A process trip happens when the protection instrumented system (SIS) places the process in its protected state by commanding an automated valve (Figure 1) or other tools to its journey state in response to an irregular process condition. In some cases, a spurious trip happens due to a valve malfunction. In that case, the valve meeting is examined under actual operating conditions, which supplies a chance to seize priceless valve diagnostic knowledge.
However, such diagnostics data 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 determine the proof take a look at credit score for an automated valve after a process trip.
Process journey
A process journey occurs when the SIS detects an irregular process condition by way of sensors such as temperature and stress, executes the logic and locations the process in its secure state by tripping the final parts corresponding to closing an automated valve, stopping a pump, and so forth. 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 เกจ์อาร์กอนsumo happens, the primary goal is often to restart the unit or equipment that has been shut down as quickly as possible. Plant engineers are busy troubleshooting and correcting issues. Maintenance technicians are busy repairing damaged gadgets. Taking the opportunity to proof test an automated valve is not going to be a prime priority and even an activity into consideration due to the tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is provided with a DVC, the proof take a look at of the valve can be considered performed with diagnostic knowledge captured by the DVC. Also, a comparability of the diagnostic knowledge with the valve’s baseline can help discover valve deficiencies and degradations that might not show up in a proof test.
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 throughout a process trip meet most of the 12 necessities of a proof check.
Process journeys versus proof checks
How can proof test credit be claimed for an automated valve that has failed and brought on a process trip? A proof take a look at is a periodic take a look at performed on SIS sensors, logic solvers and last components — similar to automated valves — to find hidden failures. These failures, if not corrected or removed, could impair the power of the SIS to take the process to its safe state when an abnormal process situation is detected.
A proof test should be performed as per the proof check interval specified in the Safety Requirement Specifications. This interval is often decided by way of a median chance of failure on demand calculation (PFDavg). Some customers could choose to proof test based mostly on predetermined intervals, corresponding to proof testing sensors every 24 months and final parts every forty eight months as a substitute of relying on the PFDavg calculations.
Automated valve proof tests could be accomplished offline or on-line. Offline proof checks are normally scheduled during a turnaround, when the process is completely shut down.
Proof testing an automated valve online usually requires a unit or gear to be shut down, or the valve to be bypassed to forestall a spurious trip, making it inconvenient and expensive.
But a proof take a look at may also be achieved during a course of journey.
According to IEC61511-2 Section A.16.3.1.three, “…shutdowns because of precise demand on the SIS throughout operation could additionally be given credit score as proof checks (fully or partial) underneath given conditions…the subsequent deliberate proof test may be skipped.”
These situations are
The shutdown paperwork equal info as registered during corresponding proof test.
The shutdown covers all elements of the SIS, and if not, the system or SIS subsystem not activated must be examined individually.
The shutdown occurs within a predetermined maximum time window earlier than the subsequent deliberate proof test which might then be canceled
When a process trip occurs, about 60% of a Safety Instrumented Function (SIF) proof take a look at can be thought-about carried out. A pattern record of activities 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 without an automated valve leak check, knowledge captured by the DVC alone can potentially account for an excellent amount of proof check coverage for an automatic valve.
The exact protection depends on the design of the automated valve, its DVC and its software. The coverage is estimated primarily based on the valve degradations, the chance of their incidence and the share of those degradations that can be detected by a DVC. Fortunately, knowledge acquired by a DVC throughout a process trip can often be sufficient to fulfill a significant a half of the proof test necessities.
If the process journey takes place within a predetermined maximum time window, the top user might select to leverage the method journey as a proof check by completing steps one by way of 5 in Figure 2, which are usually not accomplished in a process trip. The subsequent 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 present proof take a look at interval.
Figure 3. Data throughout a course of trip is captured by the Fisher DVC6200 SIS digital valve controller and made obtainable for evaluation.
Common causes of valve failure
According to ISA Technical Report ninety six.05.01, the more than likely root causes of automated valve degradations — which might result in a course of trip — embrace:
Partial or intermittent sticking of hydraulic or pneumatic system components such as solenoids, pilots, velocity controller, etc., as a outcome of moisture, debris or alignment points. This causes a lack of functional margin and makes the valve slow 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, wear or looseness that reduces the stress obtainable to actuate the valve.
Minor injury to the valve obturator plug, disk or ball caused by system situations, leakage or particles, including build-up of hydrocarbon products. This causes leakage.
Complete failure of hydraulic control system elements corresponding to solenoids, pilots, speed controller, and so on., because of moisture, particles or alignment. The valve will fail to open or close.
All of these conditions could be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For instance, a valve that’s stuck open will exhibit itself as an increase in breakout pressure. Loss of seat load compared to when the valve meeting was new indicates potential valve leak. The Fisher DVC6200SIS additionally constantly monitors for internal faults as nicely as its inputs similar to provide strain. When the DVC6200SIS sees the availability pressure is too low, an alert is generated (Figure 4) in order that the end person can take corrective actions.
Figure 4. The DVC6200SIS digital valve controller detected virtually zero supply strain, so it alerted the control system.
Other process trip advantages
Diagnostic information captured throughout a course of trip may reveal valve degradations that may not be detected during a proof check. For instance, diagnostic knowledge captured during a process journey may point out an issue with the valve closing utterly against the full stress of the method, which could be an early indication of valve leakage.
The valve stroke time computed by a DVC during a process trip is more accurate underneath real operating conditions. This ends in a extra accurate SIF response time (the time from detection of an abnormal condition to final element reaching its trip state), which is compared to the Process Safety Time to make sure the SIF is still meeting its design requirements.
Although automated valves are very reliable, failures recognized in a course of trip can provide priceless information to forestall future failures. This info can assist with turnaround planning by making sure the needed parts can be found earlier than turnaround even begins to doubtlessly shorten the turnaround schedule.
Summary
A course of trip can provide the protection required to delay a valve’s next scheduled proof take a look at, thus saving money and time. Data captured by a DVC can be analyzed to leverage the method trip as a proof test. Even if the tip consumer chooses to not take proof test credits for a course of journey, the valve diagnostic knowledge offered by the DVC can help plant personnel make proactive valve upkeep selections..
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