Valve proof take a look at credit for a process journey

A process journey happens when the security instrumented system (SIS) places the process in its protected state by commanding an automatic valve (Figure 1) or other equipment to its trip state in response to an irregular course of condition. In some circumstances, a spurious trip occurs due to a valve malfunction. In that case, the valve assembly is examined underneath real working circumstances, which supplies an opportunity to capture useful valve diagnostic knowledge.
However, such diagnostics data can only 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 score for an automated valve after a process journey.
Process trip
A process journey occurs when the SIS detects an irregular process situation via sensors such as temperature and stress, executes the logic and locations the method in its safe state by tripping the final parts corresponding to closing an automatic valve, stopping a pump, etc. The SIS could communicate with the Basic Process Control System (BPCS) to shut a valve downstream of the automated valve as a secondary action.
When a course of trip happens, the principle goal is often to restart the unit or gear that has been shut down as quickly as attainable. Plant engineers are busy troubleshooting and correcting problems. Maintenance technicians are busy repairing broken gadgets. Taking the chance to proof take a look at an automated valve will not be a prime priority and even an activity into account because of the tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is supplied with a DVC, the proof take a look at of the valve may be thought of carried out with diagnostic data captured by the DVC. Also, a comparison of the diagnostic data with the valve’s baseline might help discover valve deficiencies and degradations that may not show up in a proof take a look at.
Figure 2. Proof Test vs Process Trip. Activities performed (such as repairing the device) and knowledge captured by a Fisher DVC6200 SIS digital valve controller during a process 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 credit be claimed for an automatic valve that has failed and caused a course of trip? A proof test is a periodic test performed on SIS sensors, logic solvers and ultimate parts — corresponding to automated valves — to discover hidden failures. These failures, if not corrected or eliminated, could impair the ability of the SIS to take the process to its secure state when an abnormal course of condition is detected.
A proof check should be carried out as per the proof test interval specified within the Safety Requirement Specifications. This interval is normally determined via an average chance of failure on demand calculation (PFDavg). Some customers could choose to proof take a look at based on predetermined intervals, similar to proof testing sensors every 24 months and final elements each forty eight months as an alternative of relying on the PFDavg calculations.
Automated valve proof tests may be done offline or online. Offline proof tests are normally scheduled during 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 stop a spurious trip, making it inconvenient and dear.
But a proof check may also be achieved throughout a process journey.
According to IEC61511-2 Section A.sixteen.3.1.3, “…shutdowns because of precise demand on the SIS throughout operation may be given credit score as proof tests (fully or partial) under given conditions…the subsequent planned proof take a look at may be skipped.”
These circumstances are
The shutdown paperwork equivalent information as registered during corresponding proof take a look at.
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 most time window before the following planned proof check which may then be canceled
When a process trip happens, about 60% of a Safety Instrumented Function (SIF) proof check may be thought of performed. A sample list of actions performed throughout a proof test, together with those that are carried out during a course of journey, is shown in Figure 2. Even with out an automatic valve leak test, knowledge captured by the DVC alone can probably account for a good quantity of proof take a look at coverage for an automated valve.
The precise coverage is determined by the design of the automated valve, its DVC and its utility. The protection is estimated based on the valve degradations, the likelihood of their incidence and the proportion of these degradations that may be detected by a DVC. Fortunately, knowledge acquired by a DVC during a process trip can typically be sufficient to fulfill a serious a half of the proof take a look at necessities.
If the method journey takes place inside a predetermined most time window, the end user could choose to leverage the process trip as a proof test by completing steps one by way of five in Figure 2, that are normally not completed in a course of trip. The next scheduled proof check can then be canceled. According to the Norwegian Oil Industry Association, the maximum time window ought to be the last half of the current 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 obtainable for analysis.
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 might lead to a process trip — include:
Partial or intermittent sticking of hydraulic or pneumatic system elements similar to solenoids, pilots, velocity controller, etc., due to moisture, particles or alignment points. This causes a loss of practical margin and makes the valve gradual to open or shut.
Binding, galling or different degradation of valve seats or related circulate management trim that restricts or resists valve movement.
Actuator seal degradation attributable to compression, put on or looseness that reduces the stress out there to actuate the valve.
Minor harm to the valve obturator plug, disk or ball brought on by system circumstances, leakage or particles, together with build-up of hydrocarbon merchandise. pressure gauge น้ำมัน causes leakage.
Complete failure of hydraulic control system parts corresponding to solenoids, pilots, velocity controller, etc., as a result 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 instance, a valve that is stuck open will exhibit itself as an increase in breakout force. Loss of seat load compared to when the valve meeting was new signifies potential valve leak. The Fisher DVC6200SIS also repeatedly monitors for inside faults as nicely as its inputs such as provide pressure. When the DVC6200SIS sees the provision strain is merely too low, an alert is generated (Figure 4) in order that the end user can take corrective actions.
Figure 4. The DVC6200SIS digital valve controller detected nearly zero supply strain, so it alerted the control system.
Other process journey advantages
Diagnostic knowledge captured during a course of trip could reveal valve degradations that is most likely not detected throughout a proof take a look at. For instance, diagnostic knowledge captured during a process trip may indicate an issue with the valve closing completely towards the full stress of the method, which may be an early indication of valve leakage.
The valve stroke time computed by a DVC throughout a process trip is more accurate under real working conditions. This results in a more accurate SIF response time (the time from detection of an abnormal condition to final component reaching its trip state), which is in comparability with the Process Safety Time to make sure the SIF is still assembly its design necessities.
Although automated valves are very reliable, failures identified in a process trip can provide priceless knowledge to forestall future failures. This information can assist with turnaround planning by making sure the needed parts can be found before turnaround even begins to probably shorten the turnaround schedule.
A process 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 journey as a proof take a look at. Even if the end person chooses to not take proof take a look at credits for a process trip, the valve diagnostic information provided by the DVC can help plant personnel make proactive valve upkeep choices..

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