A course of journey happens when the safety instrumented system (SIS) places the process in its safe state by commanding an automated valve (Figure 1) or other gear to its trip state in response to an abnormal course of situation. In some cases, a spurious trip happens due to a valve malfunction. In that case, the valve assembly is examined under real operating situations, which supplies a possibility to capture 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 discuss how DVCs might help determine the proof test credit score for an automatic valve after a process journey.
Process trip
A course of trip occurs when the SIS detects an irregular process condition via sensors corresponding to temperature and stress, executes the logic and locations the method in its protected state by tripping the ultimate elements corresponding to closing an automated valve, stopping a pump, etc. The SIS may talk 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 main aim is normally to restart the unit or equipment that has been shut down as quickly as possible. Plant engineers are busy troubleshooting and correcting issues. Maintenance technicians are busy repairing damaged devices. Taking the opportunity to proof take a look at an automated valve is not going to be a prime precedence and even an exercise under consideration due to the tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is equipped with a DVC, the proof test of the valve can be thought of carried out with diagnostic knowledge captured by the DVC. Also, a comparability of the diagnostic data with the valve’s baseline can help uncover valve deficiencies and degradations that may not show up in a proof check.
Figure 2. Proof Test vs Process Trip. Activities performed (such as repairing the device) and information captured by a Fisher DVC6200 SIS digital valve controller throughout a process trip meet most of the 12 requirements of a proof test.
ร้านซ่อมเครื่องวัดความดันโลหิต versus proof exams
How can proof check credit be claimed for an automated valve that has failed and brought on a process trip? A proof test is a periodic test performed on SIS sensors, logic solvers and final parts — similar to automated valves — to find hidden failures. These failures, if not corrected or removed, could impair the flexibility of the SIS to take the method to its protected state when an irregular process condition is detected.
A proof check must be carried out as per the proof check interval specified in the Safety Requirement Specifications. This interval is often decided by way of a median chance of failure on demand calculation (PFDavg). Some customers may select to proof take a look at based on predetermined intervals, such as proof testing sensors every 24 months and final parts every 48 months as an alternative of relying on the PFDavg calculations.
Automated valve proof tests can be carried out offline or online. Offline proof checks are usually scheduled throughout a turnaround, when the process is completely shut down.
Proof testing an automated valve on-line often requires a unit or tools to be shut down, or the valve to be bypassed to prevent a spurious trip, making it inconvenient and expensive.
But a proof check can be achieved throughout a process journey.
According to IEC61511-2 Section A.16.3.1.3, “…shutdowns as a result of precise demand on the SIS throughout operation may be given credit as proof exams (fully or partial) underneath given conditions…the next planned proof check may be skipped.”
These conditions are
The shutdown paperwork equal data as registered during corresponding proof take a look at.
The shutdown covers all elements 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 subsequent deliberate proof take a look at which may then be canceled
When a process journey happens, about 60% of a Safety Instrumented Function (SIF) proof check can be considered carried out. A pattern record of activities carried out during a proof take a look at, together with these which are carried out throughout a course of journey, is proven in Figure 2. Even with out an automatic valve leak take a look at, knowledge captured by the DVC alone can probably account for an excellent amount of proof test protection for an automatic valve.
The actual coverage is dependent upon the design of the automated valve, its DVC and its utility. The coverage is estimated primarily based on the valve degradations, the chance of their occurrence and the proportion of these degradations that might be detected by a DVC. Fortunately, information acquired by a DVC throughout a course of trip can usually be enough to fulfill a major part of the proof test necessities.
If the process trip takes place inside a predetermined most time window, the top person could choose to leverage the process journey as a proof take a look at by finishing steps one by way of 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 utmost time window ought to be the final half of the current proof test interval.
Figure 3. Data throughout a course of journey 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 more than likely root causes of automated valve degradations — which might lead to a process journey — include:
Partial or intermittent sticking of hydraulic or pneumatic system elements corresponding to solenoids, pilots, pace controller, etc., due to moisture, particles or alignment issues. This causes a lack of practical margin and makes the valve gradual to open or shut.
Binding, galling or other degradation of valve seats or associated circulate control trim that restricts or resists valve movement.
Actuator seal degradation brought on by compression, put on or looseness that reduces the strain out there to actuate the valve.
Minor injury to the valve obturator plug, disk or ball attributable to system situations, leakage or debris, together with build-up of hydrocarbon products. This causes leakage.
Complete failure of hydraulic control system components similar to solenoids, pilots, velocity controller, and so on., as a end result of moisture, debris or alignment. The valve will fail to open or close.
All of those circumstances can be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For instance, a valve that’s stuck open will exhibit itself as an increase 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 corresponding to supply strain. When the DVC6200SIS sees the provision stress is merely too low, an alert is generated (Figure 4) in order that the tip consumer can take corrective actions.
Figure 4. The DVC6200SIS digital valve controller detected nearly zero provide strain, so it alerted the management system.
Other process journey advantages
Diagnostic information captured during a course of trip might reveal valve degradations that will not be detected during a proof test. For instance, diagnostic information captured throughout a process journey would possibly indicate a problem with the valve closing utterly in opposition to the full pressure of the process, which can be an early indication of valve leakage.
The valve stroke time computed by a DVC during a process trip is more correct underneath real working circumstances. This ends in a more accurate SIF response time (the time from detection of an abnormal situation to ultimate factor reaching its trip state), which is compared to the Process Safety Time to ensure the SIF is still assembly its design requirements.
Although automated valves are very dependable, failures identified in a course of journey can provide valuable data to forestall future failures. This information can assist with turnaround planning by ensuring the wanted components can be found earlier than turnaround even begins to potentially shorten the turnaround schedule.
Summary
A course of trip can present the protection required to delay a valve’s subsequent scheduled proof test, thus saving time and money. Data captured by a DVC could be analyzed to leverage the process journey as a proof take a look at. Even if the end person chooses not to take proof check credit for a process journey, the valve diagnostic information supplied by the DVC can help plant personnel make proactive valve upkeep decisions..
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