In an industrial park such as a chemical plant, oil refinery or deep sea oil rig. There is a safety equipment system designed to quickly offer a safe course if a hazardous condition certain to be detected.
Safety shutdown usually involves isolating the flow of process fluid. Usually done with an ESD (emergency shutdown) valve. If ESD is dangerous, when dangerous conditions require a shutdown. And ESD does not perform its function when needed, the worst possible result can be catastrophic.
ESD valves usually consist of a solenoid valve in an ESD design with a solenoid valve activated in an open position. During normal operation and with a solenoid valve moving to a closed position to initiate a safe closure of the liquid flow process. The solenoid valve can fail (can not be closed) due to different conditions. However, the failure mode contributes the most to the failure rate of the solenoid valve that is sticky or sticky.
Due to the fact that the solenoid valve, as part of the ESD, is often “rested”, ie static. During normal operation, the fact that it has been “stuck” in an unobservable position . In other words, dangerous failure is undetectable in normal operation.
Many studies and organizations have identified the inclination, ie static friction, in different ways. However, all agree that stiction is the act of “being stuck” by static friction, blocking a surface from moving with one. Furthermore, if the external force becomes larger than static friction. The tilt between the two surfaces will be overcome and the object will begin to move again.
In normal operation, the O-ring in the solenoid valve maintains a seal. Even when the shaft or propeller is moving. Since the O-ring is in direct contact with the valve chamber wall. When the piston / bolt starts to move, it first has to overcome the tension between the O-ring and the chamber wall.
Once moved, the plunger / spool must overcome sliding friction or oscillation. Between the O-ring and the walls. But because the frictional force is usually significantly lower than the nominal slope, this is usually not worrying.
According to the design, the magnetic force and spring force in the solenoid valve are sufficient to overcome nominal static friction. And depending on the design details of the solenoid valve, there is usually excess power to overcome the distance of 2 to 2 , 5 times. Once the stiction is corrected, the plunger / spool will continue to move.
However, a common problem and trouble can occur. When the valve rests for a time because the tension between the O ring and the valve sail increases with time from its nominal level until it reaches a maximum value.
It is possible that the increase in inclination could reach. Such that the forces generated by the solenoid coil and / or spring are no longer sufficient to overcome the acceleration. Therefore, when the valve is called closed, it can not do so.
However, if the stiction excess is overcome, the stiction returns to its original nominal level. Even though it begins to rise again when the valve stops moving.
Professional knowledge: Mechanical engineers and technicians often work with the familiar solenoid valve with a “stroke” experience, ie, moving through its range of motion, the valve has been stopped for a while. Long time they reported that it is not uncommon for sub-forces to be required to move the valve. After it has been static for a month or more.
In the O-ring manual of a manufacturer, note that the “initial friction coefficient” (stiction) increases. When the O-ring stops in the middle of a week and 1 month after that the plateau stiction.
A specific chart of “friction cracking” (stiction) compared to “cyclical delays” shows electrostatic forces at approximately 300 hours. The authors of this manual have stated: “The theory has been widely proposed. And accepted that the increase in friction on the stand is due to the O-ring rubber flowing into the grooves or surface abnormalities of the insemination. ”
ISO 13849: In the statement of reliability indicators and information on the use of solenoid-operated pneumatic valves for safety standards EN ISO 13849-1. The text is ” at least once a week or once changed to ensure the intended function. “The data supporting this claim was obtained by examining the cycle in which the solenoid valve was never resting for any significant period of time.
This statement recognizes that the results of periodic inspections are only valid if periodic valve movements are maintained. In the absence of such periodic motion, the failure rate derived from the results of the test cycle can not be considered valid. When applied to components in applications where the components. This spends considerable time on the rest.
In a quasi-quasi-test, the solenoid coil does not work for a short time to see whether the spring can fully decompress the bolt in one direction through its entire motion. Then recharge, the tube must move. Upon completion of the valve stroke check, the solenoid valve has been verified to be operational. Or a dangerous failure due to stiction has been detected.
While the valve valve stroke test will have a positive effect on the average probability of failure as required (PFDavg). Although if performed infrequently, ideally we would not simply want to detect it. These failures due to stiction but also to stop them. So check the stroke once a week often enough to ensure the accumulation of stiction will be interrupted before it exceeds the residual force of the available valve.
Unlike randomized failure actually represented by a continuous failure rate. Failure due to stiction is unlikely to occur very early in the interval between tests when stiction levels do not increase a lot and. A lot of probability occurs once the stiction level has risen beyond what the excess force of the valve can exceed. Therefore, while we know that, as a rule of thumb, the maximum degree of tilting is in the range of 275-300 hours. We also need to consider the rate of stiction generated at maximum value. Its to determine the most suitable interval for check valve valve stroke.
In some end user connections there is a belief that valve valve strokes can endanger plant closures due to wrong travel. And a better safety solution than providing a safety margin. Much higher (excessive force) to overcome the acceleration. However, those experienced in the solenoid design realized that a higher safety margin to overcome stiction actually resulted in a greater false pretreatment rate.
This happens because a higher safety margin to overcome the increased tension requires a larger spring. And in turn requires more energy to provide power to the coil. Increased energy demand leads to higher roll-up capacity, resulting in higher misalignment rates. Therefore, if we try to optimize the error rate while improving safety, the valve stroke check provides a good solution.
Based on information currently available consistently across multiple sources, we recommend:
Start the solenoid valve solenoid test on a weekly basis if the valve is stuck.
After a few months of testing. If the number of times the valve is stuck seems excessive, halve the inspection time.
Repeat the process until an optimum test interval is determined. So that test checks generally check the valve movement.
Even frequent irregularity in solenoid valve. Solenoid monitoring can have significant beneficial effects on improving reliability and safety performance. However, if a solenoid valve solenoid test at a frequency once a week is feasible in a given application. Its implementation may help with dangerous failures occurring in an ESD.