A process journey occurs when the protection instrumented system (SIS) locations the process in its secure state by commanding an automatic valve (Figure 1) or different tools to its trip state in response to an abnormal course of condition. In some cases, a spurious journey occurs because of a valve malfunction. In that case, the valve assembly is tested under actual working situations, which supplies a possibility to capture valuable valve diagnostic knowledge.
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 can help decide the proof check credit for an automatic valve after a process journey.
Process journey
A process journey occurs when the SIS detects an irregular course of situation by way of sensors such as temperature and pressure, executes the logic and locations the process in its secure state by tripping the ultimate elements corresponding to closing an automated valve, stopping a pump, etc. The SIS might communicate with the Basic Process Control System (BPCS) to shut a valve downstream of the automated valve as a secondary motion.
When a course of trip happens, the main objective is usually to restart the unit or equipment that has been shut down as quickly as possible. Plant engineers are busy troubleshooting and correcting problems. Maintenance technicians are busy repairing broken gadgets. Taking the chance to proof check an automated valve won’t be a top priority 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 can be thought-about performed with diagnostic knowledge captured by the DVC. Also, a comparison of the diagnostic knowledge with the valve’s baseline may help discover valve deficiencies and degradations that might not show up in a proof take a look at.
Figure 2. Proof Test vs Process Trip. Activities performed (such as repairing the device) and data captured by a Fisher DVC6200 SIS digital valve controller throughout a process trip meet many of the 12 necessities 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 process trip? A proof test is a periodic check performed on SIS sensors, logic solvers and final parts — similar to automated valves — to find hidden failures. These failures, if not corrected or eliminated, could impair the flexibility of the SIS to take the method to its protected state when an abnormal course of situation is detected.
A proof take a look at ought to be carried out as per the proof take a look at interval specified in the Safety Requirement Specifications. ไดอะแฟรม is often determined through a median chance of failure on demand calculation (PFDavg). Some users may select to proof test based mostly on predetermined intervals, corresponding to proof testing sensors every 24 months and last elements every 48 months instead of counting on the PFDavg calculations.
Automated valve proof checks may be carried out offline or on-line. Offline proof checks are often 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 forestall a spurious journey, making it inconvenient and dear.
But a proof test can also be achieved during a course of journey.
According to IEC61511-2 Section A.16.three.1.three, “…shutdowns because of actual demand on the SIS throughout operation may be given credit score as proof checks (fully or partial) under given conditions…the next deliberate proof check may be skipped.”
These situations are
The shutdown paperwork equal information as registered during corresponding proof test.
The shutdown covers all parts of the SIS, and if not, the system or SIS subsystem not activated must be examined separately.
The shutdown occurs within a predetermined maximum time window before the next planned proof take a look at which may then be canceled
When a course of journey happens, about 60% of a Safety Instrumented Function (SIF) proof test could be thought-about performed. A sample list of actions performed during a proof take a look at, along with these that are performed during a course of trip, is shown in Figure 2. Even without an automatic valve leak check, data captured by the DVC alone can potentially account for an excellent quantity of proof check coverage for an automatic valve.
The precise protection is dependent upon the design of the automated valve, its DVC and its application. The coverage is estimated primarily based on the valve degradations, the chance of their incidence and the percentage of these degradations that could be detected by a DVC. Fortunately, information acquired by a DVC throughout a course of trip can typically be enough to fulfill a major part of the proof test requirements.
If the process trip takes place inside a predetermined most time window, the tip consumer might choose to leverage the method trip as a proof test by completing steps one via 5 in Figure 2, which are often not completed in a course of journey. The subsequent scheduled proof test can then be canceled. According to the Norwegian Oil Industry Association, the maximum time window must be the final half of the current proof take a look at interval.
Figure 3. Data during a course of journey is captured by the Fisher DVC6200 SIS digital valve controller and made available 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 can result in a course of trip — embody:
Partial or intermittent sticking of hydraulic or pneumatic system components corresponding to solenoids, pilots, velocity controller, and so forth., due to moisture, debris or alignment points. This causes a loss of practical margin and makes the valve slow to open or shut.
Binding, galling or different degradation of valve seats or related move control trim that restricts or resists valve motion.
Actuator seal degradation brought on by compression, put on or looseness that reduces the strain available to actuate the valve.
Minor injury to the valve obturator plug, disk or ball attributable to system conditions, leakage or debris, 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., because of moisture, debris or alignment. The valve will fail to open or close.
All of these circumstances can be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For example, a valve that is caught open will exhibit itself as an increase in breakout pressure. Loss of seat load compared to when the valve assembly was new signifies potential valve leak. The Fisher DVC6200SIS additionally constantly screens for inside faults as properly as its inputs corresponding to provide strain. When the DVC6200SIS sees the provision stress is simply too low, an alert is generated (Figure 4) so that the end user can take corrective actions.
Figure 4. The DVC6200SIS digital valve controller detected nearly zero supply strain, so it alerted the control system.
Other process trip benefits
Diagnostic knowledge captured during a course of trip may reveal valve degradations that will not be detected during a proof test. For instance, diagnostic data captured during a course of trip would possibly indicate a problem with the valve closing completely in opposition to the total strain of the process, which may be an early indication of valve leakage.
The valve stroke time computed by a DVC throughout a course of journey is extra accurate under real operating conditions. This ends in a more accurate SIF response time (the time from detection of an irregular situation to final factor reaching its journey state), which is compared to the Process Safety Time to ensure the SIF continues to be assembly its design requirements.
Although automated valves are very dependable, failures identified in a course of journey can present priceless data to prevent future failures. This information can help with turnaround planning by ensuring the needed components can be found earlier than turnaround even begins to doubtlessly shorten the turnaround schedule.
Summary
A process journey can present the coverage required to delay a valve’s subsequent scheduled proof take a look at, thus saving time and money. Data captured by a DVC could be analyzed to leverage the method trip as a proof check. Even if the top user chooses to not take proof take a look at credits for a course of journey, the valve diagnostic knowledge supplied by the DVC can help plant personnel make proactive valve upkeep choices..
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