Valve proof take a look at credit for a course of trip

A course of journey occurs when the safety instrumented system (SIS) places 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 journey happens due to a valve malfunction. In that case, the valve assembly is tested under actual working circumstances, which supplies an opportunity to capture useful valve diagnostic knowledge.
However, such diagnostics information can solely be captured when automated valves are fitted with digital valve controllers (DVCs). In this text, we’ll discuss how DVCs may help determine the proof check credit for an automated valve after a process journey.
Process journey
A process trip occurs when the SIS detects an abnormal course of condition through sensors similar to temperature and pressure, executes the logic and locations the process in its protected state by tripping the ultimate components corresponding to closing an automatic 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 process journey occurs, the primary goal is often to restart the unit or tools that has been shut down as quickly as potential. Plant engineers are busy troubleshooting and correcting problems. Maintenance technicians are busy repairing damaged gadgets. Taking the chance to proof take a look at an automated valve won’t be a top precedence 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 check of the valve could be thought-about performed with diagnostic knowledge captured by the DVC. Also, a comparability of the diagnostic data with the valve’s baseline can help uncover valve deficiencies and degradations that may not show up in a proof take a look at.
Figure 2. Proof Test 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 most of the 12 necessities of a proof take a look at.
Process trips versus proof exams
How can proof take a look at credit be claimed for an automatic valve that has failed and triggered a course of trip? A proof take a look at is a periodic check performed on SIS sensors, logic solvers and last components — such as automated valves — to discover hidden failures. These failures, if not corrected or eliminated, could impair the power of the SIS to take the method to its secure state when an abnormal process situation is detected.
A proof test should be carried out as per the proof check interval specified within the Safety Requirement Specifications. This interval is often decided by way of an average probability of failure on demand calculation (PFDavg). Some customers might select to proof check based on predetermined intervals, similar to proof testing sensors each 24 months and ultimate components every forty eight months as a substitute of relying on the PFDavg calculations.
Automated valve proof exams could be done offline or on-line. Offline proof checks are usually scheduled throughout a turnaround, when the process is completely shut down.
Proof testing an automated valve on-line 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 check may also be completed throughout a course of journey.
According to IEC61511-2 Section A.sixteen.three.1.3, “…shutdowns due to actual demand on the SIS during operation may be given credit score as proof exams (fully or partial) underneath given conditions…the next deliberate proof take a look at could additionally be skipped.”
These conditions are
The shutdown documents equivalent information as registered throughout corresponding proof test.
The shutdown covers all elements of the SIS, and if not, the device or SIS subsystem not activated must be examined separately.
The shutdown happens inside a predetermined maximum time window before the next deliberate proof take a look at which can then be canceled
When a course of journey happens, about 60% of a Safety Instrumented Function (SIF) proof take a look at can be thought of performed. A pattern record of actions performed throughout a proof check, along with these which are carried out during a course of trip, is proven in Figure 2. Even with out an automatic valve leak check, data captured by the DVC alone can potentially account for a great quantity of proof test coverage for an automatic valve.
The exact protection is dependent upon the design of the automated valve, its DVC and its application. The coverage is estimated based mostly on the valve degradations, the probability of their prevalence and the percentage of these degradations that can be detected by a DVC. Fortunately, knowledge acquired by a DVC throughout a process trip can typically be enough to fulfill a significant a part of the proof take a look at requirements.
If the process trip takes place inside a predetermined most time window, the tip consumer might select to leverage the method trip as a proof test by finishing steps one through five in Figure 2, that are usually not accomplished in a course of journey. The next scheduled proof take a look at 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 3. Data throughout a process trip is captured by the Fisher DVC6200 SIS digital valve controller and made out there for analysis.
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 course of trip — include:
Partial or intermittent sticking of hydraulic or pneumatic system components such as solenoids, pilots, pace controller, etc., as a result of moisture, debris or alignment points. This causes a lack of practical margin and makes the valve slow to open or shut.
Binding, galling or other degradation of valve seats or associated flow control trim that restricts or resists valve motion.
Actuator seal degradation brought on by compression, wear or looseness that reduces the stress available to actuate the valve.
Minor injury to the valve obturator plug, disk or ball caused by system conditions, leakage or particles, together with build-up of hydrocarbon products. This causes leakage.
Complete failure of hydraulic management system components similar to solenoids, pilots, velocity controller, etc., due to moisture, particles or alignment. The valve will fail to open or shut.
All of those circumstances could be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For เกจวัดแรงดันถังแก๊ส , a valve that’s caught open will exhibit itself as a rise in breakout force. Loss of seat load compared to when the valve assembly was new signifies potential valve leak. The Fisher DVC6200SIS also repeatedly displays for inside faults as well as its inputs corresponding to provide pressure. When the DVC6200SIS sees the availability stress is just too low, an alert is generated (Figure 4) so that the tip person can take corrective actions.
Figure 4. The DVC6200SIS digital valve controller detected almost zero provide stress, so it alerted the control system.
Other process trip advantages
Diagnostic knowledge captured throughout a process journey could reveal valve degradations that will not be detected throughout a proof test. For example, diagnostic knowledge captured throughout a course of journey may indicate a problem with the valve closing utterly in opposition to the total strain 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 extra accurate under actual operating situations. This ends in a more correct SIF response time (the time from detection of an abnormal condition to last component reaching its journey state), which is in comparability with the Process Safety Time to make sure the SIF is still meeting its design requirements.
Although automated valves are very reliable, failures identified in a process journey can provide useful knowledge to stop future failures. This data can assist with turnaround planning by making sure the wanted components are available earlier than turnaround even begins to potentially shorten the turnaround schedule.
A course of trip can provide the protection required to delay a valve’s next scheduled proof take a look at, thus saving money and time. Data captured by a DVC could be analyzed to leverage the method journey as a proof take a look at. Even if the tip person chooses not to take proof test credit for a process trip, the valve diagnostic data offered by the DVC might help plant personnel make proactive valve upkeep choices..

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