A course of journey happens when the security instrumented system (SIS) locations the process in its safe state by commanding an automatic valve (Figure 1) or other tools to its journey state in response to an abnormal process condition. In some cases, a spurious journey happens due to a valve malfunction. In that case, the valve meeting is examined beneath actual operating conditions, which supplies an opportunity to seize valuable valve diagnostic information.
However, such diagnostics data can solely be captured when automated valves are fitted with digital valve controllers (DVCs). In this article, we’ll focus on how DVCs can help decide the proof test credit for an automatic valve after a process journey.
A course of trip happens when the SIS detects an irregular course of situation by way of sensors such as temperature and stress, executes the logic and places the process in its secure state by tripping the final parts corresponding to closing an automated 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 principle objective is often to restart the unit or gear that has been shut down as quickly as potential. Plant engineers are busy troubleshooting and correcting problems. Maintenance technicians are busy repairing broken gadgets. Taking the chance to proof test an automated valve won’t be a high precedence or even an exercise into consideration due to the tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is equipped with a DVC, the proof check of the valve could be thought of performed with diagnostic information captured by the DVC. Also, a comparability of the diagnostic data with the valve’s baseline can help uncover valve deficiencies and degradations that gained’t present 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 throughout a course of trip meet a lot of the 12 requirements of a proof check.
Process trips versus proof tests
How can proof test credit be claimed for an automated valve that has failed and brought on a process trip? A proof take a look at is a periodic take a look at performed on SIS sensors, logic solvers and final components — corresponding to automated valves — to find hidden failures. These failures, if not corrected or removed, might impair the ability of the SIS to take the method to its safe state when an irregular course of condition is detected.
A proof test ought to be carried out as per the proof check interval specified within the Safety Requirement Specifications. This interval is often decided through a mean probability of failure on demand calculation (PFDavg). Some customers may choose to proof check primarily based on predetermined intervals, corresponding to proof testing sensors every 24 months and final parts each forty eight months instead of counting on the PFDavg calculations.
Automated valve proof tests can be accomplished offline or online. Offline proof checks are normally scheduled during 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 prevent a spurious trip, making it inconvenient and expensive.
But a proof test may also be achieved throughout a course of trip.
According to IEC61511-2 Section A.sixteen.three.1.3, “…shutdowns as a outcome of precise demand on the SIS during operation may be given credit score as proof checks (fully or partial) underneath given conditions…the subsequent deliberate proof check may be skipped.”
These circumstances are
The shutdown documents equivalent data as registered during corresponding proof take a look at.
The shutdown covers all components of the SIS, and if not, the device or SIS subsystem not activated must be tested separately.
The shutdown happens inside a predetermined maximum time window earlier than the subsequent deliberate proof check which might then be canceled
When a course of trip happens, about 60% of a Safety Instrumented Function (SIF) proof test could be thought-about performed. A sample listing of activities performed throughout a proof take a look at, together with these which are carried out throughout a course of journey, is shown in Figure 2. Even with out an automatic valve leak check, knowledge captured by the DVC alone can potentially account for an excellent quantity of proof test protection for an automatic valve.
The precise protection is dependent upon the design of the automated valve, its DVC and its software. The protection is estimated based mostly on the valve degradations, the likelihood of their incidence and the share of those degradations that can be detected by a DVC. Fortunately, data acquired by a DVC throughout a process trip can typically be enough to fulfill a significant a half of the proof take a look at necessities.
If the method trip takes place inside a predetermined most time window, the top person may choose to leverage the process journey as a proof test by finishing steps one by way of 5 in Figure 2, which are normally not accomplished in a process journey. The subsequent scheduled proof test 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 during a process journey 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 more than likely root causes of automated valve degradations — which may result in a course of trip — embrace:
Partial or intermittent sticking of hydraulic or pneumatic system components corresponding to solenoids, pilots, pace controller, and so on., because of moisture, particles or alignment points. This causes a lack of practical margin and makes the valve gradual to open or shut.
Binding, galling or pressure gauge octa of valve seats or associated circulate management trim that restricts or resists valve movement.
Actuator seal degradation attributable to compression, wear or looseness that reduces the stress obtainable to actuate the valve.
Minor harm to the valve obturator plug, disk or ball brought on by system conditions, leakage or particles, including build-up of hydrocarbon merchandise. This causes leakage.
Complete failure of hydraulic control system elements corresponding to solenoids, pilots, velocity controller, and so forth., as a result of moisture, debris or alignment. The valve will fail to open or shut.
All of these 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 compared to when the valve assembly was new signifies potential valve leak. The Fisher DVC6200SIS also repeatedly displays for internal faults in addition to its inputs corresponding to supply stress. When the DVC6200SIS sees the provision strain is simply too low, an alert is generated (Figure 4) so that the top consumer can take corrective actions.
Figure 4. The DVC6200SIS digital valve controller detected almost zero supply strain, so it alerted the management system.
Other process journey advantages
Diagnostic information captured throughout a process trip might reveal valve degradations that will not be detected during a proof take a look at. For instance, diagnostic knowledge captured during a course of journey would possibly indicate a problem with the valve closing utterly in opposition to the total strain of the process, which could be an early indication of valve leakage.
The valve stroke time computed by a DVC throughout a process journey is extra correct underneath real working situations. This leads to a more correct SIF response time (the time from detection of an irregular condition to final component reaching its trip state), which is in comparison with the Process Safety Time to make sure the SIF is still assembly its design requirements.
Although automated valves are very dependable, failures identified in a process trip can provide priceless knowledge to forestall future failures. This info can assist with turnaround planning by making sure the needed elements are available earlier than turnaround even begins to potentially shorten the turnaround schedule.
A course of trip can present the coverage required to delay a valve’s next scheduled proof test, thus saving time and money. Data captured by a DVC may be analyzed to leverage the method journey as a proof check. Even if the top person chooses not to take proof check credit for a course of trip, the valve diagnostic knowledge supplied by the DVC can help plant personnel make proactive valve maintenance selections..