A process trip occurs when the safety instrumented system (SIS) places the process in its secure state by commanding an automated valve (Figure 1) or other equipment to its trip state in response to an irregular process condition. In some circumstances, a spurious journey happens due to a valve malfunction. In that case, the valve assembly is examined beneath real operating conditions, which supplies an opportunity to seize useful valve diagnostic information.
However, such diagnostics knowledge can solely be captured when automated valves are fitted with digital valve controllers (DVCs). In this text, we’ll talk about how DVCs may help decide the proof take a look at credit score for an automatic valve after a process trip.
Process trip
A course of trip occurs when the SIS detects an irregular course of situation by way of sensors corresponding to temperature and pressure, executes the logic and places the process in its secure state by tripping the ultimate elements corresponding to closing an automatic valve, stopping a pump, etc. The SIS could talk with the Basic Process Control System (BPCS) to shut a valve downstream of the automated valve as a secondary action.
When a course of journey occurs, the primary aim is usually to restart the unit or equipment that has been shut down as quickly as potential. Plant engineers are busy troubleshooting and correcting issues. Maintenance technicians are busy repairing broken units. Taking the chance to proof test an automated valve won’t be a prime priority or even an exercise under consideration as a outcome of tight schedule after an unplanned shutdown.
Fortunately, if เกวัดแรงดันน้ำ automated valve is supplied with a DVC, the proof check of the valve can be considered performed with diagnostic information captured by the DVC. Also, a comparison of the diagnostic knowledge with the valve’s baseline may help uncover valve deficiencies and degradations that might not present up in a proof check.
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 trip meet a lot of the 12 necessities of a proof check.
Process trips versus proof checks
How can proof check credits be claimed for an automated valve that has failed and triggered a course of trip? A proof take a look at is a periodic check performed on SIS sensors, logic solvers and last components — similar to automated valves — to find hidden failures. These failures, if not corrected or removed, might impair the flexibility of the SIS to take the process to its protected state when an abnormal process condition is detected.
A proof test should be performed as per the proof check interval specified within the Safety Requirement Specifications. This interval is often decided by way of a median probability of failure on demand calculation (PFDavg). Some customers may select to proof take a look at based mostly on predetermined intervals, such as proof testing sensors each 24 months and final components each 48 months instead of relying on the PFDavg calculations.
Automated valve proof tests could be carried out offline or on-line. Offline proof tests are normally scheduled during a turnaround, when the method is totally 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 dear.
But a proof test can be achieved throughout a course of trip.
According to IEC61511-2 Section A.16.three.1.three, “…shutdowns due to actual demand on the SIS throughout operation may be given credit as proof tests (fully or partial) underneath given conditions…the subsequent planned proof take a look at may be skipped.”
These circumstances are
The shutdown paperwork equal data 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 occurs within a predetermined maximum time window earlier than the following planned proof check which can then be canceled
When a process journey occurs, about 60% of a Safety Instrumented Function (SIF) proof check could be thought of performed. A sample list of activities performed throughout a proof check, together with those that are performed throughout a process journey, is proven in Figure 2. Even with out an automatic valve leak check, information captured by the DVC alone can probably account for a good amount of proof take a look at coverage for an automatic valve.
The actual protection depends on the design of the automated valve, its DVC and its application. The coverage is estimated based on the valve degradations, the probability of their incidence and the percentage of these degradations that might be detected by a DVC. Fortunately, data acquired by a DVC throughout a course of trip can usually be sufficient to fulfill a major part of the proof check requirements.
If the process trip takes place inside a predetermined most time window, the end user may select to leverage the method trip as a proof check by finishing steps one through five in Figure 2, which are often not completed in a course of journey. The next scheduled proof check can then be canceled. According to the Norwegian Oil Industry Association, the maximum time window should be the final half of the current proof test interval.
Figure three. Data during 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 can result in a process trip — embrace:
Partial or intermittent sticking of hydraulic or pneumatic system parts similar to solenoids, pilots, speed controller, and so on., because of moisture, debris or alignment points. This causes a loss of practical margin and makes the valve slow to open or close.
Binding, galling or other degradation of valve seats or associated move control trim that restricts or resists valve motion.
Actuator seal degradation brought on by compression, put on or looseness that reduces the strain obtainable to actuate the valve.
Minor harm to the valve obturator plug, disk or ball attributable to system circumstances, leakage or particles, including build-up of hydrocarbon merchandise. This causes leakage.
Complete failure of hydraulic management system parts corresponding to solenoids, pilots, speed controller, etc., due to moisture, particles or alignment. The valve will fail to open or shut.
All of those conditions can 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 pressure. Loss of seat load in comparability with when the valve meeting was new indicates potential valve leak. The Fisher DVC6200SIS also continuously displays for internal faults as well as its inputs similar to provide pressure. When the DVC6200SIS sees the provision strain is too low, an alert is generated (Figure 4) in order that the top person can take corrective actions.
Figure 4. The DVC6200SIS digital valve controller detected nearly zero provide stress, so it alerted the control system.
Other process trip advantages
Diagnostic data captured during a course of trip might reveal valve degradations that is in all probability not detected throughout a proof test. For instance, diagnostic knowledge captured throughout a course of journey would possibly point out an issue with the valve closing utterly towards the full pressure of the process, which could be an early indication of valve leakage.
The valve stroke time computed by a DVC during a course of trip is more correct under actual operating conditions. This leads to a extra accurate SIF response time (the time from detection of an abnormal condition to ultimate factor reaching its journey state), which is in comparison with the Process Safety Time to ensure the SIF is still meeting its design necessities.
Although automated valves are very reliable, failures recognized in a course of journey can provide valuable knowledge to stop future failures. This data can help with turnaround planning by ensuring the wanted elements 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 next scheduled proof check, thus saving money and time. Data captured by a DVC can be analyzed to leverage the method journey as a proof test. Even if the top user chooses not to take proof test credit for a process trip, the valve diagnostic data supplied by the DVC might help plant personnel make proactive valve maintenance decisions..
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