**Safety Valve Float Test in a Boiler (Specific to Power Plant Context like NTPC Dadri 210 MW Boiler)**
The safety valve float test, also known as the "popping test," is a critical procedure in a power plant boiler to verify that safety valves open at their designated set pressure to prevent overpressure conditions, ensuring boiler safety and compliance with regulations like the Indian Boiler Regulations (IBR) 1950. For a 210 MW boiler at NTPC Dadri, this involves testing drum and superheater safety valves (typically set at 155-160 bar for drum and 135-140 bar for superheater). Below is a concise, step-by-step guide to performing the safety valve float test, tailored to subcritical boilers like those at NTPC Dadri, with practical insights from thermal power plant operations.
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### **Steps for Safety Valve Float Test**
1. **Preparation and Planning**
- **Obtain Permit to Work (PTW):** Ensure Lockout-Tagout (LOTO) for all relevant systems to isolate equipment safely.
- **Review Valve Specifications:** Check the set pressure, blowdown range (typically 2-5% of set pressure), and previous test records for each valve (e.g., drum and superheater valves). For Dadri’s 210 MW units, refer to BHEL or Combustion Engineering valve manuals.
- **Gather Equipment:** Prepare calibrated pressure gauges, hydraulic test kits (if used), and communication devices (e.g., walkie-talkies).
- **Safety Protocols:** Ensure all personnel wear PPE (helmets, earplugs, gloves) due to high-pressure steam release and noise (>120 dB during popping).
- **IBR Compliance:** Verify the test aligns with IBR Regulation 73, and coordinate with the Boiler Inspector if required.
2. **Pressurize the Boiler**
- Gradually increase boiler pressure to approach the set pressure of the safety valve being tested. For Dadri’s 210 MW boiler, this means raising drum pressure to ~155 bar for drum valves or ~135 bar for superheater valves.
- **Method:** Increase load via the control room or close bypass valves to build pressure in the drum. Monitor pressure using Distributed Control System (DCS) and local gauges.
- **Precaution:** Ensure pressure rise is controlled to avoid exceeding the set point prematurely.
3. **Perform the Float Test (Popping)**
- At the set pressure, the safety valve should automatically "pop" open, releasing steam with a distinct sound.
- **Record Data:** Note the exact opening pressure and closing pressure (blowdown, typically 2-4% below set pressure). For example, a 155 bar valve should close at ~149-152 bar.
- **Observation:** Ensure the valve opens cleanly without chattering (rapid opening/closing) and reseats properly without leakage.
- **Multiple Valves:** If testing multiple valves (e.g., two drum valves), gag (lock) other valves to prevent simultaneous popping, which could destabilize pressure.
4. **Adjust the Valve (If Necessary)**
- If the valve opens too early or late (outside ±1-2% of set pressure), isolate the boiler, reduce pressure to a safe level, and adjust the valve:
- **Adjustment:** Modify the spring tension by turning the adjusting screw/nut (clockwise to increase set pressure, counterclockwise to decrease), per the valve manufacturer’s manual (e.g., BHEL or Dresser Consolidated valves at Dadri).
- Retest after adjustment by repressurizing the boiler and repeating the float test.
- **Note:** Adjustments should be minimal to avoid overstressing the valve spring.
5. **Post-Test Checks**
- Verify valve reseating and check for steam leakage at the valve seat.
- Gradually reduce boiler pressure to normal operating levels (e.g., 130-140 bar for Dadri’s 210 MW units).
- Clean and inspect the valve for wear or damage, especially in aging units (Dadri’s units are 30+ years old).
- **Documentation:** Record opening/closing pressures, blowdown, and observations in the logbook and update IBR Form XVI for compliance.
6. **Reporting and Compliance**
- Submit test results to the Boiler Inspectorate as per IBR requirements.
- For NTPC Dadri, log results in the plant’s Performance Evaluation Matrix and share with the Central Power Engineering and Efficiency Program (CenPEEP) for analysis.
- Schedule the next test (typically annual, or as mandated by IBR).
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### **Key Technical Details for NTPC Dadri 210 MW Boiler**
- **Boiler Type:** Subcritical, Combustion Engineering design with high-pressure drum and superheater.
- **Valve Specifications:**
- Drum safety valves: Set pressure ~155-160 bar, blowdown ~3-7 bar.
- Superheater valves: Set pressure ~135-140 bar.
- **Frequency:** Annual testing during planned shutdowns or as per IBR Regulation 73.
- **Challenges:** Aging valves may exhibit seat wear or spring fatigue, requiring proactive maintenance or spares (BHEL valves commonly used).
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### **Safety Precautions**
- **High Noise and Steam:** Use ear protection and clear the valve area during testing to avoid exposure to high-pressure steam.
- **Pressure Control:** Maintain clear communication between control room and field operators to prevent overpressure.
- **Emergency Procedures:** Be prepared to initiate an emergency shutdown if a valve sticks open or excessive popping occurs.
- **Gagging Non-Tested Valves:** Ensure other valves are secured to avoid unintended operation.
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### **Additional Tips**
- **Hydraulic Test Kits (Optional):** For online testing without shutdown, use hydraulic assist devices (e.g., Trevitest) to simulate pressure on the valve, though this is less common at Dadri due to shutdown-based schedules.
- **Maintenance:** Regularly inspect valve seats and springs during outages to prevent sticking or leakage, critical for Dadri’s aging units.
- **Efficiency:** Use DCS data and NTPC’s CenPEEP tools to analyze valve performance trends and optimize test frequency.
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### **Expected Outcomes**
- **Successful Test:** Confirms valves open and close within design tolerances, ensuring boiler safety.
- **Compliance:** Meets IBR and NTPC safety standards, avoiding penalties.
- **Impact:** Prevents boiler damage from overpressure, maintaining reliability (Dadri ranks high in NTPC for availability).
For further guidance, refer to NTPC’s Heat Rate Improvement Guidelines, CenPEEP resources, or IBR 1950. If specific valve data or manuals are available, cross-check set pressures before testing. This process ensures safe and efficient operation of the 210 MW boiler at NTPC Dadri.
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