Crude oil arriving at a refinery is not completely clean.
It can contain water, salt, sediment, clay, rust, sand, and other contaminants picked up during production, transportation, and storage.
If all of that material went directly into the crude furnace and distillation tower, the refinery would have serious problems.
Salt can contribute to corrosion.
Solids can foul heat exchangers.
Water increases the amount of material that needs to be heated.
Contaminants can eventually reach downstream refinery units.
That is why one of the first major pieces of process equipment in many refineries is the crude oil desalter.
A desalter washes salt out of crude oil before the crude reaches the hottest parts of the refinery.
The basic idea sounds simple.
Mix clean water with crude.
Move the salt into that water.
Separate the dirty water from the oil.
Send the cleaner crude forward.
Actually making that happen reliably with thousands of barrels of crude every hour is much more difficult.
What Is a Crude Oil Desalter?
A crude oil desalter is a large separation vessel designed primarily to remove inorganic salts, water, and suspended contaminants from crude oil.
It is normally located in the crude distillation unit before the main crude furnace.
Many desalters use high voltage electrical fields to help small water droplets combine into larger droplets that can settle out of the oil.
The refinery intentionally adds wash water to the crude before it enters the vessel.
Salt transfers from the crude associated water into this wash water.
The resulting brine settles to the bottom of the desalter.
Desalted crude leaves from the upper portion of the vessel and continues through the refinery.
Why Does Crude Oil Contain Salt?
Most of the salt is associated with water present in the crude.
Oil reservoirs often contain highly saline formation water.
Production facilities separate most of this water before the crude is transported.
But complete removal is difficult.
Small water droplets can remain dispersed through the oil.
Those droplets contain dissolved salts.
Common salts can include compounds containing sodium, calcium, and magnesium.
When crude reaches the refinery, even a relatively small amount of salty water can create problems once millions of barrels are processed.
Is the Salt Actually Dissolved in the Oil?
Most inorganic salt is much more soluble in water than in crude oil.
That is important.
The refinery does not normally remove salt by trying to pull individual salt molecules directly out of the hydrocarbon phase.
Instead, it uses water.
Salt already contained in small brine droplets can be diluted.
Additional salt associated with the crude can transfer into the added wash water.
The refinery then removes the water itself.
This is why desalting and dehydration are closely connected.
Good salt removal requires good water separation.
Why Is Salt Bad for a Refinery?
Salt creates several downstream problems.
One of the most important is corrosion.
Certain chloride salts can contribute to hydrochloric acid formation under refinery conditions.
That acid can attack equipment, particularly in areas where water condenses.
Salt and solids can also contribute to deposits.
Deposits reduce heat transfer.
They can plug equipment.
They can create corrosion underneath deposits.
Contaminants can also affect downstream catalyst systems.
A desalter therefore protects far more than the crude tower.
It helps protect a large portion of the refinery.
Where Is the Desalter Located?
The desalter is normally part of the crude unit.
Crude enters the refinery and passes through part of the crude preheat system.
It is heated using energy recovered from hot refinery streams.
Wash water is added.
The crude and water are mixed.
The mixture enters the desalter.
After water and contaminants are removed, the desalted crude continues through additional heat exchangers.
It is then heated in the crude furnace before entering the atmospheric distillation tower.
The exact arrangement differs between refineries.
Some plants use one desalter stage.
Others use two stages.
Why Is Crude Heated Before Desalting?
Warm crude is usually easier to separate than cold crude.
Heating lowers crude oil viscosity.
Lower viscosity allows water droplets to move through the oil more easily.
Droplets can collide and combine more effectively.
Larger droplets also settle more quickly.
This becomes especially important with heavy crude.
Cold heavy crude can be extremely viscous.
Small water droplets may remain suspended for a long time.
Heating improves separation.
There are limits, however.
Higher temperature affects crude conductivity, vapor pressure, equipment operation, and overall process conditions.
The refinery operates within a designed temperature range.
What Is Wash Water?
Wash water is water intentionally mixed with the incoming crude.
Its purpose is to contact the salty water and contaminants in the crude.
The added water dilutes the original brine.
Dissolved salts transfer into the water phase.
The water is then separated from the oil inside the desalter.
Wash water quality is important.
Adding contaminated water containing high chloride levels would defeat part of the purpose.
Refineries therefore pay close attention to where desalter wash water comes from.
How Much Wash Water Is Added?
The amount depends on crude quality, desalter design, salt concentration, water availability, and the required outlet specification.
A refinery might use several percent wash water compared with crude flow.
Some difficult crude slates may require more.
Too little water can reduce salt removal.
More water is not automatically better.
Every additional barrel of wash water eventually becomes wastewater that has to be handled.
The desalter also needs enough capacity to separate the added water.
Refineries therefore optimize wash water rather than simply maximizing it.
Why Does the Water Need to Mix With the Crude?
Imagine pouring clean water into a crude oil pipeline without creating good contact.
Large streams of water could move through the crude without contacting many of the small salty droplets.
Salt removal would be poor.
The wash water needs to be dispersed through the crude so it can contact the contaminants.
This often happens through a mixing valve or another mixing device.
The challenge is that the refinery wants good mixing before the desalter but easy separation inside the desalter.
Those goals compete with each other.
What Is the Desalter Mixing Valve?
The mixing valve creates a pressure drop and turbulence that disperses wash water into small droplets throughout the crude.
This improves contact between the crude associated brine and the cleaner wash water.
Salt can then transfer into the water phase.
The mixture enters the desalter.
The mixing valve is therefore important for desalting efficiency.
But excessive mixing can create a very stable emulsion.
That makes oil and water harder to separate.
The correct pressure drop is a balance.
Can Too Much Mixing Make Desalter Performance Worse?
Yes.
This is a classic desalter problem.
More mixing improves contact between water and crude.
But stronger mixing also creates smaller water droplets.
Very small droplets settle slowly.
Some can become stabilized by natural surfactants, solids, and heavy crude components.
The desalter may then develop a thick emulsion layer.
So if salt removal is poor, opening the mixing valve farther is not automatically the solution.
The problem might actually be excessive mixing.
What Is an Emulsion?
An emulsion is a mixture in which small droplets of one liquid remain dispersed inside another liquid.
In a crude desalter, the difficult case is usually water droplets dispersed in oil.
Oil and water naturally want to separate.
But very small droplets can remain suspended for long periods.
Natural crude components can form films around the droplets.
Fine solids can stabilize those films.
The result can be an emulsion that refuses to separate quickly enough for refinery operation.
What Is a Rag Layer?
A rag layer is an emulsion rich region that forms near the oil and water interface inside the desalter.
Instead of having a clean oil layer above and a clean water layer below, a thick mixed layer develops between them.
The rag layer can contain oil, water, solids, asphaltenes, and other contaminants.
Some rag is normal.
Too much creates operational problems.
A growing rag layer can interfere with level measurement, reduce effective vessel volume, increase oil loss to the brine, and increase water carryover with the crude.
Operators watch interface behavior carefully.
What Causes a Large Rag Layer?
Several conditions can contribute.
Heavy crude can make separation more difficult.
Fine solids can stabilize emulsions.
Incompatible crude blends can create stability problems.
Excessive mixing can produce extremely small droplets.
Poor chemical treatment can contribute.
Incorrect temperature can affect separation.
Changing crude quality can also upset a desalter that operated normally the previous day.
Rag problems are often caused by several factors rather than one obvious failure.
What Does a Demulsifier Do?
A demulsifier is a chemical used to help water droplets separate from crude oil.
The chemical interferes with films that stabilize the emulsion.
This allows droplets to combine more easily.
Once droplets become larger, gravity can remove them more effectively.
Demulsifier selection matters.
Crude oils have different chemical characteristics.
A chemical program that works very well with one crude blend may perform poorly with another.
The refinery therefore adjusts chemical treatment as its crude slate changes.
Is More Demulsifier Always Better?
No.
Too little chemical may produce poor separation.
But blindly increasing dosage is not good troubleshooting.
Excessive chemical costs money and can sometimes produce undesirable effects.
The correct dosage depends on the crude, temperature, mixing conditions, solids, water content, and desalter operation.
Chemical treatment should be optimized with the rest of the process.
It cannot compensate for every mechanical or operating problem.
How Does Electricity Help Separate Oil and Water?
Many refinery desalters contain high voltage electrodes.
The electrical field acts on water droplets dispersed in the crude.
The droplets become polarized.
That encourages them to move toward one another, collide, and combine.
Two small droplets become a larger droplet.
Larger droplets fall through the oil more easily because of gravity.
The electrical field therefore accelerates coalescence.
It does not simply pull salt out of crude like a magnet.
The main job is helping water droplets become large enough to separate.
Why Does Water Respond to the Electric Field Better Than Oil?
Water is much more electrically conductive than hydrocarbon liquid.
This difference allows the electric field to act strongly on dispersed water droplets.
The crude oil phase acts relatively more like an electrical insulator.
The field causes forces to develop on the droplets.
They deform, attract, collide, and combine.
The exact behavior depends on the desalter technology and operating conditions.
Modern systems can use different combinations of electrical fields and electrode arrangements.
What Happens to the Water Droplets After They Combine?
Gravity takes over.
A large water droplet is denser than crude oil.
It moves downward.
As it falls, it may collide with additional droplets.
Eventually it reaches the water phase at the bottom of the vessel.
The desalter then removes this salty water as brine.
Meanwhile, the lighter crude flows toward the oil outlet.
The entire process depends on giving the droplets enough time and suitable conditions to separate.
What Is Residence Time?
Residence time is approximately how long fluid remains inside the vessel.
Longer residence time generally gives droplets more opportunity to settle.
But refinery throughput is valuable.
A desalter cannot simply become infinitely large.
When the crude rate increases, residence time decreases unless vessel size or operating conditions change.
This is why a desalter that operates perfectly at one throughput may struggle after refinery rates increase.
Hydraulic capacity matters.
What Is the Oil Water Interface?
Inside the desalter, oil occupies most of the upper region and water collects near the bottom.
The boundary between these phases is called the oil water interface.
In reality, the transition may include a rag layer rather than a perfectly sharp boundary.
The refinery controls this interface within an acceptable range.
If the water level rises too high, water may carry over with the crude.
If it is too low, oil can leave with the brine.
Stable interface control is therefore essential.
What Happens If the Interface Is Too High?
Water can approach parts of the electrical system where it should not be.
Water carryover with the crude can also increase.
Because water is electrically conductive, excessive water near energized electrodes can create electrical instability.
The desalter may experience high current or electrical trips.
Poor interface control can therefore become both a separation problem and an electrical problem.
What Happens If the Interface Is Too Low?
More oil can leave with the brine.
That wastes valuable crude.
The downstream wastewater system then receives additional hydrocarbons.
Low interface may also change internal fluid behavior and reduce separation performance.
The goal is not to drain as much water as physically possible.
The goal is to maintain the designed interface while removing brine without losing excessive oil.
How Is the Interface Measured?
Different technologies can be used.
A refinery may use level instruments designed to detect differences between oil, emulsion, and water.
The challenge is that a thick rag layer can confuse some measurements.
The instrument may not see a perfectly clean boundary.
Operators often compare instrument readings with other operating indicators.
They may consider brine appearance, electrical load, crude water content, sample results, and historical behavior.
One instrument should not always be interpreted in isolation.
What Does Desalter Brine Look Like?
Healthy brine should contain the water and contaminants removed from the crude.
Some oil may still be present.
If the brine suddenly contains large amounts of oil, something may be wrong.
Possible causes include low interface, severe rag movement, poor separation, excessive crude rate, unsuitable chemical treatment, or hydraulic disturbance.
The wastewater system may also notice the problem.
A desalter upset can therefore create problems beyond the crude unit.
Why Does the Desalter Have High Voltage?
Small water droplets need help combining quickly enough.
The electrostatic field provides that help.
Industrial desalters can operate at very high voltage while using relatively low electrical current under normal conditions.
The equipment is specifically designed for this service.
Operators should respect the electrical hazards.
Internal grids and electrical components are not ordinary low voltage equipment.
Maintenance requires proper isolation and procedures.
Why Does Desalter Current Increase?
Several conditions can increase electrical current.
More water in the electrical field can increase conductivity.
A high interface can contribute.
A severe emulsion can affect electrical behavior.
Changes in crude conductivity can influence performance.
Solids and contamination can also affect the system.
A current increase is therefore useful information.
It does not always identify the cause by itself.
Operators look at current together with voltage, interface, crude quality, water content, and process conditions.
Why Does a Desalter Transformer Trip?
An electrical protection system can trip when current or another electrical condition becomes unacceptable.
Possible causes can include excessive water near the electrodes, electrical faults, conductive crude conditions, internal contamination, or equipment failure.
Repeatedly resetting the system without understanding the cause is poor practice.
If the desalter is trying to tell you something is wrong, forcing it back into service does not remove the problem.
The electrical system protects valuable equipment.
What Happens When the Electric Field Is Lost?
The desalter can still have some gravity separation.
But removal of small water droplets can deteriorate significantly.
Salt and water in the desalted crude may rise.
The refinery may need to reduce rate, alter crude handling, adjust chemical treatment, or take other actions depending on the severity and duration.
How quickly quality deteriorates depends on the crude and desalter design.
A light crude with good natural separation may behave differently from a difficult heavy crude.
What Is One Stage Desalting?
A one stage system uses one primary desalter vessel.
Crude is mixed with wash water.
The vessel separates the crude and brine.
The crude then continues downstream.
A well designed one stage system can achieve excellent performance for suitable crude slates.
The required outlet salt specification and crude characteristics determine whether another stage is necessary.
What Is Two Stage Desalting?
Two stage desalting sends crude through two desalter stages.
The first stage removes a large portion of the salt and water.
The second stage provides additional cleaning.
Water flow can be arranged in ways that improve efficiency.
Two stage systems can achieve extremely high salt removal.
They are useful when crude salt content is high or very low outlet salt is required.
The additional equipment also increases complexity and cost.
Why Does Heavy Crude Make Desalting Harder?
Heavy crude usually has higher viscosity.
That makes water droplets settle more slowly.
Heavy crude can also contain more asphaltenes, solids, and other components that stabilize emulsions.
Density difference between oil and water may also become less favorable.
All of these factors make separation harder.
Higher temperature, improved electrostatic technology, chemical treatment, and careful mixing control can help.
This is why changing from light crude to a much heavier crude can noticeably change desalter operation.
Why Can Blending Two Crudes Create Problems?
Two crude oils that behave well separately do not always behave well when mixed.
The blend can alter asphaltene stability.
Solids can become more troublesome.
Electrical conductivity can change.
Emulsion behavior can change.
Viscosity can change.
A refinery may therefore experience desalter problems immediately after changing crude blend even though neither individual crude seems unusually difficult.
Crude compatibility matters.
Why Are Solids a Problem in a Desalter?
Fine solids can stabilize emulsions.
Particles can collect at the oil and water interface.
They can become trapped in the rag layer.
Solids can also contribute to fouling elsewhere in the crude unit.
Examples can include clay, sand, corrosion products, and other suspended material.
The desalter removes some of these contaminants with the brine.
That is another important benefit beyond salt removal.
Can a Desalter Remove Metals?
It can remove some contaminants associated with water and suspended solids.
But it does not remove every metal contained in crude oil.
Some metals are chemically associated with heavy hydrocarbon molecules and remain in the crude.
Those contaminants can later affect downstream processing catalysts.
Desalting is an important pretreatment step, but it is not complete purification.
Crude remains a complex mixture after leaving the desalter.
What Does Salt Content in PTB Mean?
Refineries often express crude salt content as pounds of salt per thousand barrels of crude.
This is commonly shortened to PTB.
For example, a crude feed might contain 10 pounds of salt per thousand barrels before desalting.
The refinery may want the outlet concentration reduced to a much lower value.
The exact target depends on refinery design and operating strategy.
Lower outlet salt generally reduces downstream corrosion and contamination risk.
How Is Desalter Efficiency Calculated?
A simple salt removal efficiency can be estimated using inlet and outlet salt concentrations.
Suppose crude enters with 12 PTB salt.
After desalting, the crude contains 1 PTB.
Salt removed equals 11 PTB.
Dividing 11 by the original 12 gives about 0.917.
That means roughly 92 percent of the measured salt was removed.
Real performance evaluation considers more than one sample because crude quality and laboratory measurements can vary.
What Is BS and W?
BS and W means basic sediment and water.
It represents water and sediment contained in crude oil.
High BS and W can indicate that too much water or solids are leaving with the crude.
A desalter is expected to reduce both salt and water to acceptable levels.
Low salt with excessive water is not necessarily good performance.
Likewise, low water with poor salt removal can still create downstream problems.
Both need to be considered.
Can Salt Removal Be Poor Even When Water Removal Looks Good?
Yes.
This can happen if the wash water did not mix effectively with the salty brine in the crude.
Imagine the original water droplets remain extremely salty.
The desalter removes most of the total water.
But the small amount of water remaining in the crude still contains very high salt concentration.
The outlet BS and W may look acceptable while the salt test remains too high.
Good desalting requires salt transfer as well as water separation.
Can Water Removal Be Poor Even When Salt Removal Is Good?
Yes.
The wash water may dilute the salt effectively.
But separation may still leave too much water in the crude.
The remaining water might contain relatively little salt, so the outlet salt result looks good.
However, excessive water going toward the furnace and tower is still undesirable.
Operators therefore evaluate the entire process rather than relying on one laboratory number.
Why Is Wash Water Quality Important?
Wash water should help remove salt rather than adding unwanted contaminants.
High chloride wash water reduces the ability to dilute the original brine effectively.
Oil in the wash water can create additional separation problems.
Suspended solids can increase contamination.
Incorrect chemistry can affect emulsions.
Some refineries reuse treated process water as desalter wash water.
That can be efficient, but the water quality needs to remain suitable.
Can Sour Water Be Used as Desalter Wash Water?
Certain treated refinery water streams can be reused for desalting when their quality is appropriate.
This reduces freshwater demand.
But contaminants need to be controlled.
Poorly treated sour water can introduce ammonia, sulfide, chloride, hydrocarbons, or other undesirable compounds.
The refinery therefore monitors the wash water source.
Water reuse is valuable only when it does not create a larger process problem.
Why Is Desalter Temperature Important?
Temperature changes several important properties.
Higher temperature generally reduces crude viscosity.
That helps droplets settle.
But temperature also changes electrical conductivity and other crude behavior.
Very high temperature may create electrical challenges or vaporization concerns.
Low temperature can make heavy crude extremely difficult to separate.
The optimum depends on the crude and desalter technology.
Operators often notice desalter performance changes when preheat exchanger performance changes because desalter inlet temperature changes with it.
Can Heat Exchanger Fouling Affect the Desalter?
Yes.
Crude is normally heated before reaching the desalter.
If upstream heat exchangers foul, desalter inlet temperature may decrease.
The crude becomes more viscous.
Water separation becomes slower.
A desalter problem can therefore begin with a heat exchanger problem.
This is an important troubleshooting lesson.
The equipment showing the symptom is not always the equipment causing the problem.
Can a Poor Desalter Cause Heat Exchanger Fouling?
Yes.
The relationship also works in the other direction.
If salt, solids, or water leave the desalter with the crude, downstream exchangers can experience increased fouling and deposition.
Certain contaminants can create deposits in the hotter portions of the preheat train.
So exchanger fouling can hurt desalter operation, and poor desalter operation can hurt the exchangers.
The crude unit operates as one connected system.
Why Does a Desalter Upset Affect the Crude Tower Overhead?
Chloride salts that remain in the crude travel downstream.
Under high temperature conditions, some can contribute to hydrochloric acid formation.
That chloride eventually reaches the crude tower overhead system.
When water condenses there, an acidic environment can develop.
The refinery uses corrosion control strategies in the overhead system.
But reducing chloride entering the unit in the first place is far better than trying to manage excessive chloride later.
The desalter is therefore an important corrosion control barrier.
Why Are Magnesium and Calcium Chlorides Particularly Important?
Different salts behave differently when heated.
Certain magnesium and calcium chloride compounds can contribute more readily to hydrochloric acid formation than sodium chloride under refinery conditions.
This makes salt composition relevant, not just total salt concentration.
Refineries still commonly monitor total salt because it provides a practical measure of crude contamination.
But the chemistry behind downstream corrosion is more complex than one number.
Can Poor Desalting Affect the Furnace?
Yes.
Water reaching the furnace creates additional heating demand.
Salts and solids can contribute to deposits in hot equipment.
Fouling reduces heat transfer.
Tube skin temperature may need to increase to achieve the same crude outlet temperature.
Higher tube temperature can accelerate fouling and damage mechanisms.
Good desalting therefore supports furnace efficiency as well as corrosion control.
Can Poor Desalting Affect an FCC Unit?
Potentially.
Contaminants that pass through the crude unit can eventually reach downstream conversion units depending on refinery configuration.
Chlorides, sodium, solids, and metals can create problems in later processing.
Catalyst systems are particularly sensitive to certain contaminants.
Removing unwanted material at the front of the refinery is generally much easier than dealing with it after it spreads through several units.
What Happens If the Crude Rate Is Increased?
Higher throughput can challenge the desalter.
Residence time decreases.
Fluid velocity increases.
Water droplets have less time to settle.
The electrical system handles more emulsion.
The mixing requirements change.
A desalter that produces excellent outlet quality at a lower crude rate may begin carrying water or salt after the refinery increases throughput.
Operators may need to optimize temperature, wash water, chemical treatment, interface, mixing, or other conditions.
At some point, the vessel itself can become the limit.
Why Can Higher Water Cut Cause Problems?
More incoming water means the desalter needs to separate a larger water volume.
That can raise the interface.
It can increase electrical load.
The emulsion zone may grow.
Brine flow increases.
Upstream production or crude storage problems can therefore show up as desalter instability at the refinery.
A sudden increase in incoming water should be treated as a significant feed change.
What Happens If Free Water Reaches the Electrical Grid?
Water is much more conductive than oil.
If excessive water reaches the energized region, current can rise sharply.
Electrical protection may trip.
This is one reason interface control is critical.
The refinery wants the electrode system acting on dispersed water droplets in crude.
It does not want a large continuous water phase surrounding the electrical grids.
Why Can Crude Conductivity Matter?
The electrostatic system depends on electrical differences between the phases.
Some crude oils are more electrically conductive than others.
Heavy crudes and certain blends can create challenging electrical conditions.
Temperature also affects conductivity.
A desalter designed around one crude slate may struggle when the refinery begins processing a very different mixture.
Modern desalter systems can use more sophisticated electrical control to handle these variations.
Why Does the Desalter Sometimes Spark or Arc Internally?
Electrical discharge can occur when local conditions become too conductive or the distance and electrical resistance between components changes unfavorably.
Excessive water, conductive emulsions, solids, internal damage, or electrical problems can contribute.
Repeated arcing can damage equipment and disrupt separation.
Operators normally see abnormal current or protective action rather than physically seeing what is happening inside the closed vessel.
Electrical trends can therefore provide important clues.
What Does a Healthy Desalter Look Like From the Control Room?
A healthy unit generally shows stable behavior.
Crude flow is steady.
Desalter temperature is within its expected range.
Wash water flow is stable.
Mixing conditions are appropriate.
Interface is controlled.
Electrical voltage and current are relatively stable.
Brine removal is consistent.
Outlet salt and water meet target.
No single value proves the desalter is healthy.
The pattern matters.
Experienced operators know what normal looks like for the current crude slate.
What Are Signs of Poor Desalter Performance?
Possible warning signs include:
Rising outlet salt
Rising outlet water
Unstable interface
Increasing rag layer
High electrical current
Repeated electrical trips
Oil in the brine
Increasing wastewater hydrocarbon load
Downstream corrosion concerns
Changing crude tower overhead chloride
Downstream exchanger fouling
Unusual brine quality
Several of these appearing together can make troubleshooting much easier.
Why Might Outlet Salt Suddenly Increase?
Start by asking what changed.
Did the incoming crude change?
Did inlet salt increase?
Did water content increase?
Did wash water flow decrease?
Did wash water quality change?
Did desalter temperature fall?
Did the mixing valve move?
Did electrical power change?
Did chemical dosage change?
Did the interface become unstable?
A sudden performance change often follows a process change.
Finding that change is usually more useful than immediately adjusting everything.
What Happens If There Is Too Little Mixing?
Wash water does not contact the salty droplets effectively.
Salt transfer is limited.
The desalter may still remove water reasonably well.
But outlet salt can remain high.
Operators may increase mixing pressure drop within the approved operating range.
The goal is to improve contact without creating an excessively stable emulsion.
What Happens If There Is Too Much Mixing?
Droplets become extremely small.
The emulsion becomes difficult to break.
The rag layer can grow.
Water carryover may increase.
Electrical load can become unstable.
Oil can leave with the brine.
This is why desalter optimization is often a balancing exercise rather than a simple more is better problem.
What Happens If Wash Water Flow Is Too Low?
Salt dilution and extraction become less effective.
Outlet salt can increase.
The effect becomes particularly noticeable when incoming crude contains more salt than normal.
The refinery may need additional wash water to maintain the same outlet quality.
But water availability and wastewater capacity can limit how much can be added.
What Happens If Wash Water Flow Is Too High?
The vessel has more water to separate.
Brine flow increases.
Wastewater treatment load increases.
The oil water interface may become harder to control.
Electrical behavior may change.
If the vessel is already close to hydraulic capacity, extra water can make separation worse.
The optimum is enough water to achieve effective salt removal without unnecessarily overloading the system.
What Happens If the Desalter Is Too Cold?
Crude viscosity rises.
Water droplets settle more slowly.
The emulsion can become more difficult to resolve.
Heavy crude is particularly sensitive.
Outlet water and salt can increase.
Operators may increase upstream heating where equipment and procedures allow.
They should also investigate why the temperature changed.
A fouled exchanger or changed crude rate may be the real cause.
What Happens If the Desalter Is Too Hot?
Higher temperature is not always better.
Crude conductivity can increase.
Electrical performance can change.
Light hydrocarbons may approach vaporization conditions.
Equipment has temperature limits.
The process must remain within its designed operating envelope.
Desalter operation is therefore optimized rather than maximized.
Why Might Oil Appear in the Brine?
The interface may be too low.
Separation may be poor.
A large rag layer may be moving into the water outlet.
The crude rate may be too high.
Chemical treatment may be unsuitable.
Hydraulic disturbances can also pull oil downward.
Oil loss is economically undesirable and can overload downstream wastewater treatment.
The refinery wants contaminants in the brine without sending valuable crude with them.
Why Might Water Appear in the Desalted Crude?
The interface may be too high.
The emulsion may not be breaking.
Crude rate may be excessive.
Temperature may be too low.
Electrical performance may be poor.
Mixing could be too severe.
Chemical treatment may not match the crude.
Water carryover should be investigated quickly because it often accompanies poor salt removal and downstream operating problems.
Why Is Crude Sampling Important?
Laboratory results tell operators whether the process is actually achieving its purpose.
Useful measurements can include inlet salt, outlet salt, BS and W, brine properties, solids, and other crude characteristics.
A beautiful control room trend does not guarantee good crude quality.
The desalter exists to remove contaminants.
Sampling confirms whether that is happening.
Why Should Operators Be Careful With One Bad Sample?
Crude is not always perfectly uniform.
Sampling errors can occur.
Laboratory variation exists.
A single unexpected result should be taken seriously but interpreted alongside process information and repeat results where appropriate.
If the outlet salt suddenly doubles while every operating variable remains unchanged, verify the result while investigating possible causes.
Do not ignore it.
But do not destabilize the desalter by changing five operating variables based on one questionable sample.
Why Is Desalter Troubleshooting Difficult?
Because many variables interact.
Increasing mixing can improve salt transfer but worsen emulsion.
Increasing temperature can improve settling but affect conductivity.
Increasing wash water can improve dilution but increase hydraulic loading.
Changing demulsifier can improve separation but interact with crude chemistry.
Changing interface affects both oil loss and electrical stability.
A good operator makes controlled changes and watches the result.
Changing everything at once makes it impossible to learn what actually solved the problem.
A Practical Desalter Troubleshooting Sequence
Begin with the feed.
Has crude type changed?
Has incoming salt changed?
Has water content changed?
Has the refinery increased rate?
Then check temperature.
Is desalter inlet temperature where it normally is?
Next check wash water.
Is the correct amount entering?
Has its quality changed?
Then examine mixing.
Is the mixing valve operating where expected?
Look at the vessel.
Is interface stable?
Is the rag layer increasing?
What does the brine look like?
Then check electrical behavior.
Are voltage and current normal?
Have there been trips?
Finally review chemical treatment and laboratory results.
This structured approach is much better than randomly increasing chemicals and hoping the problem disappears.
A Simple Desalter Example
Imagine crude enters a refinery containing 10 PTB salt.
The refinery adds wash water upstream of the mixing valve.
The mixing valve disperses the water through the crude.
Salt transfers into the water.
The mixture enters the desalter at the designed temperature.
The electrostatic field causes small water droplets to combine.
Large droplets settle.
Brine leaves from the bottom.
Crude leaves from the top.
Laboratory testing shows the desalted crude contains 0.8 PTB salt.
The process has removed roughly 92 percent of the incoming salt.
Now imagine the crude slate changes.
A heavier crude enters the unit.
Viscosity increases.
The rag layer grows.
Electrical current becomes less stable.
Outlet salt rises to 2.5 PTB.
The desalter itself did not necessarily break.
The feed became more difficult.
Operators now need to optimize conditions for the new crude.
Why Is the Desalter More Important Than It Looks?
The desalter does not produce gasoline.
It does not produce diesel.
It does not crack heavy molecules into valuable lighter products.
It does not look as impressive as a distillation tower or catalytic cracking unit.
But poor desalting can damage the economics and reliability of everything downstream.
It can increase corrosion.
It can increase fouling.
It can increase furnace duty.
It can create wastewater problems.
It can contaminate downstream units.
In that sense, the desalter acts as one of the refinery’s first lines of defense.
Removing contamination early is almost always easier than dealing with it later.
Frequently Asked Questions
What does a crude oil desalter do?
A crude oil desalter removes salt, water, solids, and other contaminants from crude before the crude enters the hotter sections of a refinery.
Why do refineries remove salt from crude oil?
Salt can contribute to corrosion, fouling, deposits, and downstream process problems.
How is salt removed from crude?
Wash water is mixed with the crude so salt transfers into the water phase. The water is then separated from the oil.
Why does a desalter use electricity?
A high voltage electric field helps small water droplets combine into larger droplets that can settle by gravity.
Does the electric field pull salt directly out of the oil?
No. The main purpose of the electric field is to improve water droplet coalescence and separation.
What is desalter wash water?
It is water intentionally added to crude to dilute and remove salt.
Why is crude heated before desalting?
Heating lowers viscosity and helps water droplets settle more easily.
What is a desalter mixing valve?
It creates controlled turbulence so wash water contacts the salty water dispersed in crude.
Can too much mixing hurt desalter performance?
Yes. Excessive mixing can create very small water droplets and a stable emulsion that becomes difficult to separate.
What is a rag layer?
It is an emulsion rich region between the oil and water phases containing varying amounts of oil, water, solids, and other contaminants.
Why is a large rag layer bad?
It can reduce vessel capacity, confuse interface control, increase oil losses, increase water carryover, and disturb electrical operation.
What does a demulsifier do?
It helps break stabilized oil and water emulsions so droplets can combine and separate.
Is more demulsifier always better?
No. Chemical dosage needs to be optimized for the crude and operating conditions.
What does PTB mean in crude desalting?
PTB commonly means pounds of salt per thousand barrels of crude.
What does BS and W mean?
BS and W means basic sediment and water.
Can a desalter remove all salt?
No process is perfect, but efficient desalting can remove a very large percentage of incoming inorganic salt.
Why do some refineries have two desalters?
Two stage desalting can achieve greater salt removal and handle more difficult crude feeds.
Why is heavy crude harder to desalt?
Heavy crude has higher viscosity and can contain components and solids that stabilize emulsions.
Can changing crude blends upset a desalter?
Yes. Changes in viscosity, conductivity, solids, asphaltene stability, and emulsion behavior can significantly affect performance.
Why does desalter electrical current rise?
More water, conductive emulsions, changing crude conductivity, interface problems, contamination, or electrical faults can increase current.
Why would a desalter electrical system trip?
Protection can operate when current or another electrical condition becomes excessive.
What happens if the desalter loses electrical power?
Water droplet coalescence can become less effective, allowing more water and salt to leave with the crude.
Can poor desalting cause refinery corrosion?
Yes. Chloride salts carried downstream can contribute to acidic corrosion conditions, particularly in the crude tower overhead system.
Can poor desalting foul heat exchangers?
Yes. Salt, solids, and other contaminants can contribute to fouling and deposits.
Can fouled heat exchangers also hurt desalter performance?
Yes. Reduced crude preheat can lower desalter temperature and make oil and water separation more difficult.
Why would oil appear in desalter brine?
Possible causes include poor separation, a low interface, excessive rag, hydraulic disturbance, or excessive throughput.
Why would water appear in desalted crude?
Possible causes include high interface, difficult emulsion, low temperature, excessive throughput, electrical problems, or incorrect mixing conditions.
What should operators check first when desalter salt suddenly rises?
Check what changed in the crude feed, inlet salt, water content, crude rate, temperature, wash water, mixing, interface, electrical performance, and chemical treatment.
What is the most important thing to understand about a refinery desalter?
Desalting is not simply settling dirty water out of crude.
The refinery has to intentionally mix water into the oil strongly enough to wash out salt, then reverse that process and separate the water again.
Successful desalter operation is the art of getting both steps right.