Valve proof test credit score for a process journey

A course of journey occurs when the protection instrumented system (SIS) places the method in its protected state by commanding an automated valve (Figure 1) or different equipment to its trip state in response to an abnormal course of condition. In some instances, a spurious trip occurs due to a valve malfunction. In that case, the valve meeting is tested under real working circumstances, which supplies a chance to capture valuable valve diagnostic knowledge.
However, such diagnostics knowledge can solely be captured when automated valves are fitted with digital valve controllers (DVCs). In this article, we’ll focus on how DVCs may help decide the proof check credit score for an automatic valve after a process journey.
Process journey
A course of journey occurs when the SIS detects an irregular course of situation by way of sensors similar to temperature and strain, executes the logic and places the method in its safe state by tripping the final parts such as closing an automatic valve, stopping a pump, and so forth. The SIS could communicate with the Basic Process Control System (BPCS) to close a valve downstream of the automated valve as a secondary action.
When a course of journey happens, the principle goal is normally to restart the unit or tools that has been shut down as soon as attainable. Plant engineers are busy troubleshooting and correcting issues. Maintenance technicians are busy repairing broken devices. Taking the opportunity to proof test an automatic valve won’t be a high priority and even an activity under consideration due to the tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is supplied with a DVC, the proof test of the valve could be thought-about performed with diagnostic information captured by the DVC. Also, a comparison of the diagnostic knowledge with the valve’s baseline might help uncover valve deficiencies and degradations that won’t present up in a proof check.
Figure 2. pressure gauge หลักการ ทํา งาน vs Process Trip. Activities carried out (such as repairing the device) and knowledge captured by a Fisher DVC6200 SIS digital valve controller throughout a course of trip meet many of the 12 requirements of a proof take a look at.
Process trips versus proof tests
How can proof take a look at credit be claimed for an automatic valve that has failed and caused a process trip? A proof test is a periodic test carried out on SIS sensors, logic solvers and last components — such as automated valves — to find hidden failures. These failures, if not corrected or eliminated, may impair the power of the SIS to take the method to its safe state when an irregular course of condition is detected.
A proof check must be carried out as per the proof test interval specified in the Safety Requirement Specifications. This interval is usually decided by way of a median probability of failure on demand calculation (PFDavg). Some users could select to proof test based on predetermined intervals, corresponding to proof testing sensors every 24 months and last parts each forty eight months as a substitute of counting on the PFDavg calculations.
Automated valve proof checks can be carried out offline or on-line. Offline proof tests are normally scheduled throughout a turnaround, when the process is completely shut down.
Proof testing an automated valve on-line normally 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 check may also be completed during a course of trip.
According to IEC61511-2 Section A.16.3.1.three, “…shutdowns as a result of actual demand on the SIS during operation could additionally be given credit score as proof tests (fully or partial) underneath given conditions…the next planned proof check could additionally be skipped.”
These conditions are
The shutdown paperwork equal info as registered during corresponding proof check.
The shutdown covers all parts of the SIS, and if not, the system or SIS subsystem not activated must be tested individually.
The shutdown happens inside a predetermined maximum time window before the next planned proof test which can then be canceled
When a course of trip happens, about 60% of a Safety Instrumented Function (SIF) proof check may be thought-about performed. A pattern record of activities performed during a proof test, along with these which are carried out throughout a course of journey, is proven in Figure 2. Even without an automated valve leak take a look at, information captured by the DVC alone can potentially account for a great amount of proof check coverage for an automated valve.
The exact coverage is dependent upon the design of the automated valve, its DVC and its software. The coverage is estimated primarily based on the valve degradations, the likelihood of their occurrence and the share of those degradations that may be detected by a DVC. Fortunately, information acquired by a DVC during a process trip can often be sufficient to satisfy a major a half of the proof check necessities.
If the process journey takes place inside a predetermined maximum time window, the end person may choose to leverage the method journey as a proof test by completing steps one by way of 5 in Figure 2, that are normally not accomplished in a process trip. The subsequent scheduled proof test can then be canceled. According to the Norwegian Oil Industry Association, the utmost time window ought to be the final half of the present proof test 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 96.05.01, the most likely root causes of automated valve degradations — which can result in a course of trip — embody:
Partial or intermittent sticking of hydraulic or pneumatic system elements such as solenoids, pilots, velocity controller, and so forth., as a outcome of moisture, debris or alignment points. This causes a loss of useful margin and makes the valve slow to open or close.
Binding, galling or other degradation of valve seats or related move management trim that restricts or resists valve motion.
Actuator seal degradation brought on by compression, wear or looseness that reduces the strain available to actuate the valve.
Minor damage to the valve obturator plug, disk or ball caused by system conditions, leakage or debris, including build-up of hydrocarbon merchandise. This causes leakage.
Complete failure of hydraulic management system elements similar to solenoids, pilots, velocity controller, etc., due to moisture, debris or alignment. The valve will fail to open or close.
All of those situations could be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For example, a valve that is caught open will exhibit itself as a rise in breakout drive. Loss of seat load in comparability with when the valve assembly was new signifies potential valve leak. The Fisher DVC6200SIS additionally repeatedly monitors for inner faults in addition to its inputs such as supply pressure. When the DVC6200SIS sees the supply 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 almost zero supply stress, so it alerted the management system.
Other process journey advantages
Diagnostic information captured during a process trip might reveal valve degradations that will not be detected during a proof take a look at. For example, diagnostic data captured throughout a course of trip might point out a problem with the valve closing fully in opposition to the total stress of the method, which could be an early indication of valve leakage.
The valve stroke time computed by a DVC during a course of trip is extra correct underneath real working situations. This results in a extra correct SIF response time (the time from detection of an abnormal situation to last element reaching its trip state), which is in comparability with the Process Safety Time to ensure the SIF remains to be meeting its design necessities.
Although automated valves are very dependable, failures identified in a course of trip can provide priceless knowledge to stop future failures. This data can help with turnaround planning by making sure the needed elements are available before turnaround even begins to probably shorten the turnaround schedule.
Summary
A process journey can provide the protection required to delay a valve’s next scheduled proof check, thus saving money and time. Data captured by a DVC could be analyzed to leverage the method trip as a proof take a look at. Even if the tip user chooses to not take proof check credits for a course of trip, the valve diagnostic data supplied by the DVC might help plant personnel make proactive valve maintenance choices..
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