Valve proof take a look at credit score for a process trip

A course of journey occurs when the safety instrumented system (SIS) locations the process in its protected state by commanding an automatic valve (Figure 1) or other gear to its trip state in response to an irregular process situation. In some cases, a spurious trip occurs due to a valve malfunction. In that case, the valve assembly is examined under real operating situations, which offers a chance to seize priceless 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 focus on how DVCs might help decide the proof test credit score for an automated valve after a course of trip.
Process trip
A process trip happens when the SIS detects an irregular course of situation through sensors corresponding to temperature and strain, executes the logic and places the process in its secure state by tripping the final parts such as 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 course of journey occurs, the main goal is usually to restart the unit or gear that has been shut down as quickly as attainable. Plant engineers are busy troubleshooting and correcting problems. Maintenance technicians are busy repairing broken gadgets. Taking the chance to proof take a look at an automated valve is not going to be a high priority or even an exercise under consideration because of the tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is supplied with a DVC, the proof test of the valve may be thought of carried out with diagnostic knowledge captured by the DVC. Also, a comparison of the diagnostic knowledge with the valve’s baseline might help discover valve deficiencies and degradations that may 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 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 take a look at.
Process journeys versus proof tests
How can proof take a look at credits be claimed for an automated valve that has failed and brought on a course of trip? A proof take a look at is a periodic take a look at performed on SIS sensors, logic solvers and final parts — similar to automated valves — to find hidden failures. These failures, if not corrected or eliminated, might impair the power of the SIS to take the method to its secure state when an abnormal course of condition is detected.
A proof take a look at should be performed as per the proof test interval specified in the Safety Requirement Specifications. This interval is normally determined through a mean probability of failure on demand calculation (PFDavg). Some users may select to proof test primarily based on predetermined intervals, such as proof testing sensors each 24 months and ultimate components every 48 months instead of relying on the PFDavg calculations.
Automated valve proof exams could be done offline or online. Offline proof exams are usually scheduled during a turnaround, when the process is completely shut down.
Proof testing an automatic valve on-line usually 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 check can additionally be achieved during a process journey.
According to IEC61511-2 Section A.16.3.1.3, “…shutdowns because of actual demand on the SIS during operation may be given credit score as proof tests (fully or partial) underneath given conditions…the subsequent deliberate proof take a look at may be skipped.”
These circumstances are
The shutdown documents equivalent information as registered throughout corresponding proof take a look at.
The shutdown covers all elements of the SIS, and if not, the device or SIS subsystem not activated must be examined individually.
The shutdown happens within a predetermined maximum time window earlier than the next deliberate proof take a look at which can then be canceled
When a course of trip happens, about 60% of a Safety Instrumented Function (SIF) proof take a look at could be considered carried out. A pattern list of actions performed throughout a proof test, along with those which are carried out during a course of journey, is proven in Figure 2. Even without an automatic valve leak check, information captured by the DVC alone can probably account for a good amount of proof test protection for an automatic valve.
The precise protection is decided by the design of the automated valve, its DVC and its application. The protection is estimated primarily based on the valve degradations, the probability of their incidence and the proportion of these degradations that can be detected by a DVC. Fortunately, information acquired by a DVC throughout a course of trip can often be enough to satisfy a serious a half of the proof check necessities.
If the method journey takes place within a predetermined most time window, the tip person may select to leverage the process trip as a proof test by completing steps one via 5 in Figure 2, that are normally not accomplished in a course of trip. The subsequent scheduled proof take a look at can then be canceled. According to the Norwegian Oil Industry Association, the maximum time window must be the final half of the present proof take a look at interval.
Figure three. 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 probably root causes of automated valve degradations — which might lead to a course of journey — embrace:
Partial or intermittent sticking of hydraulic or pneumatic system elements similar to solenoids, pilots, pace controller, and so on., as a outcome of moisture, debris or alignment points. This causes a lack of useful margin and makes the valve slow to open or close.
Binding, galling or different degradation of valve seats or associated move control trim that restricts or resists valve movement.
Actuator seal degradation caused by compression, put on or looseness that reduces the stress obtainable to actuate the valve.
Minor damage to the valve obturator plug, disk or ball caused by system situations, leakage or debris, including build-up of hydrocarbon products. This causes leakage.
Complete failure of hydraulic control system parts corresponding to solenoids, pilots, speed controller, etc., as a outcome of moisture, debris or alignment. The valve will fail to open or close.
All of these circumstances could be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For instance, a valve that is stuck open will exhibit itself as a rise in breakout drive. Loss of seat load in comparison with when the valve meeting was new indicates potential valve leak. The Fisher DVC6200SIS additionally constantly monitors for inner faults in addition to its inputs similar to supply strain. When the DVC6200SIS sees the supply pressure is too low, an alert is generated (Figure 4) in order that the tip user can take corrective actions.
Figure 4. The DVC6200SIS digital valve controller detected almost zero supply strain, so it alerted the control system.
Building blocks captured throughout a process journey could reveal valve degradations that is probably not detected throughout a proof take a look at. For example, diagnostic data captured during a process journey would possibly point out an issue with the valve closing completely against the full stress of the process, which can be an early indication of valve leakage.
The valve stroke time computed by a DVC during a course of trip is extra accurate beneath real operating situations. This leads to a more accurate SIF response time (the time from detection of an irregular situation to last component reaching its journey state), which is compared to the Process Safety Time to ensure the SIF is still assembly its design requirements.
Although automated valves are very dependable, failures identified in a course of journey can provide useful data to stop future failures. This info can help with turnaround planning by ensuring the needed components are available earlier than turnaround even begins to doubtlessly shorten the turnaround schedule.
Summary
A course of trip can provide the coverage required to delay a valve’s subsequent scheduled proof take a look at, thus saving time and money. Data captured by a DVC can be analyzed to leverage the method journey as a proof test. Even if the tip consumer chooses to not take proof test credit for a process journey, the valve diagnostic knowledge supplied by the DVC may help plant personnel make proactive valve maintenance selections..
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