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A course of trip occurs when the safety instrumented system (SIS) locations the method in its safe state by commanding an automatic valve (Figure 1) or different tools to its journey state in response to an abnormal course of condition. In some cases, a spurious journey happens due to a valve malfunction. In that case, the valve assembly is examined under real working conditions, which offers a chance to seize valuable valve diagnostic information.
However, such diagnostics information can solely be captured when automated valves are fitted with digital valve controllers (DVCs). In this text, we’ll discuss how DVCs can help decide the proof take a look at credit for an automated valve after a process trip.
Process journey
A course of trip occurs when the SIS detects an irregular course of condition through sensors such as temperature and strain, executes the logic and places the method in its protected state by tripping the ultimate components 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 action.
When a course of trip happens, the primary objective is often to restart the unit or equipment that has been shut down as quickly as attainable. Plant engineers are busy troubleshooting and correcting problems. Maintenance technicians are busy repairing broken devices. Taking the chance to proof check an automatic valve will not be a high precedence and even an activity into account 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 can be considered performed with diagnostic information captured by the DVC. Also, a comparison of the diagnostic knowledge with the valve’s baseline can help discover valve deficiencies and degradations that will not show up in a proof take a look at.
Figure 2. Proof Test vs Process Trip. Activities carried out (such as repairing the device) and data captured by a Fisher DVC6200 SIS digital valve controller during a process trip meet a lot of the 12 necessities of a proof check.
Process journeys versus proof checks
How can proof take a look at credit be claimed for an automatic valve that has failed and triggered a course of trip? A proof check is a periodic take a look at performed on SIS sensors, logic solvers and ultimate elements — such as automated valves — to find hidden failures. These failures, if not corrected or removed, may impair the power of the SIS to take the process to its secure state when an abnormal course of condition is detected.
A proof check should be performed as per the proof check interval specified within the Safety Requirement Specifications. This interval is usually determined via a mean chance of failure on demand calculation (PFDavg). Some users could choose to proof test primarily based on predetermined intervals, such as proof testing sensors every 24 months and final components every forty eight months instead of counting on the PFDavg calculations.
Automated valve proof checks can be accomplished offline or online. Offline proof checks are normally scheduled during a turnaround, when the process is completely shut down.
Proof testing an automatic valve online normally requires a unit or tools to be shut down, or the valve to be bypassed to stop a spurious journey, making it inconvenient and dear.
But a proof take a look at can be accomplished during a process trip.
According to IEC61511-2 Section A.sixteen.three.1.three, “…shutdowns as a outcome of actual demand on the SIS throughout operation may be given credit score as proof exams (fully or partial) beneath given conditions…the subsequent planned proof take a look at could additionally be skipped.”
These situations are
The shutdown documents equal information as registered during corresponding proof take a look at.
The shutdown covers all parts of the SIS, and if not, the device or SIS subsystem not activated must be tested separately.
The shutdown occurs inside a predetermined most time window before the following deliberate proof check which might then be canceled
When a course of trip happens, about 60% of a Safety Instrumented Function (SIF) proof check may be thought-about carried out. A pattern listing of activities carried out during a proof check, together with these which are carried out throughout a course of trip, is shown in Figure 2. Even without an automatic valve leak take a look at, knowledge captured by the DVC alone can doubtlessly account for a great quantity of proof check protection for an automated valve.
The precise coverage depends on the design of the automated valve, its DVC and its software. The protection is estimated primarily based 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 course of journey can often be sufficient to fulfill a significant part of the proof test necessities.
If the method trip takes place inside a predetermined maximum time window, the end person might choose to leverage the process trip as a proof take a look at by completing steps one through five in Figure 2, which are normally not completed in a process trip. The next scheduled proof check can then be canceled. According to เกจวัดแรงอัดกระบอกสูบ , the maximum time window must be the last half of the present proof check interval.
Figure 3. Data during a process journey is captured by the Fisher DVC6200 SIS digital valve controller and made out there for evaluation.
Common causes of valve failure
According to ISA Technical Report 96.05.01, the most likely root causes of automated valve degradations — which can lead to a process trip — embody:
Partial or intermittent sticking of hydraulic or pneumatic system components corresponding to solenoids, pilots, pace controller, and so forth., as a outcome of moisture, debris or alignment issues. This causes a lack of practical margin and makes the valve sluggish to open or shut.
Binding, galling or other degradation of valve seats or related flow management trim that restricts or resists valve motion.
Actuator seal degradation brought on by compression, wear or looseness that reduces the stress out there to actuate the valve.
Minor damage to the valve obturator plug, disk or ball brought on by system circumstances, leakage or debris, together with build-up of hydrocarbon merchandise. This causes leakage.
Complete failure of hydraulic control system elements similar to solenoids, pilots, pace controller, etc., because of moisture, debris or alignment. The valve will fail to open or close.
All of those conditions can be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For example, a valve that is stuck 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 continuously monitors for inside faults in addition to its inputs corresponding to supply pressure. When the DVC6200SIS sees the supply pressure is 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 supply stress, so it alerted the control system.
Other course of journey advantages
Diagnostic information captured throughout a course of trip could reveal valve degradations that is most likely not detected during a proof check. For example, diagnostic information captured throughout a process trip might indicate an issue with the valve closing utterly in opposition to the total stress of the process, which can be an early indication of valve leakage.
The valve stroke time computed by a DVC during a process trip is more accurate beneath actual working conditions. This ends in a more accurate SIF response time (the time from detection of an irregular condition to last component reaching its trip state), which is in comparison with the Process Safety Time to make sure the SIF remains to be assembly its design necessities.
Although automated valves are very dependable, failures identified in a process trip can provide priceless information to stop future failures. This information can assist with turnaround planning by making sure the needed parts can be found before 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 test, thus saving money and time. Data captured by a DVC can be analyzed to leverage the method trip as a proof take a look at. Even if the end user chooses not to take proof take a look at credits for a process journey, the valve diagnostic knowledge supplied by the DVC may help plant personnel make proactive valve maintenance decisions..
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