Valve proof take a look at credit score for a process trip

A course of trip occurs when the safety instrumented system (SIS) locations the process in its protected state by commanding an automatic valve (Figure 1) or other gear to its trip state in response to an abnormal process situation. In some instances, a spurious journey occurs because of a valve malfunction. In that case, the valve assembly is examined under actual working situations, which provides a chance to seize useful valve diagnostic data.
However, such diagnostics knowledge can only be captured when automated valves are fitted with digital valve controllers (DVCs). In this article, we’ll discuss how DVCs may help decide the proof take a look at credit score for an automatic valve after a process journey.
Process journey
A process journey happens when the SIS detects an irregular course of situation via sensors corresponding to temperature and strain, executes the logic and locations the method in its secure state by tripping the final elements similar to closing an automated valve, stopping a pump, and so on. The SIS might communicate with the Basic Process Control System (BPCS) to close a valve downstream of the automated valve as a secondary motion.
When a process trip happens, the main objective is often to restart the unit or equipment that has been shut down as soon as attainable. Plant engineers are busy troubleshooting and correcting problems. Maintenance technicians are busy repairing broken gadgets. Taking the opportunity to proof check an automatic valve is not going to be a high priority or even an activity under consideration because of the tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is supplied with a DVC, the proof test of the valve can be considered carried out with diagnostic information captured by the DVC. Also, a comparison of the diagnostic data with the valve’s baseline might help discover valve deficiencies and degradations that gained’t show up in a proof check.
Figure 2. Proof Test vs Process Trip. Activities performed (such as repairing the device) and data captured by a Fisher DVC6200 SIS digital valve controller throughout a course of journey meet a lot of the 12 requirements of a proof take a look at.
Process journeys versus proof checks
How can proof test credits be claimed for an automatic valve that has failed and triggered a process trip? A proof check is a periodic test performed on SIS sensors, logic solvers and ultimate elements — such as automated valves — to find hidden failures. These failures, if not corrected or removed, might impair the flexibility of the SIS to take the process to its protected state when an irregular course of condition is detected.
A proof test must be carried out as per the proof take a look at interval specified in the Safety Requirement Specifications. This interval is often decided by way of a mean chance of failure on demand calculation (PFDavg). Some users might choose to proof take a look at based mostly on predetermined intervals, such as proof testing sensors every 24 months and final parts every 48 months as a substitute of relying on the PFDavg calculations.
Automated valve proof checks may be accomplished offline or online. Offline proof tests are usually scheduled during a turnaround, when the method is completely shut down.
Proof testing an automatic valve online 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 check can be completed throughout a process trip.
According to IEC61511-2 Section A.sixteen.3.1.3, “…shutdowns due to precise demand on the SIS during operation may be given credit score as proof tests (fully or partial) under given conditions…the subsequent planned proof check may be skipped.”
These conditions are
The shutdown documents equivalent data as registered throughout corresponding proof take a look at.
The shutdown covers all parts of the SIS, and if not, the device or SIS subsystem not activated must be tested separately.
The shutdown happens within a predetermined maximum time window before the subsequent deliberate proof take a look at which may then be canceled
When a process journey occurs, about 60% of a Safety Instrumented Function (SIF) proof check can be thought of carried out. A pattern list of actions carried out throughout a proof take a look at, along with these that are performed throughout a course of journey, is proven in Figure 2. Even with out an automated valve leak take a look at, knowledge captured by the DVC alone can probably account for a great quantity of proof take a look at protection for an automatic valve.
The exact coverage is determined by the design of the automated valve, its DVC and its utility. The coverage is estimated primarily based on the valve degradations, the probability of their incidence and the percentage of those degradations that may be detected by a DVC. Fortunately, knowledge acquired by a DVC during a course of trip can often be enough to satisfy a significant part of the proof test requirements.
If the method trip takes place inside a predetermined most time window, the end person could select to leverage the method journey as a proof take a look at by finishing steps one through 5 in Figure 2, which are normally not accomplished in a course of journey. The subsequent scheduled proof take a look at can then be canceled. According to the Norwegian Oil Industry Association, the maximum time window should be the last half of the present proof take a look at interval.
Figure 3. Data during a course of 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 likely root causes of automated valve degradations — which may lead to a process journey — embody:
Partial or intermittent sticking of hydraulic or pneumatic system parts such as solenoids, pilots, velocity controller, etc., due to moisture, particles or alignment issues. This causes a loss of useful margin and makes the valve gradual to open or close.
Binding, galling or different degradation of valve seats or associated flow management trim that restricts or resists valve motion.
เครื่องมือความดัน caused by compression, put on or looseness that reduces the strain available to actuate the valve.
Minor damage to the valve obturator plug, disk or ball attributable to system conditions, leakage or particles, together with build-up of hydrocarbon products. This causes leakage.
Complete failure of hydraulic control system elements such as solenoids, pilots, speed controller, etc., due to moisture, debris 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 example, a valve that’s caught open will exhibit itself as a rise in breakout force. Loss of seat load compared to when the valve meeting was new signifies potential valve leak. The Fisher DVC6200SIS also constantly displays for inner faults in addition to its inputs corresponding to provide pressure. When the DVC6200SIS sees the availability strain is 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 almost zero supply stress, so it alerted the control system.
Other course of trip benefits
Diagnostic data captured throughout a process journey may reveal valve degradations that is most likely not detected during a proof take a look at. For instance, diagnostic knowledge captured during a process trip may indicate a problem with the valve closing completely in opposition to the complete strain of the method, which may be an early indication of valve leakage.
The valve stroke time computed by a DVC during a process trip is extra correct beneath actual operating conditions. This leads to 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 ensure the SIF remains to be meeting its design necessities.
Although automated valves are very dependable, failures identified in a course of trip can present useful knowledge to forestall future failures. This info can help with turnaround planning by ensuring the wanted parts can be found earlier than turnaround even begins to doubtlessly shorten the turnaround schedule.
A course of journey can present the coverage required to delay a valve’s next scheduled proof test, thus saving money and time. Data captured by a DVC may be analyzed to leverage the process trip as a proof take a look at. Even if the end consumer chooses to not take proof take a look at credits for a process journey, the valve diagnostic data supplied by the DVC can help plant personnel make proactive valve upkeep choices..


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