How are motorized control valves calibrated?

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. 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. 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.