When evaluating control valve and its performance, you need to understand the different kinds of control valve, and what areas need to test.For no simulation control on/off valve, the test is very simple.The valve is open or closed?If you open and remain open?If it is closed, completely shut off?Mainly by observing test: to observe the valve, found during the cycle of cause and effect.
Control valve is a completely different "beast"
The proportional valve open and close, open depends on the percentage of span of 4-20 mA signal.Through the observation of the valve position, can be directly check the directions on the trip indicator, technicians or operators to know percentage of stroke in particular Settings - but does not know the valve under the condition of dynamic and changing working conditions.
Most complicated valve performance test requirements, the valve will be removed, and test "valve calibration is" on its performance.This is a very expensive test equipment, the vast majority of instrument department.Usually only use valve valve manufacturers in valve factory calibration instrument testing, or advanced field service engineers to use.This instrument offers a complete test, but does not actually for instrument department.
So, what technical personnel to use to test the control valve?
What a meaningful test can be used as a baseline?Since most valve USES 4 to 20 mA input signal, any testing tools with mA output signal can provide input signal for control valves, control its work within the scope of work.When applying 3.8 mA think normally closed valve when the input signal, the valve should be ended.Valve should remain closed when 4.0 mA, and slightly open when 4.2 mA.In the scope of work, on the other side of the 19.8 mA, the valve should be close to fully open.When 20.0 mA. The valve is fully open, and at the age of 20.2 mA forced open (stay in travel stop position).The nature of the test can determine whether the valve is open and shut properly, but still can't to control the valve throughout its range test
Many valves, including the so-called "smart" valve, have built-in components, output the actual location of the opening/closing percentage.The output can be 4 to 20 mA, or digital signalHART,Variable, on behalf of the control valve working range of 0% to 100%.By applying the mA signal of change, and monitor output mA or travel percentage signals at the same time, technical staff will be able to view the control valve throughout its range is working correctly.Through records of mA mA signal and output signal at the same time after the trip, the percentage of PV to document the performance of the valve.Documentation of the test and the result is often referred to as "characteristic" of the valve.The output should be smooth mapping of mA input signal.Any deviation, applying signals relative to the valve may be abnormal.
Maintenance policy can reduce
Fluke products manager Jim Shields said, to a greater degree to reduce control valve maintenance strategies from valves replacement for the production process, to a greater degree, reduce the risk of failure, so as to effectively reduce the cost.The following is his advice.To the practice of making such regulations, need the valve in a state of known good documentation baseline conditions.Ideally, as long as the valve after debugging or overhaul, it should be documented.Technical personnel records the valve in the ideal state of characteristic curve, drawing output mA or trip signal and the input signal, the relationship between percentage and store such information changes over time, mark the number of valves, as well as the event date.Can use the calibration management software to manage the information.
Recorded the baseline performance of the valve characteristic curve, the maintenance interval determined execution valve performance test.The existing maintenance interval as a starting point.If it had not been determined between maintenance to evaluate the use of the valve position.Relative to a lighter load, bad application maintenance interval should be shorter.Reasonable situation started with the interval of 6 months (unless the use of the valve position very hard).Some devices installed on the safety or shutdown system, need to check every 3 months, whatever its use position.
Recorded the baseline characteristic curve of the valve, the valve should be in accordance with the established intervals to test, and record characteristic curve.Characteristic curve can be compared with baseline characteristic curve to determine the performance change.If the output response curve is nonlinear characteristics, or abnormal curve, said the valve may be too viscous or hysteresis, needs to be removed for repair.
