Sample Probe Assemblies for Power Generation Applications
Sample probe assemblies for power generation — steam purity sampling, high-temperature probes, boiler water chemistry, and flue gas monitoring.
Power Generation Sampling
Power generation facilities — whether coal-fired, natural gas combined cycle (NGCC), nuclear, biomass, or waste-to-energy — rely on precise process sampling to maintain water chemistry, monitor emissions, verify fuel quality, and ensure turbine protection. Sample probe assemblies in power plants often face the most extreme temperature conditions of any industrial application, with superheated steam temperatures routinely exceeding 1,000 degF (538 degC) and flue gas temperatures reaching 2,000 degF (1,093 degC) or higher at certain measurement points.
Our probe assemblies for power generation are engineered for these demanding thermal environments, using high-temperature alloys, appropriate packing gland sealants, and robust mechanical designs verified by wake frequency analysis per ASME PTC 19.3 TW-2016.
Key Applications
Steam Purity Sampling
Maintaining steam purity is critical for protecting turbine blading, superheater tubes, and condenser tubes from corrosion, erosion, and deposition. Impurities in steam — dissolved solids, silica, sodium, chloride, iron, copper — must be monitored continuously or at frequent intervals to detect condenser leaks, makeup water upsets, or chemical treatment excursions.
Steam sampling points are typically located at:
- Superheater outlet / main steam header: The highest-temperature sampling point, often 1,000-1,100 degF at 1,800-3,500 psig in supercritical and ultra-supercritical units
- Reheat steam: Intermediate temperature (900-1,050 degF) at moderate pressure
- Saturated steam drum: Lower temperature (500-700 degF) but high moisture content
- Condensate and feedwater: Moderate temperatures (100-400 degF) at various pressures
Material and Sealant Requirements by Temperature
| Sampling Point | Typical Temperature | Probe Material | Packing Gland Sealant |
| Main steam (supercritical) | 1,000-1,100 degF | Inconel 600 | Grafoil |
| Reheat steam | 900-1,050 degF | Inconel 600 or 316 SS | Grafoil |
| Saturated steam drum | 500-700 degF | 316 SS | PEEK or Grafoil |
| Feedwater / condensate | 100-400 degF | 316 SS | Teflon (PTFE) |
| Flue gas (post-SCR) | 600-900 degF | Inconel 600 or Hastelloy C-22 | Grafoil |
| Flue gas (pre-air heater) | 300-600 degF | 316 SS or Hastelloy C-276 | PEEK |
Boiler Water Chemistry Monitoring
Boiler drum water, blowdown, and feedwater sampling probes provide the data needed to manage:
- Phosphate treatment: Monitoring PO4 residual and Na/PO4 ratio in drum boilers
- All-volatile treatment (AVT): Verifying ammonia and hydrazine (or carbohydrazide) concentrations
- Oxygenated treatment (OT): Monitoring dissolved oxygen at ppb levels in once-through and drum boilers
- Condenser leak detection: Detecting cation conductivity excursions indicating cooling water in-leakage
- Iron and copper transport: Monitoring particulate and dissolved metals in feedwater
Probe assemblies for boiler water service are typically 316 SS with Teflon or PEEK sealants, depending on the temperature at the sampling point. Electropolished probe bores are recommended for trace-level dissolved oxygen and cation conductivity sampling to minimize surface interaction.
Flue Gas Sampling and CEMS
Continuous Emission Monitoring Systems (CEMS) require sample extraction probes installed in the flue gas duct to measure:
- SO2, NOx, CO, CO2, O2, and particulate matter (PM) for environmental compliance
- Mercury (Hg) for MATS (Mercury and Air Toxics Standards) compliance
- HCl for acid gas monitoring
Flue gas sampling probes face unique challenges:
- High temperatures: Flue gas temperatures of 300-900 degF depending on location (pre- or post-air heater, post-SCR, post-FGD)
- Corrosive condensates: Sulfuric acid condensation below the acid dew point (~260 degF); HCl in chloride-bearing fuels
- Particulate erosion: Fly ash particles erode the probe tube and plug sample orifices
- Large duct dimensions: Duct cross-sections of 20-40 feet require very long probes with careful wake frequency analysis
Probe materials for flue gas service depend on the acid dew point and the presence of chlorides. Hastelloy C-276 is commonly specified for its resistance to sulfuric and hydrochloric acid condensates. Inconel 600 is used where high-temperature oxidation resistance is the primary concern.
Gas Turbine Fuel Gas Sampling
Natural gas combined cycle (NGCC) plants sample the fuel gas supply to the gas turbine for:
- Heating value (BTU) determination for heat rate calculations
- H2S and total sulfur monitoring for turbine blade protection
- Moisture content for combustion optimization
- Hydrocarbon dew point for condensation prevention
These applications are similar to oil and gas custody transfer sampling and benefit from SilcoNert 2000 coated probe assemblies for accurate trace sulfur measurement.
Design Considerations for Power Generation
Thermal Cycling
Power generation equipment experiences frequent thermal cycling — startup, shutdown, load changes, and emergency trips. These cycles impose thermal stress on the probe tube and packing gland sealant, which can cause:
- Sealant creep and loss of compression in the packing gland
- Differential thermal expansion between the probe tube and the process nozzle
- Fatigue cracking at stress concentration points
Grafoil sealant is preferred for high-temperature cycling applications because it retains its sealing properties through repeated thermal cycles better than polymer sealants. Probe tube materials with high fatigue strength (Inconel 600, Hastelloy C-22) are recommended for cyclic-duty installations.
High-Pressure Steam
Supercritical and ultra-supercritical boilers operate at pressures of 3,500 psi and above. The pressure rating of the probe tube must be verified at the operating temperature using temperature-derated allowable stress values. Heavier wall tubes (0.065" or 0.095") may be required to achieve adequate pressure ratings at these extreme conditions.
ASME Code Compliance
Power plant piping and pressure components are designed and fabricated to ASME B31.1 (Power Piping), which has different requirements than the ASME B31.3 (Process Piping) code used in refineries and chemical plants. Key differences include:
- Material allowable stresses per ASME Section II, Part D
- Weld quality and examination requirements per ASME B31.1
- Pressure testing requirements (hydrostatic at 1.5x design pressure)
- Documentation and quality assurance per the utility's QA program
Our power generation probe assemblies are supplied with documentation that meets the requirements of both ASME B31.1 and utility-specific quality programs.
Documentation Package
All power generation probe assemblies include:
- Wake frequency analysis per ASME PTC 19.3 TW-2016
- Pressure rating calculation with temperature derating
- Material test reports (MTRs) per EN 10204 Type 3.1
- Dimensional drawing with insertion depth and overall length
- Packing gland sealant specification and torque values
Contact our engineering team to discuss your power generation sampling requirements. We have extensive experience with utility-grade probe assemblies for all major boiler manufacturers and turbine OEMs.