— A Practical Guide for Engineers, EPCs, and Plant Operators
In many industrial processes, the process media is simply too harsh for a standard pressure transmitter to handle directly. Corrosive chemicals eat away at diaphragms. High-temperature fluids cook fill fluids and damage electronics. Viscous slurries clog impulse lines. Crystallising media solidify in dead legs.
The solution is the remote seal pressure transmitter—a system where the sensing element is isolated from the process by a flexible diaphragm, connected to the transmitter via a capillary tube filled with an inert fluid-. The remote seal acts as a protective barrier between the instrument and the process, while faithfully transmitting pressure signals-. This allows the transmitter to be located up to 24.3 metres (80 feet) from the point of measurement-, safely away from heat, corrosion, or physical damage.
This guide covers the applications, advantages, and key selection considerations for remote seal pressure transmitters, helping you determine when—and how—to specify them for your project.
1. What Is a Remote Seal Pressure Transmitter?
A remote seal pressure transmitter consists of three main components:
| Component | Function |
|---|---|
| Remote seal (diaphragm seal) | A flexible diaphragm mounted at the process connection. The diaphragm is exposed to the process media and transfers pressure to the fill fluid. |
| Capillary tube | A small-diameter tube filled with inert fluid (silicone oil, glycerin, etc.) that transmits pressure from the remote seal to the transmitter-. |
| Pressure transmitter | The electronic device that converts the pressure signal into a 4-20 mA or digital output-. |
The remote seal can be assembled to the transmitter either by a direct (rigid) connection (common for level measurement at the bottom of a tank) or by a capillary (for distant measuring points or high-temperature processes)-.
2. Applications: When to Use Remote Seals
Remote seal pressure transmitters are used when standard transmitters or impulse lines cannot handle the process conditions. Typical applications include-:
2.1 Corrosive Process Fluids
When the process media is chemically aggressive—acids, alkalis, salt solutions, or sour crude—standard 316L stainless steel diaphragms may not survive. Remote seals allow the use of exotic alloys (Hastelloy, Monel, Tantalum, Titanium) at the process interface while keeping the expensive transmitter safely away-.
2.2 Viscous or Solids-Bearing Fluids
Slurries, paper pulp, heavy crude, and polymer melts can clog impulse lines and coat sensor diaphragms. Remote seals with flush-mount or extended diaphragms eliminate dead legs where solids can accumulate-.
2.3 High-Temperature Processes
When process temperatures exceed the transmitter's maximum operating temperature (typically 85°C for electronics), remote seals with long capillaries allow the transmitter to be mounted in a cooler location-. The capillary dissipates heat before it reaches the transmitter.
2.4 Freezing, Crystallising or Solidifying Media
Processes that freeze, crystallise, or solidify at ambient temperatures—such as caustic soda, urea, or certain polymers—require remote seals to prevent damage to the transmitter and to allow steam tracing or flushing of the seal area.
2.5 Hygienic and Sanitary Applications
In food, beverage, and pharmaceutical processing, remote seals with flush diaphragms and FDA-compliant fill fluids eliminate crevices where bacteria can grow, enabling clean-in-place (CIP) procedures-.
2.6 Vacuum Applications
Remote seal systems can be specified for vacuum applications, but require special consideration. Mounting the pressure transmitter at or below the bottom vessel tap is an important factor to ensure a stable measurement with vacuum applications-. The static pressure limit of the fill fluid must be considered—the fill fluid must remain within the liquid phase of the vapour pressure curve-.
Common industries:
Oil and gas (upstream, midstream, refining)
Chemical and petrochemical processing-
Pulp and paper
Food and beverage-
Pharmaceutical-
Power generation-
Water and wastewater treatment
3. Advantages of Remote Seal Systems
3.1 Protection of the Transmitter
The remote seal acts as a secondary line of defence between the pressure transmitter and the process-. It protects the transmitter from corrosive, erosive, high-temperature, or hazardous media-. This extends the life of the transmitter and reduces maintenance costs.
3.2 Flexible Transmitter Location
The transmitter can be mounted up to 80 feet (24.3 metres) from the measuring point-, allowing it to be placed in a safe, accessible, and environmentally controlled location. This is particularly valuable in hazardous areas, offshore platforms, and high-temperature zones.
3.3 Elimination of Impulse Lines
Remote seals eliminate the need for long impulse lines, which are prone to plugging, freezing, and leakage-. This reduces maintenance and improves measurement reliability, especially for viscous or dirty fluids.
3.4 Wide Material Selection
Remote seals offer a much greater range of materials compared with standard transmitters-, including:
316L stainless steel (standard)
Hastelloy C-276 / C2000-
Inconel 625-
Monel
Tantalum
Titanium
Superduplex-
PTFE-lined diaphragms
Proprietary coatings-
This allows the wetted parts to be matched precisely to the process media.
3.5 Improved Safety
By isolating the transmitter from the process, remote seals reduce the risk of process leaks and exposure to hazardous media. The system can be safety-certified for use in hazardous areas-.
3.6 Versatile Configurations
Remote seal transmitters are available in a vast variety of customised designs-, including:
Single-seal configurations (for gauge pressure)
Dual-seal configurations (for differential pressure and level)
Flush-mount designs (for viscous or crystallising media)
Extended diaphragm designs (for tank nozzles)
4. Selection Considerations: Getting It Right
4.1 Diaphragm Material Selection
The diaphragm is the only part of the remote seal that contacts the process media. Selecting the correct material is critical-:
| Process Media | Recommended Diaphragm Material |
|---|---|
| Clean water, mild chemicals | 316L stainless steel |
| Seawater, chlorides, acids | Hastelloy C-276, Titanium |
| Strong acids (H₂SO₄, HCl) | Tantalum, Hastelloy |
| Sour service (H₂S) | Inconel 625, Hastelloy |
| High-temperature hydrocarbons | Inconel 625 |
| Food / pharmaceutical | 316L stainless steel (electropolished) |
4.2 Fill Fluid Selection
The fill fluid transmits pressure from the diaphragm to the transmitter. The ideal fill fluid should have a low vapour pressure, low thermal expansion coefficient, and low viscosity, while remaining stable at high temperatures and under vacuum conditions-. Common fill fluids include:
| Fill Fluid | Temperature Range | Applications |
|---|---|---|
| Silicone oil (standard) | -40°F to 400°F (-40°C to 204°C)- | General industrial |
| High-temperature silicone oil | Up to 600°F (315°C) | High-temperature processes |
| Low-temperature silicone oil | Down to -40°F (-40°C) | Cold environments |
| Glycerin / water | -10°F to 200°F (-23°C to 93°C) | Sanitary, FDA applications |
| Halocarbon (fluorinated) | Wide range | Oxygen service, aggressive chemicals- |
| Inert / specialty fluids | Custom ranges | Special applications |
Critical warning: The wrong fill fluid can boil inside the diaphragm or capillary and upset the measurement-. For vacuum applications, the fill fluid's vapour pressure must be below the process vacuum level.
4.3 Capillary Selection
Key principles-:
Choose the shortest possible capillary – Longer capillaries increase response time and temperature error
Choose the biggest possible diameter for the remote seal – A larger effective diaphragm diameter reduces temperature error-
Do not bend capillary lines sharply – Kinking can restrict fluid flow and affect measurement-
Do not rest the measurement assembly on the capillary line – This can damage the capillary-
Capillary length trade-offs:
| Capillary Length | Advantage | Disadvantage |
|---|---|---|
| Short (< 3 m) | Fast response, minimal temperature error | Limited transmitter placement options |
| Medium (3–10 m) | Good flexibility in transmitter location | Moderate response time, temperature effects |
| Long (10–24 m) | Maximum transmitter location flexibility- | Slow response, significant temperature effects |
4.4 Installation Best Practices
| Practice | Why |
|---|---|
| Mount transmitter below the lowest fill height for level applications- | Ensures stable measurement and prevents cavitation |
| Ensure the seal is centrally positioned in the flange- | Prevents diaphragm restriction and measurement errors |
| For vacuum applications, install transmitter at or below the seal height- | Ensures fill fluid remains in the liquid phase- |
| Tighten flange bolts evenly to specified torque- | Prevents leaks and diaphragm distortion |
| Ensure seals are clean and undamaged before installation- | Prevents contamination and measurement errors |
4.5 Accuracy Considerations
Remote seals introduce additional error sources compared to direct-mount transmitters:
| Error Source | Impact | Mitigation |
|---|---|---|
| Diaphragm stiffness | Adds non-linearity | Choose the largest practical diaphragm diameter- |
| Fill fluid thermal expansion | Temperature-induced drift- | Select fluid with low thermal expansion; use temperature compensation |
| Hydrostatic head from fill fluid | Zero shift | Calculate and compensate during calibration; know the specific gravity of the fill fluid- |
| Capillary length | Response delay, temperature effects- | Use the shortest practical capillary |
| Diaphragm seal effects | Added uncertainty (typically 0.1–0.2% of URL)- | Account for seal effects in total accuracy calculations |
Best practice: When specifying accuracy, always ask for the total installed performance—including the transmitter, remote seal, and capillary—not just the transmitter alone-.
5. Common Mistakes to Avoid
| Mistake | Consequence | Prevention |
|---|---|---|
| Wrong fill fluid for process temperature | Fluid boils or freezes; measurement fails- | Match fill fluid to process temperature and vacuum conditions |
| Capillary too long | Slow response, temperature errors- | Use the shortest capillary that meets installation needs |
| Diaphragm material not compatible | Corrosion, leakage, measurement failure- | Verify material compatibility with process media |
| Not accounting for seal effects in accuracy | Total error exceeds expectations- | Specify total installed performance, not just transmitter accuracy |
| Incorrect transmitter mounting for vacuum | Fill fluid vapourises, measurement fails- | Mount transmitter at or below seal height for vacuum applications |
| Guessing at fill fluid | Large and significant errors- | Determine the actual fill fluid based on process conditions |
6. Why Choose Anhui Tiankang for Remote Seal Pressure Transmitters?
Anhui Tiankang (Group) Co., Ltd. has been manufacturing industrial pressure transmitters for nearly five decades. Our remote seal solutions are trusted by major oil and gas companies, chemical plants, and power generation facilities worldwide.
Product portfolio for remote seal applications:
| Product Series | Type | Remote Seal Features |
|---|---|---|
| TK1151GP | Gauge pressure | Remote seal options, wide diaphragm material selection |
| TK1151AP | Absolute pressure | Remote seal options, suitable for vacuum applications |
| TK1151DP | Differential pressure | Dual remote seals, high-static-pressure option (32 MPa) |
| 3051 Series | Intelligent transmitter | HART protocol, temperature compensation, SIL2/SIL3 |
Remote seal options:
Diaphragm materials: 316L SS, Hastelloy C-276, Monel, Tantalum, Titanium
Fill fluids: Standard silicone oil, high-temperature silicone oil, low-temperature silicone oil, Halocarbon, FDA-compliant fluids
Process connections: Threaded, flanged (ANSI, DIN, JIS), flush-mount, extended diaphragm
Capillary lengths: Custom lengths up to 24 metres
Ex certification: CCC Ex, ATEX, IECEx
Core advantages:
Complete material selection – Match wetted materials to your process
CNAS-accredited laboratory – Full performance testing including seal effects
Comprehensive certifications – CCC Ex, ATEX, IECEx, SIL, CCS marine
Proven track record – Long-term supplier to CNPC, Sinopec, CNOOC, and international EPC projects
7. Conclusion
Remote seal pressure transmitters are essential for applications involving corrosive, viscous, high-temperature, or sanitary media. They protect the transmitter from harsh process conditions, eliminate problematic impulse lines, and provide flexibility in transmitter placement.
Key takeaways:
| Selection Factor | What to Consider |
|---|---|
| Diaphragm material | Match to process corrosiveness and temperature |
| Fill fluid | Match to process temperature and vacuum conditions- |
| Capillary length | Use the shortest practical length- |
| Diaphragm diameter | Choose the largest practical diameter to reduce temperature error- |
| Transmitter location | Mount below the seal for vacuum applications- |
| Total accuracy | Account for seal effects in total error budget- |
Remember: A remote seal system is only as good as its specification. Get the diaphragm material, fill fluid, and capillary length right, and the system will provide years of reliable service. Get them wrong, and the measurement will fail—often in ways that are difficult to diagnose.
With nearly five decades of experience and a complete range of remote seal pressure transmitters, Anhui Tiankang is your partner for reliable pressure measurement in the most challenging applications.
Contact Us
For remote seal pressure transmitter selection advice, technical documentation, or project quotations, please contact:
Yin Shuangjie
International Sales Manager
📧 Email: [email protected]
📱 WhatsApp / Zalo: +86 17856068126
🌐 Website: http://www.tiankang-global.com/
Anhui Tiankang – Your partner for reliable remote seal pressure measurement solutions.
