A process trip happens when the safety instrumented system (SIS) locations the method in its safe state by commanding an automatic valve (Figure 1) or different gear to its trip state in response to an irregular process situation. In some circumstances, a spurious trip occurs because of a valve malfunction. In that case, the valve assembly is examined beneath actual operating situations, which provides a possibility to seize valuable valve diagnostic knowledge.
However, such diagnostics knowledge 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 decide the proof take a look at credit score for an automatic valve after a course of journey.
Process trip
A course of journey occurs when the SIS detects an abnormal process condition by way of sensors such as temperature and pressure, executes the logic and locations the method in its secure state by tripping the ultimate components similar to closing an automated valve, stopping a pump, and so on. The SIS might talk with the Basic Process Control System (BPCS) to close a valve downstream of the automated valve as a secondary action.
When a process trip happens, the primary goal is normally 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 devices. Taking the chance to proof test an automated valve is not going to be a prime precedence or even an exercise under consideration due to the tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is provided with a DVC, the proof take a look at of the valve may be thought-about performed with diagnostic data captured by the DVC. Also, a comparability of the diagnostic knowledge with the valve’s baseline may help uncover valve deficiencies and degradations that received’t present up in a proof check.
Figure 2. 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 journey meet most of the 12 necessities of a proof test.
Process trips versus proof checks
How can proof check credits be claimed for an automated valve that has failed and caused a process trip? A proof check is a periodic take a look at performed on SIS sensors, logic solvers and last components — similar to automated valves — to discover hidden failures. These failures, if not corrected or eliminated, may impair the power of the SIS to take the process to its safe state when an irregular process situation is detected.
A proof check must be carried out as per the proof check interval specified within the Safety Requirement Specifications. This interval is often determined through a median chance of failure on demand calculation (PFDavg). Some customers could select to proof take a look at based mostly on predetermined intervals, such as proof testing sensors every 24 months and final parts each forty eight months instead of counting on the PFDavg calculations.
Automated valve proof exams may be done offline or on-line. Offline proof checks are usually scheduled during a turnaround, when the process is totally shut down.
Proof testing an automatic 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 be accomplished throughout a process journey.
According to IEC61511-2 Section A.sixteen.3.1.3, “…shutdowns because of precise demand on the SIS throughout operation may be given credit as proof tests (fully or partial) beneath given conditions…the subsequent planned proof test could also be skipped.”
These circumstances are
The shutdown paperwork equivalent data as registered during corresponding proof test.
The shutdown covers all components of the SIS, and if not, the system or SIS subsystem not activated needs to be examined separately.
The shutdown occurs inside a predetermined maximum time window before the following planned proof test which might then be canceled
When a process trip occurs, about 60% of a Safety Instrumented Function (SIF) proof take a look at may be thought-about performed. A pattern record of actions carried out throughout a proof check, together with these that are carried out throughout a process journey, is shown in Figure 2. Even without an automatic valve leak test, data captured by the DVC alone can potentially account for a great quantity of proof check protection for an automatic valve.
The precise coverage is dependent upon 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 those degradations that can be detected by a DVC. Fortunately, knowledge acquired by a DVC throughout a course of trip can typically be sufficient to fulfill a significant part of the proof check necessities.
If the process trip takes place inside a predetermined most time window, the top user could select to leverage the process trip as a proof test by completing steps one via 5 in Figure 2, that are normally not completed in a process journey. The subsequent scheduled proof test can then be canceled. According to the Norwegian Oil Industry Association, the maximum time window must be the final half of the current proof take a look at interval.
Figure 3. Data during a process 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 trip — include:
Partial or intermittent sticking of hydraulic or pneumatic system parts such as solenoids, pilots, pace controller, and so forth., due to moisture, debris or alignment issues. This causes a loss of functional margin and makes the valve slow to open or close.
Binding, galling or different degradation of valve seats or related move control trim that restricts or resists valve motion.
Actuator seal degradation brought on by compression, put on or looseness that reduces the strain available to actuate the valve.
Minor damage 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 control system elements corresponding to solenoids, pilots, pace controller, etc., as a end result of moisture, debris or alignment. The valve will fail to open or shut.
All of those conditions 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 drive. Loss of seat load compared to when the valve meeting was new indicates potential valve leak. The Fisher DVC6200SIS also repeatedly monitors for inner faults in addition to its inputs corresponding to provide stress. When the DVC6200SIS sees the supply strain 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 almost zero supply stress, so it alerted the management system.
Other course of trip advantages
Diagnostic data captured during a process trip may reveal valve degradations that may not be detected during a proof take a look at. For example, diagnostic information captured during a process journey might indicate an issue 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 course of trip is extra correct under actual operating conditions. This results in a extra accurate SIF response time (the time from detection of an irregular condition to ultimate element reaching its trip state), which is in comparability with the Process Safety Time to make sure the SIF remains to be assembly its design necessities.
Although automated valves are very reliable, failures identified in a process journey can present 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 doubtlessly shorten the turnaround schedule.
Summary
A process trip can provide the protection required to delay a valve’s next scheduled proof check, thus saving money and time. เกจวัดแรงดัน300psi captured by a DVC could be analyzed to leverage the process trip as a proof check. Even if the top consumer chooses to not take proof take a look at credits for a course of trip, the valve diagnostic information offered by the DVC might help plant personnel make proactive valve upkeep decisions..
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