A process trip occurs when the protection instrumented system (SIS) places the method in its safe state by commanding an automatic valve (Figure 1) or different equipment to its journey state in response to an irregular course of condition. In some instances, a spurious trip happens because of a valve malfunction. In that case, the valve assembly is tested beneath real working circumstances, which provides an opportunity to seize valuable valve diagnostic knowledge.
However, such diagnostics knowledge can only be captured when automated valves are fitted with digital valve controllers (DVCs). In this text, we’ll discuss how DVCs may help decide the proof take a look at credit for an automated valve after a process journey.
Process journey
A course of journey happens when the SIS detects an abnormal course of situation via sensors such as temperature and strain, executes the logic and places the method in its safe state by tripping the ultimate elements corresponding to closing an automated valve, stopping a pump, etc. The SIS could talk with the Basic Process Control System (BPCS) to close a valve downstream of the automated valve as a secondary motion.
When a course of trip occurs, the main objective is usually to restart the unit or gear that has been shut down as soon as potential. Plant engineers are busy troubleshooting and correcting issues. Maintenance technicians are busy repairing damaged devices. Taking the opportunity to proof take a look at an automatic valve is not going to be a top precedence and even an activity under consideration as a end result of tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is supplied with a DVC, the proof test of the valve may be thought-about carried out with diagnostic data captured by the DVC. Also, a comparison of the diagnostic knowledge with the valve’s baseline might help uncover valve deficiencies and degradations that will not show up in a proof test.
Figure 2. Proof Test vs Process Trip. Activities carried out (such as repairing the device) and data captured by a Fisher DVC6200 SIS digital valve controller during a process trip meet a lot of the 12 necessities of a proof test.
Process trips versus proof tests
How can proof test credit be claimed for an automatic valve that has failed and brought on a course of trip? A proof check is a periodic test carried out on SIS sensors, logic solvers and final components — such as automated valves — to discover hidden failures. These failures, if not corrected or removed, might impair the power of the SIS to take the process to its secure state when an irregular process 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 normally decided via a mean probability of failure on demand calculation (PFDavg). Some users might choose to proof take a look at primarily based on predetermined intervals, such as proof testing sensors each 24 months and final components each forty eight months instead of relying on the PFDavg calculations.
Automated valve proof exams may be accomplished offline or online. Offline proof checks are often scheduled throughout a turnaround, when the process is totally shut down.
Proof testing an automated valve on-line usually requires a unit or gear to be shut down, or the valve to be bypassed to stop a spurious trip, making it inconvenient and dear.
But a proof take a look at can be accomplished during a course of journey.
According to IEC61511-2 Section A.16.three.1.three, “…shutdowns as a result of actual demand on the SIS throughout operation could also be given credit score as proof tests (fully or partial) underneath given conditions…the next planned proof take a look at may be skipped.”
These conditions are
The shutdown documents equivalent info as registered during corresponding proof test.
The shutdown covers all elements of the SIS, and if not, the device or SIS subsystem not activated needs to be tested individually.
The shutdown occurs inside a predetermined maximum time window before the following planned proof take a look at which might then be canceled
When a process journey happens, about 60% of a Safety Instrumented Function (SIF) proof take a look at may be thought-about carried out. A pattern listing of activities carried out during a proof test, along with those which would possibly be performed throughout a course of trip, is shown in Figure 2. Even without an automated 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 precise protection is dependent upon the design of the automated valve, its DVC and its application. The protection is estimated primarily based on the valve degradations, the chance of their occurrence and the percentage of these degradations that can be detected by a DVC. Fortunately, data acquired by a DVC during a process journey can often be sufficient to satisfy a significant part of the proof take a look at requirements.
If the process journey takes place inside a predetermined maximum time window, the tip user could choose to leverage the method trip as a proof test by completing steps one by way of 5 in Figure 2, which are normally not completed in a process journey. The subsequent scheduled proof take a look at can then be canceled. According to the Norwegian Oil Industry Association, the utmost time window must be the final half of the current proof take a look at interval.
Figure three. Data throughout 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 may lead to a process trip — include:
Partial or intermittent sticking of hydraulic or pneumatic system components similar to solenoids, pilots, speed controller, and so forth., as a result of moisture, debris or alignment issues. This causes a loss of functional margin and makes the valve gradual to open or shut.
Binding, galling or other degradation of valve seats or related move control trim that restricts or resists valve movement.
Actuator seal degradation attributable to compression, put on or looseness that reduces the pressure available to actuate the valve.
Minor injury to the valve obturator plug, disk or ball attributable to system circumstances, leakage or debris, together with build-up of hydrocarbon merchandise. This causes leakage.
Complete failure of hydraulic control system parts corresponding to solenoids, pilots, pace controller, and so on., due to moisture, debris or alignment. The valve will fail to open or shut.
All of these situations may be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For example, a valve that is stuck open will exhibit itself as an increase in breakout pressure. Loss of seat load compared to when the valve meeting was new indicates potential valve leak. The Fisher DVC6200SIS also constantly monitors for inside faults as well as its inputs such as supply pressure. When the DVC6200SIS sees the availability stress is too low, an alert is generated (Figure 4) so that the end person can take corrective actions.
Figure four. The DVC6200SIS digital valve controller detected nearly zero provide strain, so it alerted the control system.
Other course of journey advantages
Diagnostic knowledge captured during a course of trip might reveal valve degradations that will not be detected throughout a proof check. For example, diagnostic information captured during a course of journey may indicate an issue with the valve closing completely against the total stress of the method, which can be an early indication of valve leakage.
The valve stroke time computed by a DVC during a process journey is extra correct under real working conditions. This ends in a extra correct SIF response time (the time from detection of an abnormal condition to last component reaching its trip state), which is in comparison with the Process Safety Time to ensure the SIF is still meeting its design requirements.
Although automated เกจวัดแรงดันไนโตรเจนราคา are very reliable, failures identified in a course of journey can provide valuable information to prevent future failures. This information can assist with turnaround planning by ensuring the needed elements are available earlier than turnaround even begins to potentially shorten the turnaround schedule.
Summary
A course of journey can provide the protection required to delay a valve’s next scheduled proof test, thus saving money and time. Data captured by a DVC can be analyzed to leverage the method trip as a proof check. Even if the tip user chooses to not take proof take a look at credits for a process journey, the valve diagnostic knowledge offered by the DVC can help plant personnel make proactive valve maintenance selections..
Share