— A Practical Guide for Engineers, EPCs, and Project Teams
Liquefied Natural Gas (LNG) terminals are among the most demanding environments for industrial instrumentation. The combination of ultra-low cryogenic temperatures (as low as -162°C), flammable and explosive atmospheres, large-scale storage tanks (up to 200,000 m³), and high-value custody transfer creates a unique set of engineering challenges that demand specialised instrumentation solutions--19.
As China has become the world's second-largest LNG importer, with over 20 receiving terminals built and more under construction, the need for reliable, safe, and accurate instrumentation has never been greater-29. This guide provides a practical overview of the key engineering considerations for instrumentation in LNG terminal projects—covering temperature, pressure, level, flow, control valves, gas detection, and system integration.
1. The LNG Challenge: Cryogenic Temperatures and Hazardous Atmospheres
LNG terminals present two fundamental challenges that distinguish them from conventional oil and gas facilities:
| Challenge | Impact on Instrumentation |
|---|---|
| Cryogenic temperatures (-162°C) | Standard instrument materials become brittle; fill fluids freeze or vaporise; seals fail; electronics malfunction |
| Flammable and explosive atmosphere | All electrical instruments must be Ex-certified (typically Ex ia or Ex d) |
| Boil-off gas (BOG) | Continuous vaporisation creates pressure management challenges and changes gas composition |
| Thermal cycling | Frequent cooling and warming cycles cause mechanical stress on sensors and connections |
| Large tank dimensions | Level measurement ranges exceed 40 metres, requiring high-accuracy, long-range instruments-19 |
| Custody transfer accuracy | Trade交接 measurement requires the highest accuracy and compliance with international standards- |
The key principle: LNG instrumentation must be designed for survival at cryogenic temperatures—not just signal transmission. Every instrument, from temperature sensors to control valves, must be specifically rated for cryogenic service and carry appropriate Ex certification-.
2. Applicable Standards and Design Basis
LNG terminal instrumentation design is governed by a combination of international, national, and industry-specific standards:
| Standard | Scope | Key Requirements |
|---|---|---|
| GB 51156-2015 | LNG receiving terminal engineering design (China) | 仪表及自动控制 requirements; temperature, pressure, level, flow instrument selection-62- |
| ISO 21903:2020 | Dynamic measurement of LNG and refrigerated hydrocarbon fluids | Metrological and technical requirements for flowmeters; selection and installation of Coriolis and ultrasonic flowmeters-- |
| IEC 61508 / IEC 61511 | Functional safety | SIL requirements for safety instrumented systems (SIS) and overfill prevention-17 |
| IEC 60079 / GB 3836 | Explosive atmospheres | Ex certification for all electrical instruments in hazardous areas- |
| SH/T 3005 / HG/T 20507 | Instrument selection design specifications (China) | Detailed selection criteria for process instrumentation- |
Design principle: Instruments in hazardous areas should use intrinsically safe (Ex ia) protection where possible, with isolated safety barriers. Where Ex ia is not feasible, flameproof (Ex d) or pressurised enclosures may be used-.
3. Temperature Measurement: The Foundation of LNG Operations
Temperature measurement is critical in LNG terminals. LNG is stored at approximately -162°C, and even small temperature changes affect density, volume, and vapour pressure-6. Temperature monitoring is also essential for leak detection (a sudden temperature drop indicates LNG spillage) and tank pre-cooling-29.
3.1 Sensor Selection
| Temperature Range | Recommended Sensor | Why |
|---|---|---|
| -180°C to 80°C (most LNG applications) | Pt100 sheathed RTD (3-wire or 4-wire) | Highest accuracy (±0.1°C), excellent stability, proven in cryogenic service--29 |
| Low-cost / fast-response applications | Type T thermocouple (copper-constantan) | Responds faster than RTD, but lower accuracy (±0.5°C) and long-term drift- |
| Extreme cryogenic (<-200°C) | Specialised cryogenic thermocouples | For specialised applications such as liquid hydrogen- |
Key considerations:
Pt100 is the preferred choice for LNG temperature measurement due to its linearity, stability, and accuracy-
Sheathed (armoured) RTDs are essential to protect the sensor from mechanical damage during installation and operation-29
For LNG and BOG低温介质, use remote (split) temperature transmitters to keep electronics away from the cryogenic zone; for常温介质 (nitrogen, instrument air), integrated transmitters are acceptable-29
3.2 Thermowell Material Selection
| Application | Recommended Thermowell Material |
|---|---|
| General LNG service | 316L stainless steel |
| Direct seawater exposure | Hastelloy or Monel (耐海水腐蚀)-29 |
| High-corrosion areas | Special alloys as required |
Critical requirement: Thermowells must be designed with ASME PTC 19.3 TW wake frequency calculations to prevent resonant vibration failure. The operating frequency should not exceed 40% of the thermowell's natural frequency-29.
3.3 Installation Locations
GB 51156-2015 specifies key temperature measurement points-29:
Inner tank outer wall: Surface RTDs to detect LNG leakage (temperature drop triggers alarm)
Annular space between inner and outer tank: Temperature monitoring for leak detection
Hot corner protection plates: Critical leak detection points
Inner tank bottom and walls: Pre-cooling temperature monitoring during tank cooldown
4. Level Measurement: Safety and Inventory Control
Level measurement is the single most critical measurement on an LNG storage tank. A measurement failure can lead to overfill, spills, safety hazards, inventory losses, and regulatory violations-17.
4.1 Primary Level Measurement: Radar vs Servo
Two technologies dominate LNG tank gauging: non-contacting radar and servo gauges.
| Technology | Advantages | Disadvantages |
|---|---|---|
| Non-contacting Radar | No moving parts, MTBF measured in decades, immune to density changes, penetrates vapour space, minimal maintenance--19 | Requires careful antenna selection; dielectric constant must be considered |
| Servo Gauge | High accuracy (±1 mm), proven track record- | Moving parts susceptible to mechanical wear (servo life as short as 5 years); sensitive to density changes- |
The industry trend: Modern non-contacting radar gauges are increasingly replacing servo technology in LNG applications. Radar provides greater reliability and lower total cost of ownership while matching servo accuracy-. Major oil companies often stipulate higher accuracy requirements for radar gauges than for servo-operated gauges-.
Radar advantages for LNG:
Penetrates vapour space and boil-off gas-19
Unaffected by density changes-
Range capability >40 metres for large LNG tanks-19
Accuracy sufficient for custody transfer-
4.2 Redundant Tank Gauging System
Redundancy is essential for LNG tank gauging-17. A high-reliability system typically integrates-17:
| Component | Function |
|---|---|
| Primary level gauge | Radar or servo (continuous level measurement)-17 |
| Independent high-high level switch | Separate, SIL-rated overfill protection; operates independently from main gauge-17 |
| Backup level measurement | Secondary radar or DP system for cross-validation-17 |
| Temperature and density profiling | Multi-point temperature sensors + densitometer for accurate volume/mass calculation-17 |
| Dual communication networks | Redundant protocols (Modbus TCP/IP, fieldbus)-17 |
| Independent power supplies | UPS and dual power feeds-17 |
SIL compliance: LNG tank gauging systems support SIL 2 or SIL 3 configurations, separating basic process control from safety layers-17. Overfill prevention systems must comply with IEC 61511-19.
5. Pressure Measurement: Monitoring and Safety
Pressure measurement in LNG terminals covers a wide range—from near-atmospheric in storage tanks to high-pressure in send-out pipelines.
5.1 Key Applications
| Application | Pressure Range | Recommended Technology |
|---|---|---|
| LNG storage tank pressure | Near atmospheric (vacuum to low positive) | Absolute pressure (AP) transmitter with remote seal |
| BOG compressor suction/discharge | Variable | Gauge pressure (GP) transmitter |
| Send-out pipeline pressure | High pressure (up to 100+ bar) | GP transmitter, 316L or Hastelloy wetted parts |
| Vaporiser pressure control | Process-dependent | GP transmitter with temperature compensation |
5.2 Cryogenic Considerations
Remote diaphragm seals are essential for cryogenic service—they isolate the transmitter from the -162°C LNG-
Fill fluid must be rated for cryogenic temperatures (special低温 silicone oil or inert fluids)
Wetted materials must be suitable for cryogenic service (316L stainless steel minimum)
Transmitter electronics must be mounted remotely from the cryogenic zone
5.3 Ex Certification
Pressure transmitters in LNG terminals must carry appropriate Ex certification:
Ex ia (intrinsic safety) preferred for Zone 0/1 areas-
Ex d (flameproof) for Zone 1/2 where Ex ia is not feasible-
Protection rating ≥ IP65 for outdoor installations-
6. Flow Measurement: Custody Transfer and Process Control
Flow measurement in LNG terminals serves two distinct purposes: custody transfer (贸易交接) and process control.
6.1 Custody Transfer Flow Measurement
LNG custody transfer requires the highest accuracy (±0.25% or better) and compliance with ISO 21903-. The two dominant technologies are-:
| Technology | Best For | Accuracy | Key Feature |
|---|---|---|---|
| Ultrasonic flowmeter | Large flow applications (LNG ship loading/unloading, send-out)- | High (±0.25%) | No moving parts, bidirectional, low pressure drop |
| Coriolis mass flowmeter | Low to medium flow applications, custody transfer- | Very high (±0.05% MV)- | Direct mass measurement, wide turndown, -200°C to +400°C capability- |
Why these technologies dominate LNG:
No moving parts (critical for cryogenic, low-maintenance service)-
Low pressure drop (prevents LNG vaporisation)-
Proven in cryogenic service-
6.2 Process Control Flow Measurement
For non-custody applications (BOG handling, utility flows), less expensive technologies may be suitable:
Vortex flowmeters for clean gases and liquids
DP flowmeters (orifice plates) for general service
Thermal mass flowmeters for gas flow
7. Control Valves: Cryogenic-Specific Requirements
Cryogenic control valves in LNG service are subject to significantly more stringent technical requirements than standard industrial valves-.
7.1 Key Design Requirements
| Requirement | Why |
|---|---|
| Extended bonnet | Separates the actuator from the cryogenic process; protects actuator and packing from extreme cold- |
| Special materials | Must maintain toughness at -162°C (9% Ni steel, stainless steel, special alloys)- |
| Fire-safe design | Must maintain seal integrity in fire conditions- |
| No liquid entrapment | Valve cavities must not trap LNG (trapped liquid expands, causing damage)- |
| Tight sealing | Must seal tightly even near setpoint pressure- |
| Special packing | Low-emission environmental packing designs- |
7.2 Valve Type Selection
Quarter-turn valves (ball, butterfly) are preferred for cryogenic service-
Valve body, seat, bonnet, sealing mechanisms, and materials all require special consideration-
Applicable standards: EN 12567 is one of the key standards for cryogenic valves-.
8. Gas Detection: Safety First
LNG terminals require comprehensive flammable gas detection systems to protect personnel and assets.
Key considerations-:
Laser absorption spectroscopy (tunable diode laser) is increasingly preferred for methane detection—offering fast response, immunity to other combustible gas interference, and high safety-
Infrared point and open-path detection is the industry standard—fail-safe, poison-immune, and low-maintenance-
Performance-based gas detection mapping studies should be conducted to optimise detector placement-
System requirements-:
Integration with Safety Instrumented Systems (SIS) and Emergency Shutdown (ESD) systems
Compliance with IEC 61511 and ISA-84-
Two-stage ESD: ESD1 (isolates lines, sounds alarm) and ESD2 (activates emergency release coupling)-
9. Control and Safety Systems Integration
LNG terminals require multiple integrated control and safety systems:
| System | Function | Key Standards |
|---|---|---|
| DCS (Distributed Control System) | Process control and monitoring | IEC 61131 |
| SIS (Safety Instrumented System) | Safety functions, interlocks | IEC 61508 / IEC 61511- |
| ESD (Emergency Shutdown) | Emergency isolation and shutdown | IEC 61511- |
| F&G (Fire and Gas Detection) | Fire and gas detection and alarm | NFPA 72, IEC 61511- |
| Tank Gauging System | Level, temperature, density monitoring | OIML, API MPMS-17 |
GB 51156-2015 requires that LNG receiving terminals be equipped with DCS, SIS, and F&G systems-.
10. Installation and Protection Requirements
10.1 Ingress Protection
Field instruments must have IP65 minimum protection rating--
Offshore or high-corrosion areas may require higher ratings
10.2 Temperature Range
Instruments must be rated for the full ambient temperature range of the installation location-
Cryogenic-rated instruments typically cover -196°C to +60°C-
10.3 Thermowell Frequency Analysis
All thermowells must be designed with ASME PTC 19.3 TW wake frequency calculations-29
Operating frequency must not exceed 40% of natural frequency-29
11. Why Choose Anhui Tiankang for LNG Instrumentation?
Anhui Tiankang (Group) Co., Ltd. has been manufacturing industrial instruments for nearly five decades. Our LNG terminal instrumentation solutions are trusted by major oil and gas companies and EPC contractors.
Complete LNG instrumentation portfolio:
| Category | Products | LNG-Specific Features |
|---|---|---|
| Temperature | Pt100 sheathed RTDs, thermowells, temperature transmitters | Cryogenic rating (-200°C), 316L/Hastelloy materials, ASME thermowell design |
| Pressure | GP/AP/DP transmitters, remote seals | Cryogenic fill fluids, Ex ia/Ex d certified, remote seal options |
| Level | Radar level transmitters (TKLD series), level switches | Non-contacting, 40m+ range, Ex ia/Ex d, SIL-rated switches |
| Flow | Ultrasonic, Coriolis, vortex flowmeters | Cryogenic-rated, custody transfer accuracy |
| Instrumentation cables | IS/OS/LSZH/fire-resistant cables | Low capacitance, Ex-ia certified, LSZH sheaths |
| Ex cable glands | Ex d/Ex e certified | Cryogenic-compatible materials |
Core advantages:
Complete certifications: CCC Ex, ATEX, IECEx, SIL, CCS marine
CNAS-accredited laboratory: Full cryogenic performance testing
Proven track record: Long-term supplier to CNPC, Sinopec, CNOOC, and international EPC projects
One-stop supply: From instrumentation to cables to Ex glands
12. Conclusion
LNG terminal instrumentation requires a systematic engineering approach that addresses the dual challenges of cryogenic temperatures and hazardous atmospheres.
Key takeaways:
| Parameter | Recommended Solution |
|---|---|
| Temperature | Pt100 sheathed RTD + remote transmitter; 316L/Hastelloy thermowell |
| Level (primary) | Non-contacting radar gauge (replacing servo) |
| Level (safety) | Independent SIL-rated high-high level switch |
| Pressure | Remote seal transmitter with cryogenic fill fluid |
| Flow (custody transfer) | Ultrasonic (large flow) or Coriolis (small flow) |
| Control valves | Extended bonnet, quarter-turn, fire-safe |
| Gas detection | IR point/open-path or laser spectroscopy |
| Ex protection | Ex ia preferred; Ex d where necessary |
| System integration | DCS + SIS + ESD + F&G |
Remember: The cost of the right instrument is insignificant compared to the cost of failure in an LNG terminal. Specify cryogenic-rated, Ex-certified instruments with appropriate redundancy—because in LNG service, reliability is not optional.
Contact Us
For LNG instrumentation 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 LNG terminal instrumentation solutions.
