A course of journey happens when the protection instrumented system (SIS) locations the process in its safe state by commanding an automated valve (Figure 1) or different equipment to its journey state in response to an abnormal process condition. In some cases, a spurious trip happens because of a valve malfunction. In that case, the valve meeting is examined beneath real operating conditions, which offers a possibility to capture valuable valve diagnostic knowledge.
However, such diagnostics data can only be captured when automated valves are fitted with digital valve controllers (DVCs). In this text, we’ll talk about how DVCs might help determine the proof test credit for an automated valve after a course of journey.
Process trip
A process trip happens when the SIS detects an irregular course of condition through sensors corresponding to temperature and stress, executes the logic and places the method in its protected state by tripping the final components similar to closing an automatic valve, stopping a pump, and so on. The SIS might communicate with the Basic Process Control System (BPCS) to close a valve downstream of the automated valve as a secondary motion.
When a process journey happens, the principle goal is usually to restart the unit or tools that has been shut down as soon as attainable. Plant engineers are busy troubleshooting and correcting problems. Maintenance technicians are busy repairing broken devices. Taking the chance to proof take a look at an automatic valve is not going to be a prime priority and even an activity into account as a outcome of tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is provided with a DVC, the proof take a look at of the valve can be thought-about performed with diagnostic information captured by the DVC. Also, a comparison of the diagnostic data with the valve’s baseline might help discover valve deficiencies and degradations that could not show up in a proof test.
Figure 2. Proof Test vs Process Trip. Activities carried out (such as repairing the device) and information captured by a Fisher DVC6200 SIS digital valve controller during a course of journey meet many of the 12 requirements of a proof check.
Process trips versus proof exams
How can proof check credit be claimed for an automatic valve that has failed and triggered a course of trip? A proof test is a periodic test carried out on SIS sensors, logic solvers and ultimate components — similar to automated valves — to discover hidden failures. These failures, if not corrected or removed, could impair the flexibility of the SIS to take the process to its safe state when an abnormal process condition is detected.
A proof test ought to be performed as per the proof check interval specified within the Safety Requirement Specifications. This interval is usually determined by way of a mean chance of failure on demand calculation (PFDavg). Some customers might choose to proof take a look at based on predetermined intervals, corresponding to proof testing sensors each 24 months and ultimate parts each forty eight months instead of counting on the PFDavg calculations.
Automated valve proof exams can be accomplished offline or on-line. Offline proof checks are normally scheduled throughout a turnaround, when the process is totally 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 also be achieved throughout a course of trip.
According to IEC61511-2 Section A.sixteen.three.1.3, “…shutdowns due to actual demand on the SIS throughout operation may be given credit as proof exams (fully or partial) under given conditions…the subsequent planned proof check may be skipped.”
These situations are
The shutdown documents equivalent information as registered throughout corresponding proof test.
The shutdown covers all components of the SIS, and if not, the device or SIS subsystem not activated must be examined separately.
The shutdown happens within a predetermined most time window earlier than the next deliberate proof test which can then be canceled
When a course of journey happens, about 60% of a Safety Instrumented Function (SIF) proof test could be thought of performed. A pattern listing of actions carried out throughout a proof test, together with those that are performed during a process trip, is shown in Figure 2. Even with out an automated valve leak check, knowledge captured by the DVC alone can doubtlessly account for a good quantity of proof take a look at protection for an automated valve.
The actual protection is dependent upon the design of the automated valve, its DVC and its application. The coverage is estimated based on the valve degradations, the likelihood of their incidence and the share of these degradations that may be detected by a DVC. Fortunately, information acquired by a DVC during a process trip can usually be enough to fulfill a significant a part of the proof take a look at requirements.
If the process journey takes place inside a predetermined maximum time window, the end user could select to leverage the process trip as a proof test by completing steps one through 5 in Figure 2, that are normally not accomplished in a process journey. The next scheduled proof test can then be canceled. According to the Norwegian Oil Industry Association, the utmost time window must be the last half of the present proof take a look at interval.
Figure three. Data throughout a course of journey is captured by the Fisher DVC6200 SIS digital valve controller and made available 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 can lead to a course of trip — embody:
Partial or intermittent sticking of hydraulic or pneumatic system elements similar to solenoids, pilots, velocity controller, and so on., as a outcome of moisture, particles or alignment points. This causes a lack of useful margin and makes the valve slow to open or shut.
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 harm to the valve obturator plug, disk or ball brought on by system situations, leakage or particles, including build-up of hydrocarbon products. This causes leakage.
Complete failure of hydraulic management system components similar to solenoids, pilots, velocity controller, and so on., as a outcome of moisture, particles 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 instance, a valve that’s stuck open will exhibit itself as a rise in breakout pressure. Loss of seat load compared to when the valve meeting was new indicates potential valve leak. The Fisher DVC6200SIS also repeatedly screens for internal faults in addition to its inputs such as supply pressure. When the DVC6200SIS sees the availability pressure is too low, an alert is generated (Figure 4) in order that the end user can take corrective actions.
Figure 4. The DVC6200SIS digital valve controller detected nearly zero supply pressure, so it alerted the control system.
Other course of trip advantages
Diagnostic information captured throughout a process trip could reveal valve degradations that will not be detected throughout a proof test. For example, diagnostic data captured throughout a process trip would possibly point out a problem with the valve closing completely against the full pressure of the process, which may be an early indication of valve leakage.
เกจวัดแรงดันไทวัสดุ computed by a DVC during a process trip is more accurate beneath real operating conditions. This results in a extra correct SIF response time (the time from detection of an abnormal condition to last factor reaching its journey state), which is compared to the Process Safety Time to make sure the SIF continues to be assembly its design necessities.
Although automated valves are very dependable, failures recognized in a course of trip can present priceless knowledge to prevent future failures. This information can assist with turnaround planning by ensuring the wanted components can be found before turnaround even begins to probably shorten the turnaround schedule.
Summary
A process trip can present the protection required to delay a valve’s subsequent scheduled proof check, thus saving time and money. Data captured by a DVC may be analyzed to leverage the method journey as a proof take a look at. Even if the end consumer chooses to not take proof take a look at credits for a process trip, the valve diagnostic knowledge offered by the DVC can help plant personnel make proactive valve maintenance selections..
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