What is the best way to test control valves?

Proportional smart control valves are essential in process industries. Accurately evaluating their performance can be tedious and often requires significant downtime, as well as removing the valves from the line.

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Control valves operate by adjusting their position based on a varying 4 to 20 mA input signal. Many of these valves also feature a feedback output signal that indicates their actual position as a percentage of being fully open or closed. This output may take the form of a 4 to 20 mA signal or a digital HART variable representing 0 to 100 percent of the control valve's operating range.

Another important aspect of a valve's performance is the pressure required to move it to the designated position. For example, a valve programmed to recognize that a 12 mA signal corresponds to a 50 percent open position will utilize smart electronics to manage a pressure regulator, adjusting the pressure as necessary to position the control element correctly.

By applying a fluctuating mA signal while simultaneously monitoring the output milliamp or travel percentage signal, you can determine whether the control valve functions correctly across its entire range. Vital tests include tracking and recording the pressure applied to the final control element while altering the input 4 to 20 mA signal to gauge the valve's performance, including detecting any stiction that might be present. If the measurements indicate that extra pressure is necessary due to a potential sticking issue, this can be illustrated if the data is logged and expressed visually. Such recorded signals create a performance profile commonly referred to as a valve's "signature." Traditionally, valves possess a simple manual indicator that offers a rough estimate of the percentage of travel during operation; however, this indicator may not accurately reflect performance under dynamic conditions.

Valves: A Quick Overview of Manual Indicators

The manual indicator on the side of a valve may display that it's at 50 percent. However, how do you know it's not actually 51 percent or 49 percent? Many processes require more precision in this regard. The most advanced performance tests necessitate the removal of the valve and testing it on a "valve prover." Unfortunately, this sophisticated testing equipment can be prohibitively expensive for many facilities. Other testing options exist using specialized HART instruments, but their setup and use may prove challenging.

According to a process instrumentation educator, a technician can accurately test roughly eight valves in a full day, roughly one per hour. This pace includes getting assistance to detach the valve, transport it to a testing bench, and perform the necessary evaluations, which sums to approximately two man-hours per valve.

New Innovations: Automated Testing Solutions

Fluke has recently introduced an innovative loop calibration tool, designed to swiftly assess valve performance. This tool aims to save time and minimize unnecessary removal of functional control valves. The Fluke 710H Valve Tester represents the forefront of tools that simplify expert workflows for technicians, catering to varying skill levels while delivering the expected capabilities inherent in process loop calibration tools.

The Fluke 710 features several fully automated tests, including the Signature test, which efficiently evaluates proportional smart valve performance. In many instances, this test can be executed in under five minutes, providing an easily interpretable status of the valve as good, marginal, or bad.

Technicians with lesser experience can use the 710 to assess valve conditions and determine if a specialist needs to be called in. It eliminates the need for valve disassembly, akin to the labor involved in removing a motor. After disconnecting control wires and attaching the 710 test leads, the device automatically ramps the milliamp signal from 4 to 20 and records the valve's feedback to confirm whether it moves as intended. Furthermore, it documents the pressure applied to the valve's control element across the mA input range, seeking smooth linear alterations in pressure both ascending and descending.

Assessing Valve Performance: Categorizing Status

By creating a pressure profile that moves the valve, the technician can observe any physical anomalies in the pressure requirements for various positions, even if the valve's location may appear relatively accurate.

Upon completion of the test, you receive a detailed performance assessment categorized as bad, marginal, or good. The 710 device also saves the collected data, enabling upload to applicable software for further analysis.

Once technicians become proficient with the tool and its testing procedures, they can decide if a specialist is necessary.

Best Practices: Establishing a Baseline

A best practice for maintaining valves involves establishing baseline performance metrics while in prime condition. Given that most valves operate with a 4 to 20 mA input signal, a testing tool with mA output can effectively drive the control valve across its operational spectrum. The Fluke 710 is perfectly suited for this function.

Documenting such metrics ideally occurs when a valve is commissioned or following an overhaul. The technician should record the valve's "signature" when it is in optimum shape, plotting the output mA or span percentage versus the input signal. It’s essential to store this data systematically, including the timestamp, valve tag number, and the test date. Utilizing calibration management software can aid in organizing this information.

Once the baseline performance signatures are logged, you should set a maintenance testing interval, tailored to whether the application includes rough service or light duty. Some critical devices in safety and shut-down systems should undergo bi-monthly evaluations.

When to Remove a Valve

If performed data suggests that a valve exhibits a non-linear signature or has irregular curve patterns, this may indicate increasing levels of stiction or hysteresis, warranting removal for servicing. Additionally, a "marginal" or "bad" assessment from the 710 should prompt either trim adjustments on the valve controller or consultation with a specialist for more extensive off-process testing.

Typical Expected Operation of a Valve with Applied Milliamp Signal:

• 3.8 mA – valve should be hard closed
• 4.0 mA – valve should remain closed
• 4.2 mA – valve should start to lift slightly
• 12 mA – valve should ideally be at 50 percent
• 19.8 mA – valve should be close to fully open
• 20.0 mA – valve should be entirely open
• 20.2 mA – valve should be hard open (resting on the stop)

Concluding Thoughts

In conclusion, thorough control valve testing and inspection are vital for maintaining optimal performance and safety across industrial process systems. Regular assessments can identify potential issues early, avert accidents, enhance efficiency, and ensure compliance with industry regulations. By investing in professional testing and inspection services, companies can reduce maintenance expenses, prolong the lifespan of control valves, and mitigate production downtime risks. Overall, prioritizing control valve testing and inspection is crucial to the sustained success of any operation.

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