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A course of trip happens when the security instrumented system (SIS) places the method in its secure state by commanding an automatic valve (Figure 1) or different equipment to its trip state in response to an abnormal course of situation. In some instances, a spurious journey occurs because of a valve malfunction. In that case, the valve assembly is tested beneath actual operating conditions, which supplies an opportunity to capture priceless valve diagnostic knowledge.
However, such diagnostics data can only be captured when automated valves are fitted with digital valve controllers (DVCs). In this article, we’ll focus on how DVCs can help decide the proof take a look at credit for an automated valve after a course of trip.
Process trip
A course of trip happens when the SIS detects an abnormal course of condition via sensors corresponding to temperature and stress, executes the logic and places the process in its protected state by tripping the ultimate components corresponding to closing an automated valve, stopping a pump, etc. The SIS might communicate with the Basic Process Control System (BPCS) to shut a valve downstream of the automated valve as a secondary motion.
When a process journey happens, the main goal is normally to restart the unit or equipment that has been shut down as soon as possible. Plant engineers are busy troubleshooting and correcting problems. Maintenance technicians are busy repairing broken devices. Taking the opportunity to proof check an automatic valve won’t be a high precedence or even an exercise under consideration as a end result of tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is provided with a DVC, the proof test of the valve can be considered performed with diagnostic data captured by the DVC. Also, a comparison of the diagnostic knowledge with the valve’s baseline may help uncover valve deficiencies and degradations that will not show up in a proof check.
Figure 2. Proof Test vs Process Trip. Activities performed (such as repairing the device) and knowledge captured by a Fisher DVC6200 SIS digital valve controller during a course of journey 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 automatic valve that has failed and brought on a process trip? A proof test is a periodic take a look at carried out on SIS sensors, logic solvers and final components — similar to automated valves — to find hidden failures. These failures, if not corrected or removed, could impair the flexibility of the SIS to take the method to its secure state when an irregular process situation is detected.
A proof check ought to be carried out as per the proof test interval specified within the Safety Requirement Specifications. This interval is usually determined via a mean likelihood of failure on demand calculation (PFDavg). Some users may select to proof take a look at based on predetermined intervals, similar to proof testing sensors each 24 months and ultimate parts every 48 months as a substitute of relying on the PFDavg calculations.
Automated valve proof tests could be carried out offline or online. Offline proof tests are usually scheduled during a turnaround, when the method is completely shut down.
Proof testing an automated valve on-line often requires a unit or equipment to be shut down, or the valve to be bypassed to stop a spurious journey, making it inconvenient and expensive.
But a proof check can also be completed during a process journey.
According to IEC61511-2 Section A.sixteen.three.1.three, “…shutdowns due to precise demand on the SIS during operation could additionally be given credit as proof tests (fully or partial) under given conditions…the subsequent deliberate proof take a look at could additionally be skipped.”
These conditions are
The shutdown paperwork equivalent information as registered throughout corresponding proof test.
The shutdown covers all elements of the SIS, and if not, the gadget or SIS subsystem not activated must be examined individually.
The shutdown happens inside a predetermined maximum time window earlier than the next planned proof take a look at which might then be canceled
When a process trip happens, about 60% of a Safety Instrumented Function (SIF) proof test can be thought-about carried out. A pattern listing of activities carried out throughout a proof take a look at, together with these which are performed throughout a course of trip, is shown in Figure 2. Even without an automatic valve leak take a look at, data captured by the DVC alone can probably account for an excellent quantity of proof check protection for an automated valve.
The actual 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 likelihood of their occurrence and the percentage of these degradations that can be detected by a DVC. Fortunately, data acquired by a DVC throughout a process journey can often be enough to fulfill a major part of the proof take a look at necessities.
If the method trip takes place within a predetermined maximum time window, the tip consumer may select to leverage the method trip as a proof take a look at by finishing steps one through 5 in Figure 2, which are often not accomplished in a course of trip. The next scheduled proof take a look at can then be canceled. According to the Norwegian Oil Industry Association, the maximum time window should be the last half of the current proof test interval.
Figure three. Data throughout a process trip 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 96.05.01, the most probably root causes of automated valve degradations — which may lead to a process trip — embody:
Partial or intermittent sticking of hydraulic or pneumatic system parts similar to solenoids, pilots, velocity controller, and so on., because of moisture, particles or alignment issues. This causes a lack of useful margin and makes the valve sluggish to open or shut.
Binding, galling or different degradation of valve seats or associated circulate control trim that restricts or resists valve motion.
Actuator seal degradation attributable to compression, wear or looseness that reduces the pressure obtainable to actuate the valve.
Minor injury to the valve obturator plug, disk or ball caused by system conditions, leakage or particles, together with build-up of hydrocarbon products. This causes leakage.
Complete failure of hydraulic control system elements similar to solenoids, pilots, velocity controller, and so on., as a result of moisture, particles or alignment. The valve will fail to open or shut.
เกจวัดแรงดันแก๊สlpg of these circumstances can be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For example, a valve that’s stuck open will exhibit itself as a rise in breakout force. Loss of seat load compared to when the valve meeting was new indicates potential valve leak. The Fisher DVC6200SIS also continuously screens for internal faults as well as its inputs similar to supply pressure. When the DVC6200SIS sees the provision stress is merely too low, an alert is generated (Figure 4) so that the end user can take corrective actions.
Figure four. The DVC6200SIS digital valve controller detected nearly zero supply pressure, so it alerted the control system.
Other process trip benefits
Diagnostic knowledge captured throughout a course of journey might reveal valve degradations that may not be detected throughout a proof check. For instance, diagnostic information captured during a course of journey would possibly point out a problem with the valve closing utterly towards the full stress of the method, which may be an early indication of valve leakage.
The valve stroke time computed by a DVC throughout a process trip is more accurate beneath actual operating conditions. This ends in a more correct SIF response time (the time from detection of an abnormal condition to last factor reaching its trip state), which is in comparability with the Process Safety Time to make sure the SIF continues to be meeting its design necessities.
Although automated valves are very reliable, failures recognized in a course of trip can present valuable information to stop future failures. This info can assist with turnaround planning by ensuring the wanted components can be found 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 test, thus saving time and money. Data captured by a DVC could be analyzed to leverage the process trip as a proof test. Even if the tip consumer chooses to not take proof test credits for a course of trip, the valve diagnostic information provided by the DVC may help plant personnel make proactive valve maintenance decisions..
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