Traceability and testing of digital positioners (including the valve) is a key to process control accuracy, reliability and safety. By providing this traceability, costs are reduced and flow control systems are made safer.
Today, many devices are designed to provide the feedback needed to make the valve truly traceable, including portable control valve diagnostic tools.
PROCESS CONTROL LOOPS
Process control can be broken down into three groups: the controller, the feedback and the control valve. Until now, the control valve has always been the most troublesome of the three groups because of variability within each valve. This variability comes from continually moving components, which creates wear. But all devices within the loop, including the control valves, need to be evaluated or calibrated regularly to ensure compliance and provide traceability.
The National Institute of Standards and Technology (NIST) specifies that: “Traceability of measurement requires the establishment of an unbroken chain of comparisons to stated references, each with a stated uncertainty.”
In process control loops, the 4-20 mA signal to the control valve is calibrated and traceable and the feedback from the flowmeter has been calibrated and is traceable so the loop can be closed. However, the control valve has never been traced because the mindset for years has been that to do so would be extremely difficult. That means a third of the system is not being traced.
Manufacturers of digital positioners have tried to make valves traceable by including pressure and position feedback inside. However, this creates a problem because the sensors need to be calibrated to ensure accuracy and to provide the traceability. This is where the need for a portable diagnostic control valve test system becomes crucial.
Plants continually push the engineered limits of control valves, trying to get more out of processes. Digital positioners are designed to help with this process. These positioners use inexpensive, mass-produced sensors on a 12- or 14-bit device to keep costs as low as possible. The positioners have now become instruments with sensors inside that provide information back to the control room. Because they fall within the instrumentation mandates, they must be calibrated annually to ensure they are sending accurate information.
Positioners have three components that need annual calibration: the 4-20 mA input to the positioner, the pressure sensors and the actual stroke length or displacement of the valve (inches, millimeters or degrees).
Once a digital positioner is installed on a control valve, an automatic calibration routine begins. The valve strokes up and down, which completes the calibration; however, the digital positioner is only detecting endpoints (saturation or positioner stops), which creates several possible problems. Because the feedback from a digital positioner is in percentages, no actual displacement is measured in inches, millimeters or degrees. This causes the positioner to possibly overstroke, creating a dead flow or understroke and not achieving the full flow or Cv that the valve was designed to meet.
Digital positioners were designed to gather information from the process control loops, information that can then be analyzed to make improvements in the loop including maintenance requirements and safety protections. Portable diagnostic systems can determine that positioners and valves are functioning within engineered parameters. They can then communicate with all protocols. This improves processes, ensures safety and increases output while working towards implementing a predictive and preventative maintenance program. However, this is only possible if the data gathered 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 previous tests, calibrations and repairs. Routine testing and maintenance help end users increase valve performance and longevity and reduces issues between planned plant shutdowns.
True performance testing corresponding with flowmeter feedback can only occur when attaching a traceable transducer to the stem (linear) or shaft (rotary). Using such test methodology, changes in flow can be detected. If the stem or shaft are moving, the flow is changing, which is where profits are made or lost.
This shows the difference in positioner diagnostics (from positioner feedback) and complete valve diagnostics (from stem and shaft feedback).
Three types of testing are used:
- The profile test checks the overall health of the valve (friction, loading, calibration, dynamic error, bench set, stroke length)
- A test for sensitivity and resolution simulates how the valve will control the process (small stair stepping in percentage and reverse direction—the smaller the step the valve can make, the tighter the process control.)
- Stroke speed verifies how quickly a valve can open or close, preventing trip conditions within loops that could cause safety issues and possible plant shutdown.
When these tests are run on a scheduled time frame, they provide the detailed information for decision-making on when and what to repair. This creates a cost-effective way to keep plants running longer between scheduled shutdowns.
Only by verifying that control valves and positioners are working correctly, calibrating accurately and ensuring the right configuration can predictive and preventative maintenance be truly implemented into a facility.
Portable control valve devices can calibrate the entire positioner as well as test the control valve, which makes that valve a traceable device. By providing a traceable signal, the valve can be verified as performing according to its original design specifications. Yet all too often what a valve was designed to do 10 or 20 years ago is not what it’s being tasked to do in the present. Because plants are trying to get more out of their processes, pressures and flow have changed over the years, which means actuators are pushed to their limits. This is why control valves must be tested to ensure they’re working correctly.
Until plants implement testing of all control valves and ensuring they’re correctly calibrated and traceable, those plants may be blindly searching for process solutions. Using a portable control valve diagnostic device can verify all plant control valves (linear, rotary, analog, digital and on/off solenoid valves) are working the way they should, which reduces downtime and expense as well as assuring that only necessary repairs are made. Valve diagnostics provide plants with a tool for determining the proper valves to put into specific control loop situations, verifying that those loops are safe and accurate, and alleviating trip conditions.
By using portable control valve diagnostic devices, the unbroken chain of comparisons referenced by NIST is established, providing performance testing that is truly complete valve diagnostics instead of simply positioner diagnostics.