A process trip occurs when the protection instrumented system (SIS) places the process in its secure state by commanding an automatic valve (Figure 1) or different gear to its trip state in response to an abnormal course of condition. In some cases, a spurious journey happens because of a valve malfunction. In that case, the valve assembly is examined beneath real operating circumstances, which provides a chance to seize valuable valve diagnostic information.
However, such diagnostics data can only be captured when automated valves are fitted with digital valve controllers (DVCs). In this text, we’ll focus on how DVCs may help determine the proof take a look at credit score for an automatic valve after a process trip.
A course of trip happens when the SIS detects an abnormal course of situation through sensors similar to temperature and pressure, executes the logic and places the method in its protected state by tripping the final parts similar to closing an automatic valve, stopping a pump, and so on. The SIS could talk with the Basic Process Control System (BPCS) to shut a valve downstream of the automated valve as a secondary motion.
When a course of journey happens, the main objective 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 broken devices. Taking the opportunity to proof test an automatic valve will not be a high precedence 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 test of the valve could be considered carried out with diagnostic data captured by the DVC. Also, a comparison of the diagnostic data with the valve’s baseline might help discover valve deficiencies and degradations that could 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 during a course of trip meet most of the 12 necessities of a proof check.
Process trips versus proof exams
How can proof take a look at credits be claimed for an automatic valve that has failed and caused a process trip? A proof check is a periodic check carried out on SIS sensors, logic solvers and ultimate elements — similar to automated valves — to find hidden failures. These failures, if not corrected or eliminated, could impair the flexibility of the SIS to take the method to its protected state when an irregular process situation is detected.
A proof take a look at should be carried out as per the proof check interval specified in the Safety Requirement Specifications. This interval is normally determined via a mean probability of failure on demand calculation (PFDavg). Some customers could select to proof check based on predetermined intervals, such as proof testing sensors every 24 months and ultimate elements every forty eight months instead of relying on the PFDavg calculations.
Automated valve proof tests could be carried out offline or on-line. Offline proof tests are normally scheduled during a turnaround, when the process is completely shut down.
Proof testing an automatic valve online often requires a unit or tools to be shut down, or the valve to be bypassed to forestall a spurious trip, making it inconvenient and expensive.
But a proof check can additionally be completed during a course of journey.
According to IEC61511-2 Section A.16.three.1.3, “…shutdowns due to precise demand on the SIS throughout operation may be given credit as proof exams (fully or partial) underneath given conditions…the subsequent planned proof test may be skipped.”
These conditions are
The shutdown documents equal info as registered throughout corresponding proof check.
The shutdown covers all components of the SIS, and if not, the gadget or SIS subsystem not activated needs to be tested separately.
The shutdown happens within a predetermined maximum time window earlier than the subsequent planned proof test which can then be canceled
When a process journey happens, about 60% of a Safety Instrumented Function (SIF) proof take a look at can be thought of carried out. A sample list of actions carried out throughout a proof take a look at, together with these which are performed throughout a process journey, is shown in Figure 2. Even without an automatic valve leak take a look at, data captured by the DVC alone can doubtlessly account for a good quantity of proof test protection for an automated valve.
The actual coverage is determined by the design of the automated valve, its DVC and its software. The coverage is estimated based on the valve degradations, the chance of their incidence and the share of those degradations that can be detected by a DVC. Fortunately, knowledge acquired by a DVC during a process journey can often be enough to satisfy a serious a part of the proof take a look at requirements.
If the method journey takes place inside a predetermined most time window, the tip consumer could choose to leverage the method trip as a proof test by finishing steps one via five in Figure 2, which are normally not completed in a process journey. The next scheduled proof take a look at can then be canceled. According to the Norwegian Oil Industry Association, the maximum time window must be the final half of the present proof take a look at 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 96.05.01, the most probably root causes of automated valve degradations — which may result in a process journey — include:
Partial or intermittent sticking of hydraulic or pneumatic system components corresponding to solenoids, pilots, velocity controller, etc., due to moisture, particles or alignment issues. This causes a lack of functional margin and makes the valve slow to open or shut.
Binding, galling or different degradation of valve seats or related circulate management trim that restricts or resists valve motion.
Actuator seal degradation caused by compression, put on or looseness that reduces the pressure available to actuate the valve.
Minor damage to the valve obturator plug, disk or ball caused by system situations, leakage or debris, including build-up of hydrocarbon products. This causes leakage.
เกจวัดความดันpressuregauge of hydraulic control system components corresponding to solenoids, pilots, pace controller, and so on., because of moisture, particles or alignment. The valve will fail to open or shut.
All of those circumstances could 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 force. Loss of seat load compared to when the valve meeting was new indicates potential valve leak. The Fisher DVC6200SIS additionally continuously screens for inside faults as nicely as its inputs similar to provide stress. When the DVC6200SIS sees the supply stress is merely too low, an alert is generated (Figure 4) in order that the tip user can take corrective actions.
Figure 4. The DVC6200SIS digital valve controller detected almost zero provide stress, so it alerted the control system.
Other course of journey benefits
Diagnostic knowledge captured during a process journey may reveal valve degradations that may not be detected throughout a proof take a look at. For example, diagnostic knowledge captured during a process trip would possibly point out a problem with the valve closing utterly against the total pressure of the process, which may be an early indication of valve leakage.
The valve stroke time computed by a DVC during a process journey is more accurate underneath actual operating circumstances. This ends in a more correct SIF response time (the time from detection of an abnormal situation to last component reaching its journey state), which is compared to the Process Safety Time to make sure the SIF remains to be assembly its design requirements.
Although automated valves are very reliable, failures identified in a process journey can provide useful data to prevent future failures. This data can help with turnaround planning by ensuring the wanted elements are available earlier than turnaround even begins to doubtlessly shorten the turnaround schedule.
A course of journey can provide the coverage required to delay a valve’s next scheduled proof check, thus saving money and time. Data captured by a DVC may be analyzed to leverage the process trip as a proof test. Even if the tip person chooses not to take proof take a look at credits for a course of trip, the valve diagnostic information offered by the DVC can help plant personnel make proactive valve upkeep selections..
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