A course of trip happens when the safety instrumented system (SIS) places the process in its safe state by commanding an automatic valve (Figure 1) or different tools to its journey state in response to an irregular process situation. In some instances, a spurious trip occurs due to a valve malfunction. In เกจวัดแรงดันpcp , the valve meeting is examined underneath actual working situations, which supplies a chance to capture 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 focus on how DVCs can help decide the proof test credit score for an automated valve after a course of trip.
Process journey
A process journey happens when the SIS detects an abnormal process situation via sensors corresponding to temperature and strain, executes the logic and locations the method in its secure state by tripping the final components similar to closing an automated valve, stopping a pump, and so forth. 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 trip occurs, the primary goal is often to restart the unit or equipment that has been shut down as quickly as potential. Plant engineers are busy troubleshooting and correcting problems. Maintenance technicians are busy repairing broken units. Taking the chance to proof check an automatic valve will not be a prime precedence and even an activity into account as a end result of tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is supplied with a DVC, the proof check of the valve may be thought-about carried out with diagnostic information captured by the DVC. Also, a comparison of the diagnostic knowledge with the valve’s baseline can help discover valve deficiencies and degradations that won’t 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 course of trip meet a lot of the 12 requirements of a proof test.
Process trips versus proof exams
How can proof take a look at credits be claimed for an automated valve that has failed and caused a process trip? A proof check is a periodic test carried out on SIS sensors, logic solvers and last components — similar to automated valves — to find hidden failures. These failures, if not corrected or eliminated, may impair the power of the SIS to take the process to its secure state when an irregular course of condition is detected.
A proof take a look at ought to be carried out as per the proof check interval specified in the Safety Requirement Specifications. This interval is usually decided by way of a mean probability of failure on demand calculation (PFDavg). Some users might choose to proof take a look at based on predetermined intervals, similar to proof testing sensors every 24 months and ultimate elements every 48 months as an alternative of counting on the PFDavg calculations.
Automated valve proof checks can be carried out offline or online. Offline proof exams are normally scheduled throughout a turnaround, when the method is totally shut down.
Proof testing an automated 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 costly.
But a proof take a look at may also be completed throughout a course of trip.
According to IEC61511-2 Section A.sixteen.3.1.three, “…shutdowns due to actual demand on the SIS during operation could additionally be given credit score as proof checks (fully or partial) underneath given conditions…the next deliberate proof take a look at could additionally be skipped.”
These situations are
The shutdown paperwork equivalent info 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 most time window before the next deliberate proof take a look at which can then be canceled
When a process trip happens, about 60% of a Safety Instrumented Function (SIF) proof take a look at may be thought-about carried out. A pattern listing of actions performed throughout a proof test, together with these that are performed throughout a course of journey, is shown in Figure 2. Even with out an automatic valve leak check, data captured by the DVC alone can potentially account for an excellent amount of proof take a look at protection for an automatic valve.
The exact protection depends on the design of the automated valve, its DVC and its utility. The coverage is estimated primarily based on the valve degradations, the probability of their incidence and the percentage of those degradations that can be detected by a DVC. Fortunately, knowledge acquired by a DVC during a course of journey can often be enough to satisfy a major a part of the proof take a look at requirements.
If the method journey takes place inside a predetermined most time window, the end consumer might select to leverage the method trip as a proof check by finishing steps one by way of 5 in Figure 2, which are normally not completed in a process journey. The subsequent scheduled proof test can then be canceled. According to the Norwegian Oil Industry Association, the utmost time window should be the last half of the present proof check interval.
Figure 3. Data during a course of journey 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 96.05.01, the more than likely root causes of automated valve degradations — which can lead to a process journey — embody:
Partial or intermittent sticking of hydraulic or pneumatic system elements corresponding to solenoids, pilots, pace controller, and so forth., due to moisture, debris or alignment points. This causes a lack of useful margin and makes the valve sluggish to open or close.
Binding, galling or other degradation of valve seats or associated flow management trim that restricts or resists valve motion.
Actuator seal degradation brought on by compression, wear or looseness that reduces the pressure 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 components such as solenoids, pilots, velocity controller, and so on., due to moisture, particles or alignment. The valve will fail to open or shut.
All of these circumstances could be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For example, a valve that’s caught open will exhibit itself as a rise in breakout force. Loss of seat load in comparability with when the valve meeting was new indicates potential valve leak. The Fisher DVC6200SIS additionally repeatedly monitors for inner faults in addition to its inputs such as provide strain. When the DVC6200SIS sees the availability stress is merely 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 nearly zero supply strain, so it alerted the control system.
Other process journey advantages
Diagnostic data captured during a process trip may reveal valve degradations that will not be detected during a proof test. For instance, diagnostic information captured throughout a process journey might indicate an issue with the valve closing fully towards the full strain 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 beneath real operating circumstances. This leads to a more accurate SIF response time (the time from detection of an abnormal condition to final component reaching its journey state), which is in comparability with the Process Safety Time to make sure the SIF is still assembly its design requirements.
Although automated valves are very reliable, failures identified in a course of trip can present priceless data to stop future failures. This information can help with turnaround planning by making sure the needed elements are available before turnaround even begins to probably shorten the turnaround schedule.
Summary
A process 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 could be analyzed to leverage the process trip as a proof check. Even if the tip user chooses not to take proof test credit for a course of journey, the valve diagnostic data offered by the DVC may help plant personnel make proactive valve upkeep decisions..
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