Industrial plants are tasked with reducing costs and increasing production. To achieve this, they seek plant efficiencies anywhere they can, while increasingly facing reductions in their experienced workforce. The situation forces them to rely on their vendors for know-how in reaching their goals.
In this scenario, the users that will thrive are those choosing equipment and vendors that offer value. These users quickly realize achieving their goals is not just about cutting costs—it’s about maximizing operational efficiencies.
One of the ways that companies today can find value from valves and actuators is to seek good diagnostics capabilities from positioners.
Historically, valve diagnostics were provided as a service through vendors that used special tools and equipment designed to accurately assess valve performance. This service could take days and more often weeks to perform because the valve data had to be interpreted and processed. With the introduction of digital valve positioners, however, users now have the capability of monitoring valve performance themselves and relaying current status of equipment via communication protocols or alarm cards.
Two key ways valve diagnostics improve plant efficiency and processes are through valve performance tests and partial stroke tests.
VALVE PERFORMANCE TESTING
To improve valve efficiency as well as the overall process operation, a key valve positioner diagnostic feature is the ability to “see into the valve” to monitor its performance over time. In general, this is known as a valve performance test.
The test provides a reading of a baseline view or a “signature” of a valve’s performance in real time as it operates. The user can then compare the baseline against other baseline readings over a period of time,―for example, over the course of a week, a month or a year. The following data points are some of the critical ones to assess1: hysteresis, non-linearity and non-repeatability. Also valuable are maximum measured error and inaccuracy.
Hysteresis: To determine valve hysteresis, the valve is provided a specific input signal three consecutive times. The valve positioner will monitor the difference in valve position each time. This test shows the difference between the up-stroke and down-stroke over the course of those three test cycles.
Non-linearity: Linearity is a measure of how close to a straight line the valve travels measured against the input signal that is provided. Non-linearity is measured from a curve plotted using the overall average of upstroke and downstroke errors. The non-linearity is the maximum positive or negative deviation between the average curve and the selected straight line. This is independent of dead-band and hysteresis.
Non-repeatability: Repeatability is the measurement of a valve’s ability to achieve identical results across multiple tests. Non-repeatability will measure the difference in the position of the valve (the output) when receiving the same input signal. This test is done consecutively so the measurements fall under the same operating conditions, and those conditions are approached from the same direction. The results are typically expressed in percentage of ideal output span, not including hysteresis.
Maximum Measured Error: This measurement is fairly simple: As the valve performance test runs, a list of average upstroke and downstroke percentages at different inputs is recorded. To determine what the maximum measured error was during the test, the diagnostic selects the greatest positive or negative value.
Inaccuracy: Inaccuracy is determined by selecting the greatest positive and negative deviations from any of the measured values and reporting it in percentage of ideal output span. In other words, the deviation is any measured value different from the ideal value for increasing and decreasing inputs on any test cycle
After running a valve performance test, a baseline view of the valve is attained. Monitoring the valve’s performance, then, is as simple as running the tests again and comparing the new tests with the baselines. Such features are available automatically within select digital valve positioners.
PARTIAL STROKE TESTING
Emergency shutdown valves are integral to plants across all industries. They are engineered to operate and reduce the impact of failure/emergency situations. In the event of an emergency, the failure of these valves to operate correctly carries huge financial and logistical consequences. To ensure proper emergency valve operation, partial stroke tests are conducted.
How this test works
This test is designed to ensure proper valve stem movement in the event of failure. The valve positioner introduces a small valve stroke and relays to the user the time it takes to stroke the valve. Much like a valve performance test, the user runs the partial stroke test to generate a baseline view of the valve. This baseline view can be used to compare with results from the same test at a later date.
This test can be programmed to run daily, weekly, monthly, yearly or any combination. The positioner also has the ability to generate an alarm if the test fails or exceeds given thresholds.
Valve performance and partial stroke tests are just two key diagnostic features industrial plants can use to increase valve performance and maximize operation efficiencies. In today’s plants, these tests can be performed easily via some of the digital valve positioners offered today. As a result, these positioners add value to valve and actuator products.
1. Citation: IEC 61298-2