Stroke checking of control valve

What is Stroke Checking of Control Valves?

All control valves need to be stroked after some time in service. This is due to the fact that the valve is a mechanical mechanism that has physical components. These components must constantly be in excellent functioning condition in order to prevent the valve from failing.

Stroking the valve is the term used to describe the process of determining whether or not a control valve is functioning properly.

With the use of this exercise, the valve’s parts may be tested for functionality, allowing for quick identification and prompt correction of any issues.

The frequency of proof tests is mandated by SIS requirements for the final control element of a safety system. During the stroke checking, a part of the valve’s full range of motion is used to make sure it isn’t stuck and to find other dangerous problems. It tests the control valve to make sure that it will work and move to its safe position if safety appears to require it.

Purpose and scope:

This procedure provides stroke check instructions for control valves calibrated with standard reference weights in the workshop.

Tools required for stroke checking:

• Necessary hand tools.
• Multimeter.
• Hart communicator of handheld configurable communicator if it is an electronic smart positioner.
• Loop calibrator / multipurpose calibrator for feeding 4 to 20 mA signal.
• Soft Cloth for cleaning.

Safety 

• For basic safety and general rules, as well as information on calibrating operations in process industries, please refer to the link provided below.

Basic Safety and General Consideration While Executing Calibration Process in process industries

• Request that the panel operator set the controller to manual mode for the control loop and MOS for the ESD loop.
• Verify the isolation of the control valve from the process.

Control valve Stroke Checking Setup

• Assume that the valve is a pneumatic one that is actuated by 4-20mA. 
• Additionally, it features a valve position feedback output that is a 4-20mA signal.
• Connect the control valve as well as all of its components according to the connection diagram.

Control valve Stroke Checking Procedure on field

• Ensure that the air supply pressure can be reached by the Regulator at all times.

• Adjust the regulator until the pressure meets the specifications shown in the datasheet for the Valve positioner.
• Remove and insulate the analog signal output wire and analog signal input from the control terminal box.

• Connect the loop calibrator (mA source) to the control valve terminal.
• Connect the multimeter to the control valve’s output line for valve position feedback in the mA mode.
• Select the mode in the Loop Calibrator (mA source mode) such that it supplies the positioner with the current input signal (4-20mA).
• Set the calibrator at 4mA. The control valve will indicate that it is in the 0% position.  May use the scale on the control valve to check the valve’s position.

• The valve also includes a 4 to 20 mA output that shows valve position feedback.
• In order to determine this valve position feedback, we attached a multimeter.
• The valve position feedback range is 0 to 100%, which corresponds to 4 to 20mA. The multimeter in this situation will display 4mA.
• Examine the movement of the stem. It should be smooth and has to be in the minimum position.
• Carefully release the locknut for zero and span adjustment if there is any error.

• Then zero calibrating screw in the positioner needs to be adjusted.

• Now raise the current (mA) in the calibrator all the way up to 20mA from its previous setting of 4mA.
• Check the position of the control valve stem. It should be at the 100% mark on the control valve scale. If there is any deviation from this, adjust the span screw on the positioner.

• Confirming the linearity of the control valve by applying 0%, 25%, 50%, 75%, and 100% of the 4 – 20mA signal and noting the corresponding stroke of the valve at each of these percentages.
• Now, we tested the control valve in an upward direction from 4 to 20 milliamps. Now repeat the procedure in reverse, decreasing in stages from 20 mA to 4 mA.
• The calibration locknut should be carefully tightened.
• Normalize the field wire connection of both analog input and output signal of control valve.

For control valve with SMART Positioner

• We are able to calibrate the SMART Control Valve using a HART communicator. 

• Provide the supply pressure in accordance with the valve’s nameplate specifications. 
• Connect the valve positioner and the HART communicator, then type 0% in the valve simulation command and watch the valve position.
• Enter 25%, 50%, 75%, and 100% in the HART communicator in a similar manner, then watch the valve stem travel position as needed.
• To confirm the valve travel position, look for a manual scale (0% to 100%) with a pointer on the valve.
• Verify the signal’s state and remove any simulations and forces that were applied before the release the control to operation.

Specific Errors Encountered during Stroke Checking

Hysteresis

• Hysteresis, as it relates to valves, is the difference between the position of the valve during the upstroke and the position during the down stroke for any given input signal.
• In order for it to be real hysteresis, the valve must always be in motion. The most frequent source of hysteresis in valves is a high level of static friction.

Deadband

• On the other side, a deadband is a period of complete immobility that often follows a valve direction shift.
• If you can visualize a pin and slotted linkage.
• The pin is forced against the side of the slot when the actuator moves in one direction, moving the valve.
• Prior to the valve changing direction when the actuator does, the pin must push against the opposing side of the slot.
• If there is a lot of play in the slot, there will be a time when the valve does not move.

Linearity

The linearity of a valve is proportional to the degree to which the graph of travel Vs signal is similar to a straight line.

• To maintain process control, the control valve must respond to commands in a linear way. 
• The controller output becomes disorganized as a result of the valve stem’s erratic movement.
• The potential reasons are listed below.

1. It is possible to remove or loosen the location transmitter link. It caused the positioner to get the incorrect signal.
2. This issue could be resolved by calibrating the positioner.
3. Check the I/P converter’s accuracy. Replace it even if the issue is not resolved.

 Recording Stroke checking

• Apply input corresponding to 0 %, 25 %, 50 %, 75 % and 100 % according to the control valve in the upscale and downscale direction from the test standards.
• If the result of the output value is not within an acceptable limit, then calibration must be performed. All the output values are within acceptable limits (+/- %), then further calibration is not required.
• Record the resultant output values in the as found / as left column of blank stroke check report.

Sample stroke check report

The image that follows demonstrates that the control valve’s sample report of calibration was performed using loop calibrator as the reference.

The following link provides access to a downloaded file of the excel template used in the preparation of the stroke check report for control valve.