Valve proof check credit for a process journey

A course of trip occurs when the protection instrumented system (SIS) places the process in its secure state by commanding an automated valve (Figure 1) or different tools to its trip state in response to an irregular process situation. In some circumstances, a spurious trip occurs because of a valve malfunction. In that case, the valve assembly is examined beneath real working conditions, which offers an opportunity to seize priceless valve diagnostic data.
However, such diagnostics information can only be captured when automated valves are fitted with digital valve controllers (DVCs). In this article, we’ll talk about how DVCs may help decide the proof take a look at credit score for an automatic valve after a course of journey.
Process trip
A process journey occurs when the SIS detects an abnormal course of situation through sensors corresponding to temperature and stress, 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 forth. The SIS might communicate with the Basic Process Control System (BPCS) to close a valve downstream of the automated valve as a secondary action.
When a course of journey happens, the primary goal is usually to restart the unit or gear that has been shut down as soon as attainable. Plant engineers are busy troubleshooting and correcting issues. Maintenance technicians are busy repairing damaged units. Taking the chance to proof check an automated valve won’t be a high precedence or even an exercise under consideration because of the tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is provided with a DVC, the proof test of the valve could be thought-about performed with diagnostic knowledge captured by the DVC. Also, a comparison of the diagnostic data with the valve’s baseline can help discover valve deficiencies and degradations that might not present up in a proof test.
Figure 2. Proof Test vs Process Trip. Activities performed (such as repairing the device) and information captured by a Fisher DVC6200 SIS digital valve controller during a course of journey meet most of the 12 requirements of a proof take a look at.
Process journeys 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 test is a periodic check performed on SIS sensors, logic solvers and ultimate components — similar to automated valves — to find hidden failures. These failures, if not corrected or eliminated, may impair the flexibility of the SIS to take the process to its protected state when an abnormal course of condition is detected.
A proof check ought to be carried out as per the proof take a look at interval specified within the Safety Requirement Specifications. This interval is usually determined by way of a median likelihood of failure on demand calculation (PFDavg). Some customers might select to proof check based on predetermined intervals, similar to proof testing sensors every 24 months and final components every 48 months instead of counting on the PFDavg calculations.
Automated valve proof tests could be carried out offline or on-line. Offline proof tests are often scheduled during a turnaround, when the process is completely shut down.
Proof testing an automatic valve online normally requires a unit or tools to be shut down, or the valve to be bypassed to stop a spurious journey, making it inconvenient and expensive.
But a proof take a look at can be completed during a course of journey.
According to IEC61511-2 Section A., “…shutdowns due to precise demand on the SIS throughout operation could additionally be given credit as proof exams (fully or partial) underneath given conditions…the subsequent planned proof check may be skipped.”
These situations are
The shutdown documents equal info as registered during corresponding proof check.
The shutdown covers all components of the SIS, and if not, the system or SIS subsystem not activated needs to be examined separately.
The shutdown happens within a predetermined maximum time window earlier than the subsequent deliberate proof test which can then be canceled
When a course of trip occurs, about 60% of a Safety Instrumented Function (SIF) proof check can be thought of carried out. pressure gauge แบบ น้ำมัน of actions carried out during a proof take a look at, together with these which may be carried out throughout a course of journey, is proven in Figure 2. Even with out an automatic valve leak check, information captured by the DVC alone can doubtlessly account for a great amount of proof take a look at protection for an automated valve.
The precise coverage depends on the design of the automated valve, its DVC and its utility. เกจวัดแรงดันถังแก๊ส is estimated based on the valve degradations, the probability of their incidence and the percentage of these degradations that can be detected by a DVC. Fortunately, data acquired by a DVC throughout a course of trip can usually be enough to fulfill a serious part of the proof take a look at requirements.
If the process trip takes place within a predetermined most time window, the top user might select to leverage the method trip as a proof test by finishing steps one by way of 5 in Figure 2, which are normally not accomplished in a course of trip. The next scheduled proof check can then be canceled. According to the Norwegian Oil Industry Association, the maximum time window must be the last half of the current proof take a look at interval.
Figure three. Data throughout a course of trip is captured by the Fisher DVC6200 SIS digital valve controller and made available for analysis.
Common causes of valve failure
According to ISA Technical Report 96.05.01, the more than likely root causes of automated valve degradations — which might result in a course of trip — embrace:
Partial or intermittent sticking of hydraulic or pneumatic system elements such as solenoids, pilots, velocity controller, and so on., due to moisture, debris or alignment issues. This causes a loss of functional margin and makes the valve slow to open or shut.
Binding, galling or other degradation of valve seats or related flow control trim that restricts or resists valve movement.
Actuator seal degradation attributable to compression, put on or looseness that reduces the stress obtainable to actuate the valve.
Minor damage to the valve obturator plug, disk or ball attributable to system situations, leakage or particles, including build-up of hydrocarbon merchandise. This causes leakage.
Complete failure of hydraulic management system parts similar to solenoids, pilots, velocity controller, and so forth., because of moisture, debris or alignment. The valve will fail to open or shut.
All of those situations may be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For example, a valve that’s stuck open will exhibit itself as a rise in breakout drive. Loss of seat load in comparability with when the valve assembly was new indicates potential valve leak. The Fisher DVC6200SIS additionally repeatedly monitors for internal faults in addition to its inputs corresponding to provide pressure. When the DVC6200SIS sees the provision strain is merely too low, an alert is generated (Figure 4) in order that the end consumer can take corrective actions.
Figure four. The DVC6200SIS digital valve controller detected almost zero supply stress, so it alerted the control system.
Other course of trip benefits
Diagnostic knowledge captured during a course of journey may reveal valve degradations that is in all probability not detected during a proof test. For example, diagnostic knowledge captured during a course of trip may indicate a problem with the valve closing completely against the full strain of the method, which can be an early indication of valve leakage.
The valve stroke time computed by a DVC throughout a course of trip is extra correct under real working circumstances. This ends in a extra accurate SIF response time (the time from detection of an abnormal situation to final factor 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 valves are very dependable, failures recognized in a process journey can provide priceless data to forestall future failures. This info can assist with turnaround planning by making sure the wanted parts are available earlier than turnaround even begins to probably shorten the turnaround schedule.
A process 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 can be analyzed to leverage the method trip as a proof check. Even if the end consumer chooses not to take proof check credits for a process journey, the valve diagnostic data provided by the DVC might help plant personnel make proactive valve upkeep choices..

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