— A Practical Guide for Engineers, EPCs, and Project Teams
Waste Heat Recovery (WHR) power plants capture otherwise wasted thermal energy from industrial processes—cement kilns, steel mills, glass furnaces, gas turbine exhaust, and chemical plants—and convert it into electricity. Unlike conventional thermal power plants with dedicated fired boilers, WHR plants operate on variable, often unpredictable heat sources. The flue gas temperature, flow rate, and composition can fluctuate significantly with the upstream process, making instrumentation and control far more challenging than in a conventional power plant.
This guide provides a practical overview of the instrumentation requirements for WHR power plants, covering the key measurement points, instrument selection criteria, control system architecture, and applicable standards.
1. The WHR Challenge: Variable Heat Sources and Harsh Conditions
WHR power plants present several unique challenges that distinguish them from conventional thermal power plants:
| Challenge | Impact on Instrumentation |
|---|---|
| Variable flue gas temperature and flow | Inlet conditions fluctuate with the upstream process; instrumentation must maintain accuracy across a wide operating range |
| High particulate loading | Flue gas from cement kilns, steel mills, and other industrial processes contains dust and abrasive particles that can damage sensors |
| Corrosive flue gas components | Sulphur compounds, chlorides, and other corrosive species attack instrument wetted parts |
| High temperatures | Flue gas temperatures can reach 760–1,000°C at the WHR boiler inlet- |
| Limited space | WHR systems are often retrofitted into existing plants with space constraints |
| Start-up and shutdown cycles | WHR plants may start and stop frequently, requiring robust instrumentation that withstands thermal cycling |
The key principle: WHR plant instrumentation must be selected not just for steady-state operation, but for survival during start-up, shutdown, upset conditions, and exposure to particulate-laden, corrosive flue gas.
2. Applicable Standards and Design Basis
WHR power plant instrumentation design is governed by a combination of international, national, and industry-specific standards:
| Standard | Scope | Key Requirements |
|---|---|---|
| ASME Boiler and Pressure Vessel Code (BPVC) Section I & IX | Boiler design, fabrication, and testing | Pressure gauges, level gauges, safety valves- |
| ASME B31.1 | Power piping | Piping design affecting instrument tap locations- |
| ASME B40.100 | Pressure gauges and gauge attachments | Gauge accuracy requirements (Grade 2A, Grade A)- |
| GB 50588-2017 | Cement plant waste heat recovery design (China) | Thermal automation, electrical measurement, protection systems- |
| GB 50569-2010 | Iron and steel plant thermal facility design (China) | Electrical and instrumentation for converter WHR boiler systems- |
| GB/T 51413-2020 | Non-ferrous metals industry waste heat utilisation design (China) | Instrumentation and control for WHR systems- |
| DL/T 5000-2000 | Thermal power plant design code | General instrumentation and control requirements- |
| HG/T 20507-2000 | Automation instrument selection design code | Instrument selection guidelines for pressure, temperature, flow, level- |
| NB/T 42047-2015 | Coke dry quenching WHR boiler technical conditions | Detection and monitoring instruments, control devices- |
| IEC 60079 / GB 3836 | Explosive atmospheres | Ex certification for instruments in hazardous areas (fuel gas systems) |
| IEC 61508 / IEC 61511 | Functional safety | SIL requirements for safety instrumented systems |
| IEC 60332 | Cable flame retardance | Fire performance of instrumentation cables |
Design principle: WHR plant instrumentation should comply with both the general power plant standards and any industry-specific standards applicable to the upstream process (cement, steel, chemical, etc.).
3. Key Instrumentation Systems by Measurement Type
3.1 Temperature Measurement
Temperature measurement is critical throughout the WHR plant—from flue gas inlet to steam outlet. WHR boilers operate across a wide temperature range: primary combustion chamber flue gas can reach 760°C, while steam turbine temperatures are around 260°C-.
Key temperature measurement points:
| Application | Temperature Range | Recommended Sensor | Why |
|---|---|---|---|
| Flue gas inlet (WHR boiler) | 400–1,000°C | Type K or N thermocouple with ceramic thermowell | High-temperature resistance; ceramic protects against abrasion and corrosion |
| Superheater outlet | 400–540°C | Type K thermocouple or Pt100 RTD (if <500°C) | Accuracy for steam temperature control |
| Economiser inlet/outlet | 100–300°C | Pt100 RTD | Accuracy, stability at moderate temperatures |
| Steam turbine inlet | 400–540°C | Type K thermocouple or Pt100 RTD | Critical for turbine protection |
| Condensate system | 30–80°C | Pt100 RTD | Low-temperature accuracy |
| Bearing temperature (turbine, pumps) | Ambient–100°C | Pt100 RTD (bearing sensor) | Vibration and temperature monitoring for rotating equipment |
| Exhaust gas (stack) | 100–200°C | Pt100 RTD | Emissions monitoring and efficiency calculation |
Selection criteria for thermowells:
Material: 310S stainless steel or Inconel for high-temperature flue gas zones; 316L stainless steel for lower temperature areas
Thermowell design: Must comply with ASME PTC 19.3 TW wake frequency calculations to prevent resonant vibration failure
Protection: Ceramic or silicon carbide thermowells for abrasive, high-temperature flue gas service
Tiankang offering: Anhui Tiankang offers Type K and N thermocouples with ceramic thermowells for high-temperature flue gas measurement, and Pt100 RTDs for moderate-temperature applications, all with Ex ia IIC certification where required.
3.2 Pressure Measurement
Pressure measurement is essential for boiler safety, turbine protection, and process control.
Key pressure measurement points:
| Application | Pressure Range | Recommended Technology | Why |
|---|---|---|---|
| Drum pressure (HP/LP) | Up to 100 bar | GP transmitter with remote seal | High-temperature steam; remote seal isolates transmitter |
| Superheater outlet pressure | Up to 100 bar | GP transmitter | Critical for turbine inlet pressure control |
| Feedwater pressure | 50–150 bar | GP transmitter | Feedwater pump discharge monitoring |
| Steam turbine inlet pressure | Up to 100 bar | GP transmitter | Turbine protection and control |
| Condenser vacuum | Near vacuum | AP transmitter (absolute) | Condenser performance monitoring |
| Fuel gas pressure | Variable | GP transmitter with Ex ia IIC | Fuel gas system safety |
| Instrument air pressure | 5–8 bar | GP transmitter | Pneumatic instrument supply |
Selection criteria:
Wetted materials: 316L stainless steel for steam and water service; Hastelloy or Monel for corrosive flue gas condensate
Remote seals: Essential for high-temperature steam applications to protect the transmitter from heat
Ex certification: Ex ia IIC for instruments in fuel gas or hazardous areas
Accuracy: ±0.25% for general control; ±0.075% for critical applications
Tiankang offering: Tiankang TK1151/3051 series pressure and differential pressure transmitters are available with remote seals, 316L/Hastelloy wetted parts, and Ex ia/Ex d IIC certification.
3.3 Level Measurement
Drum level measurement is the single most critical measurement on a WHR boiler. In WHR plants, drum level control is particularly challenging due to variable heat input and rapid load changes. As a cement industry case study notes, the advanced control system for WHR plants comprises "SP furnace water level controller, AQC furnace water level controller, condenser water level controller"-.
Key level measurement points:
| Application | Recommended Technology | Why |
|---|---|---|
| HP drum level | Redundant DP level transmitters with remote seals + guided wave radar or conductivity probe | Critical for boiler safety; redundancy is essential- |
| LP drum level | DP level transmitter with remote seals | Similar to HP drum, but less critical |
| Deaerator tank level | DP level transmitter or magnetic level gauge | Feedwater system control |
| Condenser hotwell level | DP level transmitter or guided wave radar | Condensate system control |
| Feedwater tank level | DP level transmitter | Feedwater storage |
Drum level measurement technologies for WHR boilers:
| Technology | Advantages | Disadvantages | Suitability for WHR |
|---|---|---|---|
| Differential pressure (DP) | Proven, reliable, cost-effective | Density compensation required; impulse line maintenance | Most common; suitable with remote seals |
| Guided wave radar (GWR) | No density compensation, immune to steam | Higher cost; contact with process | Growing adoption; excellent for variable density |
| Conductivity probe | Simple, direct measurement | Point-level only (not continuous) | Used for high-high/low-low alarms |
| Direct-reading gauges (glass, reflex, bicolor) | Visual verification | Local indication only; no signal output | Required by code for direct visual indication- |
Redundancy requirements: ASME requires two independent level measurement systems on boilers with operating pressure above 500 psig. WHR boilers typically use two DP transmitters plus a direct-reading gauge-.
Tiankang offering: Tiankang offers TK1151/3051 DP level transmitters with remote seals for drum level measurement, guided wave radar level transmitters, and vibrating fork level switches for high-high/low-low alarm protection.
3.4 Flow Measurement
Flow measurement in WHR plants serves multiple purposes: feedwater control, steam flow monitoring, and performance calculation.
Key flow measurement points:
| Application | Fluid | Recommended Technology | Why |
|---|---|---|---|
| Feedwater flow | Deionised water | Orifice plate + DP transmitter, or magnetic flowmeter | Three-element drum level control requires feedwater flow |
| Main steam flow | Superheated steam | Flow nozzle or Venturi + DP transmitter (with temperature/pressure compensation) | Turbine control and performance monitoring |
| Attemperator spray water flow | Deionised water | Orifice plate + DP transmitter or magnetic flowmeter | Steam temperature control accuracy- |
| Condensate flow | Condensate | Magnetic flowmeter or vortex flowmeter | Condensate system balance |
| Cooling water flow | Cooling water | Electromagnetic or ultrasonic flowmeter | Heat rejection monitoring |
| Flue gas flow | Flue gas | Pitot tube or averaging pitot tube | Emissions monitoring and performance |
| Fuel gas flow | Fuel gas (if applicable) | Vortex flowmeter or thermal mass flowmeter | Fuel gas system control |
Steam flow measurement considerations:
Steam flow must be temperature and pressure compensated to obtain mass flow
DP flowmeters (flow nozzles, Venturis) are preferred for high-temperature, high-pressure steam
Straight pipe requirements must be met upstream and downstream of the primary element
Tiankang offering: Tiankang offers orifice plates, flow nozzles, Venturi tubes, and vortex flowmeters for steam and gas flow measurement, with DP transmitters for compensated flow measurement.
4. Flue Gas Monitoring and Analytical Instrumentation
Flue gas monitoring is essential for emissions compliance and process control.
Key analytical measurements:
| Measurement | Technology | Purpose |
|---|---|---|
| Oxygen (O₂) in flue gas | Zirconia or paramagnetic analyser | Combustion efficiency optimisation- |
| Carbon monoxide (CO) | Infrared or electrochemical | Combustion efficiency and safety |
| Nitrogen oxides (NOx) | Chemiluminescence or ultraviolet | Emissions compliance- |
| Sulphur dioxide (SO₂) | Infrared or ultraviolet | Emissions compliance |
| Flue gas temperature | Thermocouple or RTD | Efficiency calculation |
| Flue gas flow | Pitot tube or averaging pitot tube | Mass balance and emissions reporting |
Sampling system requirements:
Sample conditioning (cooling, filtering, drying) is essential for flue gas analysers
Sample ports must be provided with access platforms and ladders per statutory regulations-
Multiple sampling points may be required for representative measurement
5. Control and Safety Systems
WHR power plants typically use a centralised DCS for monitoring and control. As documented in a typical cement industry WHR plant, "the whole WHR power plant adopts centralized DCS control: Centralized monitoring of boiler, turbine and electrical systems"-.
5.1 DCS (Distributed Control System)
Key DCS functions:
Process monitoring and control (boiler, turbine, electrical systems)
Data acquisition and archiving
Alarm management
Performance calculation
Historical trending
Control loops in WHR plants:
| Control Loop | Measurement | Manipulated Variable | Criticality |
|---|---|---|---|
| Drum level (three-element control) | Drum level, steam flow, feedwater flow | Feedwater control valve | Critical (boiler safety) |
| Steam temperature control | Superheater outlet temperature | Attemperator spray water | High (turbine protection) |
| Drum pressure control | Drum pressure | Steam turbine inlet valve or bypass | High |
| Feedwater pressure control | Feedwater header pressure | Feedwater pump speed or valve | Medium |
| Condenser vacuum control | Condenser vacuum | Cooling water flow, air extraction | Medium |
| Deaerator pressure/level | Deaerator pressure and level | Steam supply, feedwater flow | Medium |
Control system hardware:
Redundant controllers for critical loops
Redundant power supplies (UPS-backed)
Redundant communication networks
Operator workstations with HMI
Engineering workstation for configuration
5.2 Safety Instrumented Systems (SIS)
WHR boilers require safety instrumented systems for protection against hazardous conditions. The Boiler Safety Technical Supervision Regulations require safety accessories and instruments for waste heat boilers-.
Key SIS functions:
| Safety Function | Sensors | Actuators | Typical SIL |
|---|---|---|---|
| Low drum level trip | Level transmitters (2oo3 voting) | Feedwater isolation, burner trip | SIL 2 |
| High drum level trip | Level transmitters (2oo3 voting) | Feedwater isolation | SIL 1 |
| High steam pressure trip | Pressure transmitters (2oo3 voting) | Turbine trip, bypass | SIL 2 |
| High steam temperature trip | Temperature transmitters | Turbine trip | SIL 2 |
| Condenser vacuum low trip | Vacuum transmitters | Turbine trip | SIL 1 |
| Turbine overspeed trip | Speed probes (2oo3 voting) | Emergency stop valve | SIL 3 |
ESD (Emergency Shutdown) : The WHR plant should have a dedicated ESD system (often integrated with the SIS) that can initiate a controlled shutdown of the boiler and turbine in emergency conditions.
5.3 Emergency Trip Systems
Local and remote emergency trip systems are required for both the gas turbine (if applicable) and the steam turbine-. The WHR boiler should be equipped with:
Local emergency trip pushbuttons at the boiler and turbine
Remote emergency trip from the control room
Automatic trips triggered by the SIS
5.4 Boiler Management System
Some WHR boilers are controlled by a dedicated Boiler Management System (BOMS), a boiler-dedicated PLC system that includes boiler control system, boiler protection system, and burner management system-.
6. Local Indication and Field Instruments
In addition to transmitters connected to the DCS, WHR plants require local indication for operator inspection and maintenance.
Required local instruments:
| Instrument | Application | Code Requirement |
|---|---|---|
| Pressure gauges | Drum pressure, steam header, feedwater, fuel gas | Minimum 4½" local mounted, 3½" board mounted- |
| Temperature gauges | Bimetal thermometers for local temperature indication | Critical temperature points |
| Level gauges | Drum level (direct-reading glass or reflex gauge) | ASME BPVC requirement- |
| Flow indicators | Local flow indicators for critical flows | Operator verification |
Pressure gauge accuracy: ASME B40.100 specifies Grade 2A accuracy (typically ±0.5% of span) for most gauges, with Grade A (±1%) permitted for diaphragm-actuated, liquid-filled, and compound gauges-.
7. Instrumentation Cables
Instrumentation cables in WHR plants must withstand the environmental conditions of the installation: high temperatures, potential chemical exposure, and fire risk.
Key cable requirements:
| Requirement | Specification | Why |
|---|---|---|
| Shielding | IS+OS (individual + overall) for analogue signals; OS for digital | Protects against EMI from VFDs and large motors |
| Temperature rating | 90°C minimum (XLPE insulation) | High ambient temperatures in boiler areas |
| Fire performance | IEC 60332-1 (flame retardant) minimum; IEC 60331 (fire resistant) for safety circuits | Fire safety in power plants |
| Sheath material | LSZH for enclosed spaces (control rooms); oil-resistant PVC for machinery areas | Personnel safety, chemical resistance |
| Armour | SWA or STA for direct burial; braid for flexibility | Mechanical protection |
Tiankang offering: Tiankang offers a full range of instrumentation cables for power plant applications, including IS/OS shielded cables, LSZH cables, fire-resistant cables, and armoured cables.
8. Why Choose Anhui Tiankang for WHR Plant Instrumentation?
Anhui Tiankang (Group) Co., Ltd. has been manufacturing industrial instruments for nearly five decades. Our WHR power plant instrumentation solutions are trusted by EPC contractors and plant operators worldwide.
Complete WHR plant instrumentation portfolio:
| Category | Products | WHR-Specific Features |
|---|---|---|
| Temperature | Type K/N thermocouples, Pt100 RTDs, ceramic thermowells, temperature transmitters | High-temperature resistance, ceramic protection for abrasive flue gas |
| Pressure | TK1151/3051 GP/AP/DP transmitters, remote seals | High-temperature steam capability, Ex ia/Ex d IIC |
| Level | DP level transmitters (remote seal), guided wave radar, vibrating fork switches | Drum level redundancy, SIL-rated switches |
| Flow | Orifice plates, flow nozzles, Venturi tubes, vortex flowmeters | Steam flow measurement, temperature/pressure compensation |
| Analytical | O₂, CO, NOx, SO₂ analysers (partnership with leading analyser manufacturers) | Emissions monitoring, combustion efficiency |
| Instrumentation cables | IS/OS/LSZH/fire-resistant cables | Power plant-grade fire performance, EMI protection |
Core advantages:
Complete certifications: CCC, ATEX, IECEx, SIL, CCS marine
CNAS-accredited laboratory: Full performance testing for power plant applications
Proven track record: Long-term supplier to power generation and industrial projects
One-stop supply: From instrumentation to cables to control systems
9. Conclusion
WHR power plant instrumentation requires a systematic engineering approach that addresses the unique challenges of variable heat sources, high-temperature flue gas, particulate loading, and potential corrosion.
Key takeaways:
| System | Critical Measurements | Recommended Solutions |
|---|---|---|
| WHR boiler | Drum level, steam temperature, steam pressure, feedwater flow | Redundant DP level (remote seal), Type K/N TC, GP/DP transmitters, orifice+DP flow |
| Turbine | Inlet steam temperature/pressure, speed, vibration, condenser vacuum | Type K TC, GP/AP transmitters, speed probes, vibration monitors |
| Flue gas | O₂, CO, NOx, temperature, flow | Zirconia/IR analysers, thermocouples, pitot tubes |
| Control system | DCS + SIS + ESD | Redundant controllers, 2oo3 voting for safety functions |
| Local indication | Pressure gauges, level gauges, temperature gauges | ASME-compliant gauges, direct-reading level gauges |
Remember: WHR plants operate on variable, often unpredictable heat sources. Instrumentation must be robust enough to withstand thermal cycling, particulate-laden flue gas, and potentially corrosive conditions—while maintaining accuracy across a wide operating range. The cost of the right instrument is insignificant compared to the cost of unplanned downtime in a WHR plant.
Contact Us
For WHR plant 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 WHR power plant instrumentation solutions.
