Process Control Loops

Process control loops have three primary groups; the controller, the feedback, and the control valve. Historically, control valves have been the most troublesome within the three groups because of variability within each valve due to its continually moving components which increase wear. All devices within the loop, including control valves, must be evaluated or calibrated regularly to ensure compliance and provide traceability which is where valve diagnostics have come into play.

At SofTek Engineering, we refer to the NIST definition when we refer to traceability. “Traceability of measurement requires the establishment of an unbroken chain of comparisons to stated references, each with a stated uncertainty.”

In every process control loop, the 4-20 mA signal to the control valve is calibrated and traceable, and feedback from the flowmeter has been calibrated and is traceable so that the loop can be closed; however, the control valve has never been. We are leaving plants with a third of their system un-calibrated and untraceable.

Why? The industry mindset for years has been that a control valve is not a traceable device because this would have been extremely difficult to do in the past. Manufacturers of digital positioners have tried to make valves traceable by including pressure and position feedback within the positioner, which has created another issue because the sensors within the positioner now need to be calibrated to ensure accuracy and provide traceability. Furthering the need for a portable diagnostic control valve test system becomes crucial.

Digital Positioners

Plants continually push the engineered limits of control valves, trying to get more out of processes; because of this, digital positioners became popular. Digital positioners use inexpensive, mass-produced sensors on a 12 or 14-bit device to keep costs as low as possible. Positioners have become instruments, providing information to the control room.

Digital positioners now fall within the instrumentation criteria and must be calibrated annually to ensure they send accurate information. Positioners have three components that require annual calibration, the 4-20 mA input to the positioner, the pressure sensors, and the actual stroke length or displacement of the valve (inches, mm or degrees).

Once a digital positioner is installed on a control valve, the automatic calibration routine will start. At this time, the valve will stoke up and down, completing the “calibration” of the new Positioner; however, it can only detect endpoints (saturation or positioner stops), creating several possible problems.

Because the feedback from a digital Positioner is in percent, no actual displacement is measured, causing the positioner to possibly over stroke, creating a dead flow or Understroke – not achieving the valve’s full flow or Cv.

Digital Positioners gather information from the process control loops and, when analyzed, can be used to improve the loop by showing maintenance and safety concerns.

A portable diagnostic system is needed to determine if Positioners and valves are functioning within their engineered parameters to ensure plant safety and increase output. A diagnostic test system is vital in implementing predictive and preventative maintenance programs, but only if the data is from a traceable source.

Predictive and Preventative Maintenance

Once a valve is tested and verified, it becomes traceable. If future problems arise, processing plants now have a record of its previous tests, calibrations, and repairs. Routine testing and maintenance help customers increase their valve performance and valve lifespan, which will reduce issues between planned plant shutdowns.

Accurate performance testing that corresponds with flow meter feedback can only occur when attaching to the stem (linear) or shaft (rotary). With this test methodology, we can now see changes in flow, if the stem or shaft is moving, the flow is changing, which is where profits are made or lost — establishing the difference in “Positioner Diagnostics” (from Positioner feedback) and “Complete Valve Diagnostics” (feedback from stem and shaft).

There are three types of testing:

Profile Test

Confirms the overall health of the valve (friction, loading, calibration, dynamic error, bench set, stroke length).

Sensitivity and Resolution

Simulates how the valve will control in the process (small (%) stair stepping and reverse direction). The smaller the step made, the tighter the process control will be.

Stroke Speed

Verifies how quickly a valve can open or close, preventing trip conditions within loops that could cause a safety issue and possible plant shutdown.

When these three tests are routinely run, they provide detailed information to help determine when and what to repair, which is very cost-effective in keeping plants running longer between scheduled shutdowns.

Only when we can verify that control valves and Positioner’s are working correctly, calibrated accurately, set up, and installed properly can we genuinely say that we can implement predictive and preventative maintenance into a facility.

Control Valve

Portable control valve devices can calibrate the Positioner and test the control valve making it a traceable device. By providing a traceable signal to the valve, we can verify that it performs to its original design specifications. More often than not, what a valve was designed to do 10 or 20 years ago differs from what it’s being tasked to do now. Pressures and flow have changed because plants are trying to get more out of their processes, pushing actuators to their limits. Control valves should be tested regularly to ensure they’re working correctly.

Until plants implement the testing of all control valves, ensuring that they’re correctly calibrated (traceable), they are blindly searching for process solutions.

Using a portable control valve diagnostic device to verify all plant control valves (linear, rotary, analog, digital, and on/off solenoid valves) ultimately reduces downtime and expensive, most often unnecessary repairs. Valve diagnostics also provide plants with a tool to help them determine the proper valves to put into specific control loop situations – verifying that loops are safe, accurate, and alleviating trip conditions.

By using portable control valve diagnostic device, we established the unbroken chain of comparisons, providing performance testing by attaching to the positioner stem (linear) or positioner shaft (rotary) which is the difference between “Positioner diagnostics” and “Complete Valve Diagnostics” — saving customers valuable resources, reducing time and lost revenue.