A course of trip happens when the security instrumented system (SIS) places the process in its safe state by commanding an automated valve (Figure 1) or other tools to its journey state in response to an irregular course of condition. In some instances, a spurious trip happens due to a valve malfunction. In that case, the valve meeting is examined underneath real operating situations, which provides a chance to capture valuable valve diagnostic knowledge.
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 might help determine the proof check credit for an automated valve after a course of journey.
Process trip
A course of journey occurs when the SIS detects an irregular course of condition by way of sensors corresponding to temperature and stress, executes the logic and places the process in its secure state by tripping the ultimate parts corresponding to closing an automatic valve, stopping a pump, etc. The SIS may communicate with the Basic Process Control System (BPCS) to shut a valve downstream of the automated valve as a secondary action.
When a course of trip happens, the main goal is often to restart the unit or gear that has been shut down as soon as attainable. Plant engineers are busy troubleshooting and correcting problems. Maintenance technicians are busy repairing damaged gadgets. Taking the opportunity to proof take a look at an automatic valve won’t be a top priority and even an exercise into account as a outcome of tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is supplied with a DVC, the proof check of the valve may be considered performed with diagnostic data captured by the DVC. Also, a comparability of the diagnostic knowledge with the valve’s baseline may help uncover valve deficiencies and degradations that might not present up in a proof take a look at.
nuova fima pressure gauge ราคา . Proof Test vs Process Trip. Activities carried out (such as repairing the device) and knowledge captured by a Fisher DVC6200 SIS digital valve controller during a process trip meet a lot of the 12 requirements of a proof test.
Process trips versus proof checks
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 take a look at is a periodic take a look at carried out on SIS sensors, logic solvers and ultimate components — corresponding to automated valves — to discover hidden failures. These failures, if not corrected or removed, may impair the flexibility of the SIS to take the process to its safe state when an abnormal course of situation is detected.
A proof check ought to be carried out as per the proof check interval specified within the Safety Requirement Specifications. This interval is often decided by way of a median probability of failure on demand calculation (PFDavg). Some users may choose to proof check based mostly on predetermined intervals, such as proof testing sensors every 24 months and ultimate elements each forty eight months instead of counting on the PFDavg calculations.
Automated valve proof tests could be done offline or on-line. Offline proof checks are usually scheduled during a turnaround, when the process is completely shut down.
Proof testing an automated valve online often requires a unit or equipment 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 additionally be completed during a course of journey.
According to IEC61511-2 Section A., “…shutdowns because of precise demand on the SIS during operation may be given credit as proof tests (fully or partial) under given conditions…the next planned proof test could also be skipped.”
These circumstances are
The shutdown paperwork equal info as registered during corresponding proof test.
The shutdown covers all parts of the SIS, and if not, the system or SIS subsystem not activated must be examined individually.
The shutdown occurs inside a predetermined maximum time window earlier than the subsequent deliberate proof check which might then be canceled
When a process journey happens, about 60% of a Safety Instrumented Function (SIF) proof take a look at can be considered carried out. A pattern list of activities carried out during a proof take a look at, along with these which may be carried out throughout a course of trip, is proven in Figure 2. Even without an automatic valve leak check, information captured by the DVC alone can probably account for a great amount of proof take a look at protection for an automatic valve.
The precise protection is decided by the design of the automated valve, its DVC and its application. The coverage is estimated based mostly on the valve degradations, the likelihood of their incidence and the proportion of those degradations that might be detected by a DVC. Fortunately, information acquired by a DVC during a process trip can typically be enough to satisfy a major a half of the proof check requirements.
If the process journey takes place inside a predetermined maximum time window, the end user might choose to leverage the process trip as a proof test by finishing steps one by way of 5 in Figure 2, which are normally not completed in a course of trip. The subsequent scheduled proof test 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 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 out there for analysis.
Common causes of valve failure
According to ISA Technical Report ninety six.05.01, the most probably root causes of automated valve degradations — which may lead to a course of journey — embody:
Partial or intermittent sticking of hydraulic or pneumatic system parts similar to solenoids, pilots, velocity controller, and so forth., due to moisture, particles or alignment issues. This causes a loss of useful margin and makes the valve slow to open or shut.
Binding, galling or other degradation of valve seats or related move management trim that restricts or resists valve motion.
Actuator seal degradation attributable to compression, wear 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 corresponding to solenoids, pilots, pace controller, etc., due to moisture, particles or alignment. The valve will fail to open or close.
All of those conditions can be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For example, a valve that is caught open will exhibit itself as a rise in breakout drive. Loss of seat load in comparability with when the valve meeting was new signifies potential valve leak. The Fisher DVC6200SIS also repeatedly monitors for inside faults as well as its inputs similar to supply pressure. When the DVC6200SIS sees the provision strain is too low, an alert is generated (Figure 4) in order that the top person can take corrective actions.
Figure 4. The DVC6200SIS digital valve controller detected virtually zero supply pressure, so it alerted the control system.
Other course of trip advantages
Diagnostic knowledge captured during a process journey may reveal valve degradations that may not be detected during a proof check. For example, diagnostic knowledge captured during a course of trip may indicate an issue with the valve closing completely towards the total pressure of the method, which can be an early indication of valve leakage.
The valve stroke time computed by a DVC during a course of journey is extra accurate under real operating conditions. This ends in a extra correct SIF response time (the time from detection of an abnormal situation to ultimate factor reaching its journey state), which is compared to the Process Safety Time to ensure the SIF remains to be assembly its design requirements.
Although automated valves are very dependable, failures identified in a course of trip can present priceless knowledge to stop future failures. This information can assist with turnaround planning by making sure the needed components can be found 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 could be analyzed to leverage the process trip as a proof check. Even if the tip person chooses to not take proof test credits for a process journey, the valve diagnostic data supplied by the DVC might help plant personnel make proactive valve upkeep decisions..