Valve proof check credit score for a course of journey

A course of trip happens when the protection instrumented system (SIS) locations the process in its safe state by commanding an automatic valve (Figure 1) or different equipment to its journey state in response to an abnormal course of condition. In some instances, a spurious trip happens because of a valve malfunction. In that case, the valve meeting is examined underneath real operating circumstances, which provides a chance to capture priceless valve diagnostic information.
However, such diagnostics data can solely be captured when automated valves are fitted with digital valve controllers (DVCs). In เกจวัดแรงดันน้ำประปา , we’ll talk about how DVCs can help decide the proof take a look at credit score for an automated valve after a process trip.
Process trip
A course of journey happens when the SIS detects an abnormal process condition through sensors corresponding to temperature and strain, executes the logic and places the process in its secure state by tripping the final components similar to closing an automatic valve, stopping a pump, and so on. The SIS could talk 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 main goal is normally to restart the unit or equipment that has been shut down as soon as potential. Plant engineers are busy troubleshooting and correcting problems. Maintenance technicians are busy repairing damaged units. Taking the opportunity to proof test an automatic valve is not going to be a high precedence and even an exercise into consideration as a end result of tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is supplied with a DVC, the proof take a look at of the valve may be thought-about performed with diagnostic knowledge captured by the DVC. Also, a comparison of the diagnostic information with the valve’s baseline may help discover valve deficiencies and degradations that will not show up in a proof test.
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 journey meet a lot of the 12 necessities of a proof check.
Process trips versus proof checks
How can proof test credit be claimed for an automatic valve that has failed and brought on a process trip? A proof test is a periodic take a look at carried out on SIS sensors, logic solvers and ultimate components — such as automated valves — to find hidden failures. These failures, if not corrected or removed, may impair the flexibility of the SIS to take the process to its protected state when an abnormal process condition is detected.
A proof test ought to be performed as per the proof take a look at interval specified within the Safety Requirement Specifications. This interval is usually determined through an average probability of failure on demand calculation (PFDavg). Some customers could choose to proof check primarily based on predetermined intervals, similar to proof testing sensors each 24 months and last parts each forty eight months as a substitute of relying on the PFDavg calculations.
Automated valve proof checks can be carried out offline or online. Offline proof exams are often scheduled during a turnaround, when the method is completely shut down.
Proof testing an automated valve online usually requires a unit or tools to be shut down, or the valve to be bypassed to prevent a spurious journey, making it inconvenient and expensive.
But a proof take a look at can also be completed during a course of journey.
According to IEC61511-2 Section A.sixteen.three.1.three, “…shutdowns due to precise demand on the SIS during operation may be given credit score as proof tests (fully or partial) under given conditions…the subsequent planned proof check may be skipped.”
These circumstances are
The shutdown documents equal data as registered during corresponding proof take a look at.
The shutdown covers all parts of the SIS, and if not, the system or SIS subsystem not activated needs to be examined separately.
The shutdown occurs within a predetermined maximum time window before the subsequent planned proof check which may then be canceled
When a process trip occurs, about 60% of a Safety Instrumented Function (SIF) proof test may be thought-about performed. A pattern list of activities performed during a proof take a look at, along with those which might be performed during a course of trip, is shown in Figure 2. Even with out an automated valve leak check, information captured by the DVC alone can potentially account for a good quantity of proof test coverage for an automated valve.
The actual protection is determined by the design of the automated valve, its DVC and its utility. The protection is estimated primarily based on the valve degradations, the probability of their occurrence and the share of these degradations that can be detected by a DVC. Fortunately, data acquired by a DVC throughout a course of journey can often be sufficient to satisfy a significant part of the proof take a look at necessities.
If the method journey takes place within a predetermined most time window, the end person could select to leverage the method journey as a proof take a look at by finishing steps one through 5 in Figure 2, which are usually not accomplished in a process journey. The subsequent scheduled proof check can then be canceled. According to the Norwegian Oil Industry Association, the utmost time window should be the final half of the present proof check interval.
Figure three. Data throughout a process journey 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 ninety six.05.01, the more than likely root causes of automated valve degradations — which might result in a process journey — embrace:
Partial or intermittent sticking of hydraulic or pneumatic system parts such as solenoids, pilots, pace controller, and so on., because of moisture, particles or alignment points. This causes a lack of practical margin and makes the valve gradual to open or close.
Binding, galling or other degradation of valve seats or related circulate control trim that restricts or resists valve motion.
Actuator seal degradation caused by compression, put on or looseness that reduces the strain available 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 merchandise. This causes leakage.
Complete failure of hydraulic management system elements similar to solenoids, pilots, pace controller, and so on., due to moisture, particles or alignment. The valve will fail to open or shut.
All of these circumstances can 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 compared to when the valve assembly was new signifies potential valve leak. The Fisher DVC6200SIS also repeatedly screens for inner faults as nicely as its inputs such as provide stress. When the DVC6200SIS sees the supply pressure is too low, an alert is generated (Figure 4) so that the end user can take corrective actions.
Figure four. The DVC6200SIS digital valve controller detected virtually zero provide strain, so it alerted the control system.
Other course of trip advantages
Diagnostic knowledge captured throughout a course of trip might reveal valve degradations that is most likely not detected during a proof take a look at. For instance, diagnostic knowledge captured throughout a process journey would possibly point out an issue with the valve closing utterly towards the complete stress of the method, which may be an early indication of valve leakage.
The valve stroke time computed by a DVC during a course of journey is more accurate under real working situations. This leads to a extra correct SIF response time (the time from detection of an irregular 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 necessities.
Although automated valves are very reliable, failures identified in a course of trip can present useful knowledge to prevent future failures. This data 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 coverage required to delay a valve’s next scheduled proof check, thus saving time and money. Data captured by a DVC could be analyzed to leverage the method journey as a proof check. Even if the end person chooses not to take proof check credits for a process trip, the valve diagnostic data provided by the DVC may help plant personnel make proactive valve maintenance choices..
Share

Scroll to Top