A process journey occurs when the protection instrumented system (SIS) places the process in its secure state by commanding an automatic valve (Figure 1) or different tools to its journey state in response to an irregular course of situation. In some instances, a spurious trip happens because of a valve malfunction. In that case, the valve assembly is tested underneath actual working circumstances, which offers an opportunity to capture priceless valve diagnostic information.
However, such diagnostics knowledge can only be captured when automated valves are fitted with digital valve controllers (DVCs). In this article, we’ll discuss how DVCs might help decide the proof take a look at credit score for an automatic valve after a process trip.
Process journey
A process journey occurs when the SIS detects an abnormal process condition by way of sensors corresponding to temperature and stress, executes the logic and locations the method in its protected state by tripping the ultimate parts such as closing an automated 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 course of trip happens, the principle aim is normally to restart the unit or equipment that has been shut down as soon as attainable. Plant engineers are busy troubleshooting and correcting issues. Maintenance technicians are busy repairing damaged devices. Taking the chance to proof check an automated valve will not be a prime priority or even an exercise into consideration due to the tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is provided with a DVC, the proof check of the valve could be considered carried out with diagnostic knowledge captured by the DVC. Also, a comparison of the diagnostic data with the valve’s baseline can help uncover valve deficiencies and degradations that gained’t show up in a proof take a look at.
Figure 2. Proof Test vs Process Trip. Activities performed (such as repairing the device) and information captured by a Fisher DVC6200 SIS digital valve controller during a course of trip meet most of the 12 necessities of a proof test.
Process journeys versus proof exams
How can proof test credits be claimed for an automated valve that has failed and triggered a course of trip? A proof check is a periodic check performed on SIS sensors, logic solvers and last parts — corresponding to automated valves — to discover hidden failures. These failures, if not corrected or removed, could impair the flexibility of the SIS to take the process to its protected state when an abnormal process condition is detected.
A proof check must be carried out as per the proof check interval specified within the Safety Requirement Specifications. This interval is usually determined by way of a median likelihood of failure on demand calculation (PFDavg). Some users might select to proof check based on predetermined intervals, similar to proof testing sensors each 24 months and ultimate components each 48 months as a substitute of relying on the PFDavg calculations.
Automated valve proof tests may be carried out offline or on-line. Offline proof exams are often scheduled throughout a turnaround, when the process is totally 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 forestall a spurious trip, making it inconvenient and expensive.
But a proof take a look at may also be accomplished during a process journey.
According to IEC61511-2 Section A.sixteen.three.1.3, “…shutdowns as a end result of precise demand on the SIS during operation may be given credit score as proof tests (fully or partial) underneath given conditions…the subsequent deliberate proof check could also be skipped.”
These circumstances are
The shutdown documents equal information 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 separately.
The shutdown happens within a predetermined maximum time window before the following planned proof check which might then be canceled
When a course of journey occurs, about 60% of a Safety Instrumented Function (SIF) proof check could be thought of performed. A sample listing of actions carried out throughout a proof check, together with those which are carried out during a process journey, is shown in Figure 2. Even with out an automated valve leak take a look at, information captured by the DVC alone can doubtlessly account for a great amount of proof check coverage for an automatic valve.
The precise coverage depends on the design of the automated valve, its DVC and its application. The protection is estimated based on the valve degradations, the likelihood of their prevalence and the proportion of these degradations that could be detected by a DVC. Fortunately, data acquired by a DVC throughout a course of trip can usually be sufficient to fulfill a significant a half of the proof take a look at necessities.
If the method journey takes place within a predetermined maximum time window, the top user might select to leverage the method trip as a proof take a look at by finishing steps one via 5 in Figure 2, which are normally not completed in a process journey. The subsequent scheduled proof take a look at can then be canceled. According to the Norwegian Oil Industry Association, the utmost time window should be the final half of the present proof check interval.
Figure 3. Data during a process trip is captured by the Fisher DVC6200 SIS digital valve controller and made obtainable for evaluation.
Common causes of valve failure
According to ISA Technical Report 96.05.01, the most probably root causes of automated valve degradations — which can lead to a process journey — embody:
Partial or intermittent sticking of hydraulic or pneumatic system parts similar to solenoids, pilots, speed controller, etc., as a outcome of moisture, debris or alignment issues. This causes a lack of useful margin and makes the valve slow to open or shut.
Binding, galling or other degradation of valve seats or associated circulate control trim that restricts or resists valve motion.
Actuator seal degradation brought on by compression, put on or looseness that reduces the pressure obtainable to actuate the valve.
Minor injury to the valve obturator plug, disk or ball attributable to system situations, leakage or debris, including build-up of hydrocarbon merchandise. This causes leakage.
Complete failure of hydraulic management system elements corresponding to solenoids, pilots, velocity controller, etc., as a outcome of moisture, debris or alignment. The valve will fail to open or shut.
All of those situations could be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For instance, a valve that is caught open will exhibit itself as an increase in breakout pressure. Loss of seat load compared to when the valve meeting was new signifies potential valve leak. The Fisher DVC6200SIS additionally continuously monitors for inner faults as well as its inputs similar to supply pressure. When the DVC6200SIS sees the provision stress is simply too low, an alert is generated (Figure 4) in order that the end consumer can take corrective actions.
Figure four. The DVC6200SIS digital valve controller detected almost zero provide strain, so it alerted the control system.
Other course of journey benefits
Diagnostic knowledge captured during a process trip might reveal valve degradations that is in all probability not detected throughout a proof take a look at. For instance, diagnostic knowledge captured throughout a process trip may point out an issue with the valve closing completely against the total strain of the process, which can be an early indication of valve leakage.
The valve stroke time computed by a DVC during a course of trip is more correct under real operating situations. This ends in a more accurate SIF response time (the time from detection of an abnormal condition to final factor reaching its trip state), which is in comparability with the Process Safety Time to ensure the SIF continues to be assembly its design requirements.
Although automated เกจวัดแรงดันน้ำไทวัสดุ are very reliable, failures recognized in a process trip can provide valuable information to forestall future failures. This info can help with turnaround planning by making sure the needed components are available before turnaround even begins to potentially shorten the turnaround schedule.
Summary
A process journey can provide the protection required to delay a valve’s next scheduled proof test, thus saving money and time. Data captured by a DVC could be analyzed to leverage the process journey as a proof take a look at. Even if the top person chooses not to take proof take a look at credits for a course of trip, the valve diagnostic knowledge supplied by the DVC can help plant personnel make proactive valve upkeep selections..
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