A course of trip occurs when the safety instrumented system (SIS) places the method in its protected state by commanding an automatic valve (Figure 1) or other equipment to its trip state in response to an abnormal process situation. In some instances, a spurious journey happens due to a valve malfunction. In that case, the valve meeting is tested beneath real working circumstances, which offers an opportunity to capture priceless valve diagnostic data.
However, such diagnostics information can solely be captured when automated valves are fitted with digital valve controllers (DVCs). In this article, we’ll focus on how DVCs may help decide the proof test credit score for an automatic valve after a process journey.
Process journey
A course of trip occurs when the SIS detects an irregular course of condition through sensors corresponding to temperature and strain, executes the logic and locations the process in its secure state by tripping the final elements such as closing an automated valve, stopping a pump, and so forth. The SIS may communicate with the Basic Process Control System (BPCS) to close a valve downstream of the automated valve as a secondary motion.
When a process trip happens, the principle aim is usually to restart the unit or gear that has been shut down as soon as possible. Plant engineers are busy troubleshooting and correcting problems. Maintenance technicians are busy repairing broken devices. Taking the opportunity to proof take a look at an automatic valve will not be a top priority or even an activity into account as a result of tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is provided with a DVC, the proof test of the valve may be considered carried out with diagnostic data captured by the DVC. Also, a comparison of the diagnostic data with the valve’s baseline might help uncover valve deficiencies and degradations that will not show up in a proof take a look at.
Figure 2. Proof Test vs Process Trip. Activities performed (such as repairing the device) and knowledge captured by a Fisher DVC6200 SIS digital valve controller during a course of trip meet many of the 12 requirements of a proof check.
Process journeys versus proof tests
How can proof test credits be claimed for an automated valve that has failed and triggered a process trip? A proof take a look at is a periodic test carried out on SIS sensors, logic solvers and ultimate elements — similar to automated valves — to discover hidden failures. These failures, if not corrected or eliminated, might impair the flexibility of the SIS to take the process to its protected state when an irregular process condition is detected.
A proof test must be carried out as per the proof take a look at interval specified in the Safety Requirement Specifications. This interval is often determined through a median probability of failure on demand calculation (PFDavg). Some users could select to proof take a look at based mostly on predetermined intervals, corresponding to proof testing sensors each 24 months and final parts every 48 months instead of relying on the PFDavg calculations.
Automated valve proof exams may be accomplished offline or online. Offline proof checks are often scheduled throughout a turnaround, when the method is completely shut down.
Proof testing an automatic valve online often requires a unit or tools to be shut down, or the valve to be bypassed to stop a spurious trip, making it inconvenient and costly.
But a proof check may additionally be achieved throughout a process trip.
According to IEC61511-2 Section A.sixteen.three.1.three, “…shutdowns as a end result of precise demand on the SIS throughout operation could also be given credit score as proof exams (fully or partial) beneath given conditions…the next deliberate proof test could additionally be skipped.”
These circumstances are
The shutdown documents equivalent info as registered throughout corresponding proof take a look at.
The shutdown covers all parts of the SIS, and if not, the gadget or SIS subsystem not activated needs to be examined individually.
The shutdown happens within a predetermined most time window before the subsequent planned proof check which might then be canceled
When a process trip occurs, about 60% of a Safety Instrumented Function (SIF) proof test may be considered performed. A pattern record of actions carried out throughout a proof check, along with these which are performed throughout a course of trip, is proven in Figure 2. Even without an automatic valve leak take a look at, knowledge captured by the DVC alone can probably account for a good quantity of proof test coverage for an automatic valve.
The exact protection depends on the design of the automated valve, its DVC and its software. The coverage is estimated based on the valve degradations, the probability of their incidence and the proportion of these degradations that can be detected by a DVC. Fortunately, data acquired by a DVC during a course of trip can typically be enough to satisfy a major a part of the proof check requirements.
If the process trip takes place within a predetermined maximum time window, the top user may select to leverage the process trip as a proof check by completing steps one by way of five in Figure 2, that are often not accomplished in a course of trip. The subsequent scheduled proof check can then be canceled. According to the Norwegian Oil Industry Association, the maximum time window should be the final half of the current proof check interval.
Figure three. Data during a course of journey is captured by the Fisher DVC6200 SIS digital valve controller and made obtainable for analysis.
Common causes of valve failure
According to ISA Technical Report ninety six.05.01, the most likely root causes of automated valve degradations — which can result in a course of trip — include:
Partial or intermittent sticking of hydraulic or pneumatic system components corresponding to solenoids, pilots, pace controller, and so on., because of moisture, particles or alignment issues. This causes a lack of practical margin and makes the valve slow to open or shut.
Binding, galling or other degradation of valve seats or associated flow management trim that restricts or resists valve movement.
Actuator seal degradation attributable to compression, put on or looseness that reduces the pressure obtainable to actuate the valve.
เครื่องมือใช้วัดความดัน to the valve obturator plug, disk or ball caused by system situations, leakage or particles, including build-up of hydrocarbon merchandise. This causes leakage.
Complete failure of hydraulic control system components corresponding to solenoids, pilots, velocity controller, etc., as a end result of moisture, particles or alignment. The valve will fail to open or close.
All of those conditions may be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For example, a valve that is caught open will exhibit itself as a rise in breakout drive. Loss of seat load compared to when the valve meeting was new signifies potential valve leak. The Fisher DVC6200SIS additionally continuously screens for inner faults as well as its inputs similar to supply pressure. When the DVC6200SIS sees the availability pressure is just too low, an alert is generated (Figure 4) in order that the end consumer can take corrective actions.
Figure 4. The DVC6200SIS digital valve controller detected nearly zero provide stress, so it alerted the control system.
Other process journey advantages
Diagnostic information captured throughout a course of journey may reveal valve degradations that is probably not detected throughout a proof test. For instance, diagnostic information captured during a process trip may point out an issue with the valve closing completely towards the total stress of the method, which may be an early indication of valve leakage.
The valve stroke time computed by a DVC during a process trip is extra accurate beneath real working conditions. This results in a extra accurate SIF response time (the time from detection of an abnormal condition to final component reaching its trip state), which is compared to the Process Safety Time to ensure the SIF continues to be meeting its design requirements.
Although automated valves are very dependable, failures recognized in a process journey can provide useful knowledge to prevent future failures. This information can assist with turnaround planning by ensuring the needed components can be found earlier than turnaround even begins to doubtlessly shorten the turnaround schedule.
Summary
A process trip can provide the protection required to delay a valve’s next scheduled proof test, thus saving money and time. Data captured by a DVC can be analyzed to leverage the method journey as a proof take a look at. Even if pressure gauge น้ำมัน chooses not to take proof check credit for a process journey, the valve diagnostic information offered by the DVC can help plant personnel make proactive valve upkeep choices..
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