A course of journey occurs when the security instrumented system (SIS) locations the method in its safe state by commanding an automatic valve (Figure 1) or different equipment to its trip state in response to an irregular process condition. In some circumstances, a spurious trip occurs because of a valve malfunction. In that case, the valve assembly is examined underneath actual operating circumstances, which supplies a chance to seize useful valve diagnostic data.
However, such diagnostics data can solely be captured when automated valves are fitted with digital valve controllers (DVCs). In this article, we’ll focus on how DVCs may help determine the proof take a look at credit for an automatic valve after a process trip.
Process trip
A process journey occurs when the SIS detects an abnormal course of condition by way of sensors similar to temperature and strain, executes the logic and places the process in its protected state by tripping the ultimate parts such as closing an automated valve, stopping a pump, etc. The SIS might talk 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 happens, the primary objective is often to restart the unit or tools that has been shut down as quickly as potential. Plant engineers are busy troubleshooting and correcting issues. Maintenance technicians are busy repairing broken gadgets. Taking the opportunity to proof test an automatic valve will not be a top precedence and even an activity 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 could be considered carried out with diagnostic data captured by the DVC. Also, a comparison of the diagnostic knowledge with the valve’s baseline might help uncover valve deficiencies and degradations that may not show up in a proof check.
Figure 2. Proof Test vs Process Trip. Activities carried out (such as repairing the device) and data captured by a Fisher DVC6200 SIS digital valve controller during a process trip meet most of the 12 necessities of a proof check.
Process journeys versus proof tests
How can proof take a look at credit be claimed for an automatic valve that has failed and caused a process trip? A proof take a look at is a periodic take a look at carried out on SIS sensors, logic solvers and last elements — similar to automated valves — to discover hidden failures. These failures, if not corrected or eliminated, may impair the ability of the SIS to take the process to its safe state when an irregular process situation is detected.
A proof take a look at should be performed as per the proof test interval specified within the Safety Requirement Specifications. This interval is usually determined through an average likelihood of failure on demand calculation (PFDavg). Some users might select to proof check primarily based on predetermined intervals, similar to proof testing sensors each 24 months and ultimate parts every forty eight months instead of relying on the PFDavg calculations.
Automated valve proof tests can be done offline or on-line. Offline proof checks are normally scheduled throughout a turnaround, when the method 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 forestall a spurious trip, making it inconvenient and expensive.
But a proof check can also be accomplished throughout a process trip.
According to IEC61511-2 Section A.sixteen.3.1.3, “…shutdowns because of precise demand on the SIS during operation could additionally be given credit score as proof tests (fully or partial) underneath given conditions…the subsequent deliberate proof take a look at could additionally be skipped.”
These circumstances are
The shutdown paperwork equivalent info as registered throughout corresponding proof test.
The shutdown covers all components of the SIS, and if not, the system or SIS subsystem not activated needs to be examined separately.
The shutdown happens within a predetermined maximum time window earlier than the next deliberate proof check which may then be canceled
When a course of trip happens, about 60% of a Safety Instrumented Function (SIF) proof test could be considered performed. เกจวัดแรงดันแก๊สlpg of activities performed throughout a proof take a look at, together with these which are carried out during a course of trip, is shown in Figure 2. Even with out an automated valve leak check, information captured by the DVC alone can probably account for an excellent quantity of proof test protection for an automated valve.
The exact protection is dependent upon the design of the automated valve, its DVC and its utility. The coverage is estimated based mostly on the valve degradations, the probability of their incidence and the proportion of those degradations that can be detected by a DVC. Fortunately, information acquired by a DVC throughout a process journey can typically be enough to fulfill a major a half of the proof test necessities.
If the process trip takes place within a predetermined most time window, the end consumer could choose to leverage the process trip as a proof take a look at by completing steps one via 5 in Figure 2, that are usually not accomplished in a process trip. The next scheduled proof test can then be canceled. According to the Norwegian Oil Industry Association, the maximum time window ought to be the final half of the current proof check interval.
Figure 3. Data during a course of journey is captured by the Fisher DVC6200 SIS digital valve controller and made obtainable 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 result in a process trip — include:
Partial or intermittent sticking of hydraulic or pneumatic system elements such as solenoids, pilots, velocity controller, etc., due to moisture, particles or alignment points. This causes a loss of practical margin and makes the valve gradual to open or close.
Binding, galling or other degradation of valve seats or related move control trim that restricts or resists valve movement.
Actuator seal degradation caused by compression, put on or looseness that reduces the pressure out there to actuate the valve.
Minor injury to the valve obturator plug, disk or ball caused by system circumstances, leakage or particles, including build-up of hydrocarbon products. This causes leakage.
Complete failure of hydraulic control system components corresponding to solenoids, pilots, speed controller, etc., as a outcome of moisture, debris or alignment. The valve will fail to open or shut.
All of those situations could be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For example, a valve that’s caught open will exhibit itself as an increase in breakout drive. Loss of seat load compared to when the valve assembly was new signifies potential valve leak. The Fisher DVC6200SIS additionally constantly monitors for inside faults as properly as its inputs such as provide stress. When the DVC6200SIS sees the supply 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 almost zero supply strain, so it alerted the control system.
Other process trip advantages
Diagnostic data captured during a process journey may reveal valve degradations that is most likely not detected during a proof take a look at. For instance, diagnostic information captured throughout a process journey would possibly point out a problem with the valve closing completely in opposition to the total pressure 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 more accurate beneath actual operating situations. This results in a extra accurate SIF response time (the time from detection of an irregular situation to last factor reaching its journey state), which is in comparison with the Process Safety Time to make sure the SIF remains to be assembly its design requirements.
Although automated valves are very reliable, failures identified in a course of trip can present valuable data to stop future failures. This data can assist with turnaround planning by making sure the needed parts are available earlier than turnaround even begins to potentially shorten the turnaround schedule.
Summary
A course of journey can provide the protection required to delay a valve’s next scheduled proof check, thus saving time and money. Data captured by a DVC can be analyzed to leverage the process journey as a proof take a look at. Even if the end person chooses to not take proof take a look at credits for a process trip, the valve diagnostic knowledge offered by the DVC might help plant personnel make proactive valve upkeep decisions..
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