Test smart process control valves in minutes
Differentiating between misbehaving process control valves and good ones can be a confusing, time- and labor-intensive task. Most proportional smart control valves have simple manual indicators for the percent of travel for a specific valve setting. That provides some performance information, but it isn’t the most accurate reading, and it doesn’t indicate how the valve performs under dynamic conditions. If the indicator is off even one or two percentage points, it can make a big difference in critical processes.
A more accurate valve performance picture often requires pulling the valve from the process and testing it on a valve prover or with specialty test instruments. Valve provers carry a hefty price tag, and specialty test instruments can be difficult for less skilled technicians to use.
Time is a costly factor
Given all that, time becomes the real cost issue. Removing a valve from the process can take about an hour; setting it up on the prover and testing adds another hour; and reinstalling the valve takes another hour. And that’s one valve. In continuous processes, when a valve is removed with no installed process bypass, the process must be taken down.
The relevant tests typically are relegated to times when the plant is in an outage, or in the case of batch processing, in between batches. Typically, technicians need to test anywhere from tens to hundreds of valves in an outage, so time adds up fast. So does the cost of extended downtime and lost production.
The reality is only a fraction of these valves requires removal and full testing. It’s difficult, however, to determine which fraction needs attention.
An automated loop calibration tool now available can quickly screen valves and separate the good ones from those that require more testing and should be pulled.
Using this handheld loop valve tester, it takes an average of only five minutes for a technician to assess the performance of a proportional smart valve (see Figure 1). Users can run several automated valve performance tests, including a signature test, speed test, step test, and bump/partial stroke test, all in less than half an hour. However, only one or two tests are typically required for screening, so even less time is required.
To set up the test tool, the technician removes control wires and connects the loop calibrator test leads to the valve. Once the tester is connected and HART communication is established, the technician can select the most common performance test, which is the signature test. The tool then ramps the signal from 4 to 20 mA and back to 4 mA. The tool automatically records the pressure applied to the final control element and the feedback from the valve’s position to determine the valve’s movement and position in response to that pressure. Ideally, a good valve exhibits smooth linear changes of pressure during both ascending and descending movement of the valve.
If the results show spikes, that means the valve is sticking. At the end of the roughly five-minute test, the backlit screen of the valve tester displays the results ─ good, marginal or bad. The full results are also recorded to memory and can be uploaded to application software for further review and sent to supervisors for help in determining whether the valve should be pulled.
The quick-check “go/no-go” testing expedites the ability to quickly identify good valves that don’t need further attention, and bad valves that need to be pulled for further testing or immediate replacement. The additional data helps guide decisions on which marginal valves to pull for further testing and which to keep an eye on. It also provides baseline performance data for comparison use in subsequent tests.
The tool makes it practical to record baseline measurements for each valve when it is installed or after a repair. That way, technicians can quickly identify a sudden change in performance that may point to a cause beyond just the valve.