Valve proof check credit for a process journey

A course of journey occurs when the security instrumented system (SIS) locations the process in its secure state by commanding an automatic valve (Figure 1) or other gear to its trip state in response to an irregular course of situation. In some circumstances, a spurious trip occurs because of a valve malfunction. In that case, the valve assembly is tested underneath actual working circumstances, which offers 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 talk about how DVCs can help determine the proof take a look at credit score for an automated valve after a process trip.
Process trip
A process journey occurs when the SIS detects an irregular course of situation through sensors similar to temperature and strain, executes the logic and places the method in its secure state by tripping the final components similar to closing an automatic valve, stopping a pump, and so on. 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 process journey happens, the primary aim is normally to restart the unit or tools that has been shut down as soon as possible. Plant engineers are busy troubleshooting and correcting problems. Maintenance technicians are busy repairing broken devices. Taking the opportunity to proof take a look at an automated valve is not going to be a top precedence or even an activity into account 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 can be considered performed 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 show up in a proof check.
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 course of trip meet a lot of the 12 requirements of a proof test.
Process trips versus proof checks
How can proof check credits 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 final elements — similar to automated valves — to find hidden failures. These failures, if not corrected or removed, might impair the ability of the SIS to take the process to its secure state when an abnormal process situation is detected.
A proof test ought to be carried out as per the proof test interval specified in the Safety Requirement Specifications. This interval is often decided by way of a median likelihood of failure on demand calculation (PFDavg). Some customers may select to proof test based on predetermined intervals, corresponding to proof testing sensors each 24 months and final parts each 48 months as a substitute of relying on the PFDavg calculations.
Automated valve proof exams could be carried out offline or online. Offline proof checks are often scheduled during a turnaround, when the process is completely shut down.
Proof testing an automatic valve online normally requires a unit or gear to be shut down, or the valve to be bypassed to prevent a spurious journey, making it inconvenient and expensive.
But a proof take a look at may additionally be completed throughout a course of trip.
According to IEC61511-2 Section A.16.three.1.3, “…shutdowns due to precise demand on the SIS throughout operation may be given credit score as proof checks (fully or partial) underneath given conditions…the subsequent planned proof check may be skipped.”
These situations are
The shutdown paperwork equal info as registered throughout corresponding proof take a look at.
The shutdown covers all components of the SIS, and if not, the device or SIS subsystem not activated needs to be examined individually.
The shutdown occurs inside a predetermined maximum time window before the following deliberate proof test which can then be canceled
When a process trip happens, about 60% of a Safety Instrumented Function (SIF) proof take a look at can be thought of carried out. A pattern record of actions carried out during a proof check, together with those which may be carried out throughout a course of trip, is proven in Figure 2. Even with out an automated valve leak take a look at, data captured by the DVC alone can probably account for a good amount of proof check coverage for an automated valve.
The precise coverage depends on the design of the automated valve, its DVC and its application. The coverage is estimated based mostly on the valve degradations, the chance of their incidence and the proportion of those degradations that can be detected by a DVC. Fortunately, knowledge acquired by a DVC throughout a process journey can often be sufficient to fulfill a major a part of the proof check necessities.
If the method journey takes place within a predetermined maximum time window, the end consumer may select to leverage the method journey as a proof take a look at by completing steps one via 5 in Figure 2, which are often not completed in a process journey. The subsequent scheduled proof check can then be canceled. According to the Norwegian Oil Industry Association, the utmost time window ought to be the last half of the present proof test interval.
Figure three. Data during a course of trip 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 96.05.01, the most likely root causes of automated valve degradations — which might lead to a course of trip — include:
Partial or intermittent sticking of hydraulic or pneumatic system elements corresponding to solenoids, pilots, pace controller, and so on., because of moisture, particles or alignment points. This causes a loss of functional margin and makes the valve slow to open or close.
Binding, galling or different degradation of valve seats or related move control trim that restricts or resists valve motion.
Actuator seal degradation attributable to compression, wear or looseness that reduces the strain out there to actuate the valve.
Minor damage to the valve obturator plug, disk or ball attributable to system circumstances, leakage or particles, including build-up of hydrocarbon merchandise. This causes leakage.
Complete failure of hydraulic management system parts similar to solenoids, pilots, pace controller, and so forth., as a outcome of moisture, particles or alignment. เกจ์วัดแรงดัน 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 caught open will exhibit itself as an increase in breakout force. Loss of seat load in comparison with when the valve meeting was new indicates potential valve leak. The Fisher DVC6200SIS also repeatedly displays for inner faults in addition to its inputs corresponding to provide stress. When the DVC6200SIS sees the availability strain is merely too low, an alert is generated (Figure 4) so that the tip user can take corrective actions.
Figure four. The DVC6200SIS digital valve controller detected almost zero supply strain, so it alerted the management system.
Other course of journey advantages
Diagnostic data captured during a course of trip may reveal valve degradations that will not be detected during a proof check. For instance, diagnostic information captured during a process journey would possibly point out an issue with the valve closing completely against 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 journey is extra accurate underneath real operating circumstances. This ends in a extra accurate SIF response time (the time from detection of an abnormal situation to final component reaching its journey state), which is compared to the Process Safety Time to make sure the SIF is still assembly its design requirements.
Although automated valves are very reliable, failures identified in a process journey can provide priceless information to forestall future failures. This information can help with turnaround planning by ensuring the wanted elements are available earlier than turnaround even begins to potentially shorten the turnaround schedule.
A course of trip can present the protection required to delay a valve’s next scheduled proof take a look at, thus saving money and time. Data captured by a DVC may be analyzed to leverage the process journey as a proof test. Even if the tip user chooses to not take proof test credits for a process journey, the valve diagnostic information offered by the DVC may help plant personnel make proactive valve upkeep decisions..

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