A process journey happens when the security instrumented system (SIS) places the process in its protected state by commanding an automated valve (Figure 1) or different gear to its trip state in response to an abnormal course of condition. In some instances, a spurious trip occurs because of a valve malfunction. In that case, the valve assembly is examined beneath real working circumstances, which provides an opportunity to capture useful valve diagnostic data.
However, such diagnostics knowledge can only 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 check credit for an automated valve after a process trip.
Process journey
A process journey happens when the SIS detects an abnormal course of situation by way of sensors similar to temperature and pressure, executes the logic and locations the method 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 close a valve downstream of the automated valve as a secondary action.
When a process journey occurs, the primary aim is normally to restart the unit or gear that has been shut down as quickly as potential. Plant engineers are busy troubleshooting and correcting issues. Maintenance technicians are busy repairing broken devices. Taking the opportunity to proof take a look at an automated valve won’t be a high precedence or even an activity under consideration as a end result of tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is equipped with a DVC, the proof take a look at of the valve can be considered carried out with diagnostic data captured by the DVC. Also, a comparability of the diagnostic information with the valve’s baseline can help uncover valve deficiencies and degradations that will not show up in a proof test.
Figure 2. Proof Test vs Process Trip. Activities performed (such as repairing the device) and information captured by a Fisher DVC6200 SIS digital valve controller throughout a course of journey meet a lot of the 12 requirements of a proof check.
Process journeys versus proof checks
How can proof take a look at credit be claimed for an automatic valve that has failed and caused a course of trip? A proof test is a periodic check carried out on SIS sensors, logic solvers and final elements — similar to automated valves — to find hidden failures. These failures, if not corrected or removed, may impair the ability of the SIS to take the method to its secure state when an irregular process condition is detected.
A proof check must be performed as per the proof check interval specified within the Safety Requirement Specifications. This interval is usually determined through an average probability of failure on demand calculation (PFDavg). Some users may select to proof take a look at based on predetermined intervals, similar to proof testing sensors each 24 months and final elements each 48 months instead of relying on the PFDavg calculations.
Automated valve proof checks could be accomplished offline or on-line. Offline proof tests are often scheduled throughout a turnaround, when the method is completely shut down.
Proof testing an automated valve on-line often requires a unit or equipment to be shut down, or the valve to be bypassed to stop a spurious trip, making it inconvenient and costly.
But a proof take a look at may additionally be completed during a course of trip.
According to IEC61511-2 Section A.16.3.1.three, “…shutdowns because of actual demand on the SIS throughout operation could additionally be given credit as proof checks (fully or partial) under given conditions…the subsequent planned proof check could also be skipped.”
These circumstances are
The shutdown paperwork equal data as registered throughout corresponding proof test.
The shutdown covers all parts of the SIS, and if not, the device or SIS subsystem not activated must be tested individually.
The shutdown happens inside a predetermined maximum time window before the subsequent deliberate proof test which might then be canceled
When a process journey occurs, about 60% of a Safety Instrumented Function (SIF) proof check can be thought-about performed. A sample list of actions carried out during a proof check, along with those that are carried out during a process trip, is proven in Figure 2. Even without an automated valve leak test, information captured by the DVC alone can doubtlessly account for an excellent quantity of proof take a look at coverage for an automatic valve.
The actual coverage is decided by the design of the automated valve, its DVC and its application. The coverage is estimated based mostly on the valve degradations, the chance of their incidence and the proportion of these degradations that can be detected by a DVC. Fortunately, data acquired by a DVC throughout a process journey can usually be sufficient to fulfill a serious a part of the proof test necessities.
If the method journey takes place within a predetermined most time window, the top person could choose to leverage the method trip as a proof check by finishing steps one by way of five in Figure 2, that are normally 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 should be the last half of the present proof test interval.
Figure 3. Data throughout a process 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 most probably root causes of automated valve degradations — which might result in a course of journey — embody:
Partial or intermittent sticking of hydraulic or pneumatic system components corresponding to solenoids, pilots, velocity controller, and so forth., as a outcome of moisture, debris or alignment issues. This causes a loss of useful margin and makes the valve gradual to open or close.
Binding, galling or different degradation of valve seats or related flow control trim that restricts or resists valve motion.
Actuator seal degradation caused by compression, wear or looseness that reduces the stress out there to actuate the valve.
Minor damage to the valve obturator plug, disk or ball caused by system conditions, leakage or particles, including build-up of hydrocarbon merchandise. This causes leakage.
Complete failure of hydraulic control system parts such as solenoids, pilots, pace controller, and so forth., as a result of moisture, debris or alignment. เกจวัดถังแก๊ส will fail to open or close.
All of those conditions could be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For example, a valve that’s stuck open will exhibit itself as an increase in breakout drive. Loss of seat load compared to when the valve assembly was new indicates potential valve leak. The Fisher DVC6200SIS also continuously monitors for internal faults in addition to its inputs similar to supply strain. When the DVC6200SIS sees the supply strain is merely too low, an alert is generated (Figure 4) so that the tip consumer can take corrective actions.
Figure 4. The DVC6200SIS digital valve controller detected virtually zero provide pressure, so it alerted the control system.
Other course of trip benefits
Diagnostic data captured throughout a course of journey may reveal valve degradations that may not be detected throughout a proof check. For example, diagnostic knowledge captured during a course of trip would possibly point out a problem with the valve closing completely against the total strain of the method, which may be an early indication of valve leakage.
The valve stroke time computed by a DVC throughout a process trip is more accurate underneath real working circumstances. This ends in a extra correct SIF response time (the time from detection of an irregular situation to ultimate element reaching its journey state), which is in comparison with the Process Safety Time to make sure the SIF remains to be meeting its design necessities.
Although automated valves are very dependable, failures recognized in a course of journey can present valuable data to forestall future failures. This data can assist with turnaround planning by making sure the needed parts can be found before turnaround even begins to probably shorten the turnaround schedule.
Summary
A course of trip can provide the protection required to delay a valve’s next scheduled proof test, thus saving money and time. Data captured by a DVC may be analyzed to leverage the method journey as a proof check. Even if the tip consumer chooses not to take proof test credits for a process trip, the valve diagnostic knowledge provided by the DVC can help plant personnel make proactive valve maintenance choices..
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