May 27, 2025
Calibration of direct travel electric control valves is a critical operation to ensure the accuracy and stability of valve control. The following is a concise and clear calibration procedure for quick on-site operation:
I. Preparation before calibration
1. safety measures:
- Disconnect the power and gas source, relieve the pipeline pressure, and ensure the isolation of the valve before and after.
- Set up warning signs to prevent misuse.
2. Valve inspection:
- Manually open and close the valve to make sure there is no jamming, noise or leakage.
- Clean the valve stem to ensure that no impurities impede travel.
II. Zero calibration (4mA corresponds to 0% opening)
1. Connect the signal generator and input 4mA current signal. 2.
2. Observe the valve status:
- The valve should be in the fully closed position (or the lowest travel starting point).
- Measure the actual displacement of the valve stem with a meter to confirm whether it is “0 position”.
III. Full scale calibration (20mA corresponds to 100% openness)
1. Input 20mA current signal. 2.
2. Observe the valve status:
- The valve should be fully open (or reach the maximum travel).
- Measure whether the valve stem displacement is consistent with the nominal stroke (e.g. 50mm stroke should be measured 50mm).
IV. Verification of linearity (mid-point test)
1. Mid-point test:
- Input 8mA (25%), 12mA (50%), 16mA (75%) signal.
- Record the actual stroke of the valve stem and calculate the deviation (ideal value = nominal stroke x signal percentage).
2. Allowable deviation:
- General requirements for deviation ≤ ± 1%, otherwise need to recalibrate the zero point / full scale.
- Intelligent valve: Enable “linear correction” function to compensate automatically.
V. Deadband Adjustment (Sensitivity) Deadband adjustment (sensitivity calibration)
1. Test Method:
- Slowly increase the signal from 4mA until the valve starts to operate (e.g. 4.2mA), record the threshold value.
- Reverse decreasing the signal until the valve starts to move back (e.g. 19.8mA), calculate the deadband range.
2. Adjustment requirements:
- The deadband should normally be <1% (approx. 0.16mA), too much may cause control hysteresis.
- Adjustment can be made through the “Sensitivity” parameter of the positioner or through lubrication of mechanical components.
VI. Travel time test (dynamic performance)
1. Full travel time:
- Input 4mA→20mA step signal, use stopwatch to record the time from closing to opening of the valve.
- Compare with the manufacturer's nominal value (e.g. 30 seconds), deviation more than ±10% need to check the motor or gear box.
2. Return difference test:
- Measure the 50% open position difference between upstream (4→20mA) and downstream (20→4mA) respectively, it should be <2%.
VII. Post-calibration confirmation
1. Dynamic test:
- Input continuously changing signal (e.g. 0-100% triangle wave), observe whether the valve stem is smooth and without shaking.
2. Sealing test (critical occasions):
- Close the valve, inject the medium and test whether the leakage is in accordance with the standard (e.g. ANSI/FCI 70-2 Class IV).
VIII. Quick check of common problems
- Zero point drift: Check whether the feedback potentiometer or position sensor is loose.
- Valve stuck: Clean valve stem and apply high temperature grease.
- Signal not responding: check terminals for oxidation or poor contact.
IX. Precautions
- Calibration needs to be performed at room temperature and under no load condition of the valve.
- Intelligent valves (e.g. HART protocol) are prioritized to be calibrated with special software.
- After the calibration is completed, the calibration date and personnel need to be marked on the valve nameplate.
Follow the above steps to efficiently complete the calibration of direct travel electric control valves. In case of complex faults (e.g. damaged positioner module), it is recommended to contact the manufacturer's technical support.
