sulfide. Stabilization or vaporization processes, as well as a gas or steam-based stripping agent, are used during sweetening.

During the removal of dissolved gases by stabilization, level control is required in the reboiler unit. Sweetening by stage vaporization or trayed stabilization also requires level control in a series of staged separators, while sweetening by reboiled trayed stabilization requires additional level control in a reboiler.

For continuous level control in this application, a GWR or displacer transmitter will work well. A float-actuated switch or float-actuated external cage level switch can provide the point level control that this application requires.

Follow Our Crude Oil Production Discussion
Next week, we will conclude our discussion of level instrumentation for crude oil processing applications. To learn more about this topic, please download the Magnetrol® Crude Oil Processing Technology bulletin.