The image of a hapless home owner trying to fix the bathtub or kitchen sink faucet without turning off the main water supply is a well-used comedy device. The lesson: turn the pressure off at the source before starting disassembly to avoid the spray and mess. There are similar situations in chemical process plants, but most are alleviated with a pressure relief valve (PRV).
During an over pressure incident, the safety system may depend on a PRV or rupture disk blowing, rather than shutting off the pressure upstream. When the relief device opens, it releases pressure, but also product which has to be handled by a flare or other recovery mechanism.
Many production units can be equipped with an overpressure protection system (OPS) designed to shut off the pressure at the source before the relief mechanism opens, avoiding the release entirely. Understanding how this approach works and how to apply it is the theme of a new article by Erik Mathiason and Afton Coleman in the April 2019 issue of Chemical Engineering titled Prevent Product Release with Pressure Protection Systems.
In some situations, it may be more practical to change the approach from releasing the pressure to cutting it off upstream at the source as quickly as possible to minimize or prevent product release through a PRV. Although this seems like an obvious solution, it happens less than one would expect; product is still lost with all the associated costs and inconveniences. It is important to consider alternate solutions to address safety and production challenges.
As the authors point out, an OPS is a basic safety instrumented function (SIF) and has to be treated as part of the larger safety instrumented system under the appropriate standards, including IEC 61511 with its safety lifecycle approach. It should also be designed by experts. Since it is a SIF, it has to have the required elements.
Let’s focus on the OPS function itself. It is a basic SIF designed to shut down flow in the event of an excessive pressure incident. Since it is a SIF, it has three basic parts: a sensor, a logic solver and a final element. These can be simple or highly sophisticated, but any components used must be certified to operate in a SIS context, with any uncertified components justified as proven-in-use.
The article goes into more detail as to how these are normally setup and how they function during an actual incident, so it’s well worth thorough study. One of the key areas of discussion is the sophistication of the components themselves, which can range from very basic to those with advanced diagnostic capabilities which can help improve the effectiveness of the SIF. Often these diagnostic functions begin with the pressure transmitters, such as Emerson’s Rosemount 3051S Coplanar Pressure Transmitter. It is available with SIL 2/3 certification.
The sensor can be a pressure transmitter, which is safety-certified and able to provide diagnostic information in addition to a basic pressure measurement. A pressure transmitter with advanced capabilities can evaluate the complete measurement system, extending diagnostics to capture over-pressure scenarios and improve reaction time. Speed is critical because the OPS must respond immediately to initiate the pressure protection system and protect the downstream equipment. This valuable information can be passed on to the larger process-control and maintenance systems without interfering with the basic safety function.
The same concepts can apply to the logic solver, valves and valve controllers—basically all the elements of the system—each of which can have these types of advanced capabilities to help improve system performance. The article explains functions, such as partial stroke testing of the valves, which a digital valve controller can perform, and there are many more possibilities.
You can find more information like this and meet with other people looking at the same kinds of situations in the Emerson Exchange 365 community. It’s a place where you can communicate and exchange information with experts and peers in all sorts of industries around the world. Look for the Pressure Group, Chemical Group and other specialty areas for suggestions and answers.
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