Valve proof check credit for a course of journey

A process journey happens when the safety instrumented system (SIS) places the process in its secure state by commanding an automated valve (Figure 1) or different equipment to its trip state in response to an abnormal course of situation. In some instances, a spurious trip happens because of a valve malfunction. In that case, the valve meeting is tested under actual operating situations, which supplies a chance to capture priceless valve diagnostic information.
However, such diagnostics data can solely be captured when automated valves are fitted with digital valve controllers (DVCs). In this text, we’ll talk about how DVCs can help determine the proof check credit for an automated valve after a process trip.
Process trip
A course of journey occurs when the SIS detects an abnormal process situation by way of sensors similar to temperature and pressure, executes the logic and locations the method in its safe state by tripping the ultimate parts similar to closing an automated valve, stopping a pump, etc. 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 journey occurs, the primary objective is usually to restart the unit or equipment that has been shut down as quickly as potential. Plant engineers are busy troubleshooting and correcting issues. Maintenance technicians are busy repairing damaged devices. Taking the opportunity to proof check an automated valve won’t be a top priority and even an activity into consideration due to the tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is equipped with a DVC, the proof check of the valve could be considered performed with diagnostic data captured by the DVC. Also, a comparability of the diagnostic information with the valve’s baseline may help discover valve deficiencies and degradations that will not present up in a proof test.
Figure 2. Proof Test vs Process Trip. Activities performed (such as repairing the device) and data captured by a Fisher DVC6200 SIS digital valve controller during a course of trip meet a lot of the 12 requirements of a proof check.
Process trips versus proof tests
How can proof take a look at credit be claimed for an automated valve that has failed and triggered a course of trip? A proof take a look at is a periodic take a look at performed on SIS sensors, logic solvers and last parts — corresponding to automated valves — to find hidden failures. These failures, if not corrected or removed, could impair the power of the SIS to take the method to its protected state when an abnormal process condition is detected.
A proof test ought to be carried out as per the proof test interval specified within the Safety Requirement Specifications. This interval is usually decided by way of an average probability of failure on demand calculation (PFDavg). Some customers might choose to proof test based mostly on predetermined intervals, such as proof testing sensors every 24 months and final elements every forty eight months instead of relying on the PFDavg calculations.
Automated valve proof checks could be carried out offline or on-line. Offline proof exams are often scheduled during a turnaround, when the process is completely shut down.
Proof testing an automatic valve on-line often requires a unit or equipment to be shut down, or the valve to be bypassed to prevent a spurious journey, making it inconvenient and expensive.
But a proof test can be completed during a process trip.
According to IEC61511-2 Section A.16.3.1.3, “…shutdowns because of precise demand on the SIS during operation may be given credit as proof tests (fully or partial) underneath given conditions…the subsequent deliberate proof test could additionally be skipped.”
These circumstances are
The shutdown paperwork equal information as registered during corresponding proof take a look at.
The shutdown covers all components of the SIS, and if not, the system or SIS subsystem not activated must be examined separately.
The shutdown occurs inside a predetermined maximum time window earlier than the subsequent deliberate proof take a look at which might then be canceled
When a course of trip happens, about 60% of a Safety Instrumented Function (SIF) proof check can be thought-about performed. A sample list of activities carried out during a proof take a look at, together with these which may be carried out during a process trip, 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 automated valve.
The actual coverage depends on the design of the automated valve, its DVC and its software. The coverage is estimated based mostly on the valve degradations, the probability of their prevalence and the percentage of those degradations that can be detected by a DVC. Fortunately, information acquired by a DVC during a process journey can typically be sufficient to satisfy a major a part of the proof test necessities.
If the method journey takes place within a predetermined maximum time window, the tip user could choose to leverage the process journey as a proof test by completing steps one via 5 in Figure 2, which are usually not completed in a course of trip. The subsequent scheduled proof take a look at can then be canceled. According to the Norwegian Oil Industry Association, the maximum time window must be the final half of the current proof check interval.
เกจ์ลมsumo . Data throughout a course of trip 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 may lead to a process journey — include:
Partial or intermittent sticking of hydraulic or pneumatic system elements such as solenoids, pilots, speed controller, and so forth., as a outcome of moisture, debris or alignment issues. This causes a loss of practical margin and makes the valve slow to open or close.
Binding, galling or different degradation of valve seats or related circulate control trim that restricts or resists valve motion.
Actuator seal degradation attributable to compression, put on or looseness that reduces the pressure obtainable to actuate the valve.
Minor injury to the valve obturator plug, disk or ball caused by system circumstances, leakage or particles, including build-up of hydrocarbon merchandise. This causes leakage.
Complete failure of hydraulic control system elements corresponding to solenoids, pilots, speed controller, and so on., due to moisture, debris or alignment. The valve will fail to open or shut.
All of those situations can 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 force. Loss of seat load compared to when the valve assembly was new signifies potential valve leak. The Fisher DVC6200SIS additionally constantly displays for inside faults as nicely as its inputs similar to provide stress. When the DVC6200SIS sees the availability strain is merely 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 nearly zero provide strain, so it alerted the control system.
Other process trip benefits
Diagnostic data captured throughout a course of trip could reveal valve degradations that will not be detected during a proof take a look at. For example, diagnostic knowledge captured throughout a process journey may indicate an issue with the valve closing fully towards the total stress of the method, which could be an early indication of valve leakage.
The valve stroke time computed by a DVC throughout a process trip is more correct under real working circumstances. This ends in a extra correct SIF response time (the time from detection of an irregular condition to last element reaching its trip 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 recognized in a process journey can present useful information to stop future failures. This data can assist with turnaround planning by ensuring the wanted elements are available earlier than turnaround even begins to doubtlessly shorten the turnaround schedule.
Summary
A process journey can provide the protection required to delay a valve’s next scheduled proof take a look at, thus saving money and time. Data captured by a DVC may be analyzed to leverage the method trip as a proof test. Even if the top consumer chooses to not take proof test credit for a process journey, the valve diagnostic knowledge offered by the DVC may help plant personnel make proactive valve maintenance choices..
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