A process trip happens when the protection instrumented system (SIS) locations the process in its safe state by commanding an automatic valve (Figure 1) or different tools to its trip state in response to an irregular course of condition. In some instances, a spurious trip occurs because of a valve malfunction. In that case, the valve meeting is tested beneath actual operating conditions, which supplies a possibility to seize priceless valve diagnostic information.
However, such diagnostics information can solely be captured when automated valves are fitted with digital valve controllers (DVCs). In this article, we’ll talk about how DVCs might help determine the proof check credit score for an automated valve after a process trip.
Process journey
A process journey happens when the SIS detects an irregular process situation through sensors such as temperature and pressure, executes the logic and places the method in its protected state by tripping the final elements similar to closing an automated valve, stopping a pump, and so forth. The SIS might communicate with the Basic Process Control System (BPCS) to shut a valve downstream of the automated valve as a secondary motion.
When a course of trip occurs, the main goal is usually to restart the unit or equipment that has been shut down as soon as potential. Plant engineers are busy troubleshooting and correcting issues. เกจวัดแรงดันออกซิเจน are busy repairing damaged devices. Taking the opportunity to proof check an automatic valve will not be a top priority and even an activity under consideration due to the tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is provided with a DVC, the proof test of the valve may be thought-about performed with diagnostic data captured by the DVC. Also, a comparability of the diagnostic data with the valve’s baseline may help discover valve deficiencies and degradations that may not show 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 throughout a course of trip meet a lot of the 12 requirements of a proof take a look at.
Process trips versus proof checks
How can proof check credits be claimed for an automated valve that has failed and caused a course of trip? A proof check is a periodic check performed on SIS sensors, logic solvers and last parts — similar to automated valves — to discover hidden failures. These failures, if not corrected or eliminated, might impair the ability of the SIS to take the process to its secure state when an abnormal course of condition is detected.
A proof check should be performed as per the proof take a look at interval specified in the Safety Requirement Specifications. This interval is usually determined through an average likelihood of failure on demand calculation (PFDavg). Some users could select to proof take a look at primarily based on predetermined intervals, such as proof testing sensors each 24 months and ultimate parts each forty eight months as an alternative of relying on the PFDavg calculations.
Automated valve proof exams could be carried out offline or on-line. Offline proof checks are often scheduled during a turnaround, when the process is completely shut down.
Proof testing an automatic valve on-line normally requires a unit or gear to be shut down, or the valve to be bypassed to stop a spurious journey, making it inconvenient and dear.
But a proof test may also be completed during a process journey.
According to IEC61511-2 Section A.sixteen.3.1.three, “…shutdowns due to precise demand on the SIS during operation may be given credit score as proof exams (fully or partial) underneath given conditions…the subsequent deliberate proof take a look at could additionally be skipped.”
These circumstances are
The shutdown documents equal info as registered during corresponding proof test.
The shutdown covers all elements of the SIS, and if not, the gadget or SIS subsystem not activated must be tested individually.
The shutdown happens inside a predetermined most time window before the next deliberate proof take a look at which might then be canceled
When a process journey occurs, about 60% of a Safety Instrumented Function (SIF) proof take a look at may be thought-about performed. A pattern listing of activities carried out during a proof check, together with these which are carried out throughout a course of journey, is proven in Figure 2. Even without an automated valve leak test, information captured by the DVC alone can potentially account for a good quantity of proof check coverage for an automated valve.
The exact protection is dependent upon the design of the automated valve, its DVC and its application. The protection is estimated primarily based on the valve degradations, the likelihood of their incidence and the share of these degradations that could be detected by a DVC. Fortunately, information acquired by a DVC during a course of trip can usually be enough to fulfill a significant part of the proof take a look at necessities.
If the process journey takes place within a predetermined maximum time window, the top consumer might select to leverage the process journey as a proof check by finishing steps one by way of 5 in Figure 2, that are normally not completed in a course of journey. The next scheduled proof test can then be canceled. According to the Norwegian Oil Industry Association, the maximum time window should be the last half of the present proof check interval.
Figure three. Data during a course of trip 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 may result in a process trip — embody:
Partial or intermittent sticking of hydraulic or pneumatic system components similar to solenoids, pilots, pace controller, and so forth., due to moisture, particles or alignment issues. This causes a loss of useful margin and makes the valve sluggish to open or close.
Binding, galling or different degradation of valve seats or related circulate control trim that restricts or resists valve motion.
Actuator seal degradation caused by compression, wear or looseness that reduces the pressure out there to actuate the valve.
Minor harm to the valve obturator plug, disk or ball attributable to system conditions, leakage or particles, together with build-up of hydrocarbon merchandise. This causes leakage.
Complete failure of hydraulic control system parts similar to solenoids, pilots, velocity controller, and so on., as a result of moisture, debris or alignment. The valve will fail to open or close.
All of these 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 in comparability with when the valve assembly was new signifies potential valve leak. The Fisher DVC6200SIS also repeatedly screens for inner faults in addition to its inputs such as provide stress. When the DVC6200SIS sees the availability stress is too low, an alert is generated (Figure 4) so that the tip consumer can take corrective actions.
Figure 4. The DVC6200SIS digital valve controller detected virtually zero supply strain, so it alerted the management system.
Other process journey advantages
Diagnostic knowledge captured during a process trip may reveal valve degradations that is in all probability not detected during a proof take a look at. For example, diagnostic information captured during a process journey might point out a problem with the valve closing fully against the full stress of the process, which can be an early indication of valve leakage.
The valve stroke time computed by a DVC throughout a course of trip is more accurate under real working situations. This ends in a extra accurate SIF response time (the time from detection of an abnormal situation to final component reaching its journey state), which is compared to the Process Safety Time to ensure the SIF remains to be assembly its design necessities.
Although automated valves are very dependable, failures identified in a course of journey can provide valuable knowledge to stop future failures. This data 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 process trip can present the coverage required to delay a valve’s subsequent scheduled proof test, thus saving time and money. Data captured by a DVC may be analyzed to leverage the method journey as a proof take a look at. Even if the top user chooses to not take proof check credits for a process trip, the valve diagnostic information provided by the DVC can help plant personnel make proactive valve maintenance choices..
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