A course of journey occurs when the protection instrumented system (SIS) locations the method in its safe state by commanding an automatic valve (Figure 1) or other tools to its trip state in response to an irregular process situation. In some instances, a spurious trip happens due to a valve malfunction. In that case, the valve meeting is examined underneath real operating situations, which provides a chance to capture valuable valve diagnostic information.
However, such diagnostics data can only be captured when automated valves are fitted with digital valve controllers (DVCs). In this text, we’ll focus on how DVCs might help decide the proof take a look at credit for an automatic valve after a process trip.
Process journey
A process journey occurs when the SIS detects an abnormal course of situation through sensors similar to temperature and stress, executes the logic and places the process in its protected state by tripping the final components similar to closing an automatic valve, stopping a pump, and so on. The SIS could communicate with the Basic Process Control System (BPCS) to shut a valve downstream of the automated valve as a secondary motion.
When a course of journey occurs, the main objective is usually to restart the unit or equipment that has been shut down as quickly as possible. Plant engineers are busy troubleshooting and correcting problems. Maintenance technicians are busy repairing broken units. Taking the chance to proof take a look at an automatic valve is not going to be a high precedence and even an exercise under consideration because of the tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is provided with a DVC, the proof check of the valve can 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 might not show up in a proof check.
pressure gauge ไท วัสดุ . Proof Test vs Process Trip. Activities carried out (such as repairing the device) and data captured by a Fisher DVC6200 SIS digital valve controller throughout a course of trip meet a lot of the 12 necessities of a proof take a look at.
Process journeys versus proof exams
How can proof test credits be claimed for an automatic valve that has failed and brought on a process trip? A proof take a look at is a periodic take a look at performed on SIS sensors, logic solvers and last elements — corresponding 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 secure state when an abnormal course of condition is detected.
A proof test should be carried out as per the proof test interval specified in the Safety Requirement Specifications. This interval is usually determined via an average probability of failure on demand calculation (PFDavg). Some customers may choose to proof test based mostly on predetermined intervals, corresponding to proof testing sensors every 24 months and final parts each 48 months as an alternative of relying on the PFDavg calculations.
Automated valve proof exams could 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 automated valve on-line often requires a unit or tools to be shut down, or the valve to be bypassed to forestall a spurious journey, making it inconvenient and dear.
But a proof take a look at can be completed throughout a process journey.
According to IEC61511-2 Section A.sixteen.three.1.3, “…shutdowns as a result of precise demand on the SIS throughout operation could also be given credit as proof tests (fully or partial) underneath given conditions…the subsequent deliberate proof take a look at could additionally be skipped.”
These situations are
The shutdown documents equal info as registered during corresponding proof test.
The shutdown covers all elements of the SIS, and if not, the device or SIS subsystem not activated needs to be tested individually.
The shutdown occurs inside a predetermined most time window earlier than 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 can be thought-about performed. A pattern listing of activities carried out throughout a proof check, together with these which may be performed during a process trip, is shown in Figure 2. Even without an automatic valve leak take a look at, information captured by the DVC alone can doubtlessly account for a great amount of proof take a look at coverage for an automated valve.
The exact coverage is dependent upon the design of the automated valve, its DVC and its utility. The protection is estimated based mostly on the valve degradations, the likelihood 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 half of the proof check requirements.
If the process journey takes place inside a predetermined most time window, the end person may choose to leverage the method journey as a proof check by finishing steps one via five in Figure 2, that are normally not completed in a process trip. The next scheduled proof check can then be canceled. According to the Norwegian Oil Industry Association, the utmost time window ought to be the final half of the current proof check interval.
Figure 3. Data throughout a process trip is captured by the Fisher DVC6200 SIS digital valve controller and made out there 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 might result in a course of journey — embody:
Partial or intermittent sticking of hydraulic or pneumatic system elements such as solenoids, pilots, velocity controller, etc., as a end result of moisture, debris or alignment issues. This causes a lack of practical margin and makes the valve slow to open or close.
Binding, galling or other degradation of valve seats or associated flow management trim that restricts or resists valve movement.
Actuator seal degradation brought on by compression, wear or looseness that reduces the stress out there to actuate the valve.
Minor injury 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 parts similar to solenoids, pilots, velocity controller, and so on., due to moisture, particles or alignment. The valve will fail to open or close.
All of these conditions may 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 signifies potential valve leak. The Fisher DVC6200SIS additionally constantly displays for inside faults in addition to its inputs corresponding to supply strain. When the DVC6200SIS sees the provision strain is too low, an alert is generated (Figure 4) so that the tip user can take corrective actions.
Figure four. The DVC6200SIS digital valve controller detected nearly zero provide stress, so it alerted the control system.
Other process trip benefits
Diagnostic data captured during a course of journey could reveal valve degradations that is most likely not detected throughout a proof test. For example, diagnostic information captured throughout a course of journey might indicate a problem with the valve closing utterly towards the total strain of the process, which may be an early indication of valve leakage.
The valve stroke time computed by a DVC during a process journey is extra correct under real working conditions. This leads to a more accurate SIF response time (the time from detection of an abnormal condition to last component reaching its trip state), which is compared to the Process Safety Time to make sure the SIF is still meeting its design requirements.
Although automated valves are very reliable, failures recognized in a process journey can provide useful knowledge to prevent future failures. This data can assist with turnaround planning by making sure the needed components are available earlier than turnaround even begins to potentially shorten the turnaround schedule.
Summary
A process journey can present the coverage required to delay a valve’s next scheduled proof check, thus saving time and money. Data captured by a DVC could be analyzed to leverage the process journey as a proof check. Even if the end consumer chooses not to take proof check credit for a process trip, the valve diagnostic knowledge offered by the DVC may help plant personnel make proactive valve maintenance selections..
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