Valve proof test credit for a process journey

A course of journey happens when the protection instrumented system (SIS) places the process in its secure state by commanding an automated valve (Figure 1) or different tools to its journey state in response to an abnormal course of situation. In some cases, a spurious journey happens because of a valve malfunction. In that case, the valve assembly is tested underneath actual working circumstances, which supplies an opportunity to seize useful valve diagnostic data.
However, such diagnostics knowledge 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 decide the proof test credit score for an automatic valve after a process journey.
Process trip
A course of journey occurs when the SIS detects an abnormal course of situation through sensors corresponding to temperature and pressure, executes the logic and locations the process in its safe state by tripping the ultimate components 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 course of trip happens, the principle aim is normally to restart the unit or tools that has been shut down as quickly as possible. Plant engineers are busy troubleshooting and correcting problems. Maintenance technicians are busy repairing broken devices. Taking the opportunity to proof test an automatic valve is not going to be a top precedence and even an exercise into account due to the 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 performed with diagnostic knowledge captured by the DVC. Also, a comparability of the diagnostic information with the valve’s baseline can help uncover valve deficiencies and degradations that will not present up in a proof check.
Figure 2. Proof Test vs Process Trip. Activities carried out (such as repairing the device) and knowledge captured by a Fisher DVC6200 SIS digital valve controller throughout a course of journey meet a lot of the 12 necessities of a proof test.
Process journeys versus proof tests
How can proof take a look at credit be claimed for an automated valve that has failed and caused a course of trip? A proof take a look at is a periodic check performed on SIS sensors, logic solvers and last elements — similar to automated valves — to discover hidden failures. These failures, if not corrected or eliminated, may impair the ability of the SIS to take the process to its safe state when an irregular course of situation is detected.
A proof take a look at must be carried out as per the proof check interval specified within the Safety Requirement Specifications. This interval is usually decided through a median likelihood of failure on demand calculation (PFDavg). Some users may select to proof test based on predetermined intervals, such as proof testing sensors every 24 months and final elements each forty eight months as a substitute of relying on the PFDavg calculations.
Automated valve proof exams could be carried out offline or online. Offline proof tests are normally scheduled throughout a turnaround, when the process is completely shut down.
Proof testing an automatic valve online often requires a unit or equipment to be shut down, or the valve to be bypassed to forestall a spurious trip, making it inconvenient and dear.
But a proof test can be accomplished during a process journey.
According to IEC61511-2 Section A.16.3.1.three, “…shutdowns because of actual demand on the SIS throughout operation could additionally be given credit score as proof tests (fully or partial) underneath given conditions…the subsequent deliberate proof test could additionally be skipped.”
These situations are
The shutdown paperwork equivalent data as registered during corresponding proof test.
The shutdown covers all components of the SIS, and if not, the device or SIS subsystem not activated must be tested individually.
The shutdown happens within a predetermined most time window before the following deliberate proof take a look at which may then be canceled
When a course of trip occurs, about 60% of a Safety Instrumented Function (SIF) proof test may be considered performed. A sample listing of activities performed during a proof check, along with those which may be carried out during a course of trip, is shown in Figure 2. Even with out an automated valve leak test, data captured by the DVC alone can probably account for a great amount of proof check coverage for an automated valve.
The exact coverage is determined by the design of the automated valve, its DVC and its application. The coverage is estimated primarily based on the valve degradations, the likelihood of their occurrence and the proportion of those degradations that can be detected by a DVC. Fortunately, information acquired by a DVC throughout a course of journey can typically be enough to fulfill a significant part of the proof test requirements.
If the process journey takes place inside a predetermined maximum time window, the top consumer may select to leverage the process trip as a proof check by finishing steps one via 5 in Figure 2, that are usually not accomplished in a course of trip. The next scheduled proof test can then be canceled. According to the Norwegian Oil Industry Association, the maximum time window should be the last half of the current proof take a look at interval.
Figure three. Data during a course of trip is captured by the Fisher DVC6200 SIS digital valve controller and made available for evaluation.
Common causes of valve failure
According to ISA Technical Report ninety six.05.01, the most probably root causes of automated valve degradations — which may lead to a process trip — include:
Partial or intermittent sticking of hydraulic or pneumatic system components similar to solenoids, pilots, speed controller, and so forth., because of moisture, debris or alignment points. This causes a lack of functional margin and makes the valve gradual to open or close.
Binding, galling or different degradation of valve seats or related flow control trim that restricts or resists valve movement.
เกจวัดความดันแก๊ส brought on by compression, wear or looseness that reduces the stress obtainable to actuate the valve.
Minor injury to the valve obturator plug, disk or ball brought on by system circumstances, leakage or debris, including build-up of hydrocarbon products. This causes leakage.
Complete failure of hydraulic management system components similar to solenoids, pilots, pace controller, and so forth., as a outcome of moisture, particles or alignment. The valve will fail to open or close.
All of those situations can be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For example, a valve that’s stuck open will exhibit itself as an increase in breakout drive. Loss of seat load in comparability with when the valve meeting was new indicates potential valve leak. The Fisher DVC6200SIS also repeatedly screens for internal faults in addition to its inputs corresponding to supply strain. When the DVC6200SIS sees the provision strain is simply too low, an alert is generated (Figure 4) so that the end consumer can take corrective actions.
Figure 4. The DVC6200SIS digital valve controller detected virtually zero provide stress, so it alerted the control system.
Other process trip advantages
Diagnostic information captured during a process journey may reveal valve degradations that is in all probability not detected throughout a proof test. For instance, diagnostic knowledge captured during a process trip might point out a problem with the valve closing completely towards the full strain of the method, which may be an early indication of valve leakage.
The valve stroke time computed by a DVC throughout a process journey is extra accurate underneath real operating circumstances. This ends in a extra accurate SIF response time (the time from detection of an irregular condition to final component reaching its trip state), which is in comparability with the Process Safety Time to ensure the SIF is still assembly its design requirements.
Although automated valves are very dependable, failures recognized in a process journey can present priceless information to prevent future failures. This data can assist with turnaround planning by making sure the needed parts can be found earlier than turnaround even begins to doubtlessly shorten the turnaround schedule.
Summary
A process trip can present the coverage required to delay a valve’s next scheduled proof check, thus saving money and time. Data captured by a DVC could be analyzed to leverage the method journey as a proof take a look at. Even if the tip consumer chooses to not take proof take a look at credit for a course of trip, the valve diagnostic information provided by the DVC may help plant personnel make proactive valve maintenance decisions..
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