Ensuring the safety of people and processes is critical for every oil refinery today. Key to achieving these goals is the ability to accurately measure liquid levels throughout the crude oil production process. That’s why using effective level measurement controls is crucial for both process control and safety shutdown systems.
To achieve accurate level measurement, it is important to understand the application challenges within the crude oil production process. In this post, we will discuss how instrumentation can provide reliable level measurement for three critical applications at the beginning of this process – production fluid storage, chemical injection and wellstream separators.
Production Fluid Storage
During the crude oil production process, additive chemicals used to precondition the inlet fluid are stored in the field or at a processing facility. These fluids may enter via a holding tank to allow time for upstream solids and liquids to separate before production, which allows the facility to handle upset conditions more effectively and avoid costly production stops. Additive chemicals that may be stored at this stage of the process include dispersants, flocculants, surfactants, glycols, diluents and rust inhibitors.
These chemicals are usually stored in a series of outdoor steel tanks. The fluid volume within these tanks requires continuous monitoring because level variations may lead to upsets. These tanks may also contain agitated media with suspended solids that can coat floats, displacers and probes.
A guided wave radar (GWR) transmitter, such as the Eclipse® Model 706, with an enlarged coaxial probe or a pulse burst radar transmitter can provide continuous level control for this application. For point level control, a thermal dispersion switch or displacer-actuated level switch is recommended.
Crude processing can require a variety of chemical agents, including drilling fluid additives, methanol injection for freeze protection, glycol injection for hydrate inhibition, produced water treatment chemicals, foam and corrosion inhibitors, de-emulsifiers, desalting chemicals and drag reduction agents. Many refineries administer these types of chemicals via chemical injection skids.
To help control these types of chemical inventories and determine when tanks require filling, level monitoring is necessary. The careful selection and application of level controls for chemical injection systems can effectively prevent your tanks from running out of chemicals or overfilling.
A GWR transmitter or magnetostrictive level transmitter can provide a reliable continuous level control solution for this application. An ultrasonic switch or float-actuated switch can provide the point level control that this application requires.
In order to separate wellstreams into their individual components, a separator is required. Separators are large drums that are commonly designed to separate two-phase (gas/liquid) or three-phase (gas/crude/water) wellstreams. Separators are also classified according to configuration (horizontal or vertical), operating pressure, turbulent or laminar flow and test or production separation.
When interface level measurement is used within a separator, it actuates a valve to adjust vessel level. An emulsion layer along the oil/water interface can contaminate the oil with water. Foaming along the gas/liquid interface, if entrained, can also cause liquid carryover or gas blowby.
There are two principal types of separators:
- Vertical separators (pictured at right) are best for applications that experience significant liquid surges. They also work well for high sediment loads because one can attach a conical bottom to handle large volumes of sediment. In addition, vertical separators are preferred for wellstreams with large liquid-to-gas ratios and on offshore platforms where floor space is limited.
- Horizontal separators (pictured below) are well suited for three-phase separation because they provide a large interfacial area between the two liquid phases. They are preferred when wellstreams have high gas-to-oil ratios, when wellstream flow is relatively constant or when liquid surges are insignificant. These separators also have a much greater gas/liquid interface area, which aids in the release of solution gas and reduces foaming.
Effective continuous level and interface level controls for wellstream separators include GWR, displacer transmitters and magnetostrictive level transmitters, while a float-actuated external cage level switch or displacer-acutated level switch can provide point level control.
Follow Our Crude Oil Production Discussion
During the next two weeks, we will continue our discussion of level instruments for crude oil production. To learn more about this topic, please download the Magnetrol® Crude Oil Processing Technology bulletin.