Crude Oil Desalter Sensor and Interface Measurement Guide

A crude oil desalter sensor measures the oil-water and emulsion behaviour inside the vessel so operators can control separation with process evidence rather than a fixed assumption. This guide explains the measurement role and application questions.

The Agar ID-200 Interface Detector is designed specifically for this job. It provides a continuous density profile through the desalter, tracking free water, the emulsion band, clean oil and any sludge or solids. Installed externally on the vessel, it delivers stable, repeatable data in high-temperature, high-pressure, and high-voltage environments where conventional level devices struggle.

In this post, we explain how crude oil desalters work, what desalter level measurement means, the typical stages of desalting, and the principle of electrical desalting, then show why the Agar ID-200 is the best choice for crude oil desalter sensor and interface measurement.

What is a crude oil desalter, and why does it matter?

Raw crude contains water, dissolved salts and fine solids. If these salts and contaminants reach the crude distillation unit they cause corrosion, fouling, under-deposit corrosion and unplanned shutdowns.

A crude oil desalter is a separation vessel used before the atmospheric distillation column. It mixes heated crude with fresh wash water and chemical demulsifier, then separates the phases so that salty water and solids are removed from the bottom while desalted crude flows to the column.

To run this process efficiently, you must keep the oil-water interface and emulsion band in the correct position. If the interface rises too high, brine and solids can carry over into the crude tower. If it falls too low, oil can short-circuit through the water phase, leading to grid trips, poor salt removal and instability. This is why continuous interface and emulsion measurement is critical.

What is a crude oil desalter sensor?

A crude oil desalter sensor is a level and density measurement system designed for harsh refinery desalters. Its job is to detect:

• Free water level at the bottom of the vessel
• The position and thickness of the emulsion band
• The transition from tight emulsion to clean oil
• Any sludge or solids that build up near the water outlet

The Agar ID-200 uses a nuclear attenuation principle with multiple detectors mounted externally on the desalter shell. By measuring the change in gamma intensity at each point it builds a vertical density profile through the vessel. Water, tight emulsion, loose emulsion, oil and gas all have distinct densities, so the system clearly shows each layer and its movement over time.

Because the ID-200 is non-intrusive and has no moving parts inside the vessel, it is unaffected by high voltage electrical fields, changing conductivity, fouling or internals. The result is reliable interface control across a wide range of crude blends and operating conditions.

How does a crude oil desalter work?

At a high level, the crude oil desalter process follows these steps:

1. Heating
2. Crude is preheated to reduce viscosity and improve separation.
3. Mixing with wash water
4. Fresh or recycled wash water is injected into the crude line using mixing valves, static mixers or mixing pumps. Demulsifier chemicals are often added at this stage. The goal is to dilute and transfer salts from the crude into the water phase.
5. Desalter vessel
6. The crude and water mixture enters the desalter vessel and flows through an electrostatic grid system. High-voltage fields polarise the dispersed water droplets, causing them to coalesce into larger droplets that then fall under gravity.
7. Phase separation
8. Over the residence time in the vessel, three main layers form: free water at the bottom, an emulsion band in the middle and relatively clean oil at the top. Solids and sludge settle into the water layer.
9. Withdrawal and control
10. Brine and solids are drawn off from the bottom outlet while desalted crude leaves from the top. Interface and emulsion level controls adjust wash water rate, grid voltage and draw off rates to keep the process stable.

Without accurate measurement of where those layers sit, operators are essentially flying blind. This is where a high performance crude oil desalter sensor like the ID-200 is essential.

What is the desalter level measurement?

Desalter level measurement refers to the set of measurements that show the position of the main phases inside the desalter:

• Free water level at the bottom
• Oil water interface where free water meets the emulsion
• Top of emulsion band where tight emulsion becomes cleaner oil
• Overall crude level in the vessel

Traditional instruments such as single point nuclear switches, differential pressure transmitters or guided wave radar often provide only a single interface point or a bulk level. They struggle when density changes with crude blends, when the emulsion band is thick, or when solids and gas are present.

The Agar ID-200 provides continuous desalter level measurement along the full height of interest. Instead of one level number, the control room sees a live density profile and clear indication of each layer. This lets operators:

• Keep the emulsion band away from the grids.
• Avoid salt and water carryover into the crude tower.
• Detect sludge buildup before it causes blockages.
• Respond quickly to changes in crude blend or wash water quality.

What are the three stages of desalting?

People often talk about three stages in crude oil desalting. In practical refinery operation these can be thought of as:

1. Mixing and dilution
2. Crude is mixed with wash water and demulsifier to dilute salts and transfer them into the water phase. The quality of mixing strongly affects how much salt can be removed.
3. Coalescence and separation
4. The crude water emulsion passes through the electrostatic field and residence zone in the desalter vessel. Water droplets coalesce and fall to form a continuous water layer while oil rises. This is where interface and emulsion control is critical.
5. Polishing and removal
6. Brine, solids and sludge are removed from the bottom, sometimes in more than one stage, and desalted crude is sent forward. Some refineries use two-stage or even three-stage desalter trains, but the same principles apply.

At every stage, accurate interface and emulsion monitoring with the ID-200 helps maintain the correct residence time and separation, especially when throughput is pushed higher or crude quality is variable.

What is the electrical desalting of crude petroleum oil?

Electrical desalting is the most common method used in modern refineries. The principle is straightforward physics.

Water droplets dispersed in crude oil are more conductive than the oil around them. When a high-voltage alternating field is applied between the electrodes inside the desalter, these droplets become polarised. They move, rotate, and attract each other. As they collide, they merge into larger droplets. Larger droplets have more mass, so they settle out of the oil phase more quickly under gravity.

By combining electrical coalescence with washwater dilution and chemical demulsifiers, refineries can remove salts, water, and fine solids efficiently. However, the success of electrical desalting depends on keeping the electrical grids correctly immersed, avoiding excessive water in the oil phase and preventing gas pockets or sludge layers.

This again demands high-quality measurement of the interface and emulsion band, something the Agar ID-200 is built to provide.

Why the Agar ID-200 is the best crude oil desalter sensor and interface solution?

The Agar ID-200 is widely regarded as a leading choice for crude desalter interface and level measurement because it solves the real problems operators face. Key advantages include:

• Full interface profile
• Multiple detectors provide a continuous density profile from water to oil. Operators see the entire emulsion band, not just a single point.
• Non-intrusive installation
• Detectors mount externally on the vessel, with no moving parts or wetted components. This avoids process penetrations and is ideal for high-pressure, high-temperature desalters.
• Unaffected by electrical fields
• Electrical grids and high-voltage systems do not affect nuclear measurements, so accuracy is maintained under all operating conditions.
• Handles changing crudes and densities
• The system works across a wide range of densities and crude blends, giving stable measurement even when feedstock changes frequently.
• Detects sludge, foam and gas
• Because it measures density rather than a single interface reflection, the ID-200 can show sludge build up, froth layers or gas spaces that other technologies miss.
• Simple integration
• The instrument outputs can be fed directly to the DCS or PLC for automatic control of interface level, emulsion band position, and water draw-off.
• Low maintenance
• With no internals to foul or plug and long-lived source and detectors, routine maintenance is minimal compared with mechanical or radar devices in this service.

Operational benefits of using the Agar ID-200 in crude desalters

Refineries that upgrade to the ID-200 for crude oil desalter sensor and interface measurement typically see:

• Reduced salt and water carryover into the crude distillation unit
• Fewer grid trips and electrical upsets
• More stable operation at higher crude throughput
• Optimised wash water and chemical usage
• Earlier detection of sludge and solids accumulation
• Better protection of downstream equipment and improved reliability

By giving operators a clear, continuous view inside the desalter, the Agar ID-200 turns a traditionally difficult measurement into a robust, repeatable control parameter. For any refinery that wants tighter control of crude oil desalting and improved reliability, it is the logical choice for crude oil desalter sensor and interface measurement.