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A process trip happens when the safety instrumented system (SIS) places the method in its safe state by commanding an automated valve (Figure 1) or other gear to its journey state in response to an irregular course of condition. In some cases, a spurious trip happens due to a valve malfunction. In that case, the valve meeting is examined beneath actual working conditions, which supplies a possibility to capture valuable valve diagnostic data.
However, such diagnostics information can solely be captured when automated valves are fitted with digital valve controllers (DVCs). In this text, we’ll focus on how DVCs might help determine the proof test credit score for an automatic valve after a course of trip.
Process trip
A process trip occurs when the SIS detects an abnormal course of condition via sensors similar to temperature and stress, executes the logic and locations the method in its secure state by tripping the ultimate parts such as closing an automated valve, stopping a pump, and so forth. The SIS may talk with the Basic Process Control System (BPCS) to close a valve downstream of the automated valve as a secondary action.
When a process journey happens, the main objective is usually to restart the unit or gear that has been shut down as soon as possible. Plant engineers are busy troubleshooting and correcting problems. Maintenance technicians are busy repairing damaged gadgets. Taking the chance to proof test an automatic valve is not going to be a high precedence and even an exercise under consideration as a end result of tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is supplied with a DVC, the proof take a look at of the valve could be thought of carried out with diagnostic data captured by the DVC. Also, a comparability of the diagnostic data with the valve’s baseline might help discover valve deficiencies and degradations that will not present up in a proof take a look at.
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 during a process trip meet a lot of the 12 requirements of a proof take a look at.
Process journeys versus proof checks
How can proof test credit be claimed for an automatic valve that has failed and brought on a course of trip? A proof test is a periodic take a look at carried out on SIS sensors, logic solvers and ultimate elements — similar to automated valves — to discover hidden failures. These failures, if not corrected or removed, may impair the flexibility of the SIS to take the method to its safe state when an abnormal course of condition is detected.
A proof check must be carried out as per the proof take a look at interval specified in the Safety Requirement Specifications. This interval is usually decided through an average probability of failure on demand calculation (PFDavg). เกจวัดแรงดันอากาศ might choose to proof test based on predetermined intervals, corresponding to proof testing sensors every 24 months and ultimate components each forty eight months instead of counting on the PFDavg calculations.
Automated valve proof checks can be carried out offline or on-line. Offline proof tests are normally scheduled during a turnaround, when the process is completely shut down.
Proof testing an automatic valve on-line normally requires a unit or tools to be shut down, or the valve to be bypassed to prevent a spurious journey, making it inconvenient and dear.
But a proof take a look at can additionally be achieved throughout a course of trip.
According to IEC61511-2 Section A.16.3.1.three, “…shutdowns because of actual demand on the SIS throughout operation could also be given credit as proof exams (fully or partial) under given conditions…the next deliberate proof check could also be skipped.”
These circumstances are
The shutdown documents equivalent info as registered during corresponding proof take a look at.
The shutdown covers all parts of the SIS, and if not, the system or SIS subsystem not activated needs to be tested individually.
The shutdown happens within a predetermined maximum time window before the subsequent planned proof check which might then be canceled
When a process journey occurs, about 60% of a Safety Instrumented Function (SIF) proof take a look at could be considered performed. A sample listing of actions performed throughout a proof check, together with these which are carried out throughout a process trip, is proven in Figure 2. Even with out an automated valve leak take a look at, knowledge captured by the DVC alone can potentially account for an excellent quantity of proof take a look at protection for an automated valve.
The precise protection is dependent upon the design of the automated valve, its DVC and its software. The coverage is estimated primarily based on the valve degradations, the chance of their incidence and the proportion of those degradations that can be detected by a DVC. Fortunately, knowledge acquired by a DVC during a course of trip can often be enough to fulfill a major a half of the proof take a look at requirements.
If the method journey takes place within a predetermined most time window, the top user might select to leverage the method journey as a proof test by completing steps one through 5 in Figure 2, that are usually not accomplished in a process journey. The subsequent scheduled proof test can then be canceled. According to the Norwegian Oil Industry Association, the utmost time window must be the final half of the current proof test interval.
Figure three. Data during a process journey 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 likely root causes of automated valve degradations — which may result in a course of journey — embrace:
Partial or intermittent sticking of hydraulic or pneumatic system elements such as solenoids, pilots, speed controller, etc., as a end result of moisture, debris or alignment points. This causes a lack of useful margin and makes the valve sluggish to open or close.
Binding, galling or other degradation of valve seats or related circulate control trim that restricts or resists valve movement.
Actuator seal degradation caused by compression, wear or looseness that reduces the pressure out there to actuate the valve.
Minor injury to the valve obturator plug, disk or ball brought on by system conditions, leakage or particles, including build-up of hydrocarbon products. This causes leakage.
Complete failure of hydraulic management system elements corresponding to solenoids, pilots, velocity controller, etc., due to moisture, debris 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 is caught open will exhibit itself as an increase in breakout pressure. Loss of seat load in comparability with when the valve assembly was new indicates potential valve leak. The Fisher DVC6200SIS also repeatedly screens for inner faults in addition to its inputs corresponding to provide stress. When the DVC6200SIS sees the provision strain is just too low, an alert is generated (Figure 4) in order that the tip person can take corrective actions.
Figure four. The DVC6200SIS digital valve controller detected nearly zero provide strain, so it alerted the control system.
Other process journey benefits
Diagnostic knowledge captured throughout a course of trip could reveal valve degradations that is probably not detected during a proof take a look at. For instance, diagnostic knowledge captured during a process journey would possibly point out a problem with the valve closing fully towards the full stress of the process, which can be an early indication of valve leakage.
The valve stroke time computed by a DVC throughout a course of trip is extra accurate beneath actual operating conditions. This results in a more correct SIF response time (the time from detection of an abnormal situation to last component reaching its trip state), which is compared to the Process Safety Time to ensure the SIF continues to be assembly its design necessities.
Although automated valves are very dependable, failures identified in a process journey can provide valuable data to prevent future failures. This information can help with turnaround planning by making sure the needed components can be found before turnaround even begins to doubtlessly shorten the turnaround schedule.
Summary
A course of trip can present the coverage required to delay a valve’s subsequent scheduled proof check, 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 end person chooses to not take proof check credit for a process journey, the valve diagnostic data offered by the DVC can help plant personnel make proactive valve upkeep selections..
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