A process trip occurs when the safety instrumented system (SIS) places the method in its safe state by commanding an automatic valve (Figure 1) or different equipment to its journey state in response to an irregular process condition. In เครื่องมือที่ใช้วัดความดันเลือด , a spurious journey occurs because of a valve malfunction. In that case, the valve meeting is examined underneath actual operating conditions, which supplies a chance 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 talk about how DVCs might help determine the proof take a look at credit for an automatic valve after a course of trip.
Process journey
A course of trip happens when the SIS detects an abnormal course of situation by way of sensors similar to temperature and stress, executes the logic and locations the method in its safe state by tripping the final parts similar to closing an automatic valve, stopping a pump, and so on. The SIS could talk 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 primary objective is usually to restart the unit or gear that has been shut down as quickly as possible. Plant engineers are busy troubleshooting and correcting problems. Maintenance technicians are busy repairing broken units. Taking the chance to proof check an automated valve won’t be a top precedence and even an activity under consideration as a end result of tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is equipped with a DVC, the proof take a look at of the valve can be considered performed with diagnostic information captured by the DVC. Also, a comparison of the diagnostic information with the valve’s baseline might help uncover valve deficiencies and degradations that might not show up in a proof check.
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 most of the 12 requirements of a proof check.
Process journeys versus proof exams
How can proof check credit be claimed for an automatic valve that has failed and caused a course of trip? A proof check is a periodic take a look at carried out on SIS sensors, logic solvers and final parts — such as automated valves — to find hidden failures. These failures, if not corrected or eliminated, could impair the ability of the SIS to take the process to its protected state when an abnormal course of condition is detected.
A proof check ought to be performed as per the proof take a look at interval specified in the Safety Requirement Specifications. This interval is normally determined by way of an average probability of failure on demand calculation (PFDavg). Some customers might choose to proof check based on predetermined intervals, similar to proof testing sensors every 24 months and last components every 48 months as a substitute of relying on the PFDavg calculations.
Automated valve proof checks could be done offline or on-line. Offline proof checks are normally scheduled throughout a turnaround, when the method is completely shut down.
Proof testing an automated valve on-line often requires a unit or tools to be shut down, or the valve to be bypassed to stop a spurious journey, making it inconvenient and costly.
But a proof check may also be completed throughout a course of journey.
According to IEC61511-2 Section A.sixteen.three.1.three, “…shutdowns due to actual demand on the SIS throughout operation may be given credit as proof tests (fully or partial) under given conditions…the next planned proof test may be skipped.”
These conditions are
The shutdown paperwork equal info as registered throughout corresponding proof take a look at.
The shutdown covers all elements of the SIS, and if not, the system or SIS subsystem not activated needs to be tested individually.
The shutdown happens within a predetermined most time window earlier than the subsequent planned proof check which may then be canceled
When a course of trip happens, about 60% of a Safety Instrumented Function (SIF) proof take a look at may be considered carried out. A sample listing of activities carried out throughout a proof test, along with these that are performed during a course of journey, is proven in Figure 2. Even with out an automated valve leak check, knowledge captured by the DVC alone can probably account for a great amount of proof take a look at coverage for an automatic valve.
The precise protection is decided by the design of the automated valve, its DVC and its software. The protection is estimated based on the valve degradations, the likelihood of their prevalence and the percentage of these degradations that can be detected by a DVC. Fortunately, data acquired by a DVC during a course of journey can usually be sufficient to satisfy a serious a part of the proof test necessities.
If the method journey takes place inside a predetermined maximum time window, the end consumer may select to leverage the process trip as a proof check by finishing steps one by way of 5 in Figure 2, which are often not completed in a course of journey. The subsequent scheduled proof take a look at can then be canceled. According to the Norwegian Oil Industry Association, the maximum time window ought to be the final half of the present proof check interval.
Figure three. Data throughout a process trip is captured by the Fisher DVC6200 SIS digital valve controller and made available for analysis.
Common causes of valve failure
According to ISA Technical Report ninety six.05.01, the most probably root causes of automated valve degradations — which might lead to a process trip — include:
Partial or intermittent sticking of hydraulic or pneumatic system components similar to solenoids, pilots, pace controller, and so on., due to moisture, particles or alignment points. This causes a loss of useful margin and makes the valve sluggish to open or shut.
Binding, galling or different degradation of valve seats or associated circulate control trim that restricts or resists valve movement.
Actuator seal degradation caused by compression, put on or looseness that reduces the stress obtainable to actuate the valve.
Minor harm to the valve obturator plug, disk or ball caused by system conditions, leakage or particles, including build-up of hydrocarbon products. This causes leakage.
Complete failure of hydraulic management system parts such as solenoids, pilots, pace controller, and so on., because of moisture, particles or alignment. The valve will fail to open or close.
All of those situations could be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For instance, a valve that’s caught open will exhibit itself as a rise in breakout force. Loss of seat load compared to when the valve assembly was new signifies potential valve leak. The Fisher DVC6200SIS additionally repeatedly screens for inner faults as well as its inputs corresponding to provide strain. When the DVC6200SIS sees the provision stress is merely too low, an alert is generated (Figure 4) so that the top user can take corrective actions.
Figure 4. The DVC6200SIS digital valve controller detected nearly zero supply pressure, so it alerted the management system.
Other process trip advantages
Diagnostic data captured during a process journey may reveal valve degradations that may not be detected throughout a proof check. For example, diagnostic information captured throughout a process trip would possibly point out a problem with the valve closing fully towards the total stress of the process, which may be an early indication of valve leakage.
The valve stroke time computed by a DVC during a process trip is extra correct underneath actual working circumstances. This ends in a extra correct SIF response time (the time from detection of an abnormal condition to ultimate component reaching its journey state), which is compared to the Process Safety Time to ensure the SIF is still meeting its design necessities.
Although automated valves are very dependable, failures identified in a process trip can provide priceless data to forestall future failures. This data can help with turnaround planning by ensuring the needed elements can be found before turnaround even begins to probably shorten the turnaround schedule.
Summary
A course of journey can present the protection required to delay a valve’s subsequent scheduled proof test, thus saving money and time. Data captured by a DVC can be analyzed to leverage the method journey as a proof check. Even if the top consumer chooses to not take proof test credit for a course of journey, the valve diagnostic information supplied by the DVC can help plant personnel make proactive valve maintenance decisions..
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