Valve proof test credit score for a course of trip

A process trip occurs when the security instrumented system (SIS) locations the method in its protected state by commanding an automated valve (Figure 1) or different tools to its journey state in response to an abnormal course of situation. In some instances, a spurious trip happens because of a valve malfunction. In that case, the valve assembly is examined beneath real operating circumstances, which supplies an opportunity to capture valuable valve diagnostic data.
However, such diagnostics information can only be captured when automated valves are fitted with digital valve controllers (DVCs). In this article, we’ll focus on how DVCs might help decide the proof test credit score for an automated valve after a course of trip.
Process journey
A course of journey occurs when the SIS detects an abnormal course of situation through sensors such as temperature and pressure, executes the logic and places the process in its secure state by tripping the final components 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 principle goal is often to restart the unit or gear that has been shut down as soon as attainable. Plant engineers are busy troubleshooting and correcting problems. Maintenance technicians are busy repairing damaged units. Taking the chance to proof check an automatic valve is not going to be a top precedence and even an activity into consideration due to the 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 may be thought of carried out with diagnostic information captured by the DVC. Also, a comparability of the diagnostic information with the valve’s baseline can help discover valve deficiencies and degradations that will not present 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 course of journey meet a lot of the 12 requirements of a proof take a look at.
Process trips versus proof tests
How can proof take a look at credits be claimed for an automated valve that has failed and brought on a process trip? A proof test is a periodic take a look at carried out on SIS sensors, logic solvers and ultimate elements — such as automated valves — to find hidden failures. These failures, if not corrected or removed, may impair the ability of the SIS to take the process to its protected state when an abnormal course of condition is detected.
A proof check must be performed as per the proof check interval specified within the Safety Requirement Specifications. This interval is often decided by way of a mean probability of failure on demand calculation (PFDavg). Some users could select to proof check based on predetermined intervals, such as proof testing sensors each 24 months and ultimate components every forty eight months as an alternative of relying on the PFDavg calculations.
Automated valve proof exams may be carried out offline or on-line. Offline proof exams are normally scheduled throughout a turnaround, when the method is totally shut down.
Proof testing an automated valve on-line usually requires a unit or equipment 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 additionally be completed throughout a process journey.
According to IEC61511-2 Section A.16.three.1.3, “…shutdowns because of precise demand on the SIS throughout operation could additionally be given credit score as proof tests (fully or partial) underneath given conditions…the subsequent planned proof take a look at could also be skipped.”
These situations are
The shutdown documents equal data as registered throughout corresponding proof check.
The shutdown covers all elements of the SIS, and if not, the device or SIS subsystem not activated needs to be examined separately.
The shutdown occurs inside a predetermined most time window earlier than the subsequent planned proof check which might then be canceled
When a course of journey occurs, about 60% of a Safety Instrumented Function (SIF) proof test can be thought of performed. A sample list of actions performed during a proof take a look at, along with those which would possibly be carried out during a process journey, is proven in Figure 2. Even without ชนิดของpressuregauge , information captured by the DVC alone can potentially account for a great quantity of proof test coverage for an automated valve.
The actual protection depends on the design of the automated valve, its DVC and its software. The coverage is estimated based mostly on the valve degradations, the likelihood of their occurrence and the percentage of those degradations that may be detected by a DVC. Fortunately, data acquired by a DVC during a process journey can typically be sufficient to satisfy a significant part of the proof test necessities.
If the method trip takes place inside a predetermined maximum time window, the top user could select to leverage the method trip as a proof take a look at by completing steps one via five in Figure 2, that are usually not accomplished in a course of journey. The subsequent scheduled proof check 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 three. Data throughout a course of journey 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 most likely root causes of automated valve degradations — which might lead to a process trip — embody:
Partial or intermittent sticking of hydraulic or pneumatic system components such as solenoids, pilots, speed controller, and so forth., due to moisture, debris or alignment issues. This causes a lack of functional margin and makes the valve sluggish to open or shut.
Binding, galling or other degradation of valve seats or associated move control trim that restricts or resists valve motion.
Actuator seal degradation brought on by compression, put on or looseness that reduces the strain obtainable 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 control system elements corresponding to solenoids, pilots, pace controller, etc., because of moisture, debris or alignment. The valve will fail to open or close.
All of these conditions can be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For instance, a valve that is stuck open will exhibit itself as an increase in breakout drive. Loss of seat load compared to when the valve assembly was new signifies potential valve leak. The Fisher DVC6200SIS additionally constantly screens for inner faults in addition to its inputs similar to provide stress. When the DVC6200SIS sees the availability stress is too low, an alert is generated (Figure 4) so that the tip person can take corrective actions.
Figure four. The DVC6200SIS digital valve controller detected almost zero provide pressure, so it alerted the control system.
Other course of trip advantages
Diagnostic knowledge captured throughout a course of journey might reveal valve degradations that may not be detected during a proof check. For instance, diagnostic information captured during a course of journey would possibly indicate a problem with the valve closing fully towards the full pressure of the method, which can be an early indication of valve leakage.
The valve stroke time computed by a DVC during a course of journey is extra correct underneath actual working conditions. This leads to a extra accurate SIF response time (the time from detection of an abnormal condition to ultimate component reaching its journey state), which is in comparison with the Process Safety Time to make sure the SIF is still meeting its design requirements.
Although automated valves are very dependable, failures recognized in a course of trip can present valuable data to stop future failures. This data can help with turnaround planning by ensuring the wanted parts are available before turnaround even begins to doubtlessly shorten the turnaround schedule.
A process journey can present the protection required to delay a valve’s next scheduled proof take a look at, thus saving time and money. Data captured by a DVC may be analyzed to leverage the method trip as a proof check. Even if the top person chooses not to take proof check credit for a process journey, the valve diagnostic knowledge supplied by the DVC may help plant personnel make proactive valve maintenance choices..

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