We recommend three essential steps that extend beyond just safety integrity level (SIL) certification and the probability of failure on demand (PFD). Moreover, these steps help in evaluating and selecting a safety valve to ensure that your plant will be safe.
The most important step in choosing an ESD valve is to use the application-based valve selection process. This step has not been observed often enough in recent years. Instead, functional safety calculations have attracted excessive attention. There are a number of different ESD valve types available, each designed for a specific application. Correct ESD valve selection needs to consider valve type, pressure class, temperature, materials and other application details. If the valve does not match the application in question, systematic valve failures will occur. A typical systematic error that may affect the valve’s operation in a real process is incorrect seat selection.
A rough categorization includes seat types that have contact between the sealing elements all along the stroke and seat types that lose contact while operating. In polymer service, for example, the selection of seat type that loses contact leads to polymer accumulation between the surfaces. The valve does not fully work or starts to jam, which is a disaster for the ESD valve. The problem can be solved using a seat design that keeps contact between the seat and trim. Also a closed-seat design will solve the problem of seat jamming against open types of seats where impurities can enter critical areas of the seat.
Functional safety – take full advantage
Even with an incorrect valve seat type, it would be possible to calculate a high SIL and a good PFD value. But these calculations alone do not guarantee plant safety. Following the international safety standards for SIS, it is typically required to have an appropriate SIL that is based on the hazard and operability analysis (HAZOP) and safety integrity level assessment. The final elements, such as ESD valves, are a critical part of the safety loop. If they are not working properly, the safety instrumented function is not available. In recent years, there have been a number of announcements from manufacturers publishing SIL certification and advertising SIL 3 valves. The certification is a good document to prove that the valve as a component is capable of working in a SIL 3 loop, but it does not guarantee that the whole assembly is SIL 3. To know the final safety system capability, the complete final element assembly must be calculated, taking into account the SIL certifications and PFD values of each component, such as the valve body, actuator and intelligent safety solenoid.
Testing and maintenance
ESD valves are not so-called “install and forget” devices. Service and testing make up a large part of the life cycle of these valves. The only way to maintain the safety integrity is to have a test plan and follow it. ESD valve testing is typically categorized into two different types – a partial stroke test (PST) that is done while the plant is operating without causing any harm to the process, and a proof test that takes place during shutdowns. During the PST, the valve is moved just few percents of the full stroke to analyze its condition. The advantage of carrying out the PST is that the full stroke test then does not have to be done very often, whereas the life cycle of the devices is extended and the proof test interval can match the plant shutdown period. A great tool to determine the testing intervals in a simple way is Nelprof SIL.
Plant safety in one – two – three
To ensure plant safety, knowing the PFD values and having SIL certification alone is not sufficient when it comes to ESD valves. Step one is to consider the application-based valve selection requirements. This is the most important starting point for choosing the right valve. If the valve is capable of working in the specific application, the full advantage of the functional safety calculations can then be taken as step two. These calculations need to be done for the entire final element assembly. Then testing and maintenance of the valves should be carried out systematically as step three to make sure the valves work when needed and ensure a plant’s safety throughout its entire life cycle.
The full article was previously published in Hydrocarbon Engineering magazine, June 2015 issue, as ’Am I safe now?’. Please read it here