Valve proof test credit score for a process trip

A course of journey happens when the protection instrumented system (SIS) locations the process in its safe state by commanding an automatic valve (Figure 1) or other equipment to its trip state in response to an irregular process condition. In some cases, a spurious trip occurs due to a valve malfunction. In that case, the valve meeting is examined under actual working circumstances, which supplies an opportunity to capture useful valve diagnostic data.
However, such diagnostics knowledge can only be captured when automated valves are fitted with digital valve controllers (DVCs). In this text, we’ll focus on how DVCs may help decide the proof test credit score for an automatic valve after a process journey.
Process trip
A process journey happens when the SIS detects an irregular course of condition through sensors such as temperature and pressure, executes the logic and locations the method in its safe state by tripping the final elements corresponding to closing an automatic valve, stopping a pump, and so on. The SIS may talk with the Basic Process Control System (BPCS) to shut a valve downstream of the automated valve as a secondary action.
When a process trip occurs, the primary objective is normally to restart the unit or equipment that has been shut down as soon as potential. Plant engineers are busy troubleshooting and correcting issues. Maintenance technicians are busy repairing broken devices. Taking the chance to proof check an automated valve is not going to be a top priority or even an activity under consideration because of the tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is equipped with a DVC, the proof test of the valve could be thought of carried out with diagnostic data captured by the DVC. Also, a comparison of the diagnostic data with the valve’s baseline may help uncover valve deficiencies and degradations that could not show up in a proof test.
Figure 2. Proof Test vs Process Trip. Activities carried out (such as repairing the device) and information captured by a Fisher DVC6200 SIS digital valve controller during a course of trip meet most of the 12 necessities of a proof check.
Process trips versus proof tests
How can proof take a look at credit be claimed for an automatic valve that has failed and brought on a course of trip? A proof test is a periodic test carried out on SIS sensors, logic solvers and final components — similar to automated valves — to find hidden failures. These failures, if not corrected or eliminated, could impair the ability of the SIS to take the method to its protected state when an irregular course of situation is detected.
A proof check ought to be performed as per the proof test interval specified within the Safety Requirement Specifications. This interval is often determined through a mean likelihood of failure on demand calculation (PFDavg). Some customers might select to proof check primarily based on predetermined intervals, similar to proof testing sensors each 24 months and final elements each 48 months instead of counting on the PFDavg calculations.
Automated valve proof exams could be done offline or online. Offline proof exams are normally scheduled during a turnaround, when the method is completely shut down.
Proof testing an automated valve online normally requires a unit or gear to be shut down, or the valve to be bypassed to forestall a spurious journey, making it inconvenient and dear.
But a proof take a look at may additionally be completed during a process journey.
According to IEC61511-2 Section A.16.3.1.three, “…shutdowns as a end result of actual demand on the SIS throughout operation may be given credit score as proof checks (fully or partial) under given conditions…the next planned proof check may be skipped.”
These circumstances are
The shutdown documents equivalent data as registered during corresponding proof test.
The shutdown covers all components of the SIS, and if not, the gadget or SIS subsystem not activated must be tested individually.
The shutdown happens inside a predetermined most time window before the following deliberate proof test which might then be canceled
When a process trip occurs, about 60% of a Safety Instrumented Function (SIF) proof take a look at may be thought of carried out. A pattern record of actions performed during a proof test, along with these that are performed throughout a process journey, is proven in Figure 2. Even without an automated valve leak check, knowledge captured by the DVC alone can probably account for a great amount of proof take a look at 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 on the valve degradations, the chance of their prevalence and the share of those degradations that might be detected by a DVC. Fortunately, data acquired by a DVC during a course of journey can usually be enough to fulfill a major part of the proof check requirements.
If the process trip takes place inside a predetermined maximum time window, the end user might select to leverage the method trip as a proof check by completing steps one via 5 in Figure 2, that are often not completed in a process journey. The next scheduled proof test can then be canceled. According to the Norwegian Oil Industry Association, the maximum time window must be the last half of the current proof test interval.
Figure 3. Data during a process 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 96.05.01, the most probably root causes of automated valve degradations — which may lead to a process trip — embody:
Partial or intermittent sticking of hydraulic or pneumatic system elements similar to solenoids, pilots, velocity controller, etc., due to moisture, debris or alignment points. 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 management trim that restricts or resists valve movement.
Actuator seal degradation brought on by compression, wear or looseness that reduces the strain out there to actuate the valve.
Minor injury to the valve obturator plug, disk or ball attributable to system situations, leakage or debris, together with build-up of hydrocarbon merchandise. This causes leakage.
Complete failure of hydraulic management system elements similar to solenoids, pilots, pace controller, and so forth., as a outcome of moisture, particles or alignment. The valve will fail to open or close.
All of those situations could be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For instance, a valve that’s 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 additionally constantly displays for internal faults in addition to its inputs such as supply stress. When the DVC6200SIS sees the availability strain is simply too low, an alert is generated (Figure 4) so that the top consumer can take corrective actions.
Figure four. The DVC6200SIS digital valve controller detected nearly zero provide strain, so it alerted the control system.
Other process journey benefits
Diagnostic data captured during a process journey might reveal valve degradations that will not be detected during a proof take a look at. For example, diagnostic information captured during a course of trip would possibly point out a problem with the valve closing fully towards the full strain of the process, which may be an early indication of valve leakage.
The valve stroke time computed by a DVC throughout a process trip is more correct beneath actual working situations. This leads to a extra accurate SIF response time (the time from detection of an irregular condition to ultimate factor reaching its trip state), which is compared to the Process Safety Time to make sure the SIF remains to be assembly its design necessities.
Although automated valves are very dependable, failures identified in a process trip can provide priceless information to stop future failures. This information can help with turnaround planning by ensuring the wanted elements are available before turnaround even begins to probably shorten the turnaround schedule.
Summary
A course of trip can provide the protection required to delay a valve’s subsequent scheduled proof check, thus saving money and time. Studies show captured by a DVC can be analyzed to leverage the process journey as a proof test. Even if the tip consumer chooses to not take proof take a look at credit for a process trip, the valve diagnostic data supplied by the DVC may help plant personnel make proactive valve upkeep choices..
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