A course of journey happens when the protection instrumented system (SIS) places the method in its safe state by commanding an automated valve (Figure 1) or other equipment to its trip state in response to an abnormal course of situation. In some instances, a spurious trip happens due to a valve malfunction. In that case, the valve assembly is tested underneath real working circumstances, which supplies a possibility to seize valuable valve diagnostic information.
However, such diagnostics information can solely be captured when automated valves are fitted with digital valve controllers (DVCs). In this article, we’ll talk about how DVCs might help decide the proof take a look at credit for an automated valve after a process journey.
Process journey
A process journey occurs when the SIS detects an irregular process situation via sensors similar to temperature and pressure, executes the logic and places the process in its secure state by tripping the ultimate elements similar to closing an automated valve, stopping a pump, and so on. The SIS may communicate with the Basic Process Control System (BPCS) to shut a valve downstream of the automated valve as a secondary action.
When a process trip happens, the primary goal is often to restart the unit or equipment that has been shut down as quickly as attainable. Plant engineers are busy troubleshooting and correcting problems. Maintenance technicians are busy repairing damaged units. Taking the opportunity to proof check an automatic valve is not going to be a high precedence or even an activity into consideration because of the tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is supplied with a DVC, the proof check of the valve could be considered performed with diagnostic knowledge captured by the DVC. Also, a comparability of the diagnostic knowledge with the valve’s baseline can help uncover valve deficiencies and degradations that will not present up in a proof take a look at.
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 trip meet many of the 12 requirements of a proof test.
Process trips versus proof tests
How can proof take a look at credits be claimed for an automated valve that has failed and caused a course of trip? A proof check is a periodic take a look at carried out on SIS sensors, logic solvers and last elements — corresponding to automated valves — to discover hidden failures. These failures, if not corrected or removed, may impair the power of the SIS to take the process to its protected state when an irregular process situation is detected.
A proof take a look at must be performed as per the proof test 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 users may select to proof take a look at based on predetermined intervals, corresponding to proof testing sensors each 24 months and ultimate elements every forty eight months as an alternative of counting on the PFDavg calculations.
Automated valve proof checks could be done offline or online. Offline proof checks are usually scheduled throughout a turnaround, when the method is completely shut down.
Proof testing an automatic valve on-line often requires a unit or gear to be shut down, or the valve to be bypassed to forestall a spurious journey, making it inconvenient and expensive.
But a proof test can additionally be completed throughout a course of journey.
According to IEC61511-2 Section A.16.3.1.3, “…shutdowns as a outcome of actual demand on the SIS during operation may be given credit as proof checks (fully or partial) under given conditions…the next deliberate proof take a look at may be skipped.”
These conditions are
The shutdown paperwork equivalent information as registered throughout corresponding proof test.
The shutdown covers all parts of the SIS, and if not, the system or SIS subsystem not activated must be examined individually.
The shutdown happens within a predetermined maximum time window earlier than the following planned proof take a look at which might then be canceled
When a course of journey happens, about 60% of a Safety Instrumented Function (SIF) proof take a look at may be thought-about carried out. A sample list of actions performed during a proof test, together with these which would possibly be carried out throughout a course of journey, is shown in Figure 2. Even with out เกจวัดแรงดันลมราคา automated valve leak take a look at, knowledge captured by the DVC alone can probably account for a good quantity of proof test coverage for an automatic valve.
The actual protection is determined by the design of the automated valve, its DVC and its software. The protection is estimated primarily based on the valve degradations, the probability of their incidence and the percentage of these degradations that can be detected by a DVC. Fortunately, data acquired by a DVC throughout a process journey can typically be sufficient to fulfill a significant a part of the proof test requirements.
If the method journey takes place within a predetermined most time window, the tip person could select to leverage the method journey as a proof test by finishing steps one through 5 in Figure 2, which are often not completed in a process journey. The next scheduled proof check can then be canceled. According to the Norwegian Oil Industry Association, the utmost time window must be the final half of the current proof test interval.
Figure three. Data throughout a process journey is captured by the Fisher DVC6200 SIS digital valve controller and made obtainable for analysis.
เกจวัดแก๊ส 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 journey — include:
Partial or intermittent sticking of hydraulic or pneumatic system components such as solenoids, pilots, velocity controller, and so forth., because of moisture, debris or alignment points. This causes a loss of functional margin and makes the valve sluggish to open or shut.
Binding, galling or other degradation of valve seats or related flow control trim that restricts or resists valve motion.
Actuator seal degradation attributable to compression, wear or looseness that reduces the stress available to actuate the valve.
Minor harm to the valve obturator plug, disk or ball attributable to system conditions, leakage or particles, including build-up of hydrocarbon products. This causes leakage.
Complete failure of hydraulic control system elements similar to solenoids, pilots, velocity controller, etc., because of moisture, particles or alignment. The valve will fail to open or shut.
All of those conditions could be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For example, a valve that is 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 inside faults as well as its inputs corresponding to provide stress. When the DVC6200SIS sees the availability strain is simply too low, an alert is generated (Figure 4) so that the tip consumer can take corrective actions.
Figure four. The DVC6200SIS digital valve controller detected virtually zero supply pressure, so it alerted the control system.
Other process trip advantages
Diagnostic knowledge captured throughout a process trip could reveal valve degradations that may not be detected throughout a proof take a look at. For example, diagnostic data captured during a course of trip might point out a problem with the valve closing utterly in opposition to the total stress of the process, which could be an early indication of valve leakage.
The valve stroke time computed by a DVC during a process journey is extra correct under actual operating conditions. This results in a more accurate SIF response time (the time from detection of an abnormal condition to last element reaching its trip state), which is compared to the Process Safety Time to ensure the SIF is still meeting its design necessities.
Although automated valves are very dependable, failures recognized in a course of trip can present useful data to prevent future failures. This info can assist with turnaround planning by making sure the needed parts are available before turnaround even begins to doubtlessly 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 process journey as a proof take a look at. Even if the end person chooses to not take proof check credit for a course of trip, the valve diagnostic knowledge supplied by the DVC might help plant personnel make proactive valve upkeep decisions..
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