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A process journey occurs when the security instrumented system (SIS) places the method in its safe state by commanding an automatic valve (Figure 1) or other gear to its trip state in response to an abnormal process situation. In some circumstances, a spurious journey happens because of a valve malfunction. In that case, the valve assembly is examined beneath real operating situations, which offers an opportunity to seize useful valve diagnostic knowledge.
However, such diagnostics information can only be captured when automated valves are fitted with digital valve controllers (DVCs). In this text, we’ll focus on how DVCs might help decide the proof check credit for an automatic valve after a course of journey.
Process journey
A process journey happens when the SIS detects an irregular process condition via sensors corresponding to temperature and strain, executes the logic and places the method in its protected state by tripping the ultimate elements similar to closing an automatic valve, stopping a pump, and so forth. The SIS could 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 tools that has been shut down as soon as attainable. Plant engineers are busy troubleshooting and correcting issues. Maintenance technicians are busy repairing damaged units. Taking the opportunity to proof test an automatic valve will not be a high precedence and even an activity into consideration due to the tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is supplied with a DVC, the proof test of the valve could be thought-about performed with diagnostic data captured by the DVC. Also, a comparison of the diagnostic knowledge with the valve’s baseline may help discover valve deficiencies and degradations that received’t present up in a proof take a look at.
Figure 2. Proof Test vs Process Trip. Activities performed (such as repairing the device) and knowledge captured by a Fisher DVC6200 SIS digital valve controller throughout a process trip meet a lot of the 12 necessities of a proof take a look at.
Process trips versus proof checks
How can proof test credits be claimed for an automated valve that has failed and triggered a process trip? A proof check is a periodic test carried out on SIS sensors, logic solvers and last components — corresponding to automated valves — to discover hidden failures. These failures, if not corrected or eliminated, might impair the power of the SIS to take the process to its safe state when an abnormal process condition is detected.
A proof take a look at should be carried out as per the proof take a look at interval specified within the Safety Requirement Specifications. This interval is normally determined by way of a median likelihood of failure on demand calculation (PFDavg). Some users could select to proof take a look at based on predetermined intervals, corresponding to proof testing sensors each 24 months and last components each 48 months instead of relying on the PFDavg calculations.
Automated valve proof tests could be accomplished offline or online. Offline proof checks are often scheduled throughout a turnaround, when the process is completely shut down.
Proof testing an automatic valve online normally requires a unit or gear to be shut down, or the valve to be bypassed to prevent a spurious journey, making it inconvenient and dear.
But a proof check can additionally be completed during a course of trip.
According to pressure gauge nuova fima ราคา -2 Section A.16.3.1.three, “…shutdowns as a end result of precise demand on the SIS during operation may be given credit as proof tests (fully or partial) under given conditions…the subsequent planned proof take a look at may be skipped.”
These circumstances are
The shutdown paperwork equal info as registered throughout corresponding proof take a look at.
The shutdown covers all parts of the SIS, and if not, the device or SIS subsystem not activated needs to be tested individually.
The shutdown happens within a predetermined most time window earlier than the following planned proof check which might then be canceled
When a course of journey occurs, about 60% of a Safety Instrumented Function (SIF) proof test may be thought of performed. A sample record of actions carried out throughout a proof take a look at, together with these which are carried out during a process 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 quantity of proof test protection for an automated valve.
The exact coverage is determined by the design of the automated valve, its DVC and its application. The coverage is estimated primarily based on the valve degradations, the probability of their prevalence and the percentage of those degradations that can be detected by a DVC. Fortunately, data acquired by a DVC throughout a process trip can typically be enough to satisfy a significant a part of the proof take a look at requirements.
If the method journey takes place inside a predetermined maximum time window, the top consumer might choose to leverage the process trip as a proof check by finishing steps one by way of five in Figure 2, which are usually not accomplished in a course of trip. The subsequent scheduled proof test can then be canceled. According to the Norwegian Oil Industry Association, the utmost time window must be the final half of the present proof test interval.
Figure three. Data throughout a course of journey is captured by the Fisher DVC6200 SIS digital valve controller and made out there 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 may lead to a course of trip — embrace:
Partial or intermittent sticking of hydraulic or pneumatic system parts such as solenoids, pilots, speed controller, and so forth., due to moisture, particles or alignment issues. This causes a loss of useful margin and makes the valve sluggish to open or close.
Binding, galling or different degradation of valve seats or related circulate control 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 harm to the valve obturator plug, disk or ball caused by system conditions, leakage or particles, together with build-up of hydrocarbon products. This causes leakage.
Complete failure of hydraulic control system elements similar to solenoids, pilots, velocity controller, and so on., as a result of moisture, particles or alignment. The valve will fail to open or shut.
All of those circumstances can be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For instance, a valve that is stuck open will exhibit itself as a rise in breakout force. Loss of seat load in comparability with when the valve assembly was new indicates potential valve leak. The Fisher DVC6200SIS additionally continuously monitors for inside faults as well as its inputs corresponding to provide stress. When the DVC6200SIS sees the supply strain is too low, an alert is generated (Figure 4) so that the top person can take corrective actions.
Figure 4. The DVC6200SIS digital valve controller detected almost zero provide pressure, so it alerted the control system.
Other course of journey advantages
Diagnostic knowledge captured throughout a process trip could reveal valve degradations that may not be detected throughout a proof check. For instance, diagnostic information captured during a process journey might indicate an issue with the valve closing fully towards the complete strain of the method, which may be an early indication of valve leakage.
The valve stroke time computed by a DVC throughout a course of trip is more correct underneath real operating conditions. This results in a extra accurate SIF response time (the time from detection of an irregular condition to ultimate component reaching its journey state), which is in comparison with the Process Safety Time to ensure the SIF remains to be meeting its design necessities.
Although automated valves are very dependable, failures recognized in a process journey can present useful information to prevent future failures. This information can help with turnaround planning by making sure the needed elements can be found earlier than turnaround even begins to probably shorten the turnaround schedule.
Summary
A course of trip 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 process journey as a proof test. Even if the tip consumer chooses not to take proof test credits for a course of trip, the valve diagnostic data offered by the DVC may help plant personnel make proactive valve maintenance choices..
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