— A Practical Guide for Selecting Pressure Transmitters in Oil Refineries

In a refinery, pressure transmitters are an indispensable part of process control and safety systems. From crude distillation to catalytic cracking and hydrocracking, every unit relies on high-accuracy pressure measurements to ensure stable operation. However, the complexity of refining processes, the diversity of media, and the severity of operating conditions present significant challenges in pressure transmitter selection.

Incorrect selection can lead to measurement errors, control instability, or worst-case scenarios like safety incidents and unplanned shutdowns.

With nearly five decades of deep experience in the industrial automation field, Anhui Tiankang (Group) Co., Ltd. has supplied high-quality pressure measurement solutions to major domestic and international refining projects. This guide provides a comprehensive overview of the key selection requirements for each critical refinery unit, helping engineers make scientifically sound and precise decisions during the project design phase.

1. Overview of Pressure Measurement in Refineries

Refinery process units typically involve a variety of media, multiphase fluids, and extreme conditions such as high temperature and high pressure. Pressure measurements in refineries are mainly categorised into three types: Gauge Pressure, Absolute Pressure, and Differential Pressure.

• Gauge Pressure (GP) – Measures pressure relative to atmospheric pressure. Widely used for pipeline pressure monitoring, pump discharge pressure, etc.

• Absolute Pressure (AP) – Measures pressure relative to absolute vacuum. Primarily used for high‑vacuum process control, such as in vacuum distillation columns.

• Differential Pressure (DP) – Measures the pressure difference between two points. Commonly used for flow measurement (with orifices, Venturis) and level measurement in pressurised vessels.

Step 1: Identify the measurement type. For atmospheric equipment, gauge pressure transmitters are typically sufficient. To measure the vacuum at the top of a distillation column, an absolute pressure transmitter must be selected. For calculating pipeline flow rate, a differential pressure transmitter is essential.

Tiankang recommendation: For corrosive media or applications prone to crystallisation, a remote seal diaphragm transmitter should be prioritised to avoid impulse line blockage. This is emphasised again in later sections on coking and FCC units.

2. Pressure Transmitter Selection for Critical Refinery Units

2.1 Atmospheric and Vacuum Distillation Unit (CDU & VDU)

Crude distillation is the “first step” in refining, separating crude oil into gasoline, diesel, gas oil, and other fractions.

• Service conditions: High temperatures (up to 350°C at the column bottom), media include crude oil and intermediate distillates, moderately corrosive.

• Critical measuring points:

• Vacuum column top (vacuum measurement): Pressure typically in the range of a few millibars to tens of millibars absolute. This requires a transmitter with extremely high sensitivity and accuracy.

• Column bottom level (DP measurement): Double‑flange DP transmitters are used to measure bottom level, preventing diaphragm fouling by high‑temperature media.

• Selection guidelines: For the vacuum column top, a high‑accuracy absolute pressure transmitter is required, with resolution sufficient to detect millibar‑level pressure changes. For bottom level measurement, a high‑temperature remote seal diaphragm pressure transmitter filled with high‑temperature silicone oil is recommended.

Tiankang advantage: The Tiankang 3051 series intelligent absolute pressure transmitters offer a turndown ratio of up to 100:1 and accuracy as high as ±0.075%, meeting the demanding stability requirements of high‑vacuum service. For high‑temperature and fouling‑prone bottom service, remote seal DP transmitters filled with high‑temperature thermal oil are available, significantly extending service life in severe conditions.

2.2 Fluid Catalytic Cracking Unit (FCCU)

FCC is the core process for converting heavy oils into lighter products. In this unit, high‑temperature catalyst circulates in a fluidised bed to convert oil fractions.

• Service conditions: Media contain catalyst dust with high abrasiveness; reaction temperatures are very high.

• Critical measuring points:

• Reactor pressure: Controls reaction severity and equipment safety. Pressure fluctuations can easily destabilise the process.

• Regenerator pressure: A critical parameter for maintaining catalyst circulation.

• Selection guidelines: Due to the presence of catalyst particles, standard instrument diaphragms are prone to abrasion. A wear‑resistant coating (such as tungsten carbide or ceramic coating) is recommended, or a purge method using a constant supply of instrument air to prevent dust ingress into impulse lines.

• Explosion protection: FCC units are typically hazardous areas, requiring flameproof (Ex d) or intrinsically safe (Ex ia) transmitters.

Tiankang advantage: Tiankang provides specialised coating and chemical seal solutions for abrasive service such as FCC units, extending transmitter service life in catalyst‑laden environments.

2.3 Hydrocracking / Hydrotreating Unit

Hydroprocessing involves reacting oil fractions with hydrogen under high temperature and high pressure to remove sulphur, nitrogen, and other impurities while increasing the hydrogen‑to‑carbon ratio.

• Service conditions: Extremely high pressure (reactor pressure can reach 10–20 MPa or higher); media contain hydrogen sulphide (H₂S) and hydrogen, presenting risks of hydrogen embrittlement and sulphide stress corrosion cracking (SSCC).

• Critical measuring points: Reactor inlet and outlet pressure (high differential pressure), fractionator level.

• Selection guidelines:

• High pressure rating: The transmitter body must have sufficient pressure rating, typically requiring static pressure capability of 32 MPa or higher.

• Hydrogen embrittlement protection: Since hydrogen easily permeates metal diaphragms under high pressure, gold‑plated diaphragms or Hastelloy C‑276 diaphragms are required to resist hydrogen embrittlement and corrosion.

• Remote seal design: Capillary remote seal diaphragm transmitters are recommended to isolate the sensitive transmitter body from high‑temperature, corrosive process media.

• Functional safety: Hydroprocessing units are high‑risk, and transmitters with SIL2/SIL3 functional safety certification are strongly recommended.

Tiankang advantage: Tiankang high‑static‑pressure DP transmitters are capable of withstanding system static pressures up to 32 MPa. Wetted parts are available in corrosion‑resistant materials including Hastelloy C‑276, 316L SS, tantalum, and Monel. Gold‑plated diaphragm treatment is available to resist hydrogen permeation in high‑pressure hydrogen service.

2.4 Delayed Coking Unit

Delayed coking is a thermal cracking process that converts heavy residual oils into petroleum coke and lighter products.

• Service conditions: Extremely high temperatures (coker drum operating temperature approximately 450–500°C); media are highly coking residual oils that form hard coke layers on instrument diaphragms at high temperatures.

• Critical measuring points: Coker drum pressure, bottom/top level.

• Selection guidelines: Because coke hardens at ambient temperature, impulse lines and diaphragms are extremely prone to blockage. A flush‑mount flanged remote seal DP transmitter with a large diaphragm and flushing ring is strongly recommended. During shutdown, steam or solvent can be introduced through the flushing ring to remove coke deposits from the diaphragm surface. High‑temperature capillary fill fluid is also required.

• Tiankang advantage: For coking, crystallising, and high‑viscosity media, Tiankang offers extended flush‑mount double‑flanged remote seal DP transmitters with a steam flushing ring. The flush‑mount design eliminates dead zones, and the flushing ring significantly reduces maintenance frequency.

2.5 Catalytic Reforming Unit

Catalytic reforming produces high‑octane gasoline components and aromatics, including reformer reactors and catalyst regeneration systems.

• Service conditions: Media contain recycle hydrogen; high temperature and high pressure; risk of hydrogen permeation.

• Critical measuring points: Reformer reactor feed pressure (high pressure), regenerator pressure.

• Selection guidelines:

• High‑temperature / high‑pressure capability: Reactor pressure is typically high, requiring high‑temperature / high‑pressure rated absolute or gauge pressure transmitters.

• Diaphragm protection: Hydrogen embrittlement is a concern – gold‑plated diaphragms are recommended.

• Measurement accuracy: Reformer feed pressure must be precisely controlled to maintain stable catalyst bed temperature; transmitter accuracy should be no less than ±0.1%.

• Tiankang advantage: Tiankang pressure transmitters are available with ceramic diaphragms and gold‑plated treatment, combined with external cooling fins to ensure long‑term stability in high‑temperature, high‑pressure hydrogen‑rich environments.

3. Refinery Pressure Transmitter Selection Summary Table

4. Key Selection Parameters for Refinery Pressure Transmitters

4.1 Process Pressure and Range

It is recommended to select a transmitter range such that the normal operating pressure falls within 50%–80% of the full scale. Improper range selection not only affects measurement accuracy but may also cause spurious alarms or safety shutdowns due to temporary pressure fluctuations-.

Tiankang recommendation: The Tiankang 3051 series transmitters offer a 100:1 turndown ratio and bi‑directional overpressure protection. One transmitter can be ranged to cover multiple process measurement requirements, significantly reducing spare parts inventory.

4.2 Accuracy and Stability

• General process control: ±0.25% to ±0.5% FS-.

• Critical reactor pressure control / custody transfer: ±0.075% FS or higher.

• Long‑term stability: Select smart transmitters with annual drift <0.2% to extend calibration intervals.

4.3 Wetted Materials

• Standard: 316L SS – suitable for clean oil/gas and general chemical service.

• Sour / acid / chloride resistance: Hastelloy C‑276.

• High‑temperature abrasion resistance: Inconel 600/625 alloy.

• Hydrogen embrittlement resistance (high‑pressure hydrogen service): Gold‑plated diaphragm, effectively blocks hydrogen atom permeation-.

4.4 Remote Seal Diaphragm Systems

For high‑temperature, high‑viscosity, crystallising, solids‑laden, or highly corrosive media, a remote seal diaphragm system with capillary tubing is recommended.

• Fill fluids: Tiankang offers high‑temperature silicone oil for high‑temperature service, low‑temperature inert fill fluids for cold service, and special fill fluids for hygienic applications. For ultra‑high‑temperature service (up to 400°C), high‑temperature thermal oil fill is required.

• Diaphragm material: For high‑velocity abrasive media such as FCC catalyst dust, a tungsten carbide hard‑coated diaphragm is recommended.

4.5 Explosion Protection and Functional Safety

• Explosion protection: Depending on the hazardous area classification (Zone 1/Zone 2), select intrinsically safe (Ex ia) or flameproof (Ex d) transmitters. Tiankang products hold CCC, ATEX, and IECEx certification.

• Functional safety: Transmitters involved in safety interlock loops (e.g., hydrocracker overpressure protection) must be SIL2/SIL3 certified.

4.6 Output and Communication Protocols

In addition to traditional 4‑20 mA + HART, new digital refineries increasingly require digital bus communication:

• FF H1 / Profibus PA: Enables multivariable digital communication; ideal for large DCS integration.

• Modbus RTU: Convenient for RTU/PLC integration and third‑party computing systems.

• WirelessHART: Suitable for brownfield retrofits and remote monitoring locations, significantly reducing cabling costs.

Tiankang advantage: Tiankang‘s full range of intelligent pressure transmitters supports HART 7 protocol, with optional RS‑485 Modbus or Profibus PA communication. Online diagnostics and self‑verification features are supported, optimising instrument maintenance management.

5. Installation and Maintenance Best Practices

Installation in harsh refinery environments must strictly follow best practices:

• Impulse lines: For steam/gas service, slope upward toward the transmitter; for liquid service, slope downward from the tap. Slope should be no less than 1:10.

• Condensate and purging: For steam flow measurement, condensate pots are mandatory. For media prone to clogging (coking, residual oil), flushing rings and purging facilities (instrument air or steam) must be provided.

• Cold climates: Transmitters and impulse lines must be heat‑traced or insulated to prevent media solidification or water freezing in the impulse lines.

• Periodic maintenance: Perform zero and span checks periodically. If a pressure reading shows a fixed drift without pattern, the impulse lines may be blocked or leaking. Blow‑down, steam purging, or chemical cleaning methods should be applied-.

6. Conclusion

Selecting the right pressure transmitter for a refinery is a systematic process. It requires matching the transmitter‘s materials, accuracy, protection rating, and functionality to the specific process parameters of each unit – temperature, pressure, and media characteristics. Correct selection not only ensures safe and stable plant operation but also significantly reduces total life‑cycle maintenance costs.

With nearly five decades of experience in instrument manufacturing, Anhui Tiankang (Group) Co., Ltd. offers a comprehensive range of pressure, temperature, level, and flow measurement solutions. Our pressure transmitter series have been proven in long‑term service with Sinopec, CNPC, CNOOC, and numerous overseas refining projects. We are committed to providing reliable, accurate, and intelligent pressure measurement solutions to refinery customers worldwide.

For challenging applications encountered during design, selection, or maintenance, please consult the Tiankang technical team for customised solutions tailored to your process conditions.

Contact Us

For refinery pressure transmitter selection advice, technical documentation, or project quotations, please contact:

Yin Shuangjie
📧 Email: [email protected]
📱 WhatsApp / Zalo: +86 17856068126
🌐 Website: http://www.tiankang-global.com/

Anhui Tiankang – Your refinery pressure measurement expert.