A course of trip occurs when the protection instrumented system (SIS) locations the method in its safe state by commanding an automatic valve (Figure 1) or other equipment to its trip state in response to an irregular process condition. In some cases, a spurious trip occurs because of a valve malfunction. In that case, the valve meeting is tested under real operating situations, which provides a chance to capture valuable valve diagnostic data.
However, such diagnostics data can solely be captured when automated valves are fitted with digital valve controllers (DVCs). In this text, we’ll talk about how DVCs may help determine the proof test credit score 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 locations the method in its safe state by tripping the ultimate components similar to closing an automated 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 motion.
When a process journey happens, the main aim is usually to restart the unit or tools that has been shut down as soon as possible. Plant engineers are busy troubleshooting and correcting problems. Maintenance technicians are busy repairing broken gadgets. Taking the chance to proof check an automated valve is not going to be a prime priority and even an exercise under consideration as a end result 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 thought of performed with diagnostic information 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 present up in a proof check.
ไดอะแฟรม . Proof Test vs Process Trip. Activities carried out (such as repairing the device) and data captured by a Fisher DVC6200 SIS digital valve controller during a course of trip meet a lot of the 12 necessities of a proof check.
Process trips versus proof exams
How can proof check credit be claimed for an automated valve that has failed and caused a course of trip? A proof check is a periodic take a look at carried out on SIS sensors, logic solvers and last components — corresponding to automated valves — to find hidden failures. These failures, if not corrected or removed, may impair the power of the SIS to take the method to its protected state when an irregular course of condition is detected.
A proof test must be performed as per the proof take a look at interval specified within the Safety Requirement Specifications. This interval is usually decided through a median probability of failure on demand calculation (PFDavg). Some customers could choose 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 a substitute of relying on the PFDavg calculations.
Automated valve proof tests may be done offline or online. Offline proof checks are often scheduled throughout a turnaround, when the process is completely shut down.
Proof testing an automated valve on-line normally requires a unit or equipment to be shut down, or the valve to be bypassed to prevent a spurious journey, making it inconvenient and dear.
But a proof test can be completed throughout a process trip.
According to IEC61511-2 Section A.sixteen.three.1.three, “…shutdowns as a result of precise demand on the SIS during operation may be given credit score as proof checks (fully or partial) under given conditions…the subsequent deliberate proof check could additionally be skipped.”
These circumstances are
The shutdown documents equal info as registered throughout corresponding proof take a look at.
The shutdown covers all components of the SIS, and if not, the gadget or SIS subsystem not activated must be examined individually.
The shutdown occurs within a predetermined maximum time window before the next deliberate proof check which might then be canceled
When a process journey happens, about 60% of a Safety Instrumented Function (SIF) proof check can be considered performed. A pattern record of actions carried out during a proof check, together with these that are performed throughout a process journey, is shown in Figure 2. Even with out an automated valve leak check, knowledge captured by the DVC alone can potentially account for an excellent quantity of proof take a look at protection for an automatic valve.
The precise protection depends on the design of the automated valve, its DVC and its utility. The coverage is estimated based on the valve degradations, the likelihood of their incidence and the share of these degradations that can be detected by a DVC. Fortunately, information acquired by a DVC during a course of trip can typically be sufficient to fulfill a serious a half of the proof take a look at necessities.
If the method journey takes place within a predetermined maximum time window, the tip user could select to leverage the method journey as a proof check by completing steps one through five in Figure 2, which are normally not completed in a process journey. The subsequent scheduled proof check 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 take a look at interval.
Figure three. Data throughout a process journey is captured by the Fisher DVC6200 SIS digital valve controller and made obtainable for evaluation.
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 course of trip — embrace:
Partial or intermittent sticking of hydraulic or pneumatic system components corresponding to solenoids, pilots, pace controller, and so forth., because of moisture, debris or alignment issues. This causes a loss of practical margin and makes the valve gradual to open or shut.
Binding, galling or other degradation of valve seats or related move management trim that restricts or resists valve movement.
Actuator seal degradation attributable to compression, put on or looseness that reduces the pressure available to actuate the valve.
Minor damage to the valve obturator plug, disk or ball brought on by system conditions, leakage or particles, including build-up of hydrocarbon merchandise. This causes leakage.
Complete failure of hydraulic management system elements corresponding to solenoids, pilots, speed controller, and so on., as a outcome of moisture, particles or alignment. The valve will fail to open or close.
All of these conditions could be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For example, a valve that’s stuck open will exhibit itself as a rise in breakout force. Loss of seat load in comparison with when the valve meeting was new signifies potential valve leak. The Fisher DVC6200SIS also repeatedly displays for inner faults as well as its inputs such as provide strain. When the DVC6200SIS sees the availability stress is merely too low, an alert is generated (Figure 4) in order that the top person can take corrective actions.
Figure 4. The DVC6200SIS digital valve controller detected virtually zero supply pressure, so it alerted the control system.
Other course of journey benefits
Diagnostic knowledge captured throughout a process trip may reveal valve degradations that may not be detected during a proof take a look at. For instance, diagnostic information captured during a course of journey would possibly indicate a problem with the valve closing utterly towards the complete strain of the process, which could be an early indication of valve leakage.
The valve stroke time computed by a DVC throughout a process trip is more accurate underneath real operating circumstances. This results in a more accurate SIF response time (the time from detection of an irregular condition to final element reaching its journey state), which is compared to the Process Safety Time to make sure the SIF is still assembly its design necessities.
Although automated valves are very dependable, failures identified in a course of trip can provide useful data to prevent future failures. This info can assist with turnaround planning by making sure the needed parts are available earlier than turnaround even begins to potentially shorten the turnaround schedule.
Summary
A process trip can present the coverage required to delay a valve’s next scheduled proof test, 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 end person chooses to not take proof test credit for a course of journey, the valve diagnostic knowledge offered by the DVC can help plant personnel make proactive valve maintenance selections..
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