Valve proof take a look at credit score for a course of journey

A course of journey happens when the safety instrumented system (SIS) locations the method in its protected state by commanding an automated valve (Figure 1) or other equipment to its trip state in response to an abnormal course of condition. In some instances, a spurious journey happens because of a valve malfunction. In that case, the valve assembly is examined beneath actual operating circumstances, which provides a chance to seize useful valve diagnostic information.
However, such diagnostics data can solely be captured when automated valves are fitted with digital valve controllers (DVCs). In this text, we’ll discuss how DVCs can help determine the proof take a look at credit score for an automated valve after a course of journey.
Process trip
A course of journey happens when the SIS detects an abnormal process condition by way of sensors similar to temperature and pressure, executes the logic and locations the process in its protected state by tripping the ultimate components such as closing an automated 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 action.
When a process trip happens, the primary goal 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 damaged devices. Taking the chance to proof check an automated valve is not going to be a high precedence or even an activity under consideration as a result of tight schedule after an unplanned shutdown.
Fortunately, if the automated valve is supplied with a DVC, the proof check of the valve could be thought-about performed with diagnostic information captured by the DVC. Also, เกจวัดแรงดันแบบแห้ง of the diagnostic information with the valve’s baseline might help discover valve deficiencies and degradations that may not present up in a proof take a look at.
Figure 2. 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 course of journey meet most of the 12 requirements of a proof check.
Process trips versus proof tests
How can proof test credit be claimed for an automatic valve that has failed and brought on a course of trip? A proof test is a periodic test carried out on SIS sensors, logic solvers and final parts — similar to automated valves — to discover hidden failures. These failures, if not corrected or removed, may impair the ability of the SIS to take the method to its protected state when an abnormal course of situation is detected.
A proof check ought to be carried out as per the proof test interval specified within the Safety Requirement Specifications. This interval is usually determined through an average likelihood of failure on demand calculation (PFDavg). Some users may choose to proof take a look at primarily based on predetermined intervals, corresponding to proof testing sensors every 24 months and final elements every 48 months as a substitute of counting on the PFDavg calculations.
Automated valve proof checks can be done offline or online. Offline proof exams are often scheduled during a turnaround, when the process is totally shut down.
Proof testing an automatic valve on-line often requires a unit or tools to be shut down, or the valve to be bypassed to stop a spurious journey, making it inconvenient and costly.
But a proof check may also be accomplished throughout a process trip.
According to IEC61511-2 Section A.sixteen.three.1.3, “…shutdowns due to actual demand on the SIS during operation may be given credit as proof tests (fully or partial) underneath given conditions…the subsequent deliberate proof test may be skipped.”
These situations are
The shutdown paperwork equal information as registered throughout corresponding proof take a look at.
The shutdown covers all elements of the SIS, and if not, the system or SIS subsystem not activated must be examined separately.
The shutdown happens within a predetermined maximum time window before the subsequent planned proof take a look at which may then be canceled
When a course of journey happens, about 60% of a Safety Instrumented Function (SIF) proof take a look at can be considered performed. A sample record of activities performed during a proof check, along with those which are carried out during a process journey, is shown in Figure 2. Even without an automatic valve leak check, data captured by the DVC alone can potentially account for an excellent quantity of proof take a look at coverage for an automatic valve.
The actual coverage is dependent upon the design of the automated valve, its DVC and its application. The coverage is estimated based on the valve degradations, the probability of their occurrence and the percentage of these degradations that can be detected by a DVC. Fortunately, information acquired by a DVC during a course of journey can usually be sufficient to fulfill a major a part of the proof check requirements.
If the process journey takes place inside a predetermined most time window, the top user could select to leverage the method journey as a proof test by completing steps one via 5 in Figure 2, that are normally not completed in a process trip. The next scheduled proof take a look at can then be canceled. According to the Norwegian Oil Industry Association, the utmost time window must be the last half of the current proof test 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 ninety six.05.01, the most likely root causes of automated valve degradations — which can lead to a process journey — embrace:
Partial or intermittent sticking of hydraulic or pneumatic system elements corresponding to solenoids, pilots, velocity controller, etc., as a end result of moisture, debris or alignment points. This causes a lack of useful margin and makes the valve gradual to open or shut.
Binding, galling or other degradation of valve seats or related circulate control trim that restricts or resists valve motion.
Actuator seal degradation attributable to compression, put on or looseness that reduces the pressure out there to actuate the valve.
Minor damage to the valve obturator plug, disk or ball attributable to system situations, leakage or debris, including build-up of hydrocarbon merchandise. This causes leakage.
Complete failure of hydraulic management system components such as solenoids, pilots, velocity controller, and so on., because of moisture, debris or alignment. The valve will fail to open or shut.
All of these situations can be detected by a Fisher DVC6200SIS digital valve controller (Figure 3).
For example, a valve that’s caught open will exhibit itself as an increase in breakout drive. Loss of seat load compared to when the valve meeting was new signifies potential valve leak. The Fisher DVC6200SIS also repeatedly monitors for inner faults in addition to its inputs such as provide pressure. When the DVC6200SIS sees the availability stress is simply too low, an alert is generated (Figure 4) so that the end person 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 advantages
Diagnostic data captured throughout a course of journey might reveal valve degradations that may not be detected during a proof test. For example, diagnostic information captured throughout a course of journey might indicate a problem with the valve closing completely in opposition to the total stress of the process, which may be an early indication of valve leakage.
The valve stroke time computed by a DVC throughout a process trip is more accurate beneath actual operating conditions. This ends in a extra accurate SIF response time (the time from detection of an abnormal situation to final element reaching its journey state), which is in comparison with the Process Safety Time to ensure the SIF continues to be meeting its design necessities.
Although automated valves are very reliable, failures identified in a process journey can present valuable knowledge to stop future failures. This data can help with turnaround planning by making sure the needed elements are available before 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 take a look at, thus saving time and money. Data captured by a DVC can be analyzed to leverage the process journey as a proof check. Even if the top consumer chooses to not take proof take a look at credits for a course of trip, the valve diagnostic data offered by the DVC can help plant personnel make proactive valve upkeep choices..

Scroll to Top