Valve proof test credit for a course of journey

A course of journey occurs when the security instrumented system (SIS) locations the method in its secure state by commanding an automated valve (Figure 1) or different equipment to its trip state in response to an abnormal course of situation. In some cases, a spurious journey happens due to a valve malfunction. In that case, the valve meeting is tested beneath actual working circumstances, which supplies a possibility to capture useful valve diagnostic knowledge.
However, such diagnostics data can solely be captured when automated valves are fitted with digital valve controllers (DVCs). In this article, we’ll discuss how DVCs can help decide the proof take a look at credit for an automatic valve after a process trip.
Process journey
A process trip happens when the SIS detects an abnormal course of condition through sensors such as temperature and stress, executes the logic and places the method in its secure state by tripping the final elements similar to closing an automatic valve, stopping a pump, etc. The SIS could communicate with the Basic Process Control System (BPCS) to close a valve downstream of the automated valve as a secondary motion.
When a course of journey occurs, the main goal is usually to restart the unit or tools that has been shut down as soon as potential. Plant engineers are busy troubleshooting and correcting issues. Maintenance technicians are busy repairing damaged gadgets. Taking the chance to proof take a look at an automated valve will not be a prime priority 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 test of the valve could be thought-about carried out with diagnostic data captured by the DVC. Also, a comparability of the diagnostic data with the valve’s baseline can help discover valve deficiencies and degradations that will not 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 during a process trip meet most of the 12 necessities of a proof check.
Process trips versus proof checks
How can proof take a look at credit be claimed for an automated valve that has failed and triggered a process trip? A proof test is a periodic test carried out on SIS sensors, logic solvers and last parts — similar to automated valves — to discover hidden failures. These failures, if not corrected or removed, may impair the power of the SIS to take the process to its protected state when an irregular process condition is detected.
A proof test should be carried out as per the proof check interval specified within the Safety Requirement Specifications. This interval is usually determined by way of a median chance of failure on demand calculation (PFDavg). Some users might choose to proof test based mostly on predetermined intervals, similar to proof testing sensors each 24 months and last components every 48 months instead of relying on the PFDavg calculations.
Automated valve proof exams may be done offline or on-line. เกจ์วัดความดัน are normally scheduled throughout a turnaround, when the method is completely shut down.
Proof testing an automatic valve on-line normally requires a unit or tools to be shut down, or the valve to be bypassed to forestall a spurious journey, making it inconvenient and costly.
But a proof take a look at can be completed during a course of journey.
According to IEC61511-2 Section A.16.three.1.three, “…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 subsequent planned proof take a look at may be skipped.”
These conditions are
The shutdown paperwork equal information as registered during corresponding proof check.
The shutdown covers all components of the SIS, and if not, the device or SIS subsystem not activated needs to be tested individually.
The shutdown happens inside a predetermined most time window before the subsequent deliberate proof test which can then be canceled
When a process trip occurs, about 60% of a Safety Instrumented Function (SIF) proof check may be thought-about carried out. A sample record of actions performed during a proof take a look at, along with those that are carried out during a process trip, is shown in Figure 2. Even without 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 actual protection depends on the design of the automated valve, its DVC and its application. The coverage is estimated based on the valve degradations, the chance of their prevalence and the proportion of those degradations that might be detected by a DVC. Fortunately, data acquired by a DVC during a process journey can usually be sufficient to fulfill a significant a half of the proof take a look at requirements.
If the method trip takes place within a predetermined maximum time window, the end user might choose to leverage the method trip as a proof check by finishing steps one via five in Figure 2, that are normally not completed in a course of journey. The subsequent scheduled proof take a look at can then be canceled. According to the Norwegian Oil Industry Association, the utmost time window should be the last half of the present proof test interval.
Figure 3. Data throughout a course of trip is captured by the Fisher DVC6200 SIS digital valve controller and made out there 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 result in a course of journey — embody:
Partial or intermittent sticking of hydraulic or pneumatic system parts similar to solenoids, pilots, pace controller, and so on., due to moisture, particles or alignment issues. This causes a loss of functional margin and makes the valve gradual to open or close.
Binding, galling or เกจอาร์กอน of valve seats or related circulate control trim that restricts or resists valve motion.
Actuator seal degradation caused by compression, put on or looseness that reduces the stress available to actuate the valve.
Minor damage to the valve obturator plug, disk or ball brought on by system circumstances, leakage or particles, including build-up of hydrocarbon products. This causes leakage.
Complete failure of hydraulic control system parts similar to solenoids, pilots, velocity controller, etc., as a outcome of moisture, particles or alignment. The valve will fail to open or close.
All of these circumstances may be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For example, a valve that’s caught open will exhibit itself as an increase in breakout force. Loss of seat load in comparison with when the valve assembly was new indicates potential valve leak. The Fisher DVC6200SIS additionally continuously monitors for inside faults in addition to its inputs similar to supply strain. When the DVC6200SIS sees the supply strain is merely too low, an alert is generated (Figure 4) in order that the top user can take corrective actions.
Figure four. The DVC6200SIS digital valve controller detected nearly zero provide stress, so it alerted the management system.
Other process journey benefits
Diagnostic information captured during a course of trip might reveal valve degradations that is in all probability not detected during a proof take a look at. For example, diagnostic information captured during a course of trip would possibly point out an issue with the valve closing completely towards the full stress of the method, which could be an early indication of valve leakage.
The valve stroke time computed by a DVC during a process trip is extra correct beneath actual operating circumstances. This ends in a more correct SIF response time (the time from detection of an irregular situation to last element reaching its trip state), which is in comparison with the Process Safety Time to ensure the SIF continues to be meeting its design requirements.
Although automated valves are very dependable, failures identified in a course of trip can present valuable information to stop future failures. This information can help with turnaround planning by ensuring the needed parts can be found before turnaround even begins to probably shorten the turnaround schedule.
Summary
A course of journey can provide the coverage required to delay a valve’s next scheduled proof check, thus saving money and time. Data captured by a DVC can be analyzed to leverage the method trip as a proof take a look at. Even if the top person chooses to not take proof take a look at credits for a course of trip, the valve diagnostic information supplied by the DVC can help plant personnel make proactive valve maintenance selections..
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