Process Inspection and Routine Maintenance of Instrument Valves – Sewage
Regular discharge of instrument valves involves two main tasks: sewage removal and routine purging. These activities should be tailored to the specific conditions of each system. Not all process instrumentation valves require regular blowdown, so it's important to assess the necessity based on the application.
Sewage Discharge
Sewage discharge is commonly required for differential pressure transmitters, pressure transmitters, and float level gauges. The measuring medium often contains dust, grease, or fine particles that can accumulate in the pressure lines or regulating valves, affecting measurement accuracy. The frequency of discharge should be determined by the manufacturer or based on field experience.
Key Precautions for Sewage Discharge:
1. Before discharging, always consult with the process operator and obtain their approval.
2. If discharging from a flow or pressure control system, switch the control valve to manual mode to maintain the current opening position.
3. For differential pressure transmitters, close the positive and negative pressure valves before starting the discharge.
4. Place a collection container under the drain valve and slowly open the drain to allow the material and debris to enter the container, avoiding direct discharge into the environment.
5. If the sewage valve fails after repeated use, install a blind flange to prevent leakage and ensure accurate measurements.
6. After discharging, re-open the three-valve assembly and loosen the vent screw on the differential pressure transmitter to release any remaining residue. Tighten the screw afterward.
7. Monitor the local indicator until the output stabilizes. If the system is automatic, switch back to automatic mode once the readings are normal.
Purging
Purging is a method used to prevent direct contact between the measured medium and the instrument components. It involves introducing a controlled amount of gas or liquid through the measuring line to protect the equipment and ensure accurate readings. Purging is particularly useful when dealing with corrosive, viscous, crystalline, or precipitating media that cannot be isolated easily.
Important Considerations for Purging:
1. The purge medium must be compatible with the process fluid. It should meet the following criteria:
- a. No chemical reaction with the process medium;
- b. Free of solid particles;
- c. No phase change after throttling or decompression;
- d. Non-corrosive;
- e. Good fluidity.
2. The purge supply must be reliable and unaffected by process operations.
3. The purge pressure should exceed the maximum process pressure at the measurement point to ensure continuous and stable flow.
4. Use a restrictor or flow meter to measure and regulate the purge flow rate.
5. The purge entry point should be as close as possible to the measurement source to minimize pressure drop in the line.
6. To reduce measurement errors, the purge flow must remain constant. Typical values include:
- a. Fluidized bed: 0.85–3.4 nm³/h (for air or other gases);
- b. Low-pressure tank level: 0.03–0.045 nm³/h (for air or other gases);
- c. General flow: 0.03–0.14 nm³/h (gas) or 0.014–0.036 m³/h (liquid).
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