A course of trip occurs when the safety instrumented system (SIS) places the method in its protected state by commanding an automated valve (Figure 1) or other tools to its trip state in response to an abnormal process condition. In some instances, a spurious trip happens because of a valve malfunction. In that case, the valve meeting is tested under actual operating conditions, which offers a chance to seize valuable valve diagnostic information.
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 can help decide the proof take a look at credit for an automatic valve after a process journey.
Process trip
A process journey happens when the SIS detects an irregular process condition through sensors such as temperature and stress, executes the logic and places the process in its secure state by tripping the final parts such as closing an automated valve, stopping a pump, and so on. The SIS might communicate with the Basic Process Control System (BPCS) to close a valve downstream of the automated valve as a secondary action.
When a course of trip happens, the primary aim is often to restart the unit or gear that has been shut down as quickly as potential. Plant engineers are busy troubleshooting and correcting problems. Maintenance technicians are busy repairing damaged devices. Taking the opportunity to proof take a look at an automatic valve is not going to be a high priority and even an activity into consideration because of the 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 thought of performed with diagnostic information captured by the DVC. Also, a comparability of the diagnostic information with the valve’s baseline can help uncover valve deficiencies and degradations that won’t present up in a proof test.
Figure 2. Proof Test vs Process Trip. Activities carried out (such as repairing the device) and information captured by a Fisher DVC6200 SIS digital valve controller during a process journey meet a lot of the 12 necessities of a proof test.
Process trips versus proof checks
How can proof check credit be claimed for an automatic valve that has failed and caused a course of trip? A proof check is a periodic check performed on SIS sensors, logic solvers and final elements — such as automated valves — to discover hidden failures. These failures, if not corrected or eliminated, may impair the ability of the SIS to take the method to its secure state when an abnormal process situation is detected.
A proof check ought to be performed as per the proof check interval specified within the Safety Requirement Specifications. This interval is usually determined via a median chance of failure on demand calculation (PFDavg). Some customers may select to proof take a look at based mostly on predetermined intervals, corresponding 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 exams can be accomplished offline or on-line. Offline proof exams are usually scheduled throughout a turnaround, when the method is completely shut down.
Proof testing an automatic valve on-line normally requires a unit or equipment to be shut down, or the valve to be bypassed to stop a spurious journey, making it inconvenient and costly.
But เกวัดแรงดัน take a look at may additionally be achieved throughout a course of journey.
According to IEC61511-2 Section A.16.three.1.3, “…shutdowns as a outcome of precise demand on the SIS during operation may be given credit as proof checks (fully or partial) under given conditions…the next deliberate proof test could additionally be skipped.”
These circumstances are
The shutdown documents equivalent data as registered during corresponding proof check.
The shutdown covers all parts of the SIS, and if not, the system or SIS subsystem not activated needs to be examined individually.
The shutdown happens inside a predetermined most time window earlier than the following deliberate proof check which might then be canceled
When a process trip occurs, about 60% of a Safety Instrumented Function (SIF) proof check can be thought of performed. A pattern listing of actions performed throughout a proof take a look at, along with these that are performed during a process trip, is shown in Figure 2. Even with out an automated valve leak take a look at, knowledge captured by the DVC alone can doubtlessly account for a great quantity of proof check coverage for an automatic valve.
The precise coverage depends on the design of the automated valve, its DVC and its software. The coverage is estimated primarily based on the valve degradations, the probability of their occurrence and the percentage of those degradations that can be detected by a DVC. Fortunately, data acquired by a DVC throughout a process trip can often be sufficient to fulfill a significant a half of the proof take a look at requirements.
If the process trip takes place within a predetermined maximum time window, the end user might choose to leverage the process journey as a proof test by finishing steps one via 5 in Figure 2, which are usually not completed in a process journey. The next 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 current proof take a look at interval.
Figure three. Data throughout a process trip is captured by the Fisher DVC6200 SIS digital valve controller and made obtainable for analysis.
Common causes of valve failure
According to ISA Technical Report ninety six.05.01, the most probably root causes of automated valve degradations — which can lead to a course of journey — include:
Partial or intermittent sticking of hydraulic or pneumatic system parts similar to solenoids, pilots, pace controller, etc., due to moisture, particles or alignment points. This causes a lack of practical margin and makes the valve sluggish to open or shut.
Binding, galling or other degradation of valve seats or associated flow control trim that restricts or resists valve motion.
Actuator seal degradation attributable to compression, wear or looseness that reduces the strain obtainable to actuate the valve.
Minor injury to the valve obturator plug, disk or ball caused by system circumstances, leakage or debris, including build-up of hydrocarbon products. This causes leakage.
Complete failure of hydraulic control system components corresponding to solenoids, pilots, velocity controller, etc., as a end result of moisture, particles or alignment. The valve will fail to open or shut.
All of these conditions can be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For example, a valve that’s caught open will exhibit itself as an increase in breakout drive. Loss of seat load in comparison with when the valve assembly was new indicates potential valve leak. The Fisher DVC6200SIS additionally repeatedly displays for inner faults in addition to its inputs such as provide pressure. When the DVC6200SIS sees the supply strain is simply too low, an alert is generated (Figure 4) in order that the top person can take corrective actions.
Figure 4. The DVC6200SIS digital valve controller detected almost zero supply stress, so it alerted the control system.
Other process trip benefits
Diagnostic information captured during a process journey may reveal valve degradations that will not be detected throughout a proof take a look at. For example, diagnostic information captured throughout a process journey might point out a problem with the valve closing completely towards the full strain of the method, which could be an early indication of valve leakage.
The valve stroke time computed by a DVC during a course of trip is extra correct beneath real working conditions. This ends in a more accurate SIF response time (the time from detection of an irregular situation to ultimate element reaching its journey state), which is in comparability with the Process Safety Time to make sure the SIF remains to be assembly its design necessities.
Although automated valves are very dependable, failures identified in a course of journey can present useful knowledge to forestall future failures. This information can assist with turnaround planning by making sure the wanted parts can be found earlier than turnaround even begins to potentially shorten the turnaround schedule.
Summary
A process journey can provide the protection required to delay a valve’s subsequent scheduled proof take a look at, thus saving time and money. Data captured by a DVC may be analyzed to leverage the method journey as a proof check. Even if the tip user chooses to not take proof test credits for a course of trip, the valve diagnostic data offered by the DVC might help plant personnel make proactive valve upkeep selections..
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