Valve proof test credit score for a process journey

A course of journey occurs when the safety instrumented system (SIS) places the process in its secure state by commanding an automatic valve (Figure 1) or different tools to its journey state in response to an irregular course of condition. In some circumstances, a spurious trip occurs because of a valve malfunction. In that case, the valve meeting is examined beneath real operating conditions, which provides an opportunity to seize useful valve diagnostic knowledge.
However, such diagnostics information can only be captured when automated valves are fitted with digital valve controllers (DVCs). In this article, we’ll talk about how DVCs may help decide the proof test credit for an automatic valve after a course of trip.
Process trip
A process trip happens when the SIS detects an irregular process condition via sensors such as temperature and stress, executes the logic and places the process in its safe state by tripping the final components such as closing an automated valve, stopping a pump, etc. The SIS might talk with the Basic Process Control System (BPCS) to shut a valve downstream of the automated valve as a secondary motion.
When a process trip happens, the principle aim is normally to restart the unit or tools that has been shut down as soon as possible. Plant engineers are busy troubleshooting and correcting problems. Maintenance technicians are busy repairing damaged devices. Taking เกจวัดแรงดัน1บาร์ to proof check an automatic valve will not be a prime priority or even an activity into 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 considered performed with diagnostic information captured by the DVC. Also, a comparison of the diagnostic knowledge with the valve’s baseline may help uncover valve deficiencies and degradations that won’t present up in a proof check.
Figure 2. Proof Test vs Process Trip. Activities performed (such as repairing the device) and information captured by a Fisher DVC6200 SIS digital valve controller throughout a course of journey meet most of the 12 necessities of a proof test.
Process journeys versus proof exams
How can proof check credit be claimed for an automatic valve that has failed and triggered a process trip? A proof test is a periodic take a look at performed on SIS sensors, logic solvers and ultimate parts — corresponding to automated valves — to find hidden failures. These failures, if not corrected or eliminated, might impair the ability of the SIS to take the method to its safe state when an abnormal course of situation is detected.
A proof test ought to be carried out as per the proof check interval specified in the Safety Requirement Specifications. This interval is often determined by way of a median probability of failure on demand calculation (PFDavg). Some users might select to proof check based mostly on predetermined intervals, such as proof testing sensors every 24 months and ultimate components every forty eight months as a substitute of relying on the PFDavg calculations.
Automated valve proof exams could be carried out offline or online. Offline proof tests are usually scheduled throughout a turnaround, when the process is totally shut down.
Proof testing an automatic valve online often requires a unit or gear to be shut down, or the valve to be bypassed to prevent a spurious trip, making it inconvenient and dear.
But a proof test may additionally be accomplished throughout a process trip.
According to IEC61511-2 Section A.16.3.1.3, “…shutdowns because of actual demand on the SIS throughout operation may be given credit score as proof checks (fully or partial) beneath given conditions…the subsequent deliberate proof take a look at could additionally be skipped.”
These conditions are
The shutdown paperwork equal data as registered during corresponding proof take a look at.
The shutdown covers all components of the SIS, and if not, the gadget or SIS subsystem not activated must be tested separately.
The shutdown occurs inside a predetermined maximum time window before the subsequent deliberate proof check which may then be canceled
When a course of trip happens, about 60% of a Safety Instrumented Function (SIF) proof check can be thought of carried out. A sample record of actions performed throughout a proof take a look at, together with those which are performed throughout a course of trip, is proven in Figure 2. Even with out an automatic valve leak test, knowledge captured by the DVC alone can probably account for an excellent amount of proof take a look at coverage for an automatic valve.
The actual coverage is decided by the design of the automated valve, its DVC and its software. The protection is estimated primarily based on the valve degradations, the chance of their prevalence and the share of these degradations that can be detected by a DVC. Fortunately, knowledge acquired by a DVC during a course of trip can often be sufficient to fulfill a significant a part of the proof take a look at necessities.
If the process journey takes place inside a predetermined maximum time window, the tip user might choose to leverage the process journey as a proof test by completing steps one via 5 in Figure 2, which are often not accomplished in a course of journey. เกจ์วัดแรงดัน scheduled proof check can then be canceled. According to the Norwegian Oil Industry Association, the maximum time window ought to be the final half of the current proof test interval.
Figure 3. Data throughout 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 ninety six.05.01, the most probably root causes of automated valve degradations — which can result in a course of journey — include:
Partial or intermittent sticking of hydraulic or pneumatic system components similar to solenoids, pilots, pace controller, and so on., due to moisture, particles or alignment points. This causes a loss of functional margin and makes the valve slow to open or close.
Binding, galling or other degradation of valve seats or related flow management trim that restricts or resists valve movement.
Actuator seal degradation brought on by compression, put on or looseness that reduces the strain available to actuate the valve.
Minor damage to the valve obturator plug, disk or ball attributable to system circumstances, leakage or debris, including build-up of hydrocarbon products. This causes leakage.
Complete failure of hydraulic management system elements such as solenoids, pilots, speed controller, and so forth., because of moisture, particles or alignment. The valve will fail to open or shut.
All of those conditions may be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For example, a valve that’s caught open will exhibit itself as a rise in breakout drive. Loss of seat load in comparison with when the valve meeting was new indicates potential valve leak. The Fisher DVC6200SIS also repeatedly monitors for inner faults as nicely as its inputs similar to provide pressure. When the DVC6200SIS sees the supply strain is too low, an alert is generated (Figure 4) so that the tip user can take corrective actions.
Figure four. The DVC6200SIS digital valve controller detected nearly zero supply stress, so it alerted the control system.
Other course of journey benefits
Diagnostic information captured during a course of journey might reveal valve degradations that will not be detected during a proof take a look at. For example, diagnostic information captured throughout a course of trip may point out an issue with the valve closing completely towards the full stress of the process, which can be an early indication of valve leakage.
The valve stroke time computed by a DVC during a course of trip is more accurate underneath real operating situations. This ends in a extra accurate SIF response time (the time from detection of an irregular condition to final factor reaching its journey state), which is in comparison with the Process Safety Time to ensure the SIF is still assembly its design requirements.
Although automated valves are very reliable, failures identified in a course of trip can provide priceless knowledge to forestall future failures. This data can assist with turnaround planning by making sure the wanted elements are available before turnaround even begins to probably shorten the turnaround schedule.
Summary
A process trip can provide the protection required to delay a valve’s next scheduled proof check, thus saving time and money. Data captured by a DVC can be analyzed to leverage the process trip as a proof test. Even if the end user chooses not to take proof check credit for a course of journey, the valve diagnostic data supplied by the DVC can help plant personnel make proactive valve upkeep decisions..
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