Oil & Gas

Prevent your instruments from corroding

When is pH measured during the process of separating crude oil, and why are reliable calculations important for the quality of the outcomes?

Why is pH measured in the process of oil refinery?


Desalting of crude oil

pH measurements are important to ensure efficient cleaning 


Sour-water treatment

pH control to minimize risk of corrosion and avoid H2S formation.


Water treatment in the utilities

pH measurement in cooling water and in scrubber liquids


The desalter is the main “tool” to remove contaminants from crude oil including salt and sand  and to prevent downstream corrosion and fouling.

  • The separation process is optimized at 6-7 pH. pH values of 8 or above create risks of emulsions in the oil – and downstream corrosion for values of pH 6 or below
  • pH is also measured in the wash-water, which is the wastewater of desalting process

Sour water treatment

pH-measurement in sour water is very important. Incorrect pH values can result in damages of fixed assets because of corrosion or dangerous environmental impacts, should H2S be released. The challenge is that process sour water commonly contains a lot of sulfide ions(HS-). When in contact with a pH-electrode, they poison the reference system and result in the value drifting, which shortens the sensor’s lifetime and makes work at the field more time-consuming. During the distillation and oil refining processes, a large amount of water is released in the form of steam with impurities, e.g. SO2 or H2S. The two of them combined form sour-water. Low pH in the sour water may result in:

  • The release of H2S
  • Heavy corrosion downstream

Crude and vacuum distiller

A large amount of sour water extracted from the oil in combination with the injected steam and salt compounds is collected in the overhead condenser. To prevent corrosion in the top section of the tower and release of H2S, pH can be used to control the chemicals appearing into the overhead. pH is controlled to keep the value above 5.5.


This unit “cracks” diesel stock material into gasoline blending stocks using heat, catalyst and hydrogen under very high pressure. Hydrogen reacts with large quantities of nitrogen and sulphur which results in the formation of ammonia, hydrogen sulphide and ammonium bisulfide. Recommend pH value is: 5.5 < pH < 7.6. This balances the risks of corrosion at a low pH level because of formation of ammonium bisulfide. For this application, retractable sensor assembly and frequent sensor cleaning with naphtha and nitrate acid are recommended.

Fluid Catalytic Cracking Unit

Heat and catalysts are used to break or “crack” large gas oil molecules into smaller ones, e.g. gasoline. A large amount of sour water is produced and requires adjustment. To prevent corrosion in the top section of the tower and release of H2S, pH is adjusted with caustics in the overhead to keep the value above 5.5.

Delayed Coking

Delayed coking is a batch process where residuum from atmospheric distillation towers is heated and transferred to large coke drums, in order to start the slow cracking reactions until it cracks into lighter products. Vapours from the drums are returned to a fractionator where gas, naphtha, and gas oils are separated. The process generates sour water with large amount of impurities (e.g. sulphur compounds).

pH Measurement at Sour Water Stripper

To recover the components in the sour water, the “sour” components are converted into gaseous form and stripped out. This is done in gas strippers. The stripping process is controlled by pH, e.g. 4.0 – 5.5 for H2S and pH 8 – 10 for NH3.

pH in Biodiesel production

The pH value of biodiesel has to be about 9.5 for important chemical reactions to work properly. Therefore, it is essential to monitor this value constantly during production. Unfortunately, the reaction mixture can be harmful for the sensors. Highly viscous organic ingredients, dirt, and salts cover and block the diaphragm within a very short time; this leads to incorrect values that are outside the acceptable range of tolerance.

Until now, a precise and overall more reliable pH-measurement could only be achieved by regenerating and calibrating the sensor every day – a time-consuming and high-cost undertaking.

Memosens offer solutions and sensors that can help solve this problem.