A process trip occurs when the security instrumented system (SIS) locations the process in its safe state by commanding an automated valve (Figure 1) or different gear to its trip state in response to an abnormal course of condition. In some circumstances, a spurious journey happens because of a valve malfunction. In that case, the valve assembly is tested under actual operating situations, which offers a chance to seize valuable valve diagnostic information.
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 check credit for an automatic valve after a process journey.
Process journey
A process journey happens when the SIS detects an abnormal course of situation through sensors such as temperature and stress, executes the logic and places the process in its protected state by tripping the ultimate parts similar to closing an automatic valve, stopping a pump, and so on. The SIS may talk with the Basic Process Control System (BPCS) to shut a valve downstream of the automated valve as a secondary action.
When a process trip occurs, the main objective is usually to restart the unit or equipment that has been shut down as soon as possible. Plant engineers are busy troubleshooting and correcting issues. Maintenance technicians are busy repairing damaged devices. Taking the chance to proof take a look at an automated valve is not going to be a prime precedence and even an exercise into account because of 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 can be thought of performed with diagnostic data captured by the DVC. Also, a comparability of the diagnostic data with the valve’s baseline may help uncover valve deficiencies and degradations that might 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 during a process journey meet a lot of the 12 necessities of a proof take a look at.
Process trips versus proof exams
How can proof test credit be claimed for an automatic valve that has failed and brought on a process trip? A proof take a look at is a periodic take a look at carried out on SIS sensors, logic solvers and last components — such as automated valves — to find hidden failures. These failures, if not corrected or removed, might impair the power of the SIS to take the process to its secure state when an irregular course of condition is detected.
A proof test should be carried out as per the proof test interval specified within the Safety Requirement Specifications. This interval is usually determined by way of a mean chance of failure on demand calculation (PFDavg). Some customers may choose to proof test based mostly on predetermined intervals, similar to proof testing sensors each 24 months and last components every forty eight months as a substitute of relying on the PFDavg calculations.
Automated valve proof tests may be accomplished offline or online. Offline proof checks are normally scheduled throughout a turnaround, when the process is completely shut down.
Proof testing an automated valve online often requires a unit or equipment to be shut down, or the valve to be bypassed to forestall a spurious trip, making it inconvenient and costly.
But a proof take a look at may also be accomplished throughout a course of trip.
According to IEC61511-2 Section A.16.3.1.three, “…shutdowns as a outcome of actual demand on the SIS throughout operation may be given credit as proof exams (fully or partial) beneath given conditions…the subsequent deliberate proof test may be skipped.”
These conditions are
The shutdown documents equivalent data as registered throughout corresponding proof test.
The shutdown covers all elements of the SIS, and if not, the system or SIS subsystem not activated needs to be examined individually.
The shutdown happens within a predetermined maximum time window before the next planned proof take a look at which might then be canceled
When a course of trip occurs, about 60% of a Safety Instrumented Function (SIF) proof test may be thought-about carried out. เกจวัดแรงดันnuovafima of activities performed throughout a proof test, along with those which are performed during a course of journey, is shown in Figure 2. Even without an automatic valve leak take a look at, knowledge captured by the DVC alone can potentially account for a great amount of proof take a look at protection for an automated valve.
The precise protection depends on the design of the automated valve, its DVC and its application. The protection is estimated based mostly on the valve degradations, the likelihood of their prevalence and the proportion of those degradations that can be detected by a DVC. Fortunately, information acquired by a DVC during a course of journey can typically be sufficient to fulfill a serious a part of the proof test necessities.
If the process journey takes place inside a predetermined maximum time window, the tip consumer might select to leverage the process journey as a proof check by completing steps one through five in Figure 2, that are often not completed in a process trip. 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 present proof take a look at interval.
Figure 3. Data throughout a course of trip is captured by the Fisher DVC6200 SIS digital valve controller and made available for evaluation.
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 can lead to a course of journey — embody:
Partial or intermittent sticking of hydraulic or pneumatic system components corresponding to solenoids, pilots, pace controller, and so on., as a end result of moisture, particles or alignment points. This causes a lack of useful margin and makes the valve gradual to open or shut.
Binding, galling or different degradation of valve seats or associated circulate control trim that restricts or resists valve motion.
Actuator seal degradation caused by compression, put on or looseness that reduces the strain obtainable 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 products. This causes leakage.
Complete failure of hydraulic management system elements corresponding to solenoids, pilots, velocity controller, etc., because of moisture, particles or alignment. The valve will fail to open or shut.
All of those circumstances can 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 pressure. Loss of seat load compared to when the valve assembly was new signifies potential valve leak. The Fisher DVC6200SIS additionally continuously screens for inside faults as well as its inputs similar to provide strain. When the DVC6200SIS sees the provision stress is simply too low, an alert is generated (Figure 4) so that the top person can take corrective actions.
Figure four. The DVC6200SIS digital valve controller detected virtually zero supply pressure, so it alerted the control system.
Other process trip benefits
Diagnostic information captured during a course of journey might reveal valve degradations that will not be detected during a proof check. For instance, diagnostic knowledge captured throughout a course of journey would possibly indicate an issue with the valve closing fully towards the full strain of the method, which may be an early indication of valve leakage.
The valve stroke time computed by a DVC during a course of trip is more accurate under actual operating circumstances. This ends in a extra accurate SIF response time (the time from detection of an abnormal condition to final component reaching its trip state), which is compared to the Process Safety Time to make sure the SIF continues to be meeting its design requirements.
Although automated valves are very dependable, failures recognized in a process trip can provide priceless information to stop future failures. This data can assist with turnaround planning by ensuring the wanted parts are available earlier than turnaround even begins to potentially shorten the turnaround schedule.
Summary
A course of trip can provide the coverage required to delay a valve’s next scheduled proof test, thus saving time and money. Data captured by a DVC may be analyzed to leverage the method journey as a proof test. Even if the top user chooses not to take proof take a look at credit for a process trip, the valve diagnostic data provided by the DVC can help plant personnel make proactive valve maintenance choices..
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