A course of trip occurs when the protection instrumented system (SIS) locations the process in its safe 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 circumstances, a spurious journey happens because of a valve malfunction. In that case, the valve assembly is examined underneath actual working situations, which provides an opportunity to seize useful valve diagnostic information.
However, such diagnostics information can solely be captured when automated valves are fitted with digital valve controllers (DVCs). In this text, we’ll focus on how DVCs might help determine the proof check credit score for an automated valve after a course of trip.
Process trip
A course of trip occurs when the SIS detects an irregular course of situation through sensors corresponding to temperature and stress, executes the logic and places the method in its safe state by tripping the final elements corresponding 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 process journey happens, the primary objective is often to restart the unit or gear that has been shut down as soon as possible. Plant engineers are busy troubleshooting and correcting issues. Maintenance technicians are busy repairing damaged units. Taking the chance to proof test an automatic valve will not be a top priority or even an exercise under consideration as a outcome 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 information captured by the DVC. Also, a comparison of the diagnostic information with the valve’s baseline can help discover valve deficiencies and degradations that could not show up in a proof test.
pressure gauge ดิจิตอล . Proof Test vs Process Trip. Activities carried out (such as repairing the device) and data captured by a Fisher DVC6200 SIS digital valve controller throughout a course of trip meet most of the 12 requirements of a proof check.
Process trips versus proof tests
How can proof take a look at credits be claimed for an automated valve that has failed and triggered a process trip? A proof test is a periodic test performed on SIS sensors, logic solvers and final elements — corresponding to automated valves — to find hidden failures. These failures, if not corrected or removed, might impair the power of the SIS to take the process to its secure state when an irregular course of condition is detected.
A proof check must be performed as per the proof check interval specified within the Safety Requirement Specifications. This interval is normally determined by way of an average chance of failure on demand calculation (PFDavg). Some customers may choose to proof test based mostly on predetermined intervals, similar to proof testing sensors each 24 months and last components every forty eight months as an alternative of counting on the PFDavg calculations.
Automated valve proof checks may be done offline or online. Offline proof checks are normally scheduled during a turnaround, when the method is completely shut down.
Proof testing an automated valve online normally requires a unit or tools to be shut down, or the valve to be bypassed to forestall a spurious journey, making it inconvenient and costly.
But a proof check can also be achieved throughout a course of journey.
According to IEC61511-2 Section A.16.three.1.3, “…shutdowns because of actual demand on the SIS during operation may be given credit score as proof exams (fully or partial) underneath given conditions…the next planned proof test could also be skipped.”
These circumstances are
The shutdown paperwork equivalent data as registered throughout corresponding proof check.
The shutdown covers all components of the SIS, and if not, the gadget or SIS subsystem not activated must be examined separately.
The shutdown occurs within a predetermined maximum time window before the next planned proof test which might then be canceled
When a process journey happens, about 60% of a Safety Instrumented Function (SIF) proof take a look at can be thought-about carried out. A pattern list of actions performed throughout a proof test, along with those which are performed throughout a process journey, is proven in Figure 2. Even with out an automated valve leak check, data captured by the DVC alone can doubtlessly account for a great quantity of proof take a look at coverage for an automated valve.
The precise coverage depends on the design of the automated valve, its DVC and its utility. The coverage is estimated based mostly on the valve degradations, the likelihood of their incidence and the share of these degradations that might be detected by a DVC. Fortunately, knowledge acquired by a DVC throughout a course of journey can often be enough to fulfill a significant part of the proof check necessities.
If the process journey takes place inside a predetermined most time window, the end user may choose to leverage the method journey as a proof test by finishing steps one through five in Figure 2, which 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 maximum time window ought to be the final 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 available for analysis.
Common causes of valve failure
According to ISA Technical Report 96.05.01, the most probably root causes of automated valve degradations — which might lead to a process trip — include:
Partial or intermittent sticking of hydraulic or pneumatic system elements corresponding to solenoids, pilots, velocity controller, and so on., as a end result of moisture, particles or alignment points. This causes a lack of useful margin and makes the valve gradual to open or shut.
Binding, galling or other degradation of valve seats or associated move management trim that restricts or resists valve motion.
Actuator seal degradation caused by compression, wear or looseness that reduces the stress available to actuate the valve.
Minor injury to the valve obturator plug, disk or ball caused by system circumstances, leakage or debris, together with build-up of hydrocarbon merchandise. This causes leakage.
Complete failure of hydraulic control system parts similar to solenoids, pilots, velocity controller, etc., because of moisture, particles or alignment. The valve will fail to open or shut.
All of these conditions may be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For example, a valve that is caught open will exhibit itself as a rise in breakout pressure. Loss of seat load compared to when the valve assembly was new indicates potential valve leak. The Fisher DVC6200SIS additionally continuously screens for internal faults in addition to its inputs such as supply strain. When the DVC6200SIS sees the provision strain is too low, an alert is generated (Figure 4) in order that the tip user can take corrective actions.
Figure four. The DVC6200SIS digital valve controller detected virtually zero provide pressure, so it alerted the control system.
Other process journey benefits
Diagnostic information captured during a process trip may reveal valve degradations that may not be detected during a proof check. For example, diagnostic information captured during a process trip would possibly indicate a problem with the valve closing fully towards the complete pressure of the process, which can be an early indication of valve leakage.
The valve stroke time computed by a DVC throughout a course of journey is more correct underneath actual working conditions. This results in a more correct SIF response time (the time from detection of an abnormal situation to ultimate component reaching its trip state), which is in comparability with the Process Safety Time to ensure the SIF is still meeting its design requirements.
Although automated valves are very reliable, failures recognized in a process journey can present valuable information to forestall future failures. This info can assist with turnaround planning by ensuring the needed elements can be found earlier than turnaround even begins to probably shorten the turnaround schedule.
Summary
A process trip can provide the protection required to delay a valve’s next scheduled proof take a look at, thus saving money and time. Data captured by a DVC can be analyzed to leverage the process trip as a proof take a look at. Even if the tip consumer chooses not to take proof check credits for a process trip, the valve diagnostic data offered by the DVC can help plant personnel make proactive valve maintenance selections..
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